Moore, G. F., A. Taira, N. L. Bangs, S. Kuramoto, T. H. Shipley, C. M. Alex, S. P. S. Gulick, D. J. Hills, T. Ike, S. Ito, S. C. Leslie, A. J. McCutcheon, K. Mochizuki, S. Morita, Y. Nakamura, J.-O. Park, B. L. Taylor, G. Toyama, H. Yagi, and Z. Y. Zhao, Structural setting of the ODP Leg 190 Muroto transect, Proc. Ocean Drilling Prog., Init. Rept., 190, 1-14, 2001, 18 citations, #1541
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.
Shipley, T. H., and G. F. Moore, U. S. Marine Seismic Reflection Acquisition Needs for the Next Decade, Nat. Sci. Found. Workshop Rept., GeoProse, Bethesda, MD, 48 pp., 2000, #1522
Shipley, T. H., and G. F. Moore, NSF considers recommendations for marine seismic reflections, Eos, Trans. Amer. Geophys. Un., 81, 373-374, 2000, doi:10.1029/00EO00277, #1523
Shipley, T. H., and G. F. Moore, Nonindustrial marine reflection seismology capabilities and U. S. planning for the next decade, Proc. 32nd Ann. Offshore Tech. Conf., Geology, earth sciences and environmental factors, 1, 15-18, 2000, #1524
Zhao, Z. Y., G. F. Moore, N. L. Bangs, and T. H. Shipley, Spatial variations of the decollement/protodecollement zone and their implications: A 3-D seismic inversion study of the northern Barbados accretionary prism, Island Arc, 9, 219-236, 2000, 5 citations, #1476
Abstract We conducted a 3-D seismic inversion study to investigate spatial variations of physical properties of the décollement zone (DZ) and protodécollement zone (PDZ) under the northern Barbados accretionary prism. Significant spatial variations of physical properties were observed in the PDZ seaward of the thrust front from the inversion data. The density generally increases southward with a few localized low-density patches. A lower density commonly corresponds to a thicker PDZ, suggesting that the paleomorphology may at least partially control the variations of the physical properties. Similar low-density patches were also found in the DZ. These features may be inherited from those of the PDZ and enhanced after subduction through localized arrested consolidation. Under the prism toe, the density of the DZ increases landward. This trend may mainly result from shear-induced consolidation of the DZ but may also be related to landward increasing tectonic loading. Significant northâsouth differences in density and, thus, porosity and strength of the PDZ, are observed and these differences may continue into the DZ. A stronger DZ is likely responsible for a larger prism taper observed in the southern area of the prism toe. The larger taper, thus more horizontal shortening, coupled with a thinner sediment sheet above the PDZ in the southern area, may cause a relative retreat of the thrust front and a pronounced change in strike of the sequence thrusts south of seismic Line 690. The northâsouth differences may ultimately have originated in the approach of a structurally higher segment of the Tiburon Rise. The Tiburon Rise affects regional morphology and, thus, it controls the sedimentation and physical properties of the PDZ. It may also control sediment accumulation above the PDZ. Therefore, the sedimentational change induced by the structural high of the Tiburon Rise, in turn, resulted in structural change of the prism in the southern area.
Bangs, N. L., T. H. Shipley, J. C. Moore, and G. F. Moore, Fluid accumulation and channeling along the northern Barbados Ridge decollement thrust, J. Geophys. Res., 104, 20399-20414, 1999, 48 citations, #1428
A volume of three-dimensional seismic reflection data, acquired in 1992, imaged the decollement beneath the northern Barbados Ridge accretionary prism revealing reflection amplitude and waveform variations attributed to fluid accumulations along the plate boundary fault. We model the seismic reflection by inversion for seismic impedance (the product of velocity and density) throughout the 5 Ã 25 km survey area and thus map physical property variations. In 1997, Ocean Drilling Program Leg 171A penetrated the protodecollement and decollement at five sites with a logging-while-drilling (LWD) tool to log density and other physical properties of the decollement. We construct a regional map of density, and inferred porosity, within the decollement from seismic models calibrated with LWD density data. In the sediments out in front of the trench the protodecollement forms in a radiolarian-rich Miocene mudstone with an anomalously high porosity (70â75%) that appears as a pervasive, inherent characteristic of this interval seaward of the deformation front. In the decollement beneath the wedge a consolidation trend of decreasing porosity runs perpendicular to the deformation front with porosity decreasing from 70% at the wedge toe to 50% 4 km from the wedge toe. A second, distinct trend also forms along a 10-km-long, 1- to 2-km-wide, NE-SW zone in which porosity is 70%, as high as it is in the protodecollement. This zone can be explained as an area of the decollement where fluid accumulations develop by maintaining high fluid content. We postulate that high fluid content is maintained by continuous recharge flowing into and along this channel. This porosity distribution within the decollement also strongly influences fluid migration into the overlying accretionary wedge and is directly associated with fluid charging of ramps and out-of-sequence thrusts above the decollement.
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.
Phinney, E. J., P. Mann, M. F. Coffin, and T. H. Shipley, Sequence stratigraphy, structure, and tectonic history of the southwestern Ontong Java Plateau adjacent to the North Solomon Trench and Solomon Islands Arc, J. Geophys. Res., 104, 20449-20466, 1999, 28 citations, #1409
The Ontong Java Plateau (OJP) is the largest and thickest oceanic plateau on Earth and one of the few oceanic plateaus actively converging on an island arc. We present velocity determinations and geologic interpretation of 2000 km of two-dimensional, multi-channel seismic data from the southwestern Ontong Java Plateau, North Solomon Trench, and northern Solomon Islands. We recognize three megasequences, ranging in age from early Cretaceous to Quaternary, on the basis of distinct interval velocities and seismic stratigraphic facies. Megasequence OJ1 is early Cretaceous, upper igneous crust of the OJP and correlates with basalt outcrops dated at 122â125 Ma on the island of Malaita. The top of the overlying megasequence OJ2, a late Cretaceous mudstone unit, had been identified by previous workers as the top of igneous basement. Seismic facies and correlation to distant Deep Sea Drilling Project/Ocean Drilling Program sites indicate that OJ2 was deposited in a moderately low-energy, marine environment near a fluctuating carbonate compensation depth that resulted in multiple periods of dissolution. OJ2 thins south of the Stewart Arch onto the Solomon Islands where it is correlated with the Kwaraae Mudstone Formation. Megasequence OJ3 is late Cretaceous through Quaternary pelagic cover which caps the Ontong Java Plateau; it thickens into the North Solomon Trench, and seismic facies suggest that OJ3 was deposited in a low-energy marine environment. We use seismic facies analysis, sediment thickness, structural observations, and quantitative plate reconstructions of the position of the OJP and Solomon Islands to propose a tectonic, magmatic, and sedimentary history of the southwestern Ontong Java Plateau. Prior to 125 Ma late Jurassic and early Cretaceous oceanic crust formed. From 125 to 122 Ma, the first mantle plume formed igneous crust (OJ1). Between 122 and 92 Ma, marine mudstone (OJ2 and Kwaraae mudstone of Malaita, Solomon Islands) was deposited on Ontong Java Plateau. At 92 Ma a second mantle plume caused widespread volcanism on the plateau. From 92 to 15 Ma, pelagic carbonate sediment (OJ3) was deposited. At ∼15 Ma the southern Ontong Java Plateau was deformed by normal faults during its approach toward the North Solomon Trench. Finally, from 4 to 0 Ma, the Malaita Accretionary Prism formed during collision between a substantially thicker portion of the Ontong Java Plateau and the Solomon Islands arc. Flexure of the Ontong Java Plateau near the trench caused coeval normal faulting.
