Dr. Gail Christeson
Publications
Christeson, G. L., J. V. Morgan, and M. R. Warner, Shallow oceanic crust: Full waveform tomographic images of the seismic later 2A/2B boundary, J. Geophys. Res., 117, B05101, 2012, doi:10.1029/2011JB008972, #2477 
We present results of full-waveform tomographic inversions of four profiles acquired over young intermediate- and fast spreading rate oceanic crust. The mean velocity-depth functions from our study include a 0.25-0.30 km-thick low-velocity, low-gradient region beneath the seafloor overlying a 0.24-0.28-km-thick high-gradient region; together these regions compose seismic layer 2A. Mean layer 2A interval velocities are 3.0-3.2 km/s. The mean depth to the layer 2A/2B boundary is 0.49-0.54 km, and mean velocities within the upper 0.25 km of layer 2B are 4.7-4.9 km/s. Previous velocity analyses of the study areas using 1-D ray tracing underestimate the thickness of the high-gradient region at the base of layer 2A. We observe differences in the waveform inversion velocity models that correspond to imaging of the layer 2A event; regions with a layer 2A event have higher velocity gradients at the base of layer 2A. Intermittent high velocities, which we interpret as massive flows, are observed in the waveform inversion velocity models at 0.05-0.10 km below the seafloor (bsf) over 10-25% of the intermediate-spreading profiles and 20-45% of the fast spreading profiles. The high-gradient region located 0.25-0.54 km bsf at the base of layer 2A may be associated with an increased prevalence of massive flows, the first appearance of dikes (lava-dike transition zone), or with increased crack sealing by hydrothermal products. The upper portion of layer 2B, which begins at 0.49-0.54 km bsf, may correspond to sheeted dikes or the top of the transition zone of lavas and dikes.
Worthington, L. L., H. J. A. Van Avendonk, S. P. S. Gulick, G. L. Christeson, and T. L. Pavlis, Crustal structure of the Yakutat terrane and the evolution of subduction and collision in southern Alaska, J. Geophys. Res., 117, B01102, 2012, doi:10.1029/2011JB008493, #2430
We present a two-dimensional velocity model to constrain crustal thickness and composition of the Yakutat terrane in the northern Gulf of Alaska. The model was constructed using seismic reflection and refraction data along a ~455 km onshore-offshore profile. Our model shows that the crystalline crust composing the Yakutat terrane is wedge-shaped, with crustal thickness increasing west to east from ~15 km to ~30 km. Crustal velocity and structure are continuous across the terrane, with lower crustal velocities > 7 km/s, suggesting that the Yakutat terrane is an oceanic plateau across its entire offshore extent rather than a composite oceanic-continental terrane as previously proposed. The thickest Yakutat crust is entering the adjacent St. Elias orogen where elevated exhumation rates and concentrated seismicity in this vicinity are likely influenced by incipient Yakutat-North America collision. Our model includes a ~8 km thick low-velocity crustal cap extending across the eastern portion of the profile where shallow basement is imaged on marine seismic reflection data. We interpret this cap as a lithified, metamorphosed remnant accretionary prism, providing evidence of a previous attempt at Yakutat subduction along its eastern margin prior to current emplacement at the southern Alaska margin.
Aitken, T. J., P. Mann, A. Escalona, and G. L. Christeson, Evolution of the Grenada and Tobago basins and implications for arc migration, Marine Petroleum Geol., 28, 235-258, 2011, doi:10.1016/j.marpetgeo.2009.10.003, #2121
The tectonic mechanisms controlling how volcanic arcs migrate through space and geologic time within dynamic subduction environments is a fundamental tectonic process that remains poorly understood. This paper presents an integrated stratigraphic and tectonic evolution of Late Cretaceous to Recent volcanic arcs and associated basins in the southeastern Caribbean Sea using seismic reflection data, wide-angle seismic refraction data, well data, and onland geologic data. We propose a new tectonic model for the opening of the Grenada and Tobago basins and the 50-250-km eastward jump of arc volcanism from the Late Cretaceous Aves Ridge to the Miocene to Recent Lesser Antilles arc in the southeast Caribbean based on the mapping of three seismic megasequences. The striking similarity of the half-graben structure of the Grenada and Tobago basins that flank the Lesser Antilles arc, their similar smooth basement character, their similar deep-marine seismic facies, and their similar Paleogene sediment thickness mapped on a regional grid of seismic data suggest that the two basins formed as a single, saucer-shaped, oceanic crust Paleogene forearc basin adjacent to the now dormant Aves Ridge. This single forearc basin continued to extend and widen through flexural subsidence during the early to middle Eocene probably because of slow rollback of the subducting Atlantic oceanic slab. Rollback may have been accelerated by oblique collision of the southern Aves Ridge and southern Lesser Antilles arc with the South American continent. Uplift and growth of the southern Lesser Antilles arc divided the Grenada and Tobago basins by early to middle Miocene time. Inversion of normal faults and uplift effects along both edges of the Lesser Antilles arc are most pronounced in its southern zone of arc collision with the South American continent. The late Miocene to Recent depositional histories of the Grenada and Tobago basins are distinct because of isolation of the Grenada basin by growth and uplift of the Neogene Lesser Antilles volcanic ridge.
Kroehler, M. E., P. Mann, A. Escalona, and G. L. Christeson, Late Cretaceous-Miocene diachronous onset of backt hrusting along the south Caribbean deformed belt and its importance for understanding processes of arc collision and crustal growth, Tectonics, 30, TC6003, 2011, doi:10.1029/2011TC002918, #2392
The Venezuelan basin is obliquely subducted to the east-southeast beneath the continental South American plate at the east-west-trending South Caribbean deformed belt (SCDB), a 50-100-km-wide wedge of accreted sedimentary rocks derived both from offscraping of the 0.5 to 1 km-thick sedimentary cover of the subducting Venezuelan basin and from the incorporation of clastic sediments derived from the continental margin of South America. In this paper we describe the structure and sequence stratigraphy of five Late Cretaceous to recent sedimentary sequences overlying a 230,000 km2 area of the southern Venezuelan basin and Beata Ridge. The dataset includes 5900 km of 2D seismic reflection data that are tied to lithologic and age data taken from DSDP sites 146/149 and 150 in the central part of the Venezuelan basin. We use observed patterns of wedging in mapped and dated sedimentary sequences adjacent to the SCDB to determine the age of the diachronous west-to-east initiation of thrusting and subduction at the SCDB. The onset of subduction in the SCDB corresponds with the cessation of thrusting from west to east along the zone of collision between the intraoceanic Caribbean arc and the South American continent. This correlation in the age of deformation indicates that collision between the Caribbean arc and the South American continent led to subduction polarity reversal, a process that is commonly observed at various stages of tectonic evolution in other areas of active, arc-continent collision including Japan and Sunda.
