Smalley, R., I. W. D. Dalziel, M. G. Bevis, E. Kendrick, D. S. Stamps, E. C. King, F. W. Taylor, E. Lauria, A. Zakrajsek, and H. Parra, Scotia arc kinematics from GPS geodesy, Geophys. Res. Lett., 34, L21308, 2007, 6 citations, doi:10.1029/2007GL031699, #1892 
GPS crustal velocity data from the Scotia and South Sandwich plates, transform azimuths, spreading data, and an updated earthquake slip vector catalog provide the first Scotia and South Sandwich plate Euler vector estimates not dependent on closure as the GPS data tie them to the global plate circuit. Neither the GPS data, which sample limited portions of the plates, nor the geologic data, which are not tied to the global spreading circuit, are sufficient individually to define the Euler vectors. As Scotia plate GPS measurements do not sample the stable plate interior, plate boundary deformation field modeling is necessary for Euler vector estimation. Our South America-Antarctic and Scotia-South Sandwich Euler pole estimates agree with previous estimates from either GPS or geologic data. Our South America-Scotia Euler vector, however, is significantly different and near the South America-Antarctic Euler vector producing an approximately coaxial motion of Scotia between South America and Antarctica.
Dalziel, I. W. D., On the extent of the active West Antarctic rift system, Terra Antartica, 12, 193-202, 2006, #1717
Gose, W. A., R. E. Hanson, I. W. D. Dalziel, J. A. Pancake, and E. K. Seidel, Paleomagnetism of the 1.1 Ga Umkondo large igneous province in southern Africa, J. Geophys. Res., 111, B09101, 2006, 11 citations, doi:10.1029/2005JB003897, #1837
The Umkondo dolerites are present over a wide area in the Kalahari craton, southern Africa. Thirty-nine sampling sites in Botswana and South Africa yielded tightly grouped paleomagnetic directions due south with shallow inclinations and three sites of opposite polarity. The dolerites have U-Pb single-crystal baddeleyite or zircon crystallization ages of 1112 ± 0.5 to 1108 ± 0.9 Ma. These results can be combined with published data from 39 additional Umkondo sites and 33 sites in the Grunehogna Province of Antarctica after restoring East Antarctica to its position next to southern Africa. Grouping the sites geographically yields 10 site mean poles with mean at 64.0°N, 38.8°E, A95 = 3.7°. This Umkondo pole can be correlated with Keweenawan poles from Laurentia. Because both sets of poles are precisely of the same age as well as predominantly of one polarity, the relative orientation of the two cratons within the Rodinia supercontinent is fixed. This implies that the Namaqua-Natal-Maud belt which rims the southern part of the Kalahari craton, faced away from Laurentia. The Umkondo pole combined with published poles suggest that the Kalahari craton remained distinctly south of the Laurentian craton between 1.1 and 1.0 Ga, making it highly unlikely that the two cratons collided.
Hanson, R. E., R. E. Harmer, T. G. Blenkinsop, D. S. Bullen, I. W. D. Dalziel, W. A. Gose, R. P. Hall, A. B. Kampunzu, R. M. Key, J. Mukwakwami, H. Munyanyiwa, J. A. Pancake, E. K. Seidel, and S. E. Ward, Mesoproterozoic intraplate magmatism in the Kalahari Craton: A review, J. African Earth Sci., 46, 141-167, 2006, 16 citations, doi:10.1016/j.jafrearsci.2006.01.016, #1716
The Kalahari Craton was initially stabilized following cessation of Palaeoproterozoic orogenesis in southern Africa at ca. 1.8 Ga. Subsequent Mesoproterozoic intraplate magmatism at ca. 1.4ââ¬â1.35 Ga formed a series of alkaline and carbonatitic complexes in the southern part of the craton. Original volcanic structures are partly preserved in some of the complexes, and a variety of intrusive rocks (e.g., quartz syenite, nepheline syenite, pyroxenite, ijolite, carbonatite) are present. The Premier kimberlite cluster was emplaced in the same region at ca. 1.2 Ga, but available geochronology indicates that the main alkaline magmatism preceded 1.2ââ¬â1.0 Ga orogenesis in the Namaquaââ¬âNatalââ¬âMaud Belt along the southern craton margin. Another, more extensive intraplate magmatic event at ca. 1.1 Ga formed the Umkondo Igneous Province, which is recognized over an area of 2.0 Ãâ 106 km2 on the Kalahari Craton, including a detached fragment now located in Antarctica. Much of the province comprises high-level mafic intrusions, but erosional remnants of basalt lava piles and bimodal basalt/rhyolite assemblages are also present. Most of the mafic rocks are continental tholeiites, but trace-element geochemistry reveals distinct subgroups that cannot be related by crustal-level assimilation/fractional crystallization processes or by partial melting of a uniform mantle source. Geochronological and palaeomagnetic data indicate that enormous volumes of tholeiitic magma were emplaced within the province in a narrow time frame at ca. 1112ââ¬â1106 Ma, which is inferred to record uprise of a mantle plume behind the Namaquaââ¬âNatalââ¬âMaud Belt.
Hanson, R. E., J. L. Crowley, S. A. Bowring, J. Ramezani, W. A. Gose, I. W. D. Dalziel, J. A. Pancake, E. K. Seidel, T. G. Blenkinsop, and J. Mukwakwami, Coeval large-scale magmatism in the Kalahari and Laurentian cratons during Rodinia assembly, Science, 304, 1126-1129, 2004, 57 citations, doi:10.1126/science.1096329, #1702
We show that intraplate magmatism occurred 1106 to 1112 million years ago over an area of two million square kilometers within the Kalahari craton of southern Africa, during the same magnetic polarity chron as voluminous magmatism within the cratonic core of North America. These contemporaneous magmatic events occurred while the Rodinia supercontinent was being assembled and are inferred to be parts of a single large igneous province emplaced across the two cratons. Widespread intraplate magmatism during Rodinia assembly shows that mantle upwellings required to generate such provinces may occur independently of the supercontinent cycle.
Loewy, S., J. N. Connelly, and I. W. D. Dalziel, An orphaned basement block: The Arequipa-Antofalla basement of the central Andean margin of South America, Geol. Soc. Amer. Bull., 116, 171-187, 2004, 69 citations, doi:10.1130/B25226, #1668
The Arequipa-Antofalla Basement, a Proterozoic crustal block exposed along the central Andean margin, provides a key to interpreting the pre-Andean history of South America. New U/Pb geochronology and whole-rock Pb and Nd isotope geochemistry from the Arequipa-Antofalla Basement refine the tectonic history and delineate three distinct crustal domains that decrease in age from north to south. The northern domain of southern Peru and western Bolivia contains juvenile Paleoproterozoic 2.02â1.79 Ga intrusions that were metamorphosed at 1.82â1.79 Ga. The Mesoproterozoic central domain in northernmost Chile contains a significant Mesoproterozoic juvenile component that incorporates Paleoproterozoic crust from the northern domain. Rock units from both the northern and central domains were metamorphosed between 1.20 and 0.94 Ga, with coeval magmatism occurring only in the central domain. The southern domain in northern Chile and northwestern Argentina comprises Ordovician rocks, derived from a mix of juvenile material and older crust. Similar Ordovician magmatism (476â440 Ma) also occurred in the northern and central domains followed by metamorphism at ca. 440 Ma.
Barker, D. H. N., G. L. Christeson, J. A. Austin, and I. W. D. Dalziel, Backarc basin evolution and cordilleran orogenesis: Insights from new ocean-bottom seismograph refraction profiling in Bransfield Strait, Antarctica, Geology, 31, 107-110, 2003, 22 citations, doi:10.1130/0091-7613(2003)031<0107:BBEACO>2.0.CO;2, #1608
Bransfield Strait, a backarc basin off the northwestern Antarctic Peninsula, is a modern analog for Cretaceous basins inverted in the compressional tectonic regime that initiated the Andean Cordillera. Eight new refraction ocean-bottom seismograph profiles in the strait demonstrate that crustal thickness in the deep central basin increases from northeast to southwest, from ∼10 km to ∼14â16 km. This confirms multichannel seismic interpretation of upper crustal structures suggesting that the Bransfield basin is opening by northeast to southwest rift propagation within arc crust of the Antarctic Peninsula, a process also recorded in the obducted Cretaceous Rocas Verdes basin of the southernmost Andes. Thinning is most prominent along the axis of the strait, where the crust is ∼9â11 km thick. In contrast, thicknesses beneath the Antarctic Peninsula margin and the inactive South Shetland Islands pedestal are ∼18 km and ∼24 km, respectively. Seismic velocities and thicknesses suggest that new oceanic crust is not yet being generated. Extension is focused along the northwest margin, imparting the physiographic asymmetry to the strait. Comparing the Bransfield basin with the inverted Rocas Verdes basin and intraoceanic counterparts in the western Pacific suggests that rift propagation and trench-side focusing of extension may be fundamental features of young backarc basins. Resultant asymmetry may facilitate observed obduction of backarc basin floor and arc rocks onto continental margins during compressional orogenesis.
