Institute for Geophysics Theses and Dissertations

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Late Pleistocene fluvial-deltaic deposition, Texas coastal plain and shelf
(1979) Winker, Charles David; Baker, Victor R.
Deposition on the Texas coastal plain and shelf during the last Pleistocene glacial cycle has been interpreted from topographic and bathymetric maps, from borehole data including results of a detailed drilling study in Brazoria County, and from offshore sparker-profiles. The stratigraphic unit deposited during the last glacial cycle is bounded above by a largely undissected topographic surface (Beaumont), and below by a buried paleosol of stiff gray clay that correlates offshore with a persistent seismic reflector. The lower Texas coastal plain is essentially a clay-rich alluvial plain made up of coalescing low-gradient fans. The Beaumont alluvial plain onlaps an older surface (Lissie) which was tilted seaward prior to Beaumont deposition; the Lissie in turn onlaps remnants of older surfaces. During Beaumont deposition, each major coastal river deposited a branching network of meanderbelt sand-bodies by repeated avulsions. Borehole data for a meanderbelt of the ancestral Brazos River indicate that the channel was 5 to 7 m deep but that substantially greater sand thicknesses developed by stacking of point-bar sequences during fluvial aggradation. Downdip transition of fluvial deposits into deltaic and paralic sediments is inferred from shell beds, strike-oriented sand bodies, beach ridges, changes in clay color, and clinoform reflectors on sparker profiles. The updip limit of marine influence is delineated by the distribution of shell beds; the downdip limit of deltaic pro gradation is indicated by a paleobathymetric break in slope. Strike-oriented sands (Ingleside), including barrier islands and strandplains, developed contemporaneously with Beaumont fluvial aggradation. Sand thicknesses and multiple levels of sand suggest that beach-shoreface sequences are multistoried, similar to the fluvial sands. The thickest and widest strike sands formed in bights between the larger, more prominent deltaic systems. In response to falling sea level, deltas prograded from the Ingleside shoreline to the shelf edge. Sparker profiles show that deltaic thicknesses and offshore slopes increased gradually during progradation, then rapidly near the shelf edge, where deltaic sequences became stacked or imbricated. Major growth faulting and salt flowage near the shelf break were associated with the thickest deposits. Between the large Colorado and Rio Grande delta systems, reefs grew near the shelf edge. Late Pleistocene sea-level fluctuations resulted in three depositional phases: an aggradational phase (ca. 120,000 B.P.) during late rise and stillstand, dominated by fluvial and strike systems; a progradational phase (100,000 to 20,000 B.P.) during a gradual fall, dominated by deltaic systems, and a rapid transgressive phase (20,000 to 4000 B.P.). The Texas coast is now in another aggradational phase. Average rates of late Pleistocene sediment influx were similar to historic rates, and show a decrease in sediment production toward the arid southwest. Post-depositional deformation of the Beaumont and Lissie alluvial plains and Ingleside shoreline can be explained largely as an isostatic response to sedimentary loading
Seismic structural analysis of deformation in the southern Mexican Ridges
(1982-05) Pew, Elliott; Muehlberger, William R.
The southernmost region of the Mexican Ridges extends from Bryant's gap near 22.5 N latitude to the Campeche Knolls near 19.0 N latitude. Analysis of 23,030 kilometers of sparker and CDP seismic data from six surveys reveals the existence of two separate areas of folding, Zones 4a and 4b. In the Zone 4a foldbelt symmetrical folds form a gentle salient which parallels the curved outline of Isla de Tuxpan. Structural relief often in excess of 500 meters is reflected by similar bathymetric relief. Fold wavelengths average 10-12 kilometers. A detachment or decollement is interpreted in a thick Upper Cretaceous to Lower Tertiary pelagic shale sequence by the existence of relatively undeformed reflectors below this interval. The 3 to 3.5 kilometer thick allochthonous sheet has experienced approximately 1% shortening and a maximum displacement of 1 to 2 kilometers. The Zone 4a foldbelt appears to be a massive gravity slide. Folded Plio-Pleistocene strata establish the youth of these folds. A large deep-rooted structure of questionable origin is observed on GLG 22. This structure, exhibiting roughly 1500 meters of bathymetric relief, acts as a foreland buttress against which the gliding allochthonous mass deforms. The tightly appressed thrust-faulted folds up dip from the buttress exhibit anomalously short wavelengths. While no folding is observed directly down dip from the buttress, folding is observed 30 to 50 kilometers basinward of this structure just a few kilometers to the south. The boundary separating Zones 4a and 4b is a linear feature oriented transversely to regional strike and may be a tear fault. Reflections at depth are not continuous across this feature. The Zone 4b foldbelt lies directly down dip from the Veracruz Basin. Structural relief commonly doubling that observed in Zone 4a is rarely expressed as bathymetric relief. Individual folds are asymmetric, having gently dipping landward flanks and either steeply dipping or growth-faulted seaward flanks. Fold cores appear to contain diapiric material. Fold growth due to gravity sliding began in the Middle Miocene. Subsequent loading by a thick Middle-Upper Miocene section gradually halted downslope movement and initiated flowage of plastic substrata from beneath loaded synclinal troughs into anticlinal cores. This deformation has continued to the present in some folds.