Morgan, J. V., M. W. Warner, G. S. Collins, R. A. F. Grieve, G. L. Christeson, S. P. S. Gulick, and P. J. Barton, Full waveform tomographic images of the peak ring at the Chicxulub impact crater, J. Geophys. Res., 116, B06303, 2011, doi:10.1029/2010JB008015, #2348
Peak rings are a feature of large impact craters on the terrestrial planets and are generally believed to be formed from deeply buried rocks that are uplifted during crater formation. The precise lithology and kinematics of peak ring formation, however, remains unclear. Previous work has revealed a suite of bright inward dipping reflectors beneath the peak ring at the Chicxulub impact crater and that the peak ring was formed from rocks with a relatively low seismic velocity. New twoÃÆââââ¬Å¡Ã¬ÃâÃÂdimensional, full waveform tomographic velocity images show that the uppermost lithology of the peak ring is formed from a thin (ÃÆâÃâ¹Ã¢â¬Â Ãâü100ÃÆââââ¬Å¡Ã¬Ã¢ââ¬Ã
â200 m thick) layer of lowÃÆââââ¬Å¡Ã¬ÃâÃÂvelocity (ÃÆâÃâ¹Ã¢â¬Â Ãâü3000ÃÆââââ¬Å¡Ã¬Ã¢ââ¬Ã
â3200 m/s) rocks. This lowÃÆââââ¬Å¡Ã¬ÃâÃÂvelocity layer is most likely composed of highly porous, allogenic impact breccias. Our models also show that the change in velocity between lithologies within and outside the peak ring is more abrupt than previously realized and occurs close to the location of the dipping reflectors. Across the peak ring, velocity appears to correlate well with predicted shock pressures from a dynamic model of crater formation, where the rocks that form the peak ring originate from an uplifted basement that has been subjected to high shock pressures (10ÃÆââââ¬Å¡Ã¬Ã¢ââ¬Ã
â50 GPa) and lie above downthrown sedimentary rocks that have been subjected to shock pressures of <5 GPa. These observations suggest that low velocities within the peak ring may be related to shock effects and that the dipping reflectors underneath the peak ring might represent the boundary between highly shocked basement and weakly shocked sediments.
Reece, R. S., S. P. S. Gulick, B. K. Horton, G. L. Christeson, and L. L. Worthington, Tectonic and climate influence on the evolution of the Surveyor Fan and Channel system, Gulf of Alaska, Geosphere, 7, 830-844, 2011, doi:10.1130/GES00654.1, #2346
Barton, P. J., R. A. F. Grieve, J. V. Morgan, A. T. Surendra, P. M. Vermeesch, G. L. Christeson, S. P. S. Gulick, and M. W. Warner, Seismic images of Chicxulub impact melt sheet and comparisons with the Sudbury structure, in Large Meteorite Impacts and Planetary Evolution IV, Geological Society of America Special Paper 465,edited by R. L. Gibson and W. U. Reimold, 103-113, 2010, doi:10.1130/2010.2465(07), #2069
Chicxulub is the only known impact structure on Earth with a fully preserved peak ring, and it forms an important natural laboratory for the study of large impact structures and understanding of large-scale cratering on Earth and other planets. Seismic data collected in 1996 and 2005 reveal detailed images of the uppermost crater in the central basin at Chicxulub. Seismic reflection profiles show a reflective layer ~1 km beneath the apparent crater floor, topped by upwardly concave reflectors interpreted as saucer-shaped sills. The upper part of this reflective layer is coincident with a thin high-velocity layer identified by analyzing refractions on the 6 km seismic streamer data. The high-velocity layer is almost horizontal and appears to be contained within the peak ring structure. We argue that this reflective layer is the predicted coherent melt sheet formed during impact, and it may be comparable with the unit known as the Sudbury Igneous Complex at the Sudbury impact structure. The Sudbury Igneous Complex, interpreted as a differentiated impact melt sheet, appears to have a similar scale and geometry, and an uppermost lithological sequence consisting of a high velocity layer at the top and a velocity inversion beneath. This comparison suggests that the Chicxulub impact structure also contains a coherent differentiated melt sheet.
Christeson, G. L., J. A. Karson, and K. D. McIntosh, Mapping of seismic layer 2A/2B boundary above the sheeted dike unit at intermediate-spreading crust exposed near the Blanco Transform, Geochem., Geophys., Geosyst., 11, Q03015, 2010, 2 citations, doi:10.1029/2009GC002864, #2183
We present results from a study mapping the seismic layer 2A/2B boundary in young ocean crust located adjacent to the north wall of the Blanco Transform Fault (BTF). We review seafloor features, geochemical and petrologic data, seismic imaging, and seismic structure and conclude that the BTF seismic study region is representative of crust formed at the intermediate spreading Juan de Fuca Ridge. The mean layer 2A two-way travel time beneath the seafloor is 0.37 ± 0.10 s for the 42 à 12 km seismic survey area. Several regions are observed where layer 2A is consistently thin or thick over lateral distances of 5â10 km, both in a ridge-parallel and ridge-perpendicular direction. Layer 2A thicknesses appear more variable in the ridge-parallel (isochron) direction than the ridge-perpendicular (flow line) direction. There is no systematic pattern of layer 2A thickness variability with distance from the BTF, nor is there a correlation between seafloor topography and layer 2A thickness. Calculated mean layer 2A thickness for the seismic study region is 485 ± 135 m assuming an interval velocity of 2.65 km/s. The imaged layer 2A/2B boundary projects on average ∼600â650 m above the top of the sheeted dike complex mapped on the adjacent BTF north wall. We argue that the geologic context of the layer 2A/2B boundary will vary as competing processes of magmatic construction and fracturing (increasing porosity) and crustal thickening, compaction, dike intrusion, and hydrothermal sealing (decreasing porosity) vary as the crust spreads laterally from the ridge axis to ridge flanks.