Christeson, G. L., D. H. N. Barker, J. A. Austin, and I. W. D. Dalziel, Deep crustal structure of Bransfield Strait: Initiation of a back arc basin by rift reactivation and propagation, J. Geophys. Res., 108, 2492, 2003, 9 citations, doi:10.1029/2003JB002468, #1652
We present the results from a coincident seismic reflection/refraction grid conducted at the Lesser Antilles subduction zone near 16°N. This paper focuses on the seismic refraction data and constraints these data place on the three-dimensional structure of the island arc backstop. We find that the backstop in this region contains considerable topography in both the strike and dip directions. Two ridges, each 25â35 km in length and ∼10 km in width, rise 1â6 km above the adjacent basement. The eastern edge of one of the ridges deepens by ∼4â6 km over a horizontal distance of 10 km and forms the eastern edge of the backstop. In contrast to the complex nature of the backstop, the adjacent accretionary wedge displays little lateral variability at large scales. This may be a consequence of the spatial scales involved: the backstop topography is ∼10â35 km in width, while the accretionary wedge extends ∼125 km from the deformation front to the backstop. The top of the subducting oceanic crust, as identified by an increase in velocities to 6â6.5 km/s, intersects the backstop at a depth of ∼14â15 km. The updip limit of plate boundary seismicity is located 75â100 km west and downdip of the backstop. However, two earthquake clusters are observed at the intersection of the subducted Barracuda Ridge and Tiburon Ridge with the backstop, suggesting active deformation associated with the backstop edge at these locations.
Loewy, S., J. N. Connelly, I. W. D. Dalziel, and C. F. Gower, Eastern Laurentia in Rodinia: Constraints from whole-rock Pb and U/Pb geochronology, Tectonophysics, 375, 169-197, 2003, 57 citations, doi:10.1016/S0040-1951(03)00338-X, #1667
Whole-rock Pb isotopic signatures and U/Pb geochronology refute a Rodinian correlation of northeastern Laurentia and proto-Andean Amazonia. According to this previously proposed model, the LabradorâScotlandâGreenland Promontory (LSGP) of northeastern Laurentia collided with the proto-Andean margin of Amazonia, at the Arica Embayment, during the Grenville/Sunsás Orogeny (ca. 1.0 Ga). Links between the two margins were based upon the correlation of the LSGP with Arequipa-Antofalla Basement (AAB), a Proterozoic block along the proto-Andean margin of Amazonia adjacent to the Arica Embayment. Specifically, similarities in 1.8â1.0 Ga basement rocks in both regions suggested that the AAB was originally a piece of the LSGP. Furthermore, similarities in unique, post-collisional, but pre-rift, glacial sedimentary sequences also supported a link between the AAB and LSGP.
Tests of these apparent similarities fail to support correlation of the AAB and the LSGP and, thus, eliminate a direct link between northeastern Laurentia and southwestern Amazonia in Rodinia. However, Pb isotopic compositions and U/Pb geochronology provide the basis for two new correlations, namely, (1) the ca. 1.3â1.0 Ga basement in the central and southern Appalachians may be an allochthonous block that was transferred to Laurentia from Amazonia at ca. 1.0 Ga, and (2) an allochthonous AAB may be a piece of the Kalahari Craton that was transferred to Amazonia at ca. 1.0 Ga. Based on these new correlations and a previously proposed Grenvillian connection between southern Laurentia (Llano) and Kalahari, we propose that Amazonia may have collided with a contiguous southeastern Laurentia/Kalahari margin at ca. 1.0 Ga.
Macdonald, D. I. M., I. Gomez-Perez, J. Franzese, L. Spalletti, L. A. Lawver, L. M. Gahagan, I. W. D. Dalziel, C. Thomas, N. Trewin, M. Hole, and D. Paton, Mesozoic break-up of SW Gondwana: Implications for regional hydrocarbon potential of the southern South Atlantic, Marine Petroleum Geol., 20, 287-308, 2003, 35 citations, doi:10.1016/S0264-8172(03)00045-X, #1618
This work provides new palinspastic palaeofacies reconstructions of SW Gondwana incorporating rotation of a Falkland/Malvinas microplate. We discuss the implications of this for the tectonic evolution of the southern South Atlantic and hence for the regional hydrocarbon potential.
Existing Gondwana reconstructions display good fits of major continents but poorly constrained fits of microcontinents. In most continental reconstructions, the Falkland/Malvinas Plateau was assumed to be a rigid fragment of pre-Permian South American crust. However, it has been suggested, on the basis of palaeomagnetic data, that the Falkland/Malvinas Islands were rotated by 180° after 190 Ma. This rotation hypothesis has been successfully tested on the basis of Devonian stratigraphy and palaeontology, Permian stratigraphy and sedimentology and Late Palaeozoic and Early Mesozoic structure, making it unlikely that the plateau behaved as a rigid structure during breakup. We have explored the consequences of accepting this hypothesis for the tectonic evolution of SW Gondwana by compiling new palaeogeographic maps for the PermianâCretaceous of the southern Atlantic area. To achieve a realistic close fit, we have devised a pre-rift proxy for the oceanâcontinent boundary for the South Atlantic. In order to produce the best fit, it is necessary to subdivide South America into four plates. The consequences of this are far-reaching. Our work suggests that although sedimentary basins were initiated at different times, three major tectonic phases can be recognised; in regional terms these can be thought of as pre-, syn- and post-rift.
During the pre-rift time (until the Late Triassic), the area was dominated by compressional tectonism and formed part of the Gondwana foreland. The Falkland/Malvinas Islands lay east of Africa, the Falkland/Malvinas Plateau was 33% shorter and Patagonia was displaced east with respect to the rest of South America, in part along the line of the Gastre Fault System. Potential source facies are dominantly post-glacial black shales of Late Permian age deposited in lacustrine or hyposaline marine environments; these rocks would also be an effective regional seal. Sandstones deposited in the Late Permian would be dominantly volcaniclastic with poor reservoir qualities; Triassic sandstones tend to be more mature.
There was significant extension from about 210 Ma (end-Triassic) until the South Atlantic opened at about 130 Ma (Early Cretaceous). In the early syn-rift phase, extension was accompanied by strike-slip faulting and block rotation; later extension was accompanied by extrusion of large volumes of lava. Early opening of the South Atlantic was oblique, which created basins at high angle to the trend of the ocean on the Argentine margin, and resulted in microplate rotation in NE Brazil. Intermittent physical barriers controlled deposition of Upper JurassicâCretaceous anoxic sediments during breakup; some of these mudrock units are effective seals with likely regional extent. During crustal reorganisation, clastic sediments changed from a uniform volcaniclastic provenance to local derivation, with variable reservoir quality.
In the late rift and early post-rift phase, continental extension changed from oblique to normal and basins developed parallel to the continental margins of the South Atlantic. This change coincides with the main rifting in the Equatorial basins of Brazil and the early impact of the Santa Helena Plume. It resulted in widespread development of unconformities, the abandonment of the RecôncavoâTucanoâJatoba rift and the end of NE Brazil plate rotation, which remained attached to South America. There was extensive deposition of evaporites, concentrated in (but not restricted to) the area north of the Rio Grande Rise/Walvis Ridge.
Widespread deposits can be used to define potential regional elements of hydrocarbon systems and to provide a framework for relating more local elements. Our main conclusion is that the regional hydrocarbon potential of the southern South Atlantic has been constrained by the tectonic evolution.
Smalley, R., E. Kendrick, M. G. Bevis, I. W. D. Dalziel, F. W. Taylor, E. Lauria, R. Barriga, G. Casassa, E. Olivero, and E. Piana, Geodetic determination of relative plate motion and crustal deformation across the Scotia-South America plate boundary in eastern Tierra del Fuego, Geochem., Geophys., Geosyst., 4, 1070, 2003, 10 citations, doi:10.1029/2002GC000446, #1650
Global Positioning System (GPS) measurements provide the first direct measurement of plate motion and crustal deformation across the Scotia-South America transform plate boundary in Tierra del Fuego. This plate boundary accommodates a part of the overall motion between South America and Antarctica. The subaerial section of the plate boundary in Tierra del Fuego, about 160 km in length, is modeled as a two dimensional, strike-slip plate boundary with east-west strike. Along the Magallanes-Fagnano fault system, the principal fault of this portion of the plate boundary, relative plate motion is left-lateral strike-slip on a vertical fault at 6.6 ± 1.3 mm/year based on an assumed locking depth of 15 km. The site velocities on the Scotia Plate side are faster than the relative velocity by an additional 1â2 mm/yr, suggesting there may be a wider region of diffuse left-lateral deformation in southern Patagonia. The north-south components of the velocities, however, do not support the existence of active, large-scale transpression or transtension between the South America and Scotia plates along this section of the plate boundary.