Organic geochemistry of an oil and gas seep in northern Gulf of Mexico sediments
(1984) Anderson, Richard Kent; Parker, Patrick L.
During an extensive geochemical and geophysical survey of the outer slope of the northwestern Gulf of Mexico nine piston cores were recovered which had visible liquid organic deposits. In three of the cores deposits were observed concentrated in oblique fracture planes. Other occurrences included large deposits as liquid veins and smaller disseminated pockets in gassy and non-gassy sediments. The benzene soluble material (bitumen) was extracted and chemically and isotopically characterized. Bitumen content ranged as high as 8.6 percent in sediment samples. Gas chromatographic analyses of silica gel fractions showed that both the saturated and aromatic hydrocarbon components are highly biodegraded. The δ¹³C values for the whole oil and fractions were between -26.2 and -26.7 per mil on the PDB scale which closely resembles other Gulf coast oils. The δD values of the oil averaged -104 per mil relative to SMOW. Carbonate nodules found in the oil-rich zones were ¹³C depleted, indicating oxidized organic matter to be the source of the inorganic carbon. Several cores contained natural gas in concentrations high enough to result in large expansion gaps under the reduced ambient pressure at sea level. Hydrocarbon gases from methane through pentanes were sampled in nine cores. Chemical composition and δ¹³C values for methane, ethane, propane, and butanes (-30.5 to -61.9, -28.5, -24.5, -25.7 per mil) indicated that the gas has a major petrogenic component. δD values for methane, ethane, propane and butanes were -172, -101, -104, -101 per mil. Compositional variability of C₂⁺ gases among cores suggests the possible regional influence of gas hydrate formation. Compositional and isotopic variability of methane within and between cores does not conform to a two component mixing model (e.g. biogenic plus petrogenic methane). Instead, highly localized processes, possibly microbial, are implicated
Geology of the Burica Peninsula, Panama-Costa Rica : neotectonic implications for the southern Middle America convergent margin
(1986) Corrigan, Jeffrey Delon; Not available
The Burica Peninsula, situated along the Pacific coast on the Panama-Costa Rica border, extends to within 15 km of the projected trend of the Middle America trench axis and provides the thickest exposure of Neogene, marine sedimentary rocks along the Middle America margin. A thick section (~3,000 m) of gently folded, and intensely faulted, Pliocene-Early Pleistocene marine sedimentary rocks unconformably overlie a discontinuous, Paleocene-Eocene limestone unit which, in turn, overlies a highly fractured, mafic igneous basement. The basement is similar to late Cretaceous-Eocene igneous exposures (the Nicoya Complex) on the Osa and Nicoya Peninsulas, 50 and 280 km, respectively, to the northwest. The Plio-Pleistocene marine sedimentary rocks consist predominately of laterally continuous, thinly bedded (2-7 cm thick), very fine-grained volcaniclastic sandstone, siltstone, and mudstone. Benthic foraminifera in this turbidite section indicate paleo-water depths of 2-3 km at the base of the section, and ≤1.5 km at the top. Lithofacies mapping and paleocurrent patterns indicate at least two major channe-fill complexes (≥300 m wide), as well a number of smaller (3-15 m wide) channel-fill deposits. Both the large and small channel-fill deposits contain clasts up to 15 cm long (a-axis) and exhibit predominately clast-supported textures. Measurements (231 total) of sole marks (flute and groove casts), imbricated clasts in conglomerates, and aligned wood fragments suggest that paleoflow (S-SSE) was at a high-angle to the margin. Debris-flow deposits and slump horizons also occur and account for approximately 3% of the section. Facies patterns, normal-to-margin paleoflow, and paleobathymetry suggest that the sediments were deposited on the landward trench-slope of the Middle America Trench. The well-bedded, Plio-Pleistocene turbidite sequence reveals a monoclinal structure that dips to the E-NE at 10°-40° away from a basement highland defined by poorly exposed high-angle reverse(?) faults. The Medial Fault Zone (MFZ), an inferred 2-3 km wide, north-trending, right-lateral strike-slip fault system, is approximately coincident with the Panama-Costa Rica border in the study area. The MFZ separates the well-bedded turbidite sequence to the east from the pre-Paleocene basement and overlying Pliocene massive, fossiliferous, shallow-water(?) muddy siltstones to the west. Numerous normal faults throughout the Neogene sedimentary cover east of the MFZ show two dominant trends, N45°E and N80°E, and appear to have moved contemporaneously. Displacements along individual faults averages 0.5-3.0 m. I interpret the observed structures to indicate localized NNE-directed subhorizontal shortening superimposed on regional NNW-SSE directed extension in direct response to uplift of the peninsula. Based on magnetic anomaly reconstructions, impingement of the aseismic Cocos Ridge on the Panama-Costa Rica volcanic arc is spatially and temporally correlative with the uplift of the Burica Peninsula. A 2 km paleodepth, with uplift initiating at the end of the Pliocene, suggests a long-term uplift rate of 1.25 mm/yr. I suggest that the magnitude of uplift, as well as the Neogene fault patterns, are best explained in terms of isostatic adjustments resulting from subduction of the Cocos Ridge beneath the Burica Peninsula.