Shipley, T. H., N. L. Bangs, and G. F. Moore, Shallow aseismic portion of the Barbados plate boundary, Proc., Workshop on Recurrence of Great Interplate Earthquakes and Its Mechanism, Sci. and Tech. Agency, Kochi, Japan, 91-96, 1999, #1468
Shipley, T. H., N. L. Bangs, and A. T. Henning, Sediment velocity estimation using iterative 3-D migrations of short offset seismic reflection data in deep water, Marine Geophysical Researches, 20, 479-494, 1998, 3 citations, doi:10.1023/A:1004782815985, #1442
In deep ocean settings where water depth greatly exceeds the source-to-receiver length, the geometry is insufficient for accurate determinations of velocity from reflection-moveout. However, velocities are crucial for estimates of physical properties and image processing. Focusing analyses with conventional post-stack two-dimensional migration improves images, but does not produce geologically meaningful velocities except in the special case of a two-dimensional earth. For the more general case of the three-dimensional earth there is no a priori method to determine the degree of geometrical complexity. We present a technique using a short-offset three-dimensional (3-D) data set over the 5 km deep trench west of the Lesser Antilles. These data illustrate highly sensitive post-stack 3-D focusing analyses (± 20 m sâ1 interval velocities), and the relationship of these seismically derived velocities to rock velocities. In our Barbados example we were able to establish the presence of a widespread 80-160 m thick low-velocity zone at and above the main low-angle fault. This observation suggests the water-rich décollement leaks water into the overlying sections. Also evident is a low-velocity section associated with turbidite sands. These results are confirmed with sparse logging data and well samples. Deep-water short offset 3-D experiments provide a potentially effective approach for velocity estimation, replacing the operational complexity of long-offsets with simpler short-offset techniques. In areas of structural complications and abundant diffracted energy, it is a surprisingly accurate method, utilizing the high fidelity 3-D wavefield and the information carried in zero-offset diffraction ellipsoids. The velocity used to properly collapse a diffraction ellipsoid is explicitly the velocity of propagation in the media since the travel path is known exactly. Thus, the derived velocities should closely represent rock velocities, unlike the 2-D case where the propagation geometry is not known.
Stavenhagen, A. U., E. R. Flueh, C. R. Ranero, K. D. McIntosh, T. H. Shipley, G. Leandro, A. Schulze, and J. J. Danobeitia, Seismic wide-angle investigations in Costa Rica: A crustal velocity model from the Pacific to the Caribbean coast, Zbl. Geol. Palaont. Teil. I, H.3-6, 393-408, 1998, #1454
Zhao, Z. Y., G. F. Moore, and T. H. Shipley, Deformation and dewatering of the subducting plate beneath the lower slope of the northern Barbados accretionary prism, J. Geophys. Res., 103, 30431-30449, 1998, 9 citations, #1402
Detailed structural and stratigraphic analysis of a three-dimensional seismic data set reveals two scales of structural fabric in the westward dipping oceanic basement under the toe of the northern Barbados accretionary prism. The large-scale fabric consists of three NNE trending structural highs with relief of about 300 m and spacing of 5â6 km. The small-scale structure is characterized by a series of NNE trending normal faults, resulting in horsts and graben with relief of 50â200 m and spacing of 0.2â2 km. The structural fabric at both scales is preexisting. Structures in the underthrust sediment are mostly secondary, having formed by differential compaction. Three distinctive seismic stratigraphic units are defined in the underthrust sedimentary section. The upper Cretaceous to lower Eocene unit mostly fills in basement lows, resulting in great thickness variations. The thickness of the Oligocene and middle to upper Eocene units is less variable. We estimate that the upper two sedimentary units have lost 25% of their total initial void space owing to the load of the accretionary prism within the first 3.5 km landward of the thrust front. This suggests a current fluid expulsion rate of 1008 m3/yr per kilometer of strike length from the toe. The fluid discharge is expected to increase to 1092 m3/yr within 64,000 years as a thicker sediment section is subducted. Our results also suggest that the basement indirectly controls fluid movement in the underthrust Oligocene unit by creating secondary normal faults that act as major fluid conduits between the overlying décollement and the underlying, more permeable middle to upper Eocene turbidite-bearing section. Ponded sediments between basement horst blocks are not faulted and may compact more slowly.
Moore, G. F., Z. Y. Zhao, and T. H. Shipley, Integration of vertical seismic profiling, logging, and seismic data in the vicinity of the decollement, northern Barbados Ridge accretionary prism, Proc. Ocean Drill. Prog., Sci. Results, 156, 255-262, 1997, #1282
Shipley, T. H., G. F. Moore, H. J. Tobin, and J. C. Moore, Synthesis of the Barbados decollement seismic reflection response from drilling-based geophysical observations and physical properties, Proc. Ocean Drill. Prog., Sci. Results, 156, 293-302, 1997, #1311
Bangs, N. L., T. H. Shipley, and G. F. Moore, Elevated fluid pressure and fault zone dilation inferred from seismic models of the northern Barbados Ridge decollement, J. Geophys. Res., 101, 627-642, 1996, 25 citations, #1179
In 1992, a large volume of three-dimensional seismic reflection data were acquired in a 5 Ã 25 km area across the toe of the Barbados accretionary complex that covers the Deep Sea Drilling Project leg 78A and Ocean Drilling Program legs 110 and 156 drilling sites. These data are used to examine the acoustic character of the decollement seismic reflection and to qualitatively and quantitatively characterize fluid pressures within the fault zone. Seismic models have been constructed across a 6-km region of the decollement where it has been mapped as a moderate to bright polarity-reversed reflection. The models show that this segment of the decollement reflection is caused by a low-velocity interval, usually 12â16 m thick. The top of the low-velocity interval appears to be a sharp boundary that requires a decrease in velocity from 1.8 km/s to between 1.7 and 1.65 km/s, with some localized bright reflections with an even lower velocity of 1.6 km/s. The base of the low-velocity layer is less certain from modeling. The base consists of either a velocity increase that is usually approximately half the velocity contrast at the top of the layer, or the velocity increase is equal to the contrast at the top of the layer but distributed over a 10-m-thick interval. Comparison of these results to laboratory experiments on the relationship between fluid pressure and seismic velocity indicates that in this interval of the decollement, fluid pressure is at or near lithostatic. Furthermore, the reflection coefficients of the decollement are sufficiently large that some dilation of the fault zone is required. The dilation should lead to high fracture zone permeability and explain the observation of a laterally consistent decollement reflection along a 5-km segment of the decollement. It is within these segments of the fault that fluid pressure approaches lithostatic and significantly reduces fault strength.