Christeson, G. L., S. P. S. Gulick, H. J. A. Van Avendonk, L. L. Worthington, R. S. Reece, and T. L. Pavlis, The Yakutat terrane: Dramatic change in crustal thickness across the transition fault, Alaska, Geology, 38, 895-898, 2010, 3 citations, doi:10.1130/G31170.1, #2267
We present new constraints on the crustal structure of the Yakutat terrane and evidence of the role of the Transition fault in southern Alaska. The Yakutat terrane south of Yakutat Bay includes crystalline crust that is 24ââ¬â27 km thick overlain by sedimentary units that are 4.5ââ¬â7.5 km thick. The Yakutat terrane crustal thickness and velocity structure are consistent with an oceanic plateau origin. The southern edge of the Yakutat terrane is bounded by the Transition fault, which is imaged as a near-vertical fault zone ~1 km wide. The Transition fault is coincident with a dramatic change in Moho depth from 32 km for Yakutat oceanic plateau crust to 11.5 km for Pacific Ocean crust occurring over a horizontal distance of 0ââ¬â5 km. There is no evidence for underthrusting of the Pacific Ocean crust beneath the Yakutat terrane at the Transition fault. We argue that the Yakutat terrane formed on the Kula or Farallon plate and was later juxtaposed next to younger Pacific Ocean crust by the Transition fault.
Hayman, N. W., W. Bach, D. K. Blackman, G. L. Christeson, K. Edwards, R. Haymon, B. Ildefonse, P. Schulte, D. A. H. Teagle, and S. White, Future scientific drilling of oceanic crust, Eos, Trans. Amer. Geophys. Un., 91, 133-134, 2010, #2198
Schulte, P., L. Alegret, I. Arenillas, J. Antonio Arz, P. R. Barton, P. R. Bown, T. J. Bralower, G. L. Christeson, P. Claeys, C. S. Cockell, G. S. Collins, A. Deutsch, T. J. Goldin, K. Goto, J. M. Grajales-Nishimura, R. A. F. Grieve, S. P. S. Gulick, K. R. Johnson, W. Kiessling, C. Koeberl, D. A. Kring, K. G. MacLeod, T. Matsui, J. Melosh, A. Montanari, C. R. Neal, D. J. Nichols, R. D. Norris, E. Pierazzo, G. Ravizza, M. Rebolledo-Vieyra, W. U. Reimold, E. Robin, T. Salge, R. P. Speijer, A. R. Sweet, J. Urrutia-Fucugauchi, V. Vajda, M. T. Whalen, and P. S. Willumsen, The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary, Science, 327, 1214-1218, 2010, 48 citations, doi:10.1126/science.1177265, #2184
The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.
Christeson, G. L., G. S. Collins, J. V. Morgan, S. P. S. Gulick, P. J. Barton, and M. R. Warner, Mantle deformation beneath the Chicxulub impact crater, Earth Planet. Sci. Lett., 284, 249-257, 2009, 2 citations, doi:10.1016/j.epsl.2009.04.033, #2068
The surface expression of impact craters is well-known from visual images of the Moon, Venus, and other planets and planetary bodies, but constraints on deep structure of these craters is largely limited to interpretations of gravity data. Although the gravity models are non-unique, they do suggest that large impact craters are associated with structure at the base of the crust. We use seismic data to map Moho (crustâmantle interface) topography beneath the Chicxulub crater, the youngest and best preserved of the three largest known terrestrial impact craters. The Moho is upwarped by ~ 1.5â2 km near the center of the Chicxulub crater, and depressed by ~ 0.5â1.0 km at a distance of ~ 30â55 km from the crater center. A comparison with numerical modeling results reveal that immediately following impact a transient crater reached a maximum depth of at least 30 km prior to collapse, and that subsequent collapse of the transient crater uplifted target material from deep below the crater floor. These results demonstrate that deformation from large terrestrial impacts can extend to the base of the continental crust. A similar Moho topography is also modeled for some large lunar and Martian craters, which suggests that mantle deformation may play a prominent role in large crater formation.
Vermeesch, P. M., J. V. Morgan, G. L. Christeson, P. J. Barton, and A. T. Surendra, Three-dimensional joint inversion of traveltime and gravity data across the Chicxulub impact crator, J. Geophys. Res., 114, B02105, 2009, 4 citations, doi:10.1029/2008JB005776, #2063
In 2005 an extensive new seismic refraction data set was acquired over the central part of the Chicxulub impact crater, allowing us to image its structure with much better resolution than before. However, models derived from traveltime data are limited by the available ray coverage and the nonuniqueness that is inherent to all geophysical methods. Therefore, many different models can fit the data equally well. To address these issues, we have developed a new method to simultaneously invert traveltime and gravity data to obtain an integrated model. To convert velocity to density, we use a linear relationship derived from measurements on core from the Chicxulub impact basin, thus providing a reliable conversion equation that is typical for lithologies of the central part of this crater. Prior to utilizing the inversion on the observed data, we have run a suite of tests to establish the optimum weighting between traveltime and gravity constraints, using a synthetic model of central crater structure and the real experimental geometry. These synthetic tests indicate which inversion parameters lead to the best recovery of subsurface structure, as well as which parts of the model are well resolved. We applied the method to all existing gravity data and to seismic refraction data acquired in 1996 and the new, higher-resolution seismic refraction data acquired in 2005. We favor the traveltime model wherever we have sufficient ray coverage and the joint model where we have no ray coverage.
Christeson, G. L., P. Mann, A. Escalona, and T. J. Aitken, Crustal structure of the Caribbean-northeastern South America arc-continent collision zone, J. Geophys. Res., 113, B08104, 2008, 9 citations, doi:10.1029/2007JB005373, #1992
We present the results of a 568-km-long regional wide-angle seismic profile conducted in the southeastern Caribbean that crosses an active island arc, a remnant arc, two basins possibly floored by oceanic crust, an allochthonous terrane of forearc affinity, and the passive margin of northern South America. The velocity structures of the Late Cretaceous Aves Ridge remnant arc and Miocene and younger Lesser Antilles arc are remarkably similar, which implies that magmatic processes have remained moderately steady over time. Crustal thickness is ∼26 km at the Aves Ridge and ∼24 km at the Lesser Antilles arc. In comparison to the Izu-Bonin and Aleutian arcs, the Lesser Antilles arc is thinner and has no evidence for a lower crustal cumulate layer, which is consistent with the estimated low magma production rates of the Lesser Antilles arc. Crustal thickness beneath the Grenada and Tobago basins is 4â10 km, and the velocity structure suggests that these basins could be floored by oceanic crust. A decrease of ∼1 km/s in average seismic velocity of the upper crust is observed from NW to SE across the North Coast fault zone; we argue that this marks the suture between the far-traveled Caribbean arc and the passive margin of the South American continent. Current strike-slip motion between the Caribbean and South American plates is located ∼30 km to the south, and thus material originally deposited on the South American passive margin has now been transferred to the Caribbean plate.