Dalziel, I. W. D., and L. A. Lawver, The lithospheric setting of the West Antarctic Ice Sheet, in The West Anarctic Ice Sheet: Behavior and Environment, edited by R. Bindschadler, Amer. Geophys. Un., Washington, D. C., Antarctic Research Ser., 77, 29-44, 2001, #1489
Dalziel, I. W. D., and N. J. Soper, Neoproterozoic extension on the Scottish promontory of Laurentia: Paleogeograhic and tectonic implications, J. Geology, 109, 299-317, 2001, 51 citations, doi:10.1086/319974, #1528
The Hebridean shield, the northwest foreland of the Caledonian Orogen of Scotland, is a small fragment of Laurentia detached during the Cenozoic opening of the North Atlantic Ocean and is now part of Europe. The shield was at the tip of a major promontory of the ancestral core of North America, between the Newfoundland (Appalachian) and Greenland (Caledonian) margins. Its history is important to understanding late Precambrian and early Paleozoic global paleogeography and tectonics. Isotopic ages and structural complexities in the Moine and Dalradian Supergroups of the Caledonian Orogen have been interpreted as reflecting Neoproterozoic orogenic episodes overprinted by early Paleozoic deformation and metamorphism. A critical body of rock in the Scottish Highlands, the West Highland Granite Gneiss, has been viewed as a synorogenic intrusion into Moine metasedimentary rocks, and its 870‐Ma U‐Pb zircon age as dating a Riphean âKnoydartianâ orogeny. However, field evidence shows that the granitic protolith of the gneiss was emplaced before a regional suite of tholeiitic dikes was intruded into brittle fractures. The dikes carry all the ductile regional deformation. The zircon age thus reflects the crystallization of an anatectic melt, not its subsequent gneissification. Melting is thought to have resulted from advection of heat by emplacement of basaltic magma deep within the Moine sedimentary pile. In this new scenario, deformation and gneissification took place during the early (Grampian/Taconic) phase of the Caledonian Orogeny, not during the Neoproterozoic. Our interpretation is that all the Knoydartian events were extensional. This leads to a substantial simplification of the pre‐Caledonian history of the Scottish Promontory of Laurentia. Protracted rifting in the Neoproterozoic was concentrated in two phases, with episodes of major extension and bimodal magmatism in the Riphean (900â750 Ma) and Vendian (600 Ma). These episodes coincide with the two‐stage breakout of Laurentia as a discrete continent during the Neoproterozoic, hypothetically from the Rodinian and Pannotian supercontinents, respectively.
Dalziel, I. W. D., A global perspective on the Scottish Caledonides, Trans. Roy. Soc. Edinburgh, 91, 405-420, 2001, 2 citations, #1529
Dalziel, I. W. D., R. A. Astini, D. J. Fettes, and A. L. Harris, Penrose Conference report: The Iapetus Ocean, GSA Today, 11 (9), 32-35, 2001, #1561
Dalziel, I. W. D., L. A. Lawver, and J. B. Murphy, Plumes, orogenesis, and supercontinental fragmentation, Earth Planet. Sci. Lett., 178, 1-11, 2000, 67 citations, doi:10.1016/S0012-821X(00)00061-3, #1496
A timeâspace relationship between large igneous provinces (LIPS), present day hot spots, and the fragmentation of Pangea has been documented over several decades, but the cause of fragmentation has remained elusive. LIPS are regarded either as the result of impingement of a mantle plume on the base of the lithosphere, or as the initial products of adiabatic decompression melting of anomalously hot mantle. Do LIPS therefore constitute evidence of an active role for plumes from the deep mantle in supercontinental fragmentation, or are they merely the first indications of a large-scale but near-surface tectonic process? Two long recognized and enigmatic orogenic events may offer a solution to this geologically important âchicken or eggâ conundrum. The reconstructed early Mesozoic Gondwanide fold belt of South America, southern Africa, and Antarctica, could have resulted from âplume-modified orogenyâ, flattening of a downgoing lithospheric slab due to the buoyancy of a plume rising beneath a continental margin subduction zone. If so, the 180 Ma Karroo and Ferrar LIPS associated with the opening of the ocean basin between East and West Gondwanaland at 165 Ma resulted from impingement of this plume and are unrelated to the thermal insulation of the shallow mantle beneath Gondwanaland. It would then follow that the plume itself played an active, possibly critical, role in the initial breakup of the supercontinent. The Late Paleozoic âAncestral Rockiesâ deformation in the southwestern United States could be yet another example of orogeny driven by a plume that initiated the break-up of Pangea approximately 15 Myr earlier in the Central Atlantic region.
Dalziel, I. W. D., S. Mosher, and L. M. Gahagan, Laruentia-Kalahari collision and assembly of Rodinia, J. Geology, 108, 499-513, 2000, 99 citations, doi:10.1086/314418, #1503
The Llano Orogenic Belt along the present southern margin of Laurentia, regarded as continuation of the Grenvillian Orogen along the eastern Laurentian margin and exposed in basement uplifts in central and western Texas, records an 300‐m.yr. history of orogenesis culminating in arc‐continent and continent‐continent collision between 1150 and 1120 Ma and continuing until 980 Ma. The shape of the orogen and kinematics of the contractional deformation along the belt, together with the high‐P metamorphic conditions attained, indicate that a previously unidentified craton served as an indentor. It is paleomagnetically acceptable for the Kalahari Craton of southern Africa to have been opposed to this margin and within 1500 km of present‐day central Texas at 1100 Ma. Moreover, the Kalahari Craton is the correct size, and the structural and metamorphic evolution of the 1200â950 Ma Namaqua‐Natal Orogenic Belt that wraps around its present southern margin is compatible with that craton having been the indentor. The ocean basin that closed between the Laurentia and Kalahari Cratons would have been comparable to the present Pacific, with island arc/terrane accretion occurring during the Mesoproterozoic along opposing active convergent margins. The coeval 1.1 Ga Keeweenawan and Umkondo magmatic provinces of Laurentia and Kalahari, respectively, are associated with rifts at a high angle to the Llano and Namaqua Orogens. The rifts are interpreted as the result of collision‐generated extensional stresses within the two cratons. The voluminous mafic igneous rocks in both provinces, however, may reflect contemporaneous plume activity. Our reconstruction for 1.1 Ga provides a testable model for the Llano Orogenic Belt of Texas and the Namaqua Orogenic Belt of southwestern Africa as opposite sides of a Himalayan‐type collisional orogen, with the Natal Belt of southeastern Africa and the originally continuous Maudheim Belt of East Antarctica as a related Indonesian‐type ocean‐continent convergence zone. This reconstruction leads to a refinement of the paleogeography of Rodinia, with the Kalahari Craton in a position isolated from both the East Antarctic and Rio de la Plata Cratons by oceanic lithosphere. It also provides the first model for the assembly of that hypothetical early Neoproterozoic supercontinent. At least four separate cratonic entities appear to have collided along three discrete segments of the apparently anastomosing global network of âGrenvillianâ orogens: the type‐Grenville Belt of eastern North America and counterparts in South America, the Llano‐Namaqua Belt, and the Eastern Ghats‐Albany/Fraser Belt of India‐East Antarctica and Australia. Over the remarkably short interval of 200 m.yr., this first‐order composite collisional event resulted in the amalgamation of most of Earthâs continental lithosphere and defined the close of the Mesoproterozoic Era.
Mukasa, S. B., and I. W. D. Dalziel, Marie Byrd Land, West Antarctica: Evolution of Gondwana's Pacific margin constrained by zircon U-Pb geochronology and feldspar common-Pb isotopic compositions, Geol. Soc. Amer. Bull., 112, 611-627, 2000, 48 citations, doi:10.1130/0016-7606(2000)112<611:MBLWAE>2.0.CO;2, #1426
Dalziel, I. W. D., Vestiges of a beginning and the prospect of an end, in James Hutton - Present and Fugure, edited by G. Y. Craig and J. H. Hull, Geological Society, London, Special Publication, 150, 119-155, 1999, #1350
Lawver, L. A., L. M. Gahagan, and I. W. D. Dalziel, A tight fit-Early Mesozoic Gondwana, a plate reconstruction perspective, Mem. Natl. Inst. Polar Res. Spec. Issue, 53, 214-229, 1999, #1398
Dalziel, I. W. D., Reply, concerning "Neoproterozoic-Paleozoic geography and tectonics: Review, hypothesis, environmental speculation", Geol. Soc. Amer. Bull., 110, 1619-1620, 1998, doi:10.1130/0016-7606(1998)110<1615:NPGATR>2.3.CO;2, #1403
Dalziel, I. W. D., and R. A. Astini, Early Paleozoic paleogeography of Laurentia and western Gondwana: Evidence from tectonic subsidence analysis: Comment, Geology, 26, 575-576, 1998, doi:10.1130/0091-7613(1998)026<0575:EPPOLA>2.3.CO;2, #1404
Sen, V., P. L. Stoffa, I. W. D. Dalziel, D. D. Blankenship, A. M. Smith, and S. Anandakrishnan, Seismic surveys of central West Antarctica: Data and processing examples from the ANTALITH field tests (1994-1995), Terra Antartica, 5, 761-772, 1998, #1447
Dalziel, I. W. D., Overview: Neoproterozoic-Paleozoic geography and tectonics: Review, hypothesis, environmental speculation, Geol. Soc. Amer. Bull., 109, 16-42, 1997, 559 citations, doi:10.1130/0016-7606(1997)109<0016:ONPGAT>2.3.CO;2, #1244
The ever-changing distribution of continents and ocean basins on Earth is fundamental to the environment of the planet. Recent ideas regarding pre-Pangea geography and tectonics offer fresh opportunities to examine possible causative relations between tectonics and environmental and biologic changes during the Neoproterozoic and Paleozoic eras. The starting point is an appreciation that Laurentia, the rift-bounded Precambrian core of North America, could have been juxtaposed with the cratonic cores of some present-day southern continents. This has led to reconstructions of Rodinia and Pannotia, supercontinents that may have existed in early and latest Neoproterozoic time, respectively, before and after the opening of the Pacific Ocean basin.