Mann, P., M. F. Coffin, T. H. Shipley, S. Cowley, E. J. Phinney, A. Teagan, K. Suyehiro, N. Takahashi, E. Araki, M. Shinohara, and S. Miura, Researchers investigate fate of oceanic plateaus at subduction zones, Eos, Trans. Amer. Geophys. Un., 77, 282-283, 1996, #1241
Labaume, P., P. Henry, A. Rabaute, Y. Ogawa, T. H. Shipley, P. Blum, J. Ashi, W. Bruckmann, F. Filice, A. T. Fisher, D. Golberg, B. A. Housen, M.-J. Jurado, M. Kastner, T. Laier, E. C. Leitch, A. J. Maltman, A. W. Meyer, G. F. Moore, C. Moore, S. Peacock, T. H. Steiger, H. J. Tobin, M. B. Underwood, Y. S. Xu, H. Yin, Y. Zheng, and G. T. Zwart, Circulation et surpression de l'eau interstitielle dans le prisme d'accretion nord-Barbade: Resultats du Leg ODP 156, C. R. Acad. Sci. Paris, 320 (IIa), 977-984, 1995, #1228
Moore, G. F., Z. Y. Zhao, T. H. Shipley, N. L. Bangs, and J. C. Moore, Structural setting of the Leg 156 area, northern Barbados Ridge accretionary prism, Proc. Ocean Drilling Prog., Init. Rept., 156, 13-27, 1995, #1074
Moore, J. C., T. H. Shipley, D. S. Goldberg, Y. Ogawa, F. Filice, A. T. Fisher, M.-J. Jurado, A. Rabaute, H. Yin, G. T. Zwart, W. Bruckmann, P. Henry, J. Ashi, P. Blum, A. W. Meyer, B. A. Housen, M. Kastner, P. Labaume, T. Laier, E. C. Leitch, A. J. Maltman, S. Peacock, T. H. Steiger, H. J. Tobin, M. B. Underwood, Y. S. Xu, and Y. Zheng, Abnormal fluid pressures and fault-zone dilation in the Barbados accretionary prism: Evidence from logging while drilling, Geology, 23, 605-608, 1995, 70 citations, doi:10.1130/0091-7613(1995)023<0605:AFPAFZ>2.3.CO;2, #1229
Logs collected while drilling measured density in situ, through the accretionary prism and decollement zone of the northern Barbados Ridge. Consolidation tests relate void ratio (derived from density) to effective stress and predict a fluid pressure profile, assuming that the upper 100 m of the prism is at a hydrostatic pressure gradient. The calculated fluid pressure curve rises to >90% of lithostatic below thrusts in the prism, presumably due to the increase in overburden and lateral tectonic loading. Thin (0.5â2.0 m) intervals of anomalously low density and resistivity in the logs through the basal decollement zone suggest dilation and perhaps hydrofracturing. A peak in hydraulic head in the upper half of the decollement zone requires lateral influx of fluid, a conclusion consistent with previous geochemical studies. Although the calculated fluid-pressure profile is model dependent, its inherent character ties to major structural features.
Shipley, T. H., G. F. Moore, N. L. Bangs, J. C. Moore, and P. L. Stoffa, Seismically inferred dilatancy distribution, northern Barbados Ridge decollement: Implications for fluid migration and fault strength, Geology, 22, 411-414, 1994, 92 citations, doi:10.1130/0091-7613(1994)022<0411:SIDDNB>2.3.CO;2, #1025
A 5 x 25 km, three-dimensional seismic survey of the lower part of the northern Barbados Ridge accretionary prism creates a three-dimensional image of a major active decollement fault. The fault is usually a compound negative-polarity reflection modeled as a low-velocity, high-porosity zone less than ∼14 m thick. This thickness is significantly less than that defined by drilling of a >40 m zone of deformation at Ocean Drilling Program (ODP) Site 671B, located within the surveyed area. We infer that the seismically defined fault is a thin, high-porosity zone and is thus an undercompacted, high-fluid-pressure dilatant section. If these inferences are correct, then map-view variations in seismic-reflection waveform and amplitude illustrate complex patterns of fault-zone fluid content and fluid migration paths. The amplitude map suggests kilometre-wide channels of locally high porosity and thus focused fluid flow. These paths are only subparallel to the expected minimum head, as inferred from the shape of the overlying sediment wedge; other factors must modify fluid concentrations and ultimately migration. Several areas of positive-polarity fault reflections define square-kilometre-sized regions inferred to be lower porosity sections producing strong asperities in an otherwise weak fault. One, coincident with Site 671B, may explain the success of drilling through the fault here. All other holes drilled in the area were within the negative-polarity regions and were unsuccessful in penetrating through the entire fault zone, possibly because of instability associated with high fluid pressures and a weak fault. ODP Leg 156 planned for 1994 will test inferences related to fault permeability and fluid pressures.
Wood, W. T., P. L. Stoffa, and T. H. Shipley, Quantitative detection of methane hydrate through high-resolution seismic velocity analysis, J. Geophys. Res., 99, 9681-9695, 1994, 56 citations, #950
A laterally extensive, high-resolution travel time velocity analysis and acoustic wave form, inversion were used to quantitatively determine methane hydrate content in deep water sediments of the Blake Ridge off the southeast U.S. coast. The interval acoustic velocity (Vp ) analyses were performed in the τ-p domain by interactively picking the τ-p trajectories of prominent reflections in each of 50 plane wave-decomposed common midpoint gathers. The reflections correspond to seismic stratigraphic boundaries so that lateral Vp changes due to lithology changes are mitigated, and Vp changes due to changing hydrate content are enhanced. Two separate interval Vp analyses were performed, one with thick (∼0.4 km) layers which yielded lower uncertainty but also lower resolution, and one with thinner layers (∼0.1 km), yielding higher resolution but slightly larger uncertainties. Results show no correlation between low-sediment reflectivity and Vp . However, in the areas exhibiting a bottom simulating reflector (BSR) a high Vp interval (∼2.0 km/s and 0.15 km thick) is seen immediately above the BSR. Where the BSR is strongest a 256-layer, least squares acoustic wave form inversion reveals the BSR to be caused by a Vp decrease from ∼2.0 to ∼1.5 km/s, with little or no change in density. The inversion also reveals a thin (0.025 km) layer of anomalously low Vp lying immediately below the BSR. Two models of methane hydrate distribution are tested, each indicating that the volume of methane hydrate in the intervals of elevated Vp is up to ∼25% of the total volume.
Abrams, L. J., R. L. Larson, T. H. Shipley, and Y. Lancelot, Cretaceous volcanic sequences and Jurassic oceanic crust in the east Mariana and Pigafetta Basins of the western Pacific, in The Mesozoic Pacific: Geology, Tectonics, and Volcanism, edited by M. S. Pringle, Amer. Geophys. Un., Geophys. Monograph, 77, 77-101, 1993, #964
Hagen, R. A., L. A. Mayer, D. C. Mosher, L. W. Kroenke, T. H. Shipley, and E. L. Winterer, Basement structure of the northern Ontong Java Plateau, Proc. Ocean Drill. Prog., Sci. Results, 130, 23-32, 1993, #970
McIntosh, K. D., E. A. Silver, and T. H. Shipley, Evidence and mechanisms for forearc extension at the accretionary Costa Rica convergent margin, Tectonics, 12, 1380-1392, 1993, 43 citations, #1021
Seismic reflection data across the upper trench slope off the Nicoya Peninsula, Costa Rica, reveal a wide zone of nearly trench-parallel normal faults. Although work in the last decade has shown that normal faults are present at many convergent margins, most examples (e.g., Japan, Peru-Chile, and Guatemala) have been associated with margins experiencing subduction erosion or non-accretion. In contrast, extension in the Costa Rica study area apparently is coeval with frontal accretion and underplating. The normal faults across the Costa Rica forearc are striking in seismic section due to the well-layered, 2-km-thick upper slope apron. Fault plane reflections and reflector terminations show that the faults extend through the sedimentary apron and apparently into the underlying accretionary prism, indicating a deep-seated deformation process. The zone of extension is from the midslope area to within 10 km of the shelf edge, a minimum width of about 20 km; the estimated extension across the zone is at least 1.5 to 3 km. Within the apron section, spacing between the faults is generally 200â500 m, and nominal fault dip is 20°â40° and predominantly landward. Activity on the normal faults appears to have occurred over a significant period of time based on increased displacement with depth and on fault-controlled sedimentary thickening. At least some of the faults may be presently active; shallow reflectors and possibly the seafloor are displaced by faulting. Contemporary sediment accretion is documented by the same seismic reflection profiles showing offscraping and underplating near the toe of the wedge and out-of-sequence thrusting primarily below the midslope area. The consistent landward normal fault dip may be influenced by structural anisotropy in the prism and possible extensional reactivation of earlier thrust faults associated with accretion processes. With the available data it is not possible to conclusively determine the cause of the stress field leading to the upper prism and apron extension. However, the three most likely causes are underplating, changes in basal shear stress, or a brief episode of subduction erosion.