MAGANOM : a computer program for the modeling and interpretation of marine magnetic anomalies with an example from the Cayman Trough, northwest Caribbean Sea
(1987) Ross, Malcolm Ingham, 1961-; Not available
Traveltime inversion for a 3-D near surface velocity model
(1987) Simmons, James Layton, 1957-; Backus, Milo M., 1932-
The near surface environment is often the source of the most severe lateral velocity variations present in the seismic section. Near surface lateral velocity variations distort the traveltimes of deeper events and are the most serious limitation in achieving accurate structural maps. This work discusses the development of a near surface velocity model for a shallow marine data set. The near surface model consists of three components. The first is a model of the laterally variable seafloor depth and topography. Below the seafloor, the model consists of the compressional wave velocity as a function of depth which reaches a maximum depth of approximately 500 meters. The presence of vertical and lateral velocity gradients is recognized. Embedded within this slowly varying background velocity field are a number of local lens-like velocity anomalies. The lens anomalies represent the major lateral velocity variations present in the near surface. Autocorrelograms of the deeper pre-stack data are used to obtain the seafloor model. The period of the first water layer reverberation is used to estimate the water depth. These data are enhanced by a deconvolution algorithm which improves the agreement at the line intersections. Measured first arrival times from the pre-stack data are used to develop the subseafloor velocity model. A multichannel filter algorithm is devised to estimate the traveltime deviations produced by the lens anomalies and the common shot statics. These traveltime deviations are the higher spatial frequency components of the first arrival times and are produced by the higher spatial frequency components of the velocity model. The output from the algorithm consists of a sixteen layer traveltime (velocity) perturbation model. The estimates of the lens anomaly and shot static produced traveltime deviations are subtracted from the first arrival times to isolate the slowly varying background components. These data are then inverted using the Generalized Linear Inversion and Tausum algorithms to obtain the laterally varying background velocity model.
Application of extensional models to the northern Viking Graben, North Sea
(1987-05) Giltner, John Patrick, 1962-; Not available
Several previous authors have concentrated on the Central Graben in an attempt to model the North Sea as an extensional basin. Theoretical stretching models predict a certain amount of subsidence, crustal thinning, and fault displacement for a given amount of lithospheric extension. The validity of the models has been questioned since the extension suggested by subsidence data is at least twice the measured elongation on the observed faulting. In the Central Graben, the observations mentioned above are hampered by salt diapirism, extensive erosion, structural inversion, and strike-slip faulting. The Viking Graben, 300 km to the north, is a better location to test the stretching models since it appears to display extensional tectonics in a more simplified form. Subsidence curves corrected for compaction, estimated water depths of deposition, and changes in sea level from the Northern Viking Graben are matched to a uniform time-dependent extensional model. The accumulation of Lower Triassic to Recent sediments may be explained within a framework of two extensional phases that took place first during the Triassic and then again during the Late Jurassic/Early Cretaceous. Present day subsidence along the graben axis suggests a total extension of approximately 1.8, consisting of a Triassic extension of 1.5 and an Late Jurassic extension of 1.2. On the Horda Platform, Triassic rifting is even more dominant, with little expression of an Late Jurassic event. Within the graben, the model predicts subsidence very well, but the flanks are problematic. A more complicated model involving depth dependent stretching, mantle convection, or isostatic uplift of individual fault blocks may be needed to explain the uplift/non-subsidence of the graben flanks during rifting. Geometry of rotated fault blocks in the area accounts for extension of between 1.1 and 1.3, which is consistent with the Late Jurassic event. Although Triassic faulting is clearly seen on the eastern margin of the basin, early phase faulting in the graben axis has probably been rotated by the later tectonic activity and is not clearly imaged due to its depth and orientation. Crustal thinning determined from recent deep reflection and older refraction profiles suggest an extension of approximately 2.0, which is compatible with the observations of total subsidence. My modeling suggests that most of this thinning occurred during the Triassic.