Recognition that the Precordillera of northwest Argentina constitutes a terrane derived from Laurentia may provide critical longitudinal control on the relations of that craton to Gondwana during the Precambrian-Cambrian boundary transition, and in the early Paleozoic. The Precordillera was most likely derived from the general area of the Ouachita embayment, and may have been part of a hypothetical promontory of Laurentia, the âTexas plateau,â which was detached from the Cape of Good Hope embayment within Gondwana between the present-day Falkland-Malvinas Plateau and Transantarctic Mountains margins. Thus the American continents may represent geometric âtwinsâ detached from the Pannotian and Pangean supercontinents in Early Cambrian and Early Cretaceous time, respectivelyâthe new mid-ocean ridge crests of those times initiating the two environmental supercycles of Phanerozoic history 400 m.y. apart. In this scenario, the extremity of the Texas plateau was detached from Laurentia during the Caradocian Epoch, in a rift event ca. 455 Ma that followed Middle Ordovician collision with the proto-Andean margin of Gondwana as part of the complex Indonesian-style Taconic-Famatinian orogeny, which involved several island arc-continent collisions between the two major continental entities. Laurentia then continued its clockwise relative motion around the proto-Andean margin, colliding with other arc terranes, Avalonia, and Baltica en route to the Ouachita-Alleghanian-Hercynian-Uralian collision that completed the amalgamation of Pangea.
The important change in single-celled organisms at the Mesoproterozoic-Neoproterozoic boundary (1000 Ma) accompanied assembly of Rodinia along Grenvillian sutures. Possible divergence of metazoan phyla, the appearance and disappearance of the Ediacaran fauna (ca. 650â545 Ma), and the Cambrian âexplosionâ of skeletalized metazoans (ca. 545â500 Ma) also appear to have taken place within the framework of tectonic change of truly global proportions. These are the opening of the Pacific Ocean basin; uplift and erosion of orogens within the newly assembled Gondwana portion of Pannotia, including a collisional mountain range extending ≈7500 km from Arabia to the Pacific margin of Antarctica; the development of a Pannotia-splitting oceanic spreading ridge system nearly 10 000 km long as Laurentia broke away from Gondwana, Baltica, and Siberia; and initiation of subduction zones along thousands of kilometres of the South American and Antarctic-Australian continental margins. The Middle Ordovician sea-level changes and biologic radiation broadly coincided with initiation of the Appalachian-Andean mountain system along >7000 km of the Taconic and Famatinian belts. These correlations, based on testable paleogeographic reconstructions, invite further speculation about possible causative relations between the internally driven long-term tectonic evolution of the planet, its surface environment, and life.
Gose, W. A., M. A. Helper, J. N. Connelly, F. E. Hutson, and I. W. D. Dalziel, Paleomagnetic data and U-Pb isotopic age determinations from Coats Land, Antarctica: Implications for late Proterozoic plate reconstructions, J. Geophys. Res., 102, 7887-7902, 1997, 54 citations, #1022
Paleomagnetic results and isotopic age determinations for granophyre and rhyolite from small, isolated nunataks in southern Coats Land, Antarctica, are used to evaluate late Proterozoic plate reconstructions. U-Pb zircon dates for the two rock types indicate coeval crystallization at 1112 ± 4 Ma. A concordant 1106 ± 3 Ma titanite date from the granophyre overlaps the crystallization age, implying rapid cooling, and is consistent with field and petrographic evidence of no subsequent penetrative deformation, metamorphism, or hydrothermal disturbance. The mean direction of magnetization of the rhyolite at Littlewood Nunataks is statistically indistinguishable from the mean directions of five sites in the granophyre and crosscutting rhyolite dikes at Bertrab Nunataks. The group mean virtual geomagnetic pole of 22.9°N, 80.3°E (N=6, A95=6.8°) compares favorably with the only other extant Precambrian paleomagnetic poles for the East Antarctic craton, two poles from western Dronning Maud Land. The East Antarctic and Laurentian poles of 1.1 Ga do not coincide after restoration of the continents to a position suggested by the SWEAT hypothesis juxtaposing the Pacific margins of East Antarctica-Australia and Laurentia, indicating either that the hypothesis is incorrect or that Coats Land and parts of western Dronning Maud Land (herein the CMG province) were not part of the East Antarctic craton at 1.1 Ga. In support of the latter, there is reasonable agreement of the 1.1 Ga CMG poles and approximately coeval poles from the Kalahari craton of West Gondwana when the CMG is restored to a position adjacent the Kalahari craton. Such a reconstruction places the CMG in West Gondwana rather than East Gondwana, as originally implied in Rodinia reconstructions, and is consistent with previously recognized links between the geology of the Kalahari craton and western Dronning Maud Land. It further implies that the CMG did not become part of East Antarctica until latest Precambrian to Cambrian time. A new reconstruction places a partially assembled West Gondwana off the present southeastern margin of Laurentia at 1.1 Ga such that poles of the CMG and Kalahari fall on the Laurentian polar wander path for this time period.
Dalziel, I. W. D., L. H. Dalla Salda, C. A. Cingolani, and P. Palmer, Penrose Conference report: The Argentine precordillera: A Laurentian terrane?, GSA Today, 6 (2), 16-18, 1996, #1190
Dalziel, I. W. D., and L. H. Dalla Salda, Discussion on Ordovician paleogeography of Siberia and adjacent continents, J. Geol. Soc. London, 153, 329-330, 1996, 1 citation, doi:10.1144/gsjgs.153.2.0329, #1192
Dalziel, I. W. D., and L. H. Dalla Salda, Discussion, of 'The Early Paleozoic evolution of the Argentine precordillera as a Laurentian rifted, drifted, and collided terrane - A geodynamic model', by R. A. Astini, J. L. Benedetto, and N. E. Vaccari, Geol. Soc. Amer. Bull., 108, 372, 1996, 6 citations, doi:10.1130/0016-7606(1996)108<0372:TEPEOT>2.3.CO;2, #1245
DiVenere, V. J., D. V. Kent, and I. W. D. Dalziel, Summary of paleomagnetic results From West Antarctica: Implications for the tectonic evolution of the Pacific margin of Gondwana during the Mesozoic, in Weddell Sea Teconics and Gondwana Break-up, edited by B. C. Storey, E. C. King, and R. A. Livermore, Geol. Soc. London Spec. Publ., 108, 31-43, 1996, #1109
Mukasa, S. B., and I. W. D. Dalziel, Southernmost Andes and South Georgia Island, North Scotia Ridge: Zircon U-Pb and muscovite 40Ar/39Ar age constraints on kinematic evolution of southwestern Gondwanaland, J. South American Earth Sci., 9, 349-365, 1996, 34 citations, doi:10.1016/S0895-9811(96)00019-3, #1193
Zircon U-Pb and muscovite 40Ar/39Ar isotopic ages have been determined on rocks from the southernmost Andes and South Georgia Island, North Scotia Ridge, to provide absolute time constraints on the kinematic evolution of southwestern Gondwanaland, until now known mainly from stratigraphic relations. The U-Pb systematics of four zircon fractions from one sample show that proto-marginal basin magmatism in the northern Scotia arc, creating the peraluminous Darwin granite suite and submarine rhyolite sequences of the Tobifera Formation, had begun by the Middle Jurassic (164.1 ± 1.7 Ma). Seven zircon fractions from two other Darwin granites are discordant with non-linear patterns, suggesting a complex history of inheritances and Pb loss. Reference lines drawn through these points on concordia diagrams give upper intercept ages of ca. 1500 Ma, interpreted as a minimum age for the inherited zircon component. This component is believed to have been derived from sedimentary rocks in the Gondwanaland margin accretionary wedge that forms the basement of the region, or else directly from the cratonic âback stopâ of that wedge.
Ophiolitic remnants of the Rocas Verdes marginal basin preserved in the Larsen Harbour complex on South Georgia yield the first clear evidence that Gondwanaland fragmentation had resulted in the formation of oceanic crust in the Weddell Sea region by the Late Jurassic (150 ± 1 Ma). The geographic pattern in the observed age range of 8 to 13 million years in these ophiolitic materials, while not definitive, is in keeping with propagation of the marginal basin floor northwestward from South Georgia Island to the Sarmiento Complex in southern Chile.
Rocks of the Beagle granite suite, emplaced post-tectonically within the uplifted marginal basin floor, have complex zircon U-Pb systematics with gross discordances dominated by inheritances in some samples and Pb loss in others. Of eleven samples processed, only two had sufficient amounts of zircon for multiple fractions, and only one yielded colinear points. These points lie close to the lower concordia intercept for which the age is 68.9 ± 1.0 Ma, but their upper intercept is not well known. Inasmuch as this age is similar to the 40Ar/39Ar age of secondary muscovite growing in extensional fractures of pulled-apart feldspar phenocrysts in a Beagle suite granitic pluton (plateau age is 68.1 ± 0.4 Ma), we interpret the two dates as good time constraints for cooling following a period of extensional deformation probably related to the tectonic denudation of the highgrade metamorphic complex of Cordillera Darwin in Tierra del Fuego.