Moore, G. F., and T. H. Shipley, Character of the decollement in the Leg 131 area, Nankai Trough, Proc. Ocean Drill. Prog., Sci. Results, 131, 73-82, 1993, #974
Mosher, D. C., L. A. Mayer, T. H. Shipley, E. L. Winterer, R. A. Hagen, J. C. Marsters, F. Bassinot, R. H. Wilkens, and M. Lyle, Seismic stratigraphy of the Ontong-Java Plateau, Proc. Ocean Drill. Prog., Sci. Results, 130, 33-49, 1993, #975
Shipley, T. H., L. J. Abrams, Y. Lancelot, and R. L. Larson, Late Jurassic-Early Cretaceous oceanic crust and Early Cretaceous volcanic sequences of the Nauru Basin, western Pacific, in The Mesozoic Pacific: Geology, Tectonics, and Volcanism, edited by M. Pringle, W. Sager, W. Sliter, and S. Stein, Amer. Geophys. Union, Geophys. Monograph, 77, 103-119, 1993, #896
Abrams, L. J., R. L. Larson, T. H. Shipley, and Y. Lancelot, The seismic stratigraphy and sedimentary history of the East Mariana and Pigafetta Basins of the western Pacific, Proc. Ocean Drill. Prog., Sci. Results, 129, 551-569, 1992, #965
Shipley, T. H., K. D. McIntosh, E. A. Silver, and P. L. Stoffa, Three-dimensional seismic imaging of the Costa Rica accretionary prism: Structural diversity in a small volume of the lower slope, J. Geophys. Res., 97, 4439-4459, 1992, 75 citations, #901
Conventional two-dimensional seismic reflection investigations have been generally relied upon to provide images of large to medium scale structural features in accretionary prisms. We undertook a three-dimensional seismic reflection survey of a small part of a prism arcward of the Middle America Trench off Costa Rica to more correctly image structure and to use the improved structural information to examine the processes of accretion. This survey reveals small features, with dimensions of hundreds of meters, while also defining features thousands of meters in lateral extent, both of which were underappreciated in conventional two-dimensional data from the same area. We have imaged active off scraping at the trench and both duplexing and out-of-sequence faulting a few kilometers arcward of the trench. Fault spacing and reflector geometry vary dramatically over a space of several hundred meters. Some of these variations are related to visible changes in morphology of the underlying oceanic basement, but others are not so easily documented. Fault surface reflections define an architecture which may control gross fluid motion through the prism. This architecture is apparently formed by duplexing and out-of-sequence faulting and has been maintained by periodic motion on some of the out-of-sequence faults. The slope sediment apron records multiple phases of deformation. Abundant small offset reverse faults break the seafloor and indicate recent shortening of a broad region of the underlying prism. A primary result of this survey is appreciation of the structural diversity across a small width of an accretionary prism.
Stoffa, P. L., W. T. Wood, T. H. Shipley, G. F. Moore, E. Nishiyama, M. A. B. Botelho, A. Taira, H. Tokuyama, and K. Suyehiro, Deepwater high-resolution expanding spread and split spread seismic profiles in the Nankai Trough, J. Geophys. Res., 97, 1687-1713, 1992, 18 citations, #803
In deep water the source-receiver offsets that are required for accurate velocity determinations cannot be achieved with single-ship multichannel seismic methods. Two ships, one equipped with a multichannel receiving array and the other with a seismic source, have previously been employed to acquire common midpoint, expanding spread profiles, principally to determine deep crustal velocity structures. We extend this method to higher resolution in space and time to determine the velocities of sedimentary layers in deep water offshore Japan in the Nankai Trough. This high-resolution two-ship data acquisition method used a 13.1-L water gun source array; a 1.6-km, 96-channel receiving array with 0.0166-km active group; and shore-based navigation. Analysis of the data was performed in the τ-p domain by successive downward continuation of the plane wave data. Interactive velocity analysis methods for both one-dimensional and two-dimensional Earth models are described for both common source/receiver and common midpoint profiles. Results in one of two areas surveyed show a low-velocity zone below the subduction decollement which is consistent with models of low wedge taper, high pore fluid pressure, and reflection polarity reversal described previously by other researchers. The velocity profiles show the expected landward increase in velocity assumed to be due to lateral strain and porosity decrease, but the effect is small, only slightly greater than would be expected in an area of no lateral strain.
Mayer, L. A., T. H. Shipley, E. L. Winterer, D. C. Mosher, and R. A. Hagen, Seabeam and seismic reflection surveys on the Ontong Java Plateau, Proc. Ocean Drilling Prog., Init. Rept., 130, 45-75, 1991, #911
Moore, G. F., D. E. Karig, T. H. Shipley, A. Taira, P. L. Stoffa, and W. T. Wood, Structural framework of the ODP Leg 131 area, Nankai Trough, Proc. Ocean Drilling Prog., Init. Rept., 131, 15-20, 1991, #890
Stoffa, P. L., T. H. Shipley, W. Kessinger, D. F. Dean, R. Elde, E. A. Silver, D. L. Reed, and A. Aguilar, Three-dimensional seismic imaging of the Costa Rica accretionary prism: Field program and migration examples, J. Geophys. Res., 96, 21693-21712, 1991, 18 citations, #849
Seismic reflection techniques are the primary geophysical tool used to examine the structure of continental margins. On convergent plate margins, widely separated seismic reflection profiles often do not image complex structural features because out-of-plane reflections and diffractions obscure the seismic images. Yet the structural features are often critical to understanding the tectonic processes. Thus, the resulting geologic interpretations are based on images that may not accurately represent the true subsurface structure. To image the complex geologic structures of an active continental margin, a three-dimensional (3-D) seismic survey was conducted off of the Nicoya Peninsula of Costa Rica in April 1987. Over 60,000 shot points of 96-trace multichannel data were collected in a 9 Ã 22 km grid. This detailed survey was located over the accretionary prism and designed to investigate the active tectonic processes and evolution of this continental margin. In this paper we outline the data acquisition program, including the navigation processing critical to successful imaging, and the seismic processing methodology and then show several examples of images improved by 3-D surveying. The resulting 3-D seismic data were migrated both as independent lines and as a 3-D volume. Seismic images were produced as conventional depth and time sections and as cross-sectional slices in depth and time. We present comparisons of the migration results that show significant improvements of the subsurface image using 3-D techniques. To illustrate these improvements, we present images of two complex geologic areas: a mud volcano and reflections near the top of the accretionary prism, and the top of the subducting oceanic basement beneath the prism. Comparisons between the original stacked data, the data of the two-dimensional migration, and the results of the 3-D migration illustrate the value of 3-D techniques in studies of active margins. In addition to the advantages of the 3-D imaging which include better structural delineation and improved signal-to-noise ratio in the final image, we show that the ability to view the 3-D data volume in both section and plan view offers significant interpretational advantages.