The mapping of tectonic features in the ocean basins from satellite altimetry data
(1988-05) Gahagan, Lisa Marie, 1963-; Not available
Satellite altimetry data provide information on the height variations of the sea surface. The angle between a line perpendicular to the sea surface and a vertical line between the satellite and the sea surface is referred to as the deflection of the vertical and is equal to the first derivative of the sea surface. This study examines two theoretical models describing the relationship between the deflection of the vertical data and the bathymetry 1) across a fracture zone in a large age-offset, fast-spreading regime and 2) across a fracture zone in a small age-offset, slow-spreading regime. The models are respectively compared to the observed relationship 1) across the Mendocino Fracture Zone which is in a large age-offset, fast-spreading regime and 2) across the DuToit Fracture Zone which is in a medium age-offset, slow-spreading regime. The strong agreement between the theoretical models and the observed relationships suggests that the models can be used with the deflection of the vertical data to locate fracture zones in known regimes. The angle between the trend of a feature and the trend of the satellite track affects the deflection of the vertical signal. As the angle becomes smaller, the amplitude of the deflection of the vertical signal, which varies with the sine of this angle, decreases and the wavelength of the signal increases. Once the feature is parallel to the track, there is no deflection of the vertical signal. The deflection of the vertical signal is also affected by the direction the satellite travels. If the feature trends between the ascending and descending tracks of the satellite, then the satellite will cross the feature from opposite directions and the ascending and descending signals will be opposite to each other. If the feature does not trend between the ascending and descending tracks, then the satellite will cross the feature from the same side and the deflection of the vertical signal will be similar for both the ascending and descending data sets. A third factor affecting the deflection of the vertical signal is the latitude at which the feature is located. The trend of the satellite track varies as a function of latitude, ranging from 18° at 0° latitude to 64.6° at 70° latitude. Because the trend of the satellite track varies, not only does the angle between the trend of a feature and the trend of the satellite track vary with latitude, but the amplitude of the deflection of the vertical signal varies with latitude as well.
One and two dimensional velocity inversion in the domain of intercept time and ray parameter : an example in the Nankai Trough
(1989) Wood, Warren Theodore, 1962-; Stoffa, Paul L., 1948-
The intercept time equation in τ-p (Diebold and Stoffa, 1981) [mathematical equation] is used as the basis for exact 1D and 2D τ-p velocity analysis. Intercept time curves for an initial model are superimposed on the τ-p seismic data. Model parameters such as layer velocity, thickness, and dip are adjusted until the intercept time curves are coincident with the reflections in the data. Normal moveout in the domain of τ and p is applied during the analysis so as to check the picks of the reflections. When a reflection has been moved out correctly, it has been properly modeled, (i.e. the velocity, thickness, and dip of the layer have been determined). Once all the reflections have been imaged, the analysis is complete. One attractive feature of this method is that all of the calculations can be done quickly, so the analysis can be done interactively on a computer with a graphics screen. This velocity analysis method was then applied to long offset seismic data collected in the Nankai Trough. Eight expanding spread profiles (ESPs) and five split spread profiles (SSPs) were collected in two different areas and analyzed to accurately determine sediment velocities in the trough and on the accretionary wedge. The results are a series of 1D earth models in the ESP area and 2D models in the SSP area. The analysis of the ESP data clearly shows a low velocity zone associated with a bottom simulating reflector but does not show evidence of a large (200 - 300 m/sec) velocity reversal at the decollement as expected. The analysis of the SSPs in an area about 100 km away, however, does point to a trend of decreasing velocity just above the decollement with increasing distance under the wedge.