Collins, G. S., J. V. Morgan, P. J. Barton, G. L. Christeson, S. P. S. Gulick, J. Urrutia-Fucugauchi, M. R. Warner, and K. Wunnemann, Dynamic modeling suggests terrace zone asymmetry in the Chicxulub crater is caused by target heterogeneity, Earth Planet. Sci. Lett., 270, 221-230, 2008, 20 citations, doi:10.1016/j.epsl.2008.03.032, #1934
We investigate the cause of terrace zone asymmetry in the Chicxulub impact crater using dynamic models of crater formation. Marine seismic data acquired across the crater show that the geometry of the crater's terrace zone, a series of sedimentary megablocks that slumped into the crater from the crater rim, varies significantly around the offshore half of the crater. The seismic data also reveal that, at the time of impact, both the water depth and sediment thickness varied with azimuth around the impact site. To test whether the observed heterogeneity in the pre-impact target might have affected terrace zone geometry we constructed two end-member models of upper-target structure at Chicxulub, based on the seismic data at different azimuths. One model, representing the northwest sector, had no water layer and a 3-km thick sediment layer; the other model, representing the northeast sector, had a 2-km water layer above a 4-km sediment layer. Numerical models of vertical impacts into these two targets produced final craters that differ substantially in terrace zone geometry, suggesting that the initial water depth and sediment thickness variations affected the structure of the terrace zone at Chicxulub. Moreover, the differences in terrace zone geometry between the two numerical models are consistent with the observed differences in the geometry of the terrace zone at different azimuths around the Chicxulub crater. We conclude that asymmetry in the pre-impact target rocks at Chicxulub is likely to be the primary cause of asymmetry in the terrace zone.
Gulick, S. P. S., P. J. Barton, G. L. Christeson, J. V. Morgan, M. A. McDonald, K. Mendoza-Cervantes, Z. F. Pearson, A. T. Surendra, J. Urrutia-Fucugauchi, P. M. Vermeesch, and M. R. Warner, Importance of pre-impact crustal structure for the asymmetry of the Chicxulub impact crater, Nature Geoscience, 1, 131-135, 2008, 31 citations, doi:10.1038/ngeo103, #1889
Impact craters are observed on the surfaces of all rocky planets and satellites in our Solar System1; some impacts on Earth, such as the Cretaceous/Tertiary one that formed the Chicxulub impact crater2, 3, have been implicated in mass extinctions4, 5, 6, 7, 8, 9, 10, 11, 12. The direction and angle of the impactâor its trajectoryâis an important determinant of the severity of the consequent environmental damage, both in the downrange direction (direction bolide travels) and in the amount of material that enters the plume of material vaporized on impact2, 13, 14, 15. The trajectory of the Chicxulub impact has previously been inferred largely from asymmetries in the gravity anomalies over the crater2, 3. Here, we use seismic data to image the Chicxulub crater in three dimensions and demonstrate that the strong asymmetry of its subsurface correlates with significant pre-existing undulations on the end-Cretaceous continental shelf that was the site of this impact. These results suggest that for rocky planets, geological and geomorphological heterogeneities at the target site may play an important role in determining impact crater structure, in addition to impact trajectories. In those cases where heterogeneous targets are inferred, deciphering impact trajectories from final crater geometries alone may be difficult and require further data such as the distribution of ejecta.
Christeson, G. L., K. D. McIntosh, and J. A. Karson, Inconsistent correlation of seismic layer 2A and lava layer thickness in oceanic crust, Nature, 445, 418-421, 2007, 23 citations, doi:10.1038/nature05517, #1867
At mid-ocean ridges with fast to intermediate spreading rates, the upper section of oceanic crust is composed of lavas overlying a sheeted dyke complex. These units are formed by dykes intruding into rocks overlying a magma chamber, with lavas erupting at the ocean floor. Seismic reflection data acquired over young oceanic crust commonly image a reflector known as 'layer 2A', which is typically interpreted as defining the geologic boundary between lavas and dykes1, 2, 3. An alternative hypothesis is that the reflector is associated with an alteration boundary within the lava unit4, 5, 6. Many studies have used mapped variability in layer 2A thickness to make inferences regarding the geology of the oceanic crust, including volcanic construction, dyke intrusion and faulting7, 8, 9, 10. However, there has been no link between the geologic and seismological structure of oceanic crust except at a few deep drill holes. Here we show that, although the layer 2A reflector is imaged near the top of the sheeted dyke complex at fast-spreading crust located adjacent to the Hess Deep rift, it is imaged significantly above the sheeted dykes section at intermediate-spreading crust located near the Blanco transform fault. Although the lavas and underlying transition zone thicknesses differ by about a factor of two, the shallow seismic structure is remarkably similar at the two locations. This implies that seismic layer 2A cannot be used reliably to map the boundary between lavas and dykes in young oceanic crust. Instead we argue that the seismic layer 2A reflector corresponds to an alteration boundary that can be located either within the lava section or near the top of the sheeted dyke complex of oceanic crust.