Moore, G. F., T. H. Shipley, P. L. Stoffa, D. E. Karig, A. Taira, S. Kuramoto, H. Tokuyama, and K. Suyehiro, Structure of the Nankai Trough accretionary zone from multichannel seismic reflection data, J. Geophys. Res., 95, 8753-8765, 1990, 147 citations, #846
New multichannel seismic reflection data collected over the Nankai Trough image the accretionary complex in two areas: the International Program of Ocean Drilling leg 87 transect area (western area) and the region of upcoming Ocean Drilling Program leg 131 (eastern area). The incoming Shikoku Basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites. The basin section is overlain by a trench wedge that is 12â16 km wide and 350â750 m thick at the thrust front. Accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge. The zone in the western area is 4.5 km wide and is characterized by âkinkâ folds; the zone in the eastern area is only 2.5 km wide and does not exhibit such folds. The frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section. The overthrusting sediments form fault-bend folds over these ramps. Thrust spacing at the toe of the slope is 1.5â2.5 km. The second thrust cuts up from an inferred décollement within the Shikoku Basin sedimentary section. In the eastern area, a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6.3 km seaward of the thrust front and underlies the entire protothrust zone. This reflector continues with reversed polarity under the accretionary complex and is at the level of the basal décollement. The underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism. The reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface. This decrease in velocity and density may indicate that the décollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism. The reflections from the first and second thrusts are also reversed polarity, possibly indicating that they also are pathways of fluid expulsion. The critical wedge taper of the western area is greater than that of the eastern area, an observation that is consistent with the existence of an overpressured décollement in the eastern area.
Reed, D. L., E. A. Silver, J. E. Tagudin, T. H. Shipley, and P. Vrolijk, Relations between mud volcanoes, thrust deformation, slope sedimentation, and gas hydrate, offshore North Panama, Marine Petroleum Geol., 7, 44-54, 1990, 53 citations, doi:10.1016/S0264-8172(90)90055-L, #818
Shipley, T. H., P. L. Stoffa, and D. F. Dean, Underthrust sediments, fluid migration paths, and mud volcanoes associated with the accretionary wedge off Costa Rica: Middle American Trench, J. Geophys. Res., 95, 8743-8752, 1990, 75 citations, #925
Styles of deformation and tectonic responses resulting from the convergence of oceanic and continental plates are strongly dependent on fluids in the sediments. We estimate the volume of fluids and sediment underthrust beneath the toe of an accretionary wedge and describe evidence of fluid migration observed in seismic data in the Middle America Trench near Costa Rica. Reduction in normal incident travel time of the underthrust oceanic plate section arcward of the deformation front may be related to fluid expulsion. Within 4 km of the deformation front, where overburden is <800 m, more than half of the total water in the pore spaces may have been expelled. The rate of fluid transfer from the underthrust section is highest in the zone 0ââ¬â4 km arcward of the deformation front. The clarity of the reflection associated with the décollement is exceptional and shows a phase reversal, relative to the seafloor. This high-amplitude phase-reversed reflection ends about 4 km arcward of the deformation front, suggesting a rapid reduction in the density and/or velocity contrast between the dewatering underthrust section and overlying offscraped sediments at this position. Fluid migration within the wedge produces mud volcanoes or ridges in the midslope region having relief of about 50 m and extent of perhaps 500 m Ãâ 1000 m. A reflection traced beneath a seafloor mud volcano crosses through the accretionary wedge/slope cover boundary and may mark a fluid conduit. Locally, high-amplitude reflections associated with the base of slope cover are coincident with closure of structure and indicate local traps to upward organized or diffusive fluid flow. Some fluids appear to migrate along pathways marked by reflections which extend deep within the accretionary wedge but not always extending into the underthrust section. Near the trench where fluid expulsion rates are highest, water may exit to the seafloor seaward of the downslope pinch-out of the slope section.
Coltrin, G., M. Backus, T. H. Shipley, and M. Cloos, Seismic reflection imaging problems resulting from a rough surface at the top of the accretionary prism at convergent margins, J. Geophys. Res., 94, 17485-17496, 1989, 4 citations, #754
Modern seismic reflection profiling has contributed substantially to our understanding of deformation processes at actively convergent plate margins. Nonetheless, the complexity of most seismic profiles leads to great uncertainty about internal prism structures because interpretations of particular profiles can vary from one showing dominantly bedded but faulted sediments to another showing chaotic mélange. The steep dips of bedding and faults form a complex three-dimensional geometry that causes widely recognized but poorly understood imaging problems. In this report, the specific effects on seismic wave propagation of roughness of a few hundred meters amplitude at the interface between the tectonized sediments of a prism and the overlying slope sediments are systematically analyzed through normal incidence and offset synthetic modeling. For a surface roughness and velocity contrast similar to that between the prism and slope cover off Costa Rica, the modeling shows that there are important focusing and defocusing effects because of wave front distortion at the interface. The synthetic seismic profiles of simple continuous interfaces within a model prism produce discontinuous reflections beneath the interface with amplitudes reduced by a factor of ∼2, and phase changes occurring for over 30% of the rays traced through the models. The significance of these effects increases with the depth and the dip of the reflecting interface. The distortions are greater for dip lines than for strike lines as the structure changes more rapidly in the dip direction. The study demonstrates that wherever there is surface roughness comparable to that found off Costa Rica, the detection of even simple structures within the prism will be quite difficult with only two-dimensional reflection methods. Three-dimensional imaging techniques will be necessary at many convergent plate margins to differentiate between models of the prism accretion and deformation.
Laguros, G. A., and T. H. Shipley, Quantitative estimate of resedimentation in the pelagic sequence of the equatorial Pacific, Marine Geol., 89, 269-277, 1989, 2 citations, doi:10.1016/0025-3227(89)90080-7, #782
A study of deep-sea sedimentation processes was undertaken in the east-central equatorial Pacific Ocean near DSDP Site 574, using lithologic information from the site and a grid of 250 km of high-resolution seismic reflection profiles. These data are used to quantitatively evaluate pelagic drape and basin-filling processes in a pelagic carbonate environment. Sediment accumulation influenced by the relief of the depositional surface, seafloor currents, and sediment supply was examined in eight seismic-stratigraphic intervals. As expected, the thinnest sediment sections occur over topographic highs and areas of steep dips, while the thickest portions occur in lows and over gradual slopes. Syn-depositional sediment redistribution varies between about 4 and 17% of the volume of the defined intervals.
Moore, G. F., and T. H. Shipley, Behavior of the decollement at the toe of the Middle America Trench, Geologische Rundschau, 77, 275-284, 1988, 14 citations, doi:10.1007/BF01848689, #748
Seismic reflection data from three areas of the Middle America Trench provide insights into the behavior of the decollement that separates subducted and offscraped sediment. The range of responses observed along this single subduction system provides clues as to how the decollement forms and is influenced by local conditions. The location of the decollement and whether or not the subducting basement topography influences the decollement are apparently controlled by the type and thickness of sediment in the trench. Where subducting basement topography and oceanic plate hemipelagic sediments are buried by sandy terrigenous turbidites, such as in the trench axis off Mexico, the decollement is localized near the base of the turbidite section. Subducting normal fault blocks in the oceanic crust control thrust ramps, and hanging wall anticlines form above the ramps.