On apatite fission-track analysis and heat transfer processes in the upper crust
(1990) Corrigan, Jeffrey Delon; Not available
Fission-track ages and confined track-length measurements from detrital apatites recovered from Ocean Drilling Program (ODP) Leg 116 Site 717 and 718 cores of distal Bengal Fan sediments provide information on both the thermal history of sediments recovered at these two sites and the cooling rates of apatite source areas. Site 717 was drilled to a depth of 820 meters below the seafloor (mbsf) bottoming out in upper Miocene strata. Site 718, located approximately 7 km to the south of Site 717 on an adjacent fault block, was drilled to a depth of 960 mbsf bottoming out in lower Miocene strata. Fourteen composite apatite samples were derived by combining fine-grained sand samples from Site 717 and 718 cores over 70- to 120-m intervals. Thirty apatite grains per composite sample from ten samples (at least every other sampled interval) were dated and track length measurements (20 - 50 per sample) were obtained for all samples. Mean track lengths from Site 717 samples are statistically identical, ranging from 14.4 ± 0.4 to 14.8 ± 0.3 μm (all uncertainties are the standard error of the mean at the 95% confidence interval), and mean fission-track ages increase monotonically down-section from 4.8 ± 1.1 to 14.3 ± 2.2 Ma. At Site 718, mean track lengths to 560 mbsf are equivalent to those measured from Site 717 samples. A monotonic decrease in mean track length (14.6 ± 0.3 to 13.2 ± 0.4 μm) and a corresponding decrease in mean fission-track age (21.1 ± 2.9 to 15.8 ± 2.4 Ma) with depth for samples between 560 and 960 mbsf at Site 718 indicates that shortening of fission-tracks in these samples occurred at elevated temperatures after deposition. Track length shortening, relative to an unannealed mean track length of 16.3 μm, is approximately 10% for all Site 717 samples and for samples from the upper 560 m at Site 718. Mean track length reduction of the lowermost sample at Site 718 is approximately 20%. Assuming that elevated temperatures responsible for this reduction are related to the onset of deformation at approximately 7 Ma, temperatures of about 55 ± 5 °C at 860- to 960-mbsf are inferred based on extrapolation of laboratory annealing experiments. These temperature estimates imply significant advective heat transfer through the sediment pile near Site 718 because temperatures at these depths are expected to be on the order of 30-35 °C for conductive heat transfer predicted by cooling plate models for oceanic lithosphere. Dated samples have mean apatite fission-track ages that are less than 10 m.y. older than sample depositional ages. These young ages indicate that rocks in the upper crust of Bengal Fan source areas cooled at rates greater than 15 °C/m.y. These cooling rates imply source area denudation rates exceeded 300 m/m.y. These estimates indicate that source areas similar to the present Himalayas have supplied sediment to the distal Bengal Fan since at least 17 Ma.
Seismic traveltime inversion in three-dimensional heterogeneous media
(1990) Finn, Christopher Jude, 1960-; Backus, Milo M., 1932-
The measured traveltimes of specular reflection events are inverted to obtain a low spatial frequency, three-dimensional model of the reflector geometry and the compressional wave propagation speed. B-spline functions are used to describe the shapes of the interfaces and the lateral variations in velocity. The inversion is performed by optimizing a maximum likelihood criterion using a Newton based iteration. Model updates are obtained by iterative forward modeling and solution of the linearized equation set derived from the maximum likelihood criterion. In the forward problem, the ray tracing equations are solved as a two point boundary value problem with appropriate internal boundary conditions at velocity discontinuities. Analytic expressions for the Frechet derivatives necessary to obtain the model updates are given. Conventional methods are compared to the traveltime inversion technique using synthetic examples. For a relatively simple earth model containing only moderate lateral velocity variations hyperbolic moveout analysis followed by a Dix inversion produces a biased estimate of the velocity and depth. This is a consequence of the simplifying assumptions of the method. In this case, the more general traveltime analysis provides a better result. This is also true for a more complex earth model containing lateral velocity variations and interfaces with large dips and curvatures where the conventional methods fail badly. Picked traveltimes are used as the data in the inversion although the use of the data semblance or the stack power along the predicted traveltime trajectory is also explored. These criterion are shown to be more nonlinear than the least-squares data residual measure. Thus, it is difficult to converge to a global minimum using these criterion and more accurate initial guesses are necessary. An application of the traveltime inversion technique to a 3D marine data set is presented. In this application the effects of the seismic source and the recording system on the measured traveltimes are estimated. The time delay between the first break and the main pulse of the minimum phase source wavelet and the effect of the ghost reflections from the free surface are compensated for in the prediction of the measured traveltimes
Strike-slip faulting and basin formation at the Guayape Fault--Valle de Catacamas intersection, Honduras, Central America
(1990-12) Gordon, Mark Buchanan, 1961-; Muehlberger, William R.