Clark, S. A., D. S. Sawyer, J. A. Austin, G. L. Christeson, and Y. Nakamura, Characterizing the Galicia Bank-Southern Iberia Abyssal Plain rifted margin segment boundary using multichannel seismic and ocean bottom seismometer data, J. Geophys. Res., 112, B03408, 2007, 7 citations, doi:10.1029/2006JB004581, #1544
We present multichannel seismic reflection and ocean bottom seismometer reflection/refraction data from ISE-9, a margin-parallel, northâsouth oriented profile ∼200 km west of the Portuguese coast. ISE-9 images the boundary between two distinct segments of the Iberia nonvolcanic rifted margin: Galicia Bank (GB) and the Southern Iberia Abyssal Plain (SIAP). The bathymetric contrast between GB (2 km depth) and SIAP (4â5 km depth) spans only 25 km. The crustal thickness transition, however, spans 137 km, from 13â18 km thick beneath GB to <2 km thick beneath SIAP. We define this crustal thickness transition as the segment boundary. Crustal structure along the segment boundary, tilted blocks bounded by normal faults, is surprisingly similar to crustal structure observed along orthogonal, eastâwest profiles of the Iberia margin. The apparent northâsouth extension is similar in magnitude to previously calculated eastâwest extension, implying an overall northeastâsouthwest extension. However, paleoreconstructions and rift basin orientations constrain lithospheric extension to a nearly eastâwest direction. We speculate that northâsouth extension is limited to the crust and is caused by large-scale mass wasting sometime between the Tithonian and Valanginian. This rotational slump spans the 137-km-wide modern segment boundary, emplacing GB continental crust directly onto exhumed, serpentinized mantle of SIAP. Palinspastic reconstruction restores the southern edge of the blocks to coincide within <10 km of a steep Moho transition and a near-vertical fault which extends through the slump and offsets Moho. We interpret this location as the preslump segment boundary that accommodated transform motion between the two rifted margin segments.
Levander, A. R., M. Schmitz, H. Ave Lallemant, C. A. Zelt, D. S. Sawyer, M. B. Magnani, P. Mann, G. L. Christeson, J. G. Wright, G. Pavlis, and J. L. Pindell, Evolution of the southern Caribbean plate boundary, Eos, Trans. Amer. Geophys. Un., 87, 97-100, 2006, 11 citations, doi:10.1029/2006EO090001, #1818
It is generally accepted that the cores of the continents, called cratons, formed by the accretion of island arcs into proto-continents and then by proto-continental agglomeration to form the large continental masses. Mantle-wedge processes, combined with higher melting temperatures during the Archean (2.5–3.8 billion years ago) and possibly thrust stacking of highly depleted Archean oceanic lithosphere, produced a strong, buoyant, upper mantle chemical boundary layer. This stabilizing mantle layer, known as the tectosphere, has shielded the Archean cratons from most subsequent tectonic disruption and is highly depleted in iron, providing the positive buoyancy that is required to ‘float’ the continents more than four kilometers above the surrounding ocean basins. What is not clear is whether today the continental mass is growing, shrinking, or is at steady state. A number of continental growth curves have been proposed; the most widely accepted models call for rapid continental growth in the late Archean and Paleoproterozoic (between 3.0 and ˜1.7 billion years ago), followed by slow growth to the present. Whether modern continental accretion and something akin to tectosphere formation are occurring today is an open question. It is not clear how island arcs accrete to the continents, or if modern arcs contribute to continental growth. Seismic observations of arcs worldwide show that the crustal velocity structure is too fast, and hence the chemical composition too silica-poor, to generate an average continental crust without substantial chemical and/or mechanical refining during or subsequent to accretion.
Morgan, J. V., M. R. Warner, J. Urrutia-Fucugauchi, S. P. S. Gulick, G. L. Christeson, P. J. Barton, M. Rebolledo-Vieyra, and H. J. Melosh, Chicxulub crater seismic survey prepares way for future drilling, Eos, Trans. Amer. Geophys. Un., 86, 325-328, 2005, doi:10.1029/2005EO360001, #1786
Sixty-five million years ago, a large meteorite hit the Earth and formed the Ë200-km-wide Chicxulub crater in Yucatán, Mexico. The well-known, massive extinction event at the Cretaceous-Tertiary (K-T) boundary appears to have been caused, at least in part, by this impact. In the first few seconds after impact the surface of the Earth was pushed down to form a cavity Ë35 km deep, and in the next few hundred seconds this cavity collapsed to form a multi-ring basin with an inner peak ring. To examine the rings and subsurface structure of this superbly preserved impact crater, a seismic experiment was shot across the crater in January and February 2005 by a team of scientists from Mexico, the United States, and the United Kingdom (Figure 1).
Bangs, N. L., G. L. Christeson, and T. H. Shipley, Structure of the Lesser Antilles subduction zone backstop and its role in a large accretionary system, J. Geophys. Res., 108, 2358, 2003, 8 citations, doi:10.1029/2002JB002040, #1648
The role of a backstop in subduction zones has been the subject of numerous laboratory and numerical modeling studies; however, few field observations exist revealing how backstops control deformation in subduction zones and accretionary wedge construction. A seismic reflection and refraction survey acquired in 1998 with the R/V Maurice Ewing reveals the geometry of the forearc igneous crust, accretionary wedge, and forearc basin structure of the northern Guadeloupe area of the Lesser Antilles forearc. An accreted block of buoyant crust, accreted in the late Miocene, forms the toe of the overriding arc crust and forms the backstop. We imaged the top of this surface, beneath the forearc basin, to its seaward edge where it meets the subducting oceanic crust. The toe of the backstop was thrust upward and forms a steep buttress in contact with the lower half of the accretionary wedge. The steep buttress produces a narrow inner deformation zone with minimal backthrusting of the accretionary complex landward over the backstop, and a narrow <10 km transition between accreted and forearc basin sediment. Seismic reflections from the subducting crust and the decollement appear beneath the entire accretionary wedge and below the backstop toe. Separating the decollement and the subducting crust is an interval, usually between 500 and 750 m, of underthrust sediment carried underneath the accretionary wedge and subducted 15 km landward and beneath the toe of the backstop. We speculate that the upturned geometry of the toe of the backstop and a weak fluid-rich decollement may facilitate sediment subduction beneath the backstop and potentially into the mantle.
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., N. L. Bangs, and T. H. Shipley, Deep structure of an island arc backstop, Lesser Antilles subduction zone, J. Geophys. Res., 108, 2327, 2003, 8 citations, doi:10.1029/2002JB002243, #1628
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.
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.