In regions of the trench where coarse terrigenous sediment is thin or absent, the decollement is localized within the incoming sediment section. Where muddy trench turbidites bury subducting topography (e.g. off Guatemala), the decollement is approximately 100 m deep and is little affected by the underlying subducting topography. The lower 200â300 meters of trench sediment and all of the pelagic sediment are subducted. Where there is no trench sediment overlying carbonate-rich oceanic plate sediments (e.g. off Costa Rica), the decollement is located within the subducting plate sediment section. The decollement is localized at a single stratigraphic level and rides up and over subducting horst blocks.
Moore, G. F., and T. H. Shipley, Mechanisms of sediment accretion in the Middle America Trench off Mexico, J. Geophys. Res., 93, 8911-8927, 1988, 22 citations, #783
High-resolution seismic reflection and Sea Beam bathymetrie data provide insights into the processes of sediment offscraping and accretion in the Middle America Trench off southern Mexico. Thick terrigenous sediments that are transported down Ometepec Canyon and accumulate along the trench floor are scraped off the oceanic plate and accreted in thrust packets to the lower trench slope. The packets offscraped represent most of the trench strata. Underlying hemipelagic deposits that accumulate on the seafloor seaward of the trench are subducted landward of the toe of the slope. Horizontal displacement on the thrusts is less than 1 km. Leading edge folds are the surface expressions of the thrusts and strike subparallel to the base of the trench slope. The folds are continuous for as much as 10 km and have amplitudes as high as 200 m and wavelengths of 0.5 to 2 km. Folds are best developed along sections of the trench with interbedded silly turbidite and mud deposits. Folds are absent where thick coarse-grained fan deposits occur. Thickening of the thrust packets occurs by large-scale thrust duplication, by layer-parallel shortening, and by deposition of material that slumps off the leading edge of older upslope thrust blocks.
Moore, G. F., and T. H. Shipley, Marine sediments, in McGraw Hill Yearbook of Science and Technology, 1985, McGraw Hill, New York, 289-290, 1987, #1309
Shipley, T. H., and R. T. Buffler, Costa Rica continental margin: Line CR-7, in Seismic Images of Modern Convergent Margin Tectonic Structure, edited by R. von Huene, AAPG Studies in Geol., 26, 33-36, 1987, #1125
Mayer, L. A., T. H. Shipley, and E. L. Winterer, Equatorial Pacific seismic reflectors as indicators of global oceanographic events, Science, 233, 761-764, 1986, 32 citations, doi:10.1126/science.233.4765.761, #657
The origin of a series of regionally correlatable seismic horizons in the Neogene sediments of the central equatorial Pacific is examined through seismic modeling and the detailed analyses of stratigraphic and physical property relationships in Deep Sea Drilling Project cores. These regionally traceable reflectors are synchronous; the younger reflectors are the direct result of carbonate dissolution events, the older ones of stratigraphically selective diagenetic processes. The changes in ocean chemistry associated with these events appear to be linked to global reorganizations of surface and bottom-water circulation patterns, the most dramatic of which are associated with reorganizations of North Atlantic bottom waters. These deepwater seismic horizons appear to correlate with the major events on the "relative sea-level" curve of Vail et al. for the Neogene.
Moore, G. F., T. H. Shipley, and P. F. Lonsdale, Subduction erosion versus sediment offscraping at the toe of the Middle America Trench off Guatemala, Tectonics, 5, 513-523, 1986, 18 citations, #664
High-resolution surface ship and deep-tow data from the Middle America Trench off Guatemala demonstrate that structures at the base of the landward slope are most simply interpreted as resulting from the offscraping and accretion of the uppermost trench sediments. There is a 250 to 300-m-wide ridge elevated 40ââ¬â60 m above the trench floor at the toe of the trench slope. Uplifted trench sediments are resolved on the ridge in one deep-tow profiler record. Trench strata beneath the ridge are imaged on migrated seismic reflection profiles, which show evidence for folding of the trench strata. Vergence of the structures is consistent with folding above a landward dipping thrust fault. We therefore interpret the ridge at the base of the slope as the surface expression of folded trench sediments that are presently being offscraped and accreted to the toe of the trench slope. Only the upper hundred or so meters of trench strata are offscraped; the remainder of the trench strata and the underlying plate deposits are subducted beneath the toe of the slope. Trench sediment fill buries much of the horst topography, which is then passively subducted without eroding the base of the slope. Our results indicate that (1) non-steady state accretion has occurred at this margin, even though Deep Sea Drilling Project drilling suggested otherwise, and (2) the lower-most slope has been affected by compressional deformation rather than extension or slumping.
Shipley, T. H., and G. F. Moore, Sediment accretion, subduction, and dewatering at the base of the trench slope off Costa Rica: A seismic reflection view of the decollement, J. Geophys. Res., 91, 2019-2028, 1986, 62 citations, #634
Sea Beam bathymetric and water gun seismic reflection data off the west coast of Costa Rica image the base of the trench slope, its underlying structure, and the behavior of a décollement surface in the presence of relief on the subducting plate. A low-angle, landward-dipping reflection marks the master décollement, above which only about 80 m of a 350 m oceanic sedimentary section is offscraped and accreted to the lower trench slope. A small horst block at least 9 km long, 1.5 km wide, and 300 m high is being obliquely subducted beneath the landward trench slope. On five seismic lines the décollement rises over the horst and then descends into the next incoming graben as the fault zone maintains a specific stratigraphic level. The undulating geometry of the décollement implies that the subducting grabens are not sites for sediment entrapment and subduction and that the horsts are not causing tectonic erosion of the landward trench wall. Substantial dewatering within the subducting sediments is documented within about 4 km of the deformation front. The dewatering may favor development of bedding-parallel zones of low shear strength within the subducting sediments and the master décollement may seek one of these zones.
Mayer, L. A., T. H. Shipley, F. Theyer, R. F. Wilkens, and E. L. Winterer, Seismic modelling and paleoceanography at Deep Sea Drilling Project Site 574, Init. Rept., Deep Sea Drilling Proj., 85, 947-970, 1985, 19 citations, #615
Petersen, L. D., F. K. Duennebier, and T. H. Shipley, Site surveys in the western Pacific conducted aboard the Kana Keoki cruise KK810626 Leg 4, Init. Rept., Deep Sea Drilling Proj., 89, 603-628, 1985, 1 citation, #671
Shipley, T. H., and G. F. Moore, Sediment accretion and subduction in the Middle America Trench, in Formation of Active Ocean Margins, edited by N. Nasu et al, Terra Scientific, 221-225, 1985, #598
Shipley, T. H., E. L. Winterer, M. Goud, S. J. Hills, C. V. Metzler, C. K. Paull, and J. T. Shay, Sea beam bathymetric and water-gun seismic reflection surveys in the equatorial Pacific, Init. Rept., Deep Sea Drilling Proj., 85, 825-837, 1985, 10 citations, #617
Silver, E. A., M. J. Ellis, N. A. Breen, and T. H. Shipley, Comments on the growth of accretionary wedges, Geology, 13, 6-9, 1985, 81 citations, doi:10.1130/0091-7613(1985)13<6:COTGOA>2.0.CO;2, #614
Uplift within accretionary foreland wedges has been explained by the development of duplexes, which provides a mechanism for âunderplatingâ in submarine accretionary wedges with little deformation in the upper part of the wedge. We interpret duplex development beneath the Costa Rica forearc, a zone widely considered to be a prime example of nonaccretion because of the apparent absence of an imbricate fan of thrusts at the toe of the wedge. Large-scale structural geometry of accretionary wedges is compatible with low-angle, trenchward-dipping backstops in centrist to reverse-angle backstops used commonly in models of return flow within wedges, which have been invoked to explain the presence of high-P/T metamorphic rocks. We suggest that emplacement of high-P/T metamorphic rocks are better explained in a collisional setting where known uplift rates are high than by flow within noncollisional accretionary wedges.