The Valle de Catacamas forms a major basin along the central portion of the Guayape fault, the most prominent tectonic element of the Chortís block. The Guayape fault extends 290 km southwest from the Caribbean coast to the region of El Paraíso, Honduras, and may continue to the Pacific coast along a related prominent topographic feature, the Choluteca linear. Basins presently forming along the Guayape fault indicate that the fault is currently experiencing right-slip. The active features of the Valle de Catacamas displace older folds and reverse faults which apparently formed during an earlier period of sinistral shear. Thus, the Guayape fault has undergone at least two phases of movement, post-Cenomanian left-slip followed by the present right-slip. The geology of the valley suggests multiple stages of evolution. These include at least one period of thrust and reverse faulting, possibly associated with sinistral shear along the Guayape fault, and a recent episode of normal faulting associated with dextral shear on the Guayape fault. Thrusting of basement rocks over Jurassic strata on the south side of the valley was the earliest deformation to affect Mesozoic or Cenozoic rocks. The event can only be dated as post-Jurassic in age. The Cretaceous rocks of the Sierra de Agalta on the north side of the Valle de Catacamas are much more strongly deformed than similar rocks in central Honduras. In this range, the Aptian-Albian Atima Limestone commonly has a pervasive pressure solution cleavage which has not been reported from other locations on the Chortís block. The cleavage is apparently axial planar to the folds. The age of this deformation is constrained only as post-Cenomanian. SIR data indicate that these folds are deflected in sinistral shear near the Guayape fault. In addition, a major structural contact has a large left-lateral separation. The folds in the Sierra de Agalta are cut by the range-bounding normal fault of the Sierra de Agalta. Younger rocks are placed on older rocks by this normal fault, and fault slip data from small fault planes in the footwall block indicate normal faulting. The N 65° E strike of this normal fault, the N 35° E strike of the Guayape fault, and stress orientations inferred from fault slip data indicate that the present movement on the Guayape fault is right-slip. Fault slip data from the Guayape fault zone is heterogeneous as would be expected if two stage slip has occurred.
Cretaceous to Neogene tectonic control on sedimentation : Maracaibo Basin, Venezuela
(1991) Lugo Lobo, Jairo Miguel, 1955-; Salvador, Amos
The Maracaibo basin records a complex tectonic and depositional history. Compilation of seismic and well data allows me to distinguish six tectonostratigraphic episodes. The first episode is characterized by Jurassic rifting along north-northeast-trending half-grabens filled with continental red beds and volcanics rocks. The second episode is characterized by the deposition of shallow to deep marine carbonates and clastic rocks in which subsidence rates increase away from the northwest-trending Mérida arch, a mid-Late Paleozoic northwest-trending range anchored to the Guayana Shield. The third episode is distinguished by oblique collision of south-southwest-verging Caribbean terranes toward the continental platform. As collision evolved, the tectonically carried crustal flexure and associated turbiditic basin migrated south-southeastward. Evidences from geohistory analysis shows the shift of the axis of subsidence in that direction. During this episode six unconformity-bounded deltaic wedges marked an equal number of regressive stages caused by thrust propagation above the flysch basin. Seismic clinoform orientations and sandstone composition document the regional northern provenance for such paralic wedges in the northeastern portion of the basin. The fourth episode developed from Late Eocene to Middle Miocene in which transpressive tectonism reactivated the structural weaknesses developed in the earlier rift. Left-lateral north-northeast-trending strike-slip faults and related en echelon secondary structures were built in two main active phases: one during Late Eocene and the other in Middle Miocene time. The reactivation of the main Icotea and Pueblo Viejo faults influenced the distribution of intrabasinal restraining bends, pull-apart basins, and associated sediment infill. The fifth episode is characterized by diachronous orogeny that began with Oligocene uplift along the Sierra de Perijá and continued southeastward toward the Mérida Andes. These uplifts created a closed depositional basin and converted the depositional environment from marine to continental. The six episode is characterized by compression evidenced by north-striking east-vergent reverse fault in the Sierra de Perijá
Three dimensional seismic imaging of the Costa Rica accretionary margin
(1991-05) Kessinger, Walter Paul, 1963-; Stoffa, Paul L., 1948-
Studies in seismic scattering
(1992) Cardimona, Steven James, 1963-; Wilson, Clark R.
Modem methods of seismic data analysis are tending to inversion through model fitting, i.e., actually finding the best model of the Earth's subsurface which would produce the amplitude and phase variation in the observed data. An understanding of seismic scattering is fundamental to this form of data analysis. This dissertation involves the study of seismic scattering and its use in the inverse problem, applying full waveform inversion ideas in novel situations. The terminology and methodology of inverse theory may sometimes hide what is going on, and may make it difficult to connect the results with those from more familiar techniques. In Chapter 1 I show that, with the appropriate choice for the model parameters, the first iteration of the nonlinear least-squares seismic waveform inversion algorithm reduces to classical results from linear filter theory. I use the idea of the adjoint of the Frechet derivative linear operator in Chapter 2 to understand smoothing in the waveform inversion, which manifests itself as a new sensitivity function incorporating the smoothing information. This gives us physical intuition into the wave equation based inverse problem. My mathematical analysis is general; however, using sensitivity functions for the paraxial equation in ray centered coordinates, I show a specific application to full waveform imaging in a tomographic experiment where only phase information (travel-time data) is normally used. I consider an inversion of teleseismic data from some deep earthquakes in Chapter 3. I use the phase and amplitude variation in the seismic signals in an imaging technique derived from inverse theory and digital signal analysis, interpreting the coherent energy in the coda of the first arrival as due to scattering from upper mantle discontinuities. Applying an inversion through iterative forward modeling, I measure the depth variation of the spinel-perovskite upper mantle phase transition within the subduction zone region. This measurement allows me to characterize the variation of the transition with respect to pressure and temperature. My results are consistent with convection in a model of a chemically homogeneous mantle, where the presence of the phase transition at around 670 km depth disrupts the full mantle convection patterns.