Karson, J. A., and G. L. Christeson, Comparison of geologic and seismic structure of uppermost fast-spread oceanic crust, Insights from a crustal cross section of the Hess Deep Rift, in Heterogeneity in the Crust and Upper Mantle, Nature, Scaling and Seismic Properties, 99-129, 2002, #1571
Karson, J. A., E. M. Klein, S. D. Hurst, C. E. Lee, P. A. Rivizzigno, D. Curewitz, A. R. Morris, D. J. Miller, R. G. Varga, G. L. Christeson, B. Cushman, J. M. O'Neill, J. G. Brophy, K. M. Gillis, M. A. Stewart, and A. L. Sutton, Structure of uppermost fast-spread oceanic crust exposed at the Hess Deep Rift: Implications for subaxial processes at the East Pacific Rise, Geochem., Geophys., Geosyst., 3, 1002, 2002, 30 citations, doi:10.1029/2001GC000155, #1826
The uppermost 2 km of the oceanic crust created at the fast spreading (135 mm yr−1, full rate) equatorial East Pacific Rise (EPR) is exposed for tens of kilometers along escarpments bounding the Hess Deep Rift. Mosaics of large-scale digital images from the remotely operated vehicle (ROV) Argo II and direct observations from the submersible Alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading. Dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units. These rock units are characterized by extensive faulting, fine-scale fracturing, and rotations of coherent crustal blocks meters to tens of meters across. The uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces. Through most of the basaltic lava unit, however, lava flow contacts dip (20°â70°W) toward the EPR and generally increase in dip downward in the section. Dikes cutting the lavas and in the underlying sheeted dike unit generally dip (90°â40°E) away from the EPR. Deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units. We interpret this upper crustal structure as the result of subaxial subsidence within 1â2 km of the EPR that accommodated the thickening of the basaltic lava unit to ∼500 m. Variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply. These results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the Hess Deep Rift. If this type of structure is typical of uppermost oceanic crust generated at the EPR, these processes may be common along fast spreading mid-ocean ridges.
Morgan, J. V., G. L. Christeson, and C. A. Zelt, Testing the resolution of a 3D velocity tomogram across the Chicxulub crater, Tectonophysics, 355, 215-226, 2002, 18 citations, doi:10.1016/S0040-1951(02)00143-9, #1545
An integrated offshore/onshore reflection and refraction experiment was shot across the Chicxulub impact crater in 1996. The refraction data were previously inverted in 3D using first-arrival travel-time tomography. A regularized inversion, in which both data misfit and model roughness are minimized simultaneously, was used to determine a smooth velocity tomogram across the inner crater region. However, the experimental geometry for the refraction data was irregular, causing concern that this velocity model might not be well resolved. In this paper, we present a suite of checkerboard tests to investigate the lateral resolution of our velocity model. The Chicxulub crater is located partly onshore and partly offshore, with its centre close to the Yucatan coastline in Mexico. The shallow water limited acquisition of marine reflection data to distances of not closer than 25 km from the crater centre, and the centre of the structure is imaged with refraction data only. Intriguing velocity anomalies were observed across the central crater region, providing constraints on the lithological and structural form of Chicxulub. A high-velocity body within the central crater is most likely to represent lower-crustal rocks that were stratigraphically uplifted during the formation of this complex crater. The concave shape of this stratigraphic uplift suggests clues to the mechanics of large-crater collapseâthe rocks appear to have moved upward and outward. An inward-dipping zone of lowered velocity has been interpreted as delimiting the outer edge of a central zone of melt-rich rocks. The resolution tests presented here indicate that these observed velocity anomalies are likely to be real.
Christeson, G. L., Y. Nakamura, R. T. Buffler, J. V. Morgan, and M. R. Warner, Deep crustal structure of the Chicxulub impact crater, J. Geophys. Res., 106, 21751-21769, 2001, 27 citations, #1543
We present the results of a wide-angle seismic survey conducted over the Chicxulub impact crater. Profile Chicx-A/Al is a chord across the offshore portion of the crater, and Chicx-B/F is an onshore-offshore profile through the center of the crater. We use travel times recorded by 32 ocean bottom seismograph and 19 land-based receivers to model the crustal structure of the crater. The primary feature in the shallow velocity structure is a low-velocity Tertiary basin, the edge of which correlates with a steep gravity gradient in some regions of our survey. Directly beneath the Tertiary basin at the center of the crater is a region of relatively low velocities and densities that is interpreted as a unit of melt rocks (either suevite breccias with isolated melt pods or suevite breccias overlying a coherent melt sheet). This unit has a thickness of ∼1 km and a diameter of ∼100 km. Central uplift with a diameter of 40â60 km is observed on Chicx-B/F. A lower limit on the vertical extent of uplift is 9 km. Shallow basement is also observed along the northwest portion of Chicx-B/F, associated with a prominent northwest trending high in the gravity field. Superimposed on a regional trend is Moho uplift of ∼1 km near the center of the Chicx-A/Al profile, with adjacent Moho deepening of ∼1.25â1.5 km. The Moho topography may be related to deformation processes associated with the formation of the outer ring or to the excavation and collapse of the transient cavity.
Christeson, G. L., K. D. McIntosh, and T. H. Shipley, Seismic attenuation in the Costa Rica margin wedge: Amplitude modeling of ocean bottom hydrophone data, Earth Planet. Sci. Lett., 179, 391-405, 2000, 8 citations, doi:10.1016/S0012-821X(00)00118-7, #1502
Travel-time and amplitude modeling were carried out on data recorded by three ocean bottom seismometers along two strike profiles, located 10â20 km landward of the Middle America Trench offshore Costa Rica. These two profiles bracket a region of rapid change in physical properties and structural styles. The travel-time modeling indicates little structural change along the profiles, and therefore the data are suitable for amplitude modeling with the reflectivity method. For the profile 10 km from the trench, amplitude modeling indicates a margin wedge with velocities of 4â4.4 km/s and Qp=25â50. For the profile 20 km from the trench, the amplitude modeling indicates a margin wedge with velocities of 4.4â4.8 km/s and Qp=50â75. The primary new result of this analysis is the constraint that Qp=25â75 within the margin wedge at both locations. These high attenuation values are consistent with fracturing of the material making up the margin wedge, either ophiolitic Nicoya Complex rocks, dewatered and cemented sediments, or a tectonic mixture of both.