Volpe, A. M., T. H. Shipley, and G. F. Moore, A high resolution geophysical survey of Deep Sea Drilling Project Leg 84 Site 570, Init. Rept., Deep Sea Drilling Proj., 84, 851-860, 1985, #626
Whitman, J. M., T. H. Shipley, F. K. Duennebier, and L. D. Petersen, Western Pacific seismic stratigraphy, structure and sedimentation history, east Mariana Basin, Geology of Ocean Basins, Proc., 27th Int. Geol. Cong., 6 , VNU Science, 83-113, 1984, #607
Sheridan, R. E., F. M. Gradstein, L. A. Barnard, D. M. Bliefnick, D. Habib, P. D. Jenden, H. Kagami, E. M. Keenan, J. Kostecki, K. A. Kvenvolden, M. Moullade, J. G. Ogg, A. H. F. Robertson, P. H. Roth, T. H. Shipley, L. Wells, J. L. Bowdler, P. H. Cotillon, R. B. Halley, H. Kinoshita, J. W. Patton, K. A. Pisciotto, I. Premoli-Silva, M. M. Testarmata, R. V. Tyson, and D. K. Watkins, Early history of the Atlantic Ocean and gas hydrates on the Blake outer ridge: Results of the Deep Sea Drilling Project, Leg 76, Geol. Soc. Amer. Bull., 93, 876-885, 1982, 22 citations, doi:10.1130/0016-7606(1982)93<876:EHOTAO>2.0.CO;2, #549
Leg 76 of the Deep Sea Drilling Project achieved two major scientific objectives. The first objective was met at Site 533, where on the Blake Outer Ridge, gas hydrates were identified by geophysical, geochemical, and geological studies. Gas-hydrate decomposition produced a volumetric expansion of 20:1 of gas volume to pore-fluid volume; this expansion exceeded by about a factor of four the volume of gas that could be released from solution in pore water under similar conditions. The gas hydrate includes methane, ethane, propane, and isobutane but apparently excluded normal butane and higher molecular weight hydrocarbons as predicted from gas hydrate crystallography. For the first time, marine gas hydrates were tested with a pressure core barrel.
The second objective was achieved when coring at Site 534 in the Blake-Bahama Basin sampled the oldest oceanic sediments yet recovered. The sequence of oceanic basement and overlying sediments documents the geologic history of the early stages of the opening of the North Atlantic Ocean in detail. The oldest oceanic sediments are red claystones and laminated green and brown claystones of middle Callovian age. This finding supports the interpretation that the beginning of the modern North Atlantic occurred in the early Callovian (∼ 155 m.y. B.P.), as much as 20 m.y. later in time than often previously thought.
Shipley, T. H., J. W. Ladd, R. T. Buffler, and J. S. Watkins, Tectonic processes along the Middle America Trench inner slope, in Trench Forearc Geology, edited by J. K. Leggett, Geol. Soc. London Spec. Publ., 10, 95-106, 1982, #656
Shipley, T. H., K. J. McMillen, J. S. Watkins, J. C. Moore, J. H. Sandoval-Ochoa, and J. L. Worzel, Continental margin and lower slope structures of the Middle America Trench near Acapulco (Mexico), Marine Geol., 35, 65-82, 1980, 48 citations, doi:10.1016/0025-3227(80)90022-5, #344
The continental margin of southern Mexico north of the Tehuantepec Ridge and south of the Rivera Fracture Zone is a narrow crustal transition zone. The margin lacks a well-developed forearc basin and the active volcanic arc is subparallel to the trench and not clearly related in a normal fashion to the Benioff Zone. A geological survey of a small part of the margin 100 km south of Acapulco reveals significant variations in structure over short distances. Multi-channel seismic reflection data, sonobuoy refraction data and dredging indicate that the shelf and upper slope are underlain by continental crust. Geometry derived from the reflection data suggests that the continental crust is less than 10 km thick at its seaward termination.
Vail, P. R., R. M. Mitchum, T. H. Shipley, and R. T. Buffler, Unconformities of the North Atlantic, Phil. Trans. Roy. Soc. London, A294, 137-155, 1980, 55 citations, #291
Lowstands of sea level produce significant unconformities, both on the continental shelves as subaerial unconformities and on the ocean basin slopes and floors by submarine erosion and shifts in depositional patterns. This report utilizes seismic data from the eastern Atlantic off Africa and the western Atlantic off the Blake Escarpment to illustrate the recognition and dating of deep sea unconformities. Twenty-eight major and minor deep sea unconformities are identified on these seismic data and tentatively dated by means of well control and a chart showing global relative changes of sea level. The major unconformities identified are basal Sinemurian, basal Callovian, basal Valanginian, basal middle Aptian, basal middle Cenomanian, basal Thanetian, basal upper Ypresian, basal middle Chattian, basal Burdigalian, basal middle Tortonian, and basal Messinian. Unconformity identification and correlation on seismic data from the deep sea is useful for building a stratigraphic framework for palaeoenvironmental studies and correlating deep-sea stratigraphy with the stratigraphy of continental shelves and interior basins.
Shipley, T. H., M. H. Houston, R. T. Buffler, F. J. Shaub, K. J. McMillen, J. W. Ladd, and J. L. Worzel, Seismic evidence for widespread possible gas hydrate horizons on continental slopes and rises, AAPG Bull., 63, 2204-2213, 1979, 279 citations, #321
Anomalous reflections in marine seismic reflection data from continental slopes are often correlated with the base of gas hydrated sedimentary rocks. Examination of University of Texas Marine Science Institute reflection data reveals the possible presence of such gas hydrates along the east coast of the United States, the western Gulf of Mexico, the coasts of northern Colombia and northern Panama, and along the Pacific side of Central America in areas extending from Panama to near Acapulco, Mexico. Suspected hydrates are present in water depths of 700 to 4,400 m and extend from 100 to 1,100 m subbottom.
Geometric relations, reflection coefficients, reflection polarity, and pressure-temperature relations all support the identification of the anomalous reflections as the base of gas hydrated sediments. In most places, gas hydrate association is related to structural anomalies (anticlines, dipping strata), which may allow gas to concentrate and migrate updip into pressure and temperature conditions suitable for hydrate formation. The gas hydrate boundary can be used to estimate thermal gradients. In general, thermal gradients estimated from the gas hydrate phase boundary are higher than reported thermal gradients measured by conventional means.