Late Cretaceous-Cenozoic tectonics and sedimentation in southern Central America : Costa Rica and Panama
(1992) Kolarsky, Radim Antonin, 1965-; Not available
This thesis presents three integrated studies of Upper Cretaceous to Recent tectonics and sedimentation in southern Central America (Panama and Costa Rica). A short concluding Chapter presents my evaluation of the hydrocarbon potential of Panama and Costa Rica. The main sources of data used in the thesis include: 1) multi-channel seismic data, well data, and field data donated by industry and the Panama government for use in this study; and 2) field data collected in western Panama and eastern Costa Rica during 12 weeks in 1990 and 1991
Deformation mechanisms along active strike-slip faults : SeaMARC II and seismic data from the North America-Caribbean plate boundary
(1992-05) Tyburski, Stacey Ann; Muehlberger, William R.
The northwest part of the North America-Caribbean plate boundary zone is characterized by active, left-lateral strike-slip faults that are well constrained seismically and are corroborated by on- and offshore geologic mapping. The onshore plate boundary zone comprises the Motogua and Polochic fault systems of southern Guatemala which join and continue offshore as the Swan Islands fault zone along the southern edge of the Cayman trough. At the Mid-Cayman spreading center in the central Caribbean Sea, the fault motion is transferred at a 100 km wide left-step in the fault system to the Oriente fault zone. A third system, the Walton fault zone, continues east from the Mid-Cayman Spreading center to define the Gonave microplate. Seafloor features produced by strike-slip faulting along the Swan Islands and Walton fault zones have been imaged and mapped using the SeaMARC II side-scan sonar and swath bathymetric mapping system, single-channel seismic data, multichannel seismic data and 3.5 kHz depth profiles. Structures mapped along the Swan Islands and Walton fault zones include: 1) twenty-six restraining bends and five releasing bends ranging in size from several kilometers in area to several hundred kilometers in area; 2)en echelon folds which occur only within the restraining bends; 3) straight, continuous fault segments of up to several tens of kilometers in length; 4) restraining and releasing bends forming in "paired" configurations; and 5) a fault-parallel fold belt fold and thrust belt adjacent to a major restraining bend. The features observed along the Swan Islands and Walton fault systems are compared to other features observed along other strike-slip fault systems, from which empirical models have previously been derived. Based on the features observed in these strike-slip systems, a rigid plate scenario is envisioned where the geometry of the fault and the direction of plate motion have controlled the types of deformation that have occurred. In a related study, microtectonic features in an area of Neogene extension within the northwestern Caribbean plate were investigated in order to provide insight on the nature of intraplate deformation related to the motion along the plate boundary. Microtectonic features were measured in the Sula-Yojoa rift of northwestern Honduras with the intention of inverting the data to estimate stress states responsible for the observed strains. Data inversion for the estimation of stress states could not be undertaken with the available measurements, however, the observations made can be used to support several existing models for the intraplate deformation as well as to encourage the elimination of other models.
Sequence stratigraphy of the upper San Andres Formation and Cherry Canyon Tongue (Permian, Guadalupian), southern Brokeoff Mountains, New Mexico
(1992-08) Fitchen, William M.; Buffler, Richard T.