McIntosh, K. D., F. E. Akbar, C. Calderon-Macias, P. L. Stoffa, S. Operto, G. L. Christeson, Y. Nakamura, T. H. Shipley, E. R. Flueh, A. U. Stavenhagen, and G. Leandro, Large aperture seismic imaging at a convergent margin: Techniques and results from the Costa Rica seismogenic zone, Marine Geophysical Researches, 21, 451-474, 2000, 5 citations, doi:10.1023/A:1026597927732, #1514
In March and April 1995 a cooperative German, Costa Rican, and United States research team recorded onshore-offshore seismic data sets along the Pacific margin of Costa Rica using the R/V Ewing. Off the Nicoya Peninsula we used a linear array of ocean bottom seismometers and hydrophones (OBS/H) with onshore seismometers extending across much of the isthmus. In the central area we deployed an OBS/H areal array consisting of 30 instruments over a 9 km by 35-km area and had land stations on the Nicoya Peninsula adjacent to this marine array and also extending northeast on the main Costa Rican landmass. Our goal in these experiments was to determine the crustal velocity structure along different portions of this convergent margin and to use the dense instrument deployments to create migrated reflection images of the plate boundary zone and the subducting Cocos Plate. Our specific goal in the central area was to determine whether a subducted seamount is present at the location of the 1990, M 7 earthquake off the Nicoya Peninsula and can thus be linked to its nucleation. Subsequently we have processed the data to improve reflection signals, used the data to calculate crustal velocity models, and developed several wide-aperture migration techniques, based on a Kirchhoff algorithm, to produce reflection images. Along the northern transect we used the ocean bottom data to construct a detailed crustal velocity model, but reflections from the plate boundary and top and bottom of the subducting Cocos plate are difficult to identify and have so far produced poor images. In contrast, the land stations along this same transect recorded clear reflections from the top of the subducting plate or plate boundary, within the seismogenic zone, and we have constructed a clear image from this reflector beneath the Nicoya shelf. Data from the 3-D seismic experiment suffer from high-amplitude, coherent noise (arrivals other than reflections), and we have tried many techniques to enhance the signal to noise ratio of reflected arrivals. Due to the noise, an apparent lack of strong reflections from the plate boundary zone, and probable structural complexity, the resulting 3-D images only poorly resolve the top of the subducting Cocos Plate. The images are not able to provide compelling evidence of whether there is a subducting seamount at the 1990 earthquake hypocenter. Our results do show that OBS surveys are capable of creating images of the plate boundary zone and the subducting plate well into the seismogenic zone if coherent reflections are recorded at 1.8 km instrument spacing (2-D) and 5 km inline by 1 km crossline spacing for 3-D acquisition. However, due to typical high amplitude coherent noise, imaging results may be poorer than expected, especially in unfavorable geologic settings such as our 3-D survey area. More effective noise reduction in acquisition, possibly with the use of vertical hydrophone arrays, and in processing, with advanced multiple removal and possibly depth filtering, is required to achieve the desired detailed images of the seismogenic plate boundary zone.
Morgan, J. V., M. R. Warner, G. S. Collins, H. J. Melosh, and G. L. Christeson, Peak-ring formation in large impact craters: Geophysical constraints from Chicxulub, Earth Planet. Sci. Lett., 183, 347-354, 2000, 58 citations, doi:10.1016/S0012-821X(00)00307-1, #1546
A seismic reflection and three-dimensional wide-angle tomographic study of the buried, 200-km diameter, Chicxulub impact crater in Mexico reveals the kinematics of central structural uplift and peak-ring formation during large-crater collapse. The seismic data show downward and inward radial collapse of the transient cavity in the outer crater, and upward and outward collapse within the central structurally uplifted region. Peak rings are formed by the interference between these two flow regimes, and involve significant radial transport of material. Hydrocode modeling replicates the observed collapse features. Impact-generated melt rocks lie mostly inside the peak ring; the melt appears to be clast-rich and undifferentiated, with a maximum thickness of 3.5 km in the center.
Christeson, G. L., R. T. Buffler, and Y. Nakamura, Upper crustal structure of the Chicxulub impact crater from wide-angle ocean bottom seismograph data, in Large Meteorite Impacts and Planetary Evolution II, edited by Dressler, B. O. and V. L. Sharpton, Boulder, CO, Geol. Soc. Amer. Spec. Paper, 339, 291-298, 1999, 2 citations, #1393
Christeson, G. L., K. D. McIntosh, T. H. Shipley, E. R. Flueh, and H. Goedde, Structure of the Costa Rica convergent margin, offshore Nicoya Peninsula, J. Geophys. Res., 104, 25443-25468, 1999, 48 citations, #1429
We present the results of a seismic refraction survey conducted offshore Costa Rica near the Nicoya Peninsula. A dip profile and three strike profiles were carried out over 22 ocean bottom hydrophones and seismographs and were also recorded by land receivers. These data are used to construct a crustal structure model of the convergent margin from 20 km seaward of the Middle America Trench onto the Nicoya Peninsula. The best constrained portion of our model is the velocity at the top of the margin wedge immediately below the slope apron. Velocities increase from 3.5 to 4.2 to 4.6 km/s at distances of 10, 20, and 30â50 km landward of the trench. These velocities are higher than observed within margin wedges at other well-studied convergent margins but lower than the velocities within the adjacent Nicoya Complex, which are ∼5.5 km/s at similar depths below the surface. We interpret the margin wedge velocities as indicating that material similar to the Nicoya Complex extends seaward to near the lower slope but that fracturing, alteration, or accretion processes have lowered the velocity of the margin wedge with respect to the Nicoya Complex. The seismic refraction data cannot constrain the exact thickness or velocity of a possible low-velocity zone (LVZ) overlying the subducting plate; however, geologically reasonable structures are only produced with a LVZ <400 m thick. Velocities in the upper part of oceanic layer 2 are ∼3.5â4.0 km/s within the subducting slab. These velocities are unusually low for oceanic crust of this age and may correlate with a proposed highly permeable zone at the top of the subducting crust. The top of the subducted slab is well resolved, and deepens from 5 km depth at the trench to 15â16 km depth at the Nicoya Peninsula coastline. The dip angle of the subducting plate increases from 6° to 13° at a distance of ∼30 km from the trench. Interplate seismicity appears to become common ∼55 km from the trench where the plate boundary is at ∼14 km depth.