Storey, B. C., D. I. M. Macdonald, I. W. D. Dalziel, J. L. Isbell, and I. L. Millar, Early Paleozoic sedimentation, magmatism, and deformation in the Pensacola Mountains, Antartica: The significance of the Ross orogeny, Geol. Soc. Amer. Bull., 108, 685-707, 1996, 28 citations, doi:10.1130/0016-7606(1996)108<0685:EPSMAD>2.3.CO;2, #1247
Combined sedimentological, structural, and geochemical studies of a lower Paleozoic succession within the Pensacola Mountains, Antarctica, suggest that it probably formed in a foreland basin setting during the Ross-Delamerian orogen, a complex early Paleozoic convergent margin of Antarctica and Australia. The lower Paleozoic succession lies unconformably on a deformed(?) Neoproterozoic sequence (referred to here as Sequence 1) and is divided into three unconformity-bounded sequences (Sequences 2âââ‰â¬Å4). The oldest sequence, Sequence 2, comprises Middleâââ‰â¬ÅUpper Cambrian platformal limestone (Nelson Limestone) and overlying Lower Ordovician silicic volcanic rocks of the Gambacorta Formation (U-Pb zircon age of 501 Ãâñ 3 Ma). The volcanic rocks crystallized from a high-temperature anhydrous magma derived from a lower crustal igneous source and may represent magmatism on the inboard side of a magmatic arc now largely absent from this part of the margin. Sequence 3 (Wiens Formation), in part conformable with Sequence 2, represents deposition by unconfined ephemeral streams followed by a marine transgressive unit. The base of Sequence 4 (Neptune Group) is a major erosion surface marked by karstification of the exposed Nelson Limestone and by calcrete pedogenesis. The Neptune Group is an alluvial fan complex typical of many syn- and post-orogenic red beds. The predominance of nonmarine and shallow marine sequences, and the facies and paleocurrent directions within the basin, suggest that it may be more typical of a âââ¬Ãâpiggybackâââ¬Ã basin than of a foredeep basin, with the alluvial fan complexes derived from advancing thrust sheets. Growth folds, progressive unconformities, and deformed clasts of underlying strata within basal conglomerates are consistent with active deformation during sedimentation and the proposed tectonic setting. The presence of variably plunging folds, some of which are transected by a slaty cleavage, suggests that deformation was in an oblique-slip setting perhaps due to oblique convergence along this part of the Antarctic margin during the Ross-Delamerian orogeny.
Cunningham, W. D., I. W. D. Dalziel, T.-Y. Lee, and L. A. Lawver, Southernmost South America-Antarctic Peninsula relative plate motions since 84 Ma: Implications for the tectonic evolution of the Scotia Arc region, J. Geophys. Res., 100, 8257-8266, 1995, 48 citations, #1144
We have attempted to quantify the relative motion history between southernmost South America (SSA) and the Antarctic Peninsula (AP) by calculating and comparing SSA-Africa, AP-Africa and SSA-AP synthetic flow lines for 84â0 Ma. The flow lines were created using published poles of rotation and plate reconstruction software. The results indicate that since 84 Ma, SSA and AP have moved approximately westward relative to a fixed Africa; however, SSA's rate of westerly motion in that reference frame has been significantly more rapid than AP's rate. Approximately 1320 km of east-west, left-lateral strike-slip displacement and 490 km of north-south, divergent displacement have occurred between the southern tip of SSA and the northern tip of AP since 84 Ma. Increased rates of SSA-AP interplate separation and a change in the angle of plate divergence at approximately 55â40 Ma marked the onset of accelerated continental separation that eventually led to seafloor spreading in the western Scotia Sea at 30 Ma and the development of the Scotia Arc. Increased separation rates between SSA and AP at 55â40 Ma may be related to a global Eocene plate reorganization event. The northeast-southwest oriented western Scotia Sea spreading centers appear to have accommodated all of the SSA-AP interplate motion between 30 and 9 Ma. We suggest that prior to 30 Ma and the opening of Drake Passage, components of interplate strike-slip and divergent motion were accommodated by intracontinental deformation that included strike-slip faulting, counterclockwise tectonic rotation, and continental extension in the southernmost Andes. The results indicate that the opening of the Scotia Sea was caused by plate-scale motions as SSA and AP drifted away from Africa at different velocities along different, nonparallel trajectories. Subduction retreat along the South Scotia Ridge and South Sandwich arc and back arc spreading in the Scotia Sea contributed to the width of separation between SSA and AP across Drake Passage. The results place limits on how SSA-AP relative motion has been temporally and spatially partitioned in the Scotia Arc region.
Dalziel, I. W. D., Earth before Pangea, Scientific Amer., 272 (1), 58-63, 1995, 25 citations, #1100
Dalziel, I. W. D., and M. A. S. McMenamin, Are Neoproterozoic glacial deposits preserved on the margins of Laurentia related to the fragmentation of two supercontinents? Comment, Geology, 23, 959-960, 1995, 3 citations, doi:10.1130/0091-7613(1995)023<0959:ANGDPO>2.3.CO;2, #1191
Dalziel, I. W. D., Neoproterozoic and Paleozoic plate tectonics: A new scenario, Earth Scientist, 12, 3-12, 1995, #1348
Dalziel, I. W. D., Paleogeography, in McGraw-Hill Yearbook of Science and Technology, McGraw-Hill, New York, 243-247, 1995, #1349
DiVenere, V. J., D. V. Kent, and I. W. D. Dalziel, Early Cretaceous paleomagnetic results from Marie Byrd Land, West Antarctica: Implications for the Weddellia collage of crustal blocks, J. Geophys. Res., 100, 8133-8151, 1995, 14 citations, #1142
A new -117 Ma paleomagnetic pole has been defined from the study of volcanic and plutonic rocks from the eastern portion Marie Byrd Land (MBL). The new pole (185.6°E/56.8°S, A 95 = 8.7°) implies that the eastern portion of MBL was an integral part of Weddellia, which included the ancestral Antarctic Peninsula, Thurston Island, and Ellsworth-Whitmore Mountains blocks of West Antarctica. This pole is generally similar to a ∼125 Ma pole from Thurston Island. Both poles call for major clockwise rotation and poleward motion of eastern MBL and Thurston Island between the Early Cretaceous (125â117 Ma) and the mid-Cretaceous (110â100 Ma). We propose that in the Early Cretaceous, eastern MBL and the Eastern Province of New Zealand were part of a continuous active Pacific margin of Gondwana, connecting with the Antarctic Peninsula, and distinct from western MBL, the Western Province of New Zealand, and North Victoria Land. These western terranes are thought to have accreted to Gondwana in the Devonian. Eastern MBL and the Eastern Province of New Zealand amalgamated with western MBL and the Western Province of New Zealand by the mid-Cretaceous. Major Early Cretaceous motions of the Weddellia blocks postdate the estimated initiation of seafloor spreading in the Weddell Sea and therefore may be the result of plate reorganization during the Cretaceous Quiet Zone.
Kohn, M. J., F. S. Spear, T. M. Harrison, and I. W. D. Dalziel, 40Ar/39Ar geochronology and P-T-t paths from the cordillera Darwin metamorphic complex, Tierra del Fuego, Chile, J. Metamorphic Geol., 13, 251-270, 1995, 45 citations, doi:10.1111/j.1525-1314.1995.tb00217.x, #1143
Dalziel, I. W. D., L. H. Dalla Salda, and L. M. Gahagan, Paleozoic Laurentia-Gondwana interaction and the origin of the Appalachian-Andean mountain system, Geol. Soc. Amer. Bull., 106, 243-252, 1994, 229 citations, doi:10.1130/0016-7606(1994)106<0243:PLGIAT>2.3.CO;2, #939
Laurentia, the rift-bounded Precambrian nucleus of North America, may have broken out from a Neoproterozoic supercontinent between East and West Gondwana. Several lines of evidence suggest that the Appalachian margin of Laurentia subsequently collided with the proto-Andean margin of the amalgamated Gondwana supercontinent in different relative positions during early and mid-Paleozoic time, in route to final docking against northwest Africa to complete the assembly of Pangea. Hence the Appalachian and Andean orogens may have originated as a single mountain system. The overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the Paleozoic Era. Laurentia-Gondwana collisions may help to explain contemporaneous unconformities in the Paleozoic sedimentary cover of the Laurentian, Gondwanan, and Baltic cratons.