Shipley, T. H., M. H. Houston, R. T. Buffler, F. J. Shaub, K. J. McMillen, J. W. Ladd, and J. L. Worzel, Widespread occurrence of possible gas-hydrate horizons from continental slopes as identified on seismic reflection profiles, Proc., Offshore Tech. Conf., 11, OTC 3570, 1979, #322
Buffler, R. T., T. H. Shipley, and J. S. Watkins, Blake Continental Margin Seismic Section, Amer. Assoc. Petrol. Geol. Seismic Sect., 2, 1978, #87
Shipley, T. H., Sedimentation and echo characteristics in the abyssal hills of the west-central North Atlantic, Geol. Soc. Amer. Bull., 89, 397-408, 1978, 17 citations, doi:10.1130/0016-7606(1978)89<397:SAECIT>2.0.CO;2, #219
The abyssal hills province of the western North Atlantic 600 km southeast of Bermuda contains irregular hills and basins 10 to 20 km wide, with about 400 m of relief probably generated by ridge-flank volcanism and faulting. Conical(?) abyssal hills 5 to 10 km wide with about 300 m of relief are thought to represent a later stage of midplate volcanism, perhaps related to the post-Eocene uplift of the Bermuda Rise.
The 3.5-kHz echo characteristics of the surficial sediments were combined with piston core data to produce a geologic map with six main units. Pelagic brown-clay sedimentation has been fairly constant at 1 to 3 m/m.y. since mid-Cretaceous time. In some of the high areas and on the Bermuda Rise at depths less than 5,500 m, a 30-m opaque unit records carbonate sedimentation that began sometime in Pliocene time with lowering of the calcium carbonate compensation depth. Three distinct echo characters are observed from the various stratified sediment bodies. One is owing to near-outcrop of a deep reflector unit. Another unit is composed of discrete, closely spaced reflectors of the Nares Abyssal Plain and parts of the southern fracture valley. A third unit contains widely spaced (20 m) reflectors restricted to the more northerly fracture valley and certain cross valleys. The stratified sediments in both the Nares Abyssal Plain and the fracture valleys consist of brown clays at the surface and gray clays at depth. The gray clays are interpreted as distal turbidites, having 12% more silt than the pelagic brown clays and always occurring in water depths greater than 5,700 m. The turbidity currents crossed the Nares Abyssal Plain into the southern valley and over low sills into the northerly fracture valley. The veneer of brown clay indicates that turbidity currents probably have not been active in this area for at least 300,000 yr.
Shipley, T. H., R. T. Buffler, and J. S. Watkins, Seismic stratigraphy and geologic history of the Blake Plateau and adjacent western Atlantic continental margin, AAPG Bull., 62, 792-812, 1978, 49 citations, #230
A multifold seismic reflection profile across the Atlantic continental margin from the shelf off Jacksonville, Florida, northeast across the Blake Plateau, northern Blake basin, Blake Outer Ridge, and lower continental rise shows in some detail the structure, stratigraphy, and geologic history of this passive margin since the Late Triassic(?)--Early Jurassic. A seismic stratigraphic framework divides the sedimentary section into seismic intervals, each representing depositional sequences and having distinct acoustic-stratigraphic characteristics. Interval velocities calculated from 64 reflection velocity analyses were used in the geologic interpretation and to construct a geologic depth section along the seismic profile.
Dipping reflectors 7 to 10 km beneath the eastern margin of the Blake Plateau probably represent the eastern edge of the rifted North American continental basement. Up to 8 km of Jurassic(?) through Cretaceous sediments overlie the regional post-breakup unconformity beneath the Blake Plateau. Shallow-water carbonate sedimentation persisted through most of the Cretaceous. In the Late Cretaceous the shelf margin shifted from the Blake Escarpment landward to approximately its present location as a result of continued subsidence and a Late Cretaceous overall rise in sea level. An abbreviated section of deeper water carbonate sediments followed on the plateau during the Tertiary owing to many changes in sea level and currents sweeping the plateau. Subsidence of the plateau appears to be in response mainly to sediment loading.
Up to 2 km of Neocomian and older sediments were deposited in the deep sea adjacent to the Blake Plateau. Widespread seafloor erosion and the localization of depositional regimes began in the middle Cretaceous. During the Tertiary enormous quantities of mainly terrigenous material were deposited along the deep-sea continental margin as a large wedge up to 4 km thick. Currents following the seafloor contours influenced sedimentation throughout the Tertiary.
Shipley, T. H., and J. S. Watkins, Fine-scale seismic stratigraphy in the western North Atlantic, Geology, 6, 635-639, 1978, 10 citations, doi:10.1130/0091-7613(1978)6<635:FSSITW>2.0.CO;2, #289
Seismic reflection profiles from the lower continental rise off Cape Hatteras illustrate the importance of cycle-to-cycle correlation in seismic stratigraphy. The reflection data reveal that horizons β and A are unconformities with truncations of 120 m or less in the study area. These unconformities probably result from erosion, which suggests that bottom currents were active as early as middle Cretaceous time. The reflection geometries and amplitudes also show that the seaward termination of reflector X may be a facies transition from the seaward-prograding rise sediments to abyssal plain turbidites.
Watkins, J. S., R. T. Buffler, M. H. Houston, J. W. Ladd, T. H. Shipley, F. J. Shaub, J. B. Sinton, J. L. Worzel, and W. P. Dillon, Crustal velocities from marine common-depth-point reflection data, western North Atlantic, Caribbean, and Gulf of Mexico, in Earth's Crust, edited by J. G. Heacock, Amer. Geophys. Union, Geophys. Monograph, 20, 271-288, 1977, #203
Lindsay, J. F., T. H. Shipley, and J. L. Worzel, Role of canyons in the growth of the Campeche Escarpment, Geology, 3, 533-536, 1975, 2 citations, doi:10.1130/0091-7613(1975)3<533:ROCITG>2.0.CO;2, #70
At least 14 deeply incised V-shaped canyons transect the Campeche Escarpment between long. 88°00′W and 92°35′W. The canyons all appear to be erosional features that may have been initiated during Cretaceous time; considerable erosion occurred probably during Pleistocene time. It is suggested that the canyons act as a balancing mechanism in sediment dispersal. At times when sediment supply is large, such as times of Pleistocene lowering of sea level, the canyons act as a conduit for the transfer of sediment from the shelf region directly to the deep ocean. Thus the lateral rate of growth of a carbonate platform is not necessarily a linear function of sediment supply. The canyons do not appear to be transporting sediment actively at the present time.
Ewing, M., R. W. Embley, and T. H. Shipley, Observations of shallow layering utilizing the pingerprobe echo-sounding system, Marine Geol., 14, M55-M63, 1973, 6 citations, doi:10.1016/0025-3227(73)90012-1, #1
A âPingerprobeâ is a system of echo sounding in which the sound source is placed near bottom to improve resolution by restricting the area investigated. It is demonstrated that a commercially available 12-kHz âpingerâ with a synchronized shipboard receiver is useful not only in the monitoring of the positioning of a bottom or near-bottom instrument package (such as a corer) but also in making observations on the acoustic nature of the sea floor. In rough terrain the Pingerprobe has measured stratified sediments in some places where the PDR (Precision Depth Recorder) cannot. Observations on proximal abyssal plains indicate that the prolonged echo character common to these areas may result from small-scale roughness or inhomogeneity. When a suspended instrument is sent to the bottom in rough terrain, or in areas of intermittent subbottom reflections, use of a Pingerprobe improves information about the conditions at the point of contact and permits selection of the desired topographic setting.
Ewing, M., T. H. Shipley, and S. D. Connary, Intensive survey of a manganese nodule region in the North Atlantic Ocean, in Inter-University Program of Research on Ferromanganese Deposits of the Ocean Floor, edited by S. Gerard, IDOE-Nat. Sci. Found., 187-215, 1973, #88