Outcrop exposures of the upper San Andres Formation and Cherry Canyon Tongue (Permian, Guadalupian) in the Brokeoff Mountains, New Mexico provide a seismic-scale cross-section through the margin of the Northwest Shelf and adjacent Delaware Basin. Stratal patterns such as onlap, offlap, downlap, and toplap, which are commonly used by seismic interpreters to identify sequences and systems tracts in the subsurface, can be observed directly in the area and integrated with facies distributions to generate a high-resolution sequence stratigraphic framework. The upper San Andres Formation and Cherry Canyon Tongue comprise a third-or fourth-order sequence (85 to 145 m-thick) bounded by unconformities and their correlative conformities (sequence boundaries). Lowstand/shelf margin, transgressive, and highstand systems tracts within the sequence were recognized on the basis of bounding surfaces and cycle stacking patterns. Cycle stacking patterns were analyzed with respect to geometry, component facies distribution, thickness, and the nature of lateral termination (e.g. onlap). The basal sequence boundary of the upper San Andres-Cherry Canyon Tongue sequence overlies the middle San Andres highstand platform. A basinward shift in facies tracts across this sequence boundary is evidenced by the vertical progression from offlapping ramp margin and slope strata of the middle San Andres highstand systems tract to onlapping, ramp crest strata of the upper San Andres lowstand/shelf margin systems tract. Erosion of underlying slope and toe-of-slope strata is evident along the boundary; carbonate megabreccias locally overlie the sequence boundary at the toe-of-slope. As a result of the strongly offlapping and toplapping character of the upper San Andres sequence, the upper sequence boundary intersects early highstand systems tract strata in the platform interior and late highstand systems tract strata closer to the terminal ramp margin. The sequence boundary is marked locally by a karst horizon; the best development of karst occurs in ramp margin strata of the early highstand systems tract. The karst is characterized by sandstone-filled rundkarren, grikes, and caverns that extend up to 30 m downward into San Andres strata. Toplap and minor stratal truncation mark the sequence boundary along the top of the late highstand systems tract. Subtidal to peritidal cycles of the Grayburg Formation (lowstand/shelf margin systems tract and transgressive systems tract) onlap the upper sequence boundary. The lowstand/shelf margin systems tract of the upper San Andres sequence is composed of a thin (6 to 25 m-thick) aggradational to slightly progradational cycle set that onlaps the basal sequence boundary along the platform and downlaps the sequence boundary for a distance of 100 m along the slope. Peritidal sandstone and carbonate facies of the systems tract pass basinward within a few hundred meters into a distinctive bryozoan-sponge-crinoid shelf margin buildup, which in turn passes basinward into toe-of-slope allodapic carbonates and discontinuous carbonate megabreccias derived from the underlying sequence. Relatively thin ramp crest cycles of the lowstand/shelf margin systems tract are capped by a transgressive surface that is locally erosional. The stacking pattern of cycles in the lowstand/shelf margin systems tract reflects relatively low rates of accommodation and sediment production. Siliciclastic sediment bypass across the shelf and slope may have been active during deposition of this systems tract. The transgressive systems tract of the upper San Andres sequence comprises 1) an aggradational to weakly progradational cycle set of ramp crest/ramp margin carbonates and siliciclastics (40-45 m-thick), 2) a proximal, onlapping slope apron complex of carbonates and siliciclastics (25-30 m-thick), and 3) a distal, marine onlap-wedge of basinal siliciclastics (the lower Cherry Canyon Tongue, 30-80 m-thick). Ramp margin/upper slope carbonates of the transgressive systems tract downlap the transgressive surface along the platform. On the platform, the transgressive systems tract is composed of relatively thick ramp crest cycles containing a high ratio of subtidal vs. supratidal facies. The systems tract is capped by three to four sandstone-based cycles, the upper two of which can be traced downslope into the slope apron complex. The top of the slope apron complex is downlapped by strongly progradational cycles of the overlying highstand systems tract. The downlap or maximum-flooding surface marks the top of the transgressive systems tract. Towards the basin, allodapic carbonates of the slope apron complex thin and pinch out into the body of the lower Cherry Canyon Tongue. These relationships indicate that lower Cherry Canyon Tongue sandstones were bypassed to the basin during high-frequency lowstand intervals superimposed on a longer term relative sea level rise. In contrast, allodapic carbonates within the slope apron were supplied to the slope during high-frequency highstand intervals. The highstand systems tract of the upper San Andres sequence (15-85 m-thick) comprises strongly progradational cycle sets composed of ramp crest through slope carbonates and sandstones. Clinoform cycles downlap onto the slope and basinal segments of the transgressive systems tract (lower Cherry Cherry Canyon Tongue). The highstand can be subdivided into two major cycle sets, termed the "early highstand" and "late highstand" respectively. The early highstand is characterized by carbonate-dominated sigmoid progradational clinoform cycles that can be traced from the ramp crest to the toe-of-slope. In the platform interior, early highstand cycles are thin and contain a higher ratio of supratidal facies relative to cycles of the transgressive systems tract. Early highstand cycles aggraded the ramp crest some 15-20 m and prograded 1-2 km into the basin. The late highstand is characterized by mixed carbonate-siliciclastic, sigmoid to oblique progradational clinoform cycles that toplap shelfward into the upper sequence boundary at low (1-2°) to high (10-15°) angles. Late highstand cycles did not aggrade the platform, rather, the former ramp crest area was a subaerial sediment bypass zone during deposition of these cycles. Late highstand cycles prograded 5-6 km into the basin. A progressive decrease in accomodation rates through the highstand systems tract is inferred from a decrease in the ratio of aggradation to progradation, a concommitant change from