Dalziel, I. W. D., Precambrian Scotland as a Laurentia-Gondwana link: Origin and significance of cratonic promontories, Geology, 22, 589-592, 1994, 36 citations, doi:10.1130/0091-7613(1994)022<0589:PSAALG>2.3.CO;2, #1024
Refinement of the supercontinental reconstruction for the latest Precambrian that places the Labrador-Greenland promontory of Laurentia within the Arica embayment along the margin of the Gondwana craton juxtaposes the Rockall microcontinent and northwestern British Isles with the continental margin of Peru. The conjugate cratonic margin to the northwest Caledonian foreland, the Hebridean shield, may have been Amazonia. Possible South American correlatives of the Precambrian Moinian and Dalradian complexes of the Scottish Highlands can be identified in the reconstruction; notably, there are possible equivalents of the "older granites" within the Moinian and of Dalradian glacial deposits. Therefore, the well-studied Precambrian rocks of the Scottish Highlands provide critical tests for the suggested reconstruction. The latest Precambrian setting of the Arequipa massif with respect to the Gondwana craton margin and the Paleozoic intracratonic basin of Peru-Bolivia bears a striking similarity to the early Mesozoic setting of the Rockall Plateau and northwestern British Isles with respect to the European continental margin and the North Sea-Western Approaches graben system. In each case rifting appears to have been controlled by the youngest, and presumably weakest, lithosphere, but the extremity of the Labrador-Scotland-Greenland promontory was detached during final continental separation in Vendian to earliest Cambrian time to form the Arequipa massif and in Jurassic time to form the Hebridean shield. The promontory may have played an important tectonic role through at least 1 b.y. of Earth history including influence on the development of the Andean Cordillera. Features of this type likely have played a major role in the development of orogenic belts of all ages.
Dalziel, I. W. D., A. Knoll, and E. M. Moores, Late Precambrian tectonics and the dawn of the Phanerozoic, GSA Today, 4 (1), 8-9, 1994, #1362
DiVenere, V. J., D. V. Kent, and I. W. D. Dalziel, Mid-Cretaceous paleomagnetic results From Marie Byrd Land, West Antarctica: A test of post-100 Ma relative motion between East and West Antarctica, J. Geophys. Res., 99, 15115-15139, 1994, 43 citations, #1037
A new ∼117 Ma paleomagnetic pole has been defined from the study of volcanic and plutonic rocks from the eastern portion Marie Byrd Land (MBL). The new pole (185.6°E/56.8°S, A 95 = 8.7°) implies that the eastern portion of MBL was an integral part of Weddellia, which included the ancestral Antarctic Peninsula, Thurston Island, and Ellsworth-Whitmore Mountains blocks of West Antarctica. This pole is generally similar to a ∼125 Ma pole from Thurston Island. Both poles call for major clockwise rotation and poleward motion of eastern MBL and Thurston Island between the Early Cretaceous (125â117 Ma) and the mid-Cretaceous (110â100 Ma). We propose that in the Early Cretaceous, eastern MBL and the Eastern Province of New Zealand were part of a continuous active Pacific margin of Gondwana, connecting with the Antarctic Peninsula, and distinct from western MBL, the Western Province of New Zealand, and North Victoria Land. These western terranes are thought to have accreted to Gondwana in the Devonian. Eastern MBL and the Eastern Province of New Zealand amalgamated with western MBL and the Western Province of New Zealand by the mid-Cretaceous. Major Early Cretaceous motions of the Weddellia blocks postdate the estimated initiation of seafloor spreading in the Weddell Sea and therefore may be the result of plate reorganization during the Cretaceous Quiet Zone.
Dalla Salda, L. H., and I. W. D. Dalziel, Evolucion paleogeografica del occidente del Gondwana durante el Neoproterozoico-Paleozoico Medio, Primer Simposio Int. del Neoproterozoico-Cambrico de la Cuenca del Plata, 1, 1993, #1033
Dalla Salda, L. H., and I. W. D. Dalziel, O Supercontinente Neoproterozóico e as interações Gondwana-Laurentia durante o Paleozóico Inferior-Médio, Revista Brasileira de Geofisica, 23, 183-186, 1993, #1153
Dalziel, I. W. D., Tectonic tracers and the origin of the proto-Andean margin, XII Cong. Geologico Argentino y II Conggreso de Exploracion de Hidrocarburos, 3, 367-374, 1993, #996
Dalziel, I. W. D., Tectonics, Geotimes, 38, 22, 1993, #1032
Dalziel, I. W. D., (Book review): Geology and Paleontology of the Ellsworth Mountains, West Antarctica, edited by G. F. Webbers, C. Craddock, and J. F. Splettstoesser, Econ. Geol., 88, 1917, 1993, #1141
Kohn, M. J., F. S. Spear, and I. W. D. Dalziel, Metamorphic P-T paths from Cordillera Darwin, a core complex in Tierra del Fuego, Chile, J. Petrology, 34, 519-542, 1993, 47 citations, #1065
PâT conditions and prograde PâT paths have been calculated for amphibolite-grade pelites and amphibolites from Cordillera Darwin, Tierra del Fuego, Chile. Peak PâT conditions are nearly all within the kyanite stability field; temperatures generally show an increase with increasing grade, but pressures have a less consistent trend, possibly increasing slightly from garnet to kyanite grade. PâT paths from pelites show heating by 80â100°C during loading of 0â¢2â3 kbar. Textural analysis and previous structural work indicate that this segment of the path correlates with back-folding deformation. PâT paths from two Mg-rich garnet amphibolites suggest a decrease in pressure of as much as 3 kbar with 25â50°C of heating from the kyanite stability field to the sillimanite, and are consistent with pervasive, minor development of fibrolitic sillimanite along plagioclase grain boundaries. Together, the PâT path segments from pelites and amphibolites constitute a clockwise PâT trajectory.
The proposed clockwise PâT paths are consistent with the interpretation that Cordillera Darwin represents an extensionally exhumed metamorphic core complex, in which loading during garnet growth in the pelitic rocks was succeeded by differential uplift during garnet growth in magnesian amphibolites.
Lawver, L. A., I. W. D. Dalziel, and D. T. Sandwell, Antarctic plate: Tectonics from a gravity anomaly and infrared satellite image, GSA Today, 3 (5), 117-122, 1993, #992
Dalla Salda, L. H., I. W. D. Dalziel, C. A. Cingolani, and R. Varela, Did the Taconic Appalachians continue into southern South America?, Geology, 20, 1059-1062, 1992, 10 citations, doi:10.1130/0091-7613(1992)020<1059:DTTACI>2.3.CO;2, #900
The Appalachian Mountains, now terminating abruptly at the Gulf of Mexico coastal plain, may have formerly continued into southern South America. Rocks forming the basement of the Argentine Andes can be interpreted as remnants of an early Paleozoic orogen, the Famatinian belt, not unlike the Taconic Appalachians. Both orogens are bordered to the west (present coordinates) by lower Paleozoic carbonate platforms bearing the Olenellid trilobite fauna that is characteristic of Laurentia. Paleomagnetic and geologic data indicate that they could have formed as one continuous mountain chain, possibly extending into Antarctica, during Ordovician closure of an ocean basin ("southern" Iapetus) between Laurentia and Gondwana. The Taconic and Famatinian segments of the chain may have been truncated during Late Ordovician separation of Laurentia and Gondwana along the preexisting (late Neoproterozoic to Cambrian) rift system that initiated formation of the Ouachita embayment and the southern margin of North America.
Dalziel, I. W. D., Antarctica; A tale of two supercontinents?, Ann. Rev. Earth Planet. Sci., 20, 501-526, 1992, 132 citations, doi:10.1146/annurev.ea.20.050192.002441, #889
Dalziel, I. W. D., and A. M. Grunow, Late Gondwanide tectonic rotations within Gondwanaland, Tectonics, 11, 603-606, 1992, 42 citations, #892
Geologic and paleomagnetic evidence from the Ellsworth-Whitmore mountains crustal block of West Antarctica, and from the Falkland Islands of South America, indicates that tectonic displacement of major portions of the âSamfrau geosynclineâ occurred after the early Mesozoic Gondwanide folding but prior to seafloor spreading. Comparison with continental deformation in the collisional Alpine-Himalayan belt offers a mechanism for the displacements while supporting the hypothesis that the Gondwanian orogeny resulted from collision of a fore-arc and magmatic arc terrane with the Panthalassic margin of the Gondwana craton during closure of a marginal basin. The displacements had major consequences for the ensuing evolution of the South Atlantic-Weddell Sea-Ross Sea region.
Dalziel, I. W. D., Tectonics, Geotimes, 37 (2), 27-28, 1992, #916
Dalziel, I. W. D., Reply, to "Pacific margins of Laurentia and East Antarctica-Australia as a conjugate rift pair: Evidence and implications for an Eocambrian supercontinent", Geology, 20, 190-191, 1992, doi:10.1130/0091-7613(1992)020<0191:C>2.3.CO;2, #917
Dalziel, I. W. D., The future of scientific drilling in Antarctic waters, Antarctic Sci., 4, 1, 1992, doi:10.1017/S0954102092000014, #918
Over the past twenty years, nine legs of the Ocean Drilling Programme (ODP) and its predecessor the Deep Sea Drilling Project have been conducted at high southern latitudes (>45°S). Only four have taken place near the margins of the Antarctic continent (>60°S), the last off the Amery Ice Shelf in 1988. At present, JOIDES Resolution is drilling on the Chile Rise-Chile Trench triple junction (46°S), but she will return to lower latitudes at the end of this leg (#141). The Planning Committee of ODP has already approved a schedule that precludes a return to the Antarctic prior to the 1994â95 austral summer at the earliest. Few proposals for Antarctic drilling have even been submitted in recent years; none has received high ranking. This should be a matter of considerable concern to the Antarctic earth sciences community. The JOIDES Resolution is an international asset with a unique sampling capability but the lifetime of the ODP may not extend beyond 1998.