Christeson, G. L., P. R. Shaw, and J. D. Garmany, Shear and compressional wave structure of the East Pacific Rise, 9°-10°N, J. Geophys. Res., 102, 7821-7835, 1997, 24 citations, #1273
We use shear and compressional arrivals recorded by 45 ocean bottom seismograph receivers to model regional structure of the 9°â10°N region of the East Pacific Rise. Modeling indicates that shear conversion occurs at the base of layer 2A, producing Ps (energy that travels through oceanic crust as P and converts to S at the base of layer 2A on upgoing path) and pS (energy that travels as P through layer 2A and converts to S at the base of layer 2A on downgoing path) arrivals. The travel times of these arrivals require a shear wave velocity within layer 2A of 0.4â0.8 km/s (Poisson's ratio of 0.46â0.49). Waveform inversion was used to model the Poisson's ratio structure of layers 2B and 3 for nine instruments with good shear arrivals. Poisson's ratio within layer 2B was highly variable, with values as low as 0.24 and a mean value of 0.263. Some of this variability might be due to lateral variability in layer 2A structure. The mean Poisson's ratio of layer 3 was 0.271. According to the cracking model of Shaw [1994], the low Poisson's ratios within the upper portion of layer 2B indicate that thick cracks (aspect ratio α = 0.1) extend to depths of ∼ 1.5â1.7 km in this region. Two-dimensional travel time modeling indicates that the southwestern portion of our study area is associated with anomalously low seismic velocities, with layer 2B and layer 3 velocities reduced by up to 11% from the regional values. Cracking theory suggests that these low velocities could be caused by porosities of 0.3â5.5%, depending on the crack aspect ratio; a maximum porosity of 1.5% is predicted from our one Poisson's ratio measurement in this area. The anomalous velocities are located near the western discordant zone left by the 9°03′N overlapping spreading center. We suggest that shearing associated with on-axis rotation of the overlap basins is responsible for the low seismic velocities. The pattern of anomalies indicates that faulting extends to distances of 10â15 km from the basins.
Morgan, J. V., M. R. Warner, the Chicxulub Working Group, J. Brittan, R. T. Buffler, A. Camargo, G. L. Christeson, P. Denton, A. Hildebrand, R. W. Hobbs, H. Macintyre, G. MacKenzie, P. Maguire, L. Marin, Y. Nakamura, M. Pilkington, V. L. Sharpton, D. B. Snyder, G. Suarez, and A. Trejo, Size and morphology of the Chicxulub impact crater, Nature, 390, 472-476, 1997, 148 citations, doi:10.1038/37291, #1330
The Chicxulub impact in Mexico has been linked to the mass extinction of species at the end of the Cretaceous period. From seismic data collected across the offshore portion of the impact crater, the diameter of the transient cavity is determined to be about 100 km. This parameter is critical for constraining impact-related effects on the Cretaceous environment, with previous estimates of the cavity diameter spanning an order of magnitude in impact energy. The offshore seismic data indicate that the Chicxulub crater has a multi-ring basin morphology, similar to large impact structures observed on other planets, such as Venus.
Christeson, G. L., G. M. Kent, G. M. Purdy, and R. S. Detrick, Extrusive thickness variability at the East Pacific Rise, 9°-10°N: Constraints from seismic techniques, J. Geophys. Res., 101, 2859-2873, 1996, 49 citations, #1183
We calculate synthetic shot gathers and their corresponding common depth point (CDP) profiles over plausible East Pacific Rise (EPR) shallow velocity structures, based on the structures obtained from high-resolution on-bottom seismic refraction experiments. We then use these results to analyze the variability in layer 2A thickness at the EPR 9°â10°N region, as measured by CDP, wide-aperture profile (WAP), on-bottom seismic refraction experiments, and conventional air gun refraction data. The synthetics indicate that the accuracy of correlating the prominent shallow reflector observed in CDP and wide-angle data with the layer 2A/2B boundary is strongly dependent on the structure within layer 2A. If layer 2A consists of a surficial low-velocity layer overlying a steep velocity gradient (our gradient model), then there is an excellent correspondence between the two-way travel times to the shallow reflector and the base of layer 2A. However, the shallow reflector may originate from a gradient within layer 2A if the upper crust contains more than one high-gradient region (our step model). This implies that independent estimates of layer 2A velocity structure are needed to properly interpret CDP and wide-angle data. We also determine that the travel time to the layer 2A reflector, for identical velocity structure, can vary by as much as 50 ms (about 125 m) for differing experimental geometries. This can explain the discrepancy in two-way travel time to the layer 2A reflector imaged on zero-age CDP and WAP lines. The depths to a shallow reflector calculated from CDP and wide-angle data in the 9°â10°N region of the EPR generally correlate with estimated layer 2A thicknesses from on-bottom refraction profiles and conventional air gun refraction lines, which suggests that the upper crustal structure in this area is similar to the gradient model. WAP and conventional air gun refraction data indicate that there is a 100â200 m decrease in off-axis layer 2A thickness at 9°35′N on the EPR, the present-day location of a deviation in axial linearity (deval). There is no bathymetric expression of the 50% decrease in layer 2A thickness. Layer 2A can be interpreted to consist of the extrusive section and transition zone, with the layer 2A/2B boundary corresponding to the top of the sheeted dikes. We suggest that buoyancy forces associated with the axial-magma chamber (AMC) are supporting the extrusive layer and sheeted dikes at the neovolcanic zone. With distance from the rise axis, the AMC solidifies, the crust cools, the buoyancy forces are reduced, and the sheeted dike complex subsides. Concurrently, the extrusive layer thickens resulting in significantly less subsidence of the seafloor. We speculate that the 50% decrease in dike subsidence and extrusive thickness at the 9°35′N deval is due to a local reduction in magma supply within the axial magma chamber. The off-axis pattern of layer 2A thickness suggests that the 9°35′N deval has persisted for 175,000â275,000 years.
Christeson, G. L., Y. Nakamura, K. D. McIntosh, and P. L. Stoffa, Effect of shot interval on ocean bottom seismograph and hydrophone data, Geophys. Res. Lett., 23, 3783-3786, 1996, 10 citations, #1246
Data collected by 18 ocean bottom receivers for a seismic line shot at both 50‐m (∼24 s shot interval) and 125‐m (∼58 s shot interval) shot spacing provide a direct field comparison of the effect of shot interval on marine wide‐angle seismic data. Our results indicate that both shot spacings produce high‐quality refraction data in shallow water (<1000 m) on hydrophone and vertical channel data. In deeper water, the data quality of the 50‐m line is adequate for the vertical channel, but it is often poor at large offsets for the hydrophone channel in comparison to the 125‐m shot spacing data. A theoretical model to explain these observations provides further information useful for designing an experiment using ocean‐bottom receivers.