Dalziel, I. W. D., On the organization of American plates in the Neoproterozoic and the breakout of Laurentia, GSA Today, 2 (2), 237-241, 1992, #940
Dalziel, I. W. D., D. V. Kent, and S. B. Mukasa, The southern rim of the Pacific Ocean: Preliminary geologic report of the Amundsen Sea-Bellingshausen Sea cruise of the Polar Sea, 12 February - 21 March 1992, Antarctic J. of the U. S., Review, 27 (2), 11-14, 1992, #959
Dalziel, I. W. D., Orogenic belts, in Chapter 3, Arthur Holmes' Physical Geology (Fourth Edition), edited by P.M. Duff, Chapman and Hall, New York, 724-777, 1992, #1030
Grunow, A. M., I. W. D. Dalziel, T. M. Harrison, and M. T. Heizler, Structural geology and geochronology of subduction complexes along the margin of Gondwanaland: New data from the Antarctic Peninsula and southernmost Andes, Geol. Soc. Amer. Bull., 104, 1497-1514, 1992, 47 citations, doi:10.1130/0016-7606(1992)104<1497:SGAGOS>2.3.CO;2, #906
Subduction complexes along the Andean margin in central and southern Chile yield mid-Paleozoic to lower Mesozoic ages, yet they crop out within 100 km of the modern trench that shows evidence of active accretion along much of its length. The scarcity of uplifted subduction-complex rocks younger than mid-Mesozoic along the Chilean margin and in parts of the Scotia Arc suggests to us that these old, crystalline rocks, uplifted in the Triassic and Jurassic, represent a boundary in the forearc beyond which tectonic erosion does not easily occur.
Greenschist-, blueschist-, and amphibolite-facies subduction-complex rocks from the Scotia Arc were originally thought to be a simple continuation of the subduction complexes in Chile. Based on new 40Ar/39Ar ages, the Scotia Arc subduction complexes reveal a complex history related to distinct local tectonic events and are not a simple continuation of the old accretionary prism in Chile. Structural and metamorphic analysis indicates the earliest and most penetrative deformation in the subduction complexes around the Scotia Arc occurred at some depth in a subduction zone or zones, certainly below the brittle-ductile transition, and in some cases under blueschist-facies conditions. We believe that the early subduction-related deformation and metamorphism in the greenschist- and blueschist-facies rocks of the Scotia Arc to be overprinted by mid-Cretaceous transpression along the South America-Antarctica plate boundary in the case of Elephant Island, transpression and subsequent localized transtension in the earliest Cenozoic in the case of the Darwin Complex, a mid-Cenozoic spreading-rate change in the case of Smith Island, and early Neogene initiation of Drake Passage opening/Shackleton Fracture Zone formation in the case of the Gibbs Island subgroup.
Moores, E. M., and I. W. D. Dalziel, Reply,
to "Southwest U.S.-east Antarctic (SWEAT) connection: A hypothesis" and "Pacific margins of Laurentia and East Antarctica-Australia as a conjugate rift pair: Evidence and implications for an Eocambrian supercontinent", Geology, 20, 87-88, 1992, 3 citations, doi:10.1130/0091-7613(1992)020<0087:CAROSU>2.3.CO;2, #919
Sandwell, D. T., L. A. Lawver, I. W. D. Dalziel, W. H. F. Smith, and M. Wiederspahn, Antarctica: Gravity Anomaly and Infrared Satellite Image, (map), Scripps Inst. Oceanography and Inst. for Geophys., Univ. Texas, 1992, #962
Storey, B. C., T. Alabaster, D. I. M. Macdonald, I. L. Millar, R. J. Pankhurst, and I. W. D. Dalziel, Upper Proterozoic rift-related rocks in the Pensacola Mountains, Antarctica: Precursors to supercontinent breakup?, Tectonics, 11, 1392-1405, 1992, 27 citations, #1031
Sedimentological and structural studies in the Pensacola Mountains, Antarctica, suggest that upper Precambrian clastic sedimentary rocks of the Patuxent Formation and associated bimodal volcanic rocks formed in an intracontinental rift setting. The turbidites of the Patuxent Formation are part of a large depositional system, derived from a continental source. Interbedded pillow basalts and basaltic sills have trace and rare earth element signatures enriched relative to mid-ocean ridge basalt and similar to some rift-related tholeiitic suites. Nd and Sr isotopic values are compatible with derivation from a lithospheric mantle source in a continental setting. Associated felsic volcanic rocks have crustal trace element and isotopic characteristics. The rifting may have been a prelude to the fragmentation of a supercontinent and, according to recent hypotheses, the separation of Laurentia from Antarctica. Comparisons between the late Precambrian and Cambrian records of western North America and Antarctica suggest that, if these were conjugate margins, separation must have been Neoproterozoic rather than Cambrian in age.
Barker, P. F., I. W. D. Dalziel, and B. C. Storey, Tectonic development of the Scotia arc region, in Geology of Antarctica, edited by R. J. Tingey, Oxford Univ. Press, 215-248, 1991, #831
Cunningham, W. D., K. A. Klepeis, W. A. Gose, and I. W. D. Dalziel, The Patagonian orocline: New paleomagnetic data from the Andean magmatic arc in Tierra del Fuego, Chile, J. Geophys. Res., 96, 16061-16067, 1991, 19 citations, #875
The Hardy Formation is a 1300-m-thick succession of Upper Jurassic-Lower Cretaceous volcaniclastic sedimentary rocks interbedded with lava flows on Hoste Island at the southernmost tip of South America (55.5°S, 291.8°E). The strata are gently folded and metamorphosed to the prehnite-pumpellyite grade. A well-defined characteristic direction of magnetization, carried by magnetite, was readily identified in 95 samples from seven sites. At a given site, the directions group slightly better without structural correction. However, the means of the seven sites cluster better without tilt correction at the 99% significance level, implying that the magnetization postdates the folding event. It is most likely that the magnetization was acquired during the mid- to Late Cretaceous Andean orogeny that involved the folding and emplacement of the Patagonian Batholith. The fact that all samples are normally magnetized supports this age assignment. The pole position of 42.9°N, 156.6°E, α95=3.3° implies that the sampling area has rotated counterclockwise relative to cratonic South America by 90.1 ± 11.9° with no significant flattening of inclination (F=1.9 ± 3.7°). Geologic considerations indicate that the rotation involved the entire Andean magmatic arc in Tierra Del Fuego. The results support interpretation of the Hardy Formation as part of the Andean magmatic arc deposited on the Pacific side of the Late Jurassic-Early Cretaceous Rocas Verdes marginal basin. Oroclinal bending of the arc in southernmost South America accompanied inversion of the marginal basin and the development of a Late Cretaceous-Cenozoic left-lateral transform system (South America-Antarctica) that later developed into the North Scotia Ridge.
Dalziel, I. W. D., Pacific margins of Laurentia and East Antarctica-Australia as a conjugate rift pair: Evidence and implications for an Eocambrian supercontinent, Geology, 19, 598-601, 1991, 513 citations, doi:10.1130/0091-7613(1991)019<0598:PMOLAE>2.3.CO;2, #866
Evidence supports the hypothesis that the Laurentian and East Antarctic-Australian cratons were continuous in the late Precambrian and that their Pacific margins formed as a conjugate rift pair. Both margins extend for approximately 40° of latitude. They have a similar rift history throughout their lengthâi.e., Late Proterozoic rifting and Early Cambrian carbonate platform development. A geometrically acceptable computer-generated reconstruction for the latest Precambrian juxtaposes and aligns the Grenville front that is truncated at the Pacific margin of Laurentia and a closely comparable tectonic boundary in East Antarctica that is truncated along the Weddell Sea margin. These may prove to be critical, perhaps even unique, "piercing points" for relating the northern and southern continents. Geologic and paleomagnetic evidence also suggests that the Atlantic margin of Laurentia rifted from the proto-Andean margin of South America in earliest Cambrian time. Early Phanerozoic sea-floor spreading that isolated Laurentia from South America and East Antarctica-Australia in an Eocambrian supercontinent appears to balance convergence along the Mozambique suture which resulted in final amalgamation of the smaller Gondwana supercontinent at ∼500 Ma.
Grunow, A. M., D. V. Kent, and I. W. D. Dalziel, New paleomagnetic data From Thurston Island: Implications for the tectonics of West Antarctica and Weddell Sea opening, J. Geophys. Res., 96, 17935-17954, 1991, 60 citations, #888
Paleomagnetic data from three West Antarctic crustal blocks (Antarctic Peninsula (AP), Thurston Island-Eights Coast (TI), and the Ellsworth-Whitmore Mountains (EWM) indicate that there has been motion between the individual blocks and motion relative to East Antarctica during the Mesozoic. A Triassic paleomagnetic pole from the TI block (116°E, 61°S, A 95 = 19.4°, N = 3 VGPs) appears to indicate that the block has rotated ∼90° relative to East Antarctica between 230 Ma and 110 Ma. Our previously reported Middle Jurassic paleomagnetic pole from the EWM block indicates that a 90° rotation relative to East Antarctica occurred sometime between the Cambrian and 175 Ma. We believe that the 90° counterclockwise EWM rotation occurred between ∼220 Ma and 175 Ma related to the development of post-Gondwanide Orogeny shear zones. The motion of the AP, TI, and EWM blocks appears to be linked during the mid- to late Mesozoic to three major events in the evolution of the southern ocean basins. Opening in the Mozambique-Somali-Weddell Sea basins may have produced major counterclockwise rotation of the TI block with respect to East Antarctica between the Jurassic and Early Cretaceous based on new Late Jurassic (145°E, 64.5°S, A 95 = 7°,N = 5 VGPs) poles. We believe that the TI rotation, as well as deformation in the southern AP block, was caused by collision and shearing of the EWM block against the other two as the EWM block moved southward with East Antarctica. An Early Cretaceous paleomagnetic pole (232°E, 49°S, A 95 = 7.9°, N = 5 VGPs) from the TI block requires that between the Early and mid- Cretaceous there was clockwise rotation, with respect to East Antarctica, of the AP-TI-EWM blocks (an entity we call Weddellia). A change in the opening history of the Weddell Sea basin caused by initiation of spreading in the South Atlantic ocean basin at ∼130 Ma probably started Weddellia's clockwise rotation. Two new ∼110 and ∼90 Ma poles from the TI block (210°E, 73°S, A 95 = 7.6°,N = 7 VGPs and 161°E, 81°S, A 95 = 3.9°,N = 18 VGPs, respectively) are similar to equivalent age poles from the AP block and East Antarctica and indicate that Weddellia was at or near its present-day position with respect to East Antarctica by ∼110 Ma. This corresponds to a time of major plate reorganization in the South Atlantic and southeast Indian Oceans. Based on both the new TI paleomagnetic data and previously reported data from Marie Byrd Land (MBL), dextral shearing would be expected to have occurred between MBL and Weddellia since the mid-Cretaceous. Pine Island Bay, the area between the TI and MBL blocks, marks a fundamental and complex tectonic boundary in West Antarctica that we propose has largely been a zone of transcurrent shearing.
Storey, B. C., R. J. Pankhurst, I. L. Millar, I. W. D. Dalziel, and A. M. Grunow, A new look at the geology of Thurston Island, in Geologic Evolution of Antarctica, edited by M. R. A. Thomson, J. A. Crame, and J. M. Thomson, Cambridge Univ. Press, 399-403, 1991, #1137
Dalziel, I. W. D., Last unspoiled continent, 1990 World Book Yearbook, World Book, 124-139, 1990, #839
Dalziel, I. W. D., and R. L. Brown, Tectonic denudation of the Darwin metamorphic core complex in the Andes of Tierra del Fuego, southernmost Chile: Implications for cordilleran orogenesis, Geology, 17, 699-703, 1989, 59 citations, doi:10.1130/0091-7613(1989)017<0699:TDOTDM>2.3.CO;2, #762
Cordillera Darwin in the southernmost Chilean Andes is a tectonically denuded metamorphic core complex similar to those widely developed in the North American Cordillera. Detailed comparison can be drawn with the Shuswap metamorphic core complex in British Columbia and Washington. Analysis of the geotectonic history of the Darwin complex reinforces the view that the presence of tectonically thickened continental crust is critical for development of most, if not all, known core complexes and also suggests that inversion of a marginal basin during a compressional regime is one way of initiating such a crustal welt. The Cordillera Darwin complex is unique in the Andes and Antarctandes. This indicates that although low-angle normal faulting occurs at high levels in convergent plate settings such as the Central Andes and the Himalayas, extensional denudation of deep crustal levels in active Andean-type orogens may require special tectonic circumstances. Uplift of the Darwin complex began at about 70 Ma in a localized extensional setting within the developing transform zone between the South American and Antarctic plates. This strike-slip setting is analogous to some metamorphic core complexes in North America and possibly to a recently identified "core complex" in New Zealand. It is likely to be a common one for core complexes in convergent orogens.
Dalziel, I. W. D., Tectonics of the Scotia Arc, Antarctica, Guidebook to Field Trip T180, 28th Int. Geol. Cong., Washington, D.C., Amer. Geophys. Union, 206 pp., 1989, #781
Dalziel, I. W. D., Circum-Pacific orogenic processes: A view from the southernmost Andes and the Antarctandes, in Geology of the Andes and its Relation to Hydrocarbon and Mineral Resources, edited by G. E. Ericksen, M. T. Canas Pinochet, and J. A. Reinemund, Circum-Pacific Council for Energy & Mineral Resources, Earth Sci. Series, 11, 13-22, 1989, #838
Dalziel, I. W. D., Field trip launches year of the IGC, Geotimes, 34 (4), 10-13, 1989, #1131
Dalziel, I. W. D., and H. Zimmerman, Antarctic geoscience initiative, Eos, Trans. Amer. Geophys. Un., 70, 546, 1989, doi:10.1029/89EO00132, #1132
Storey, B. C., I. W. D. Dalziel, S. W. Garrett, A. M. Grunow, R. J. Pankhurst, and W. R. Vennum, West Antarctica in Gondwanaland: Crustal blocks, reconstruction and breakup processes, Tectonophysics, 155, 381-390, 1988, 33 citations, doi:10.1016/0040-1951(88)90276-4, #1130
A combined BAS-USARP West Antarctic Tectonic Project has provided new constraints on the crustal structure and geological evolution of West Antarctica and its relationship to the rest of Gondwanaland. Rb-Sr age dating has confirmed the presence of 1200-1000 m.y. old Precambrian gneisses within West Antarctica and aeromagnetic data have defined the extent of this Precambrian crustal block. Paleomagnetic data have constrained the position of four of the five geologically distinctive West Antarctic crustal blocks within Gondwanaland. They can be restored to their original position prior to breakup by 15°â25° counter clockwise rotation with little or no relative displacement between the blocks. A major period of within-plate middle Jurassic magmatism, associated with an important crustal melting event that is clearly related to the breakup of the supercontinent, has also been identified based on new isotopic and geochemical data.
Dalziel, I. W. D., S. W. Garrett, A. M. Grunow, R. J. Pankhurst, B. C. Storey, and W. R. Vennum, The Ellsworth-Whitmore mountains crustal block: Its role in the tectonic evolution of West Antarctica, in Gondwana Six: Structure, Tectonics & Geophysics, edited by G. D. McKenzie, Geophys. Monog. 40, Amer. Geophys. Union, 173-182, 1987, #1008
Dalziel, I. W. D., B. C. Storey, S. W. Garrett, A. M. Grunow, L. D. B. Herrod, and R. J. Pankhurst, Extensional tectonics and the fragmentation of Gondwanaland, in Continental Extensional Tectonics, edited byM. P. Coward, J. F. Dewey, and P. L. Hancock, Geol. Soc. (London), Spec. Publ., 28, 433-441, 1987, #1128
Dalziel, I. W. D., and R. J. Pankhurst, Joint UK-US West Antarctic tectonics project: An introduction, in Gondwana Six: Structure, Tectonics, and Geophysics, edited by G. D. Mckenzie, American Geophysical Union, Geophys. Monoraph, 40, 107-108, 1987, #1363
Grunow, A. M., I. W. D. Dalziel, and D. V. Kent, Ellsworth-Whitmore mountains crustal block, western Antarctica: New paleomagnetic results and their tectonic significance, in Gondwana Six: Structure, Tectonics, and Geophysics, edited by G. D. McKenzie, Amer. Geophys. Union, Geophys. Monog. , 40, 161-171, 1987, #739
Grunow, A. M., D. V. Kent, and I. W. D. Dalziel, Mesozoic evolution of West Antarctica and the Weddell Sea Basin: New paleomagnetic constraints, Earth Planet. Sci. Lett., 86, 16-26, 1987, 29 citations, doi:10.1016/0012-821X(87)90184-1, #777
Paleomagnetic data from the Antarctic Peninsula and our recent results from the Ellsworth-Whitmore Mountains block suggest that since the Middle Jurassic these two West Antarctic blocks have undergone little relative movement and together have rotated relative to the East Antarctic craton. New data from Lower Cretaceous rocks from the Thurston Island region of West Antarctica suggest that on the basis of paleomagnetic constraints, the Antarctic Peninsula, Ellsworth-Whitmore Mountains and Thurston Island blocks define a single entity which we call Weddellia; some motion between these blocks is possible within the limits of the paleomagnetic data.
Between the Middle Jurassic and Early Cretaceous, Weddellia remained attached to West Gondwanaland while East Antarctica moved southward (dextrally) relative to Weddellia. From the Early Cretaceous to mid-Cretaceous, Weddellia rotated clockwise 30° and moved sinistrally approximately 2500 km relative to East Antarctica, to its present-day position. We suggest the Early to mid-Cretaceous to be the time of the main if not initial opening of the Weddell Sea.
Storey, B. C., and I. W. D. Dalziel, Outline of the structural and tectonic history of the Ellsworth Mountains - Thiel Mountains Ridge, West Antarctica, in Gondwana Six: Structure, Tectonics, and Geophysics, edited by G. D. McKenzie, Amer. Geophys. Un., Geophys. Monograph, 40, 117-128, 1987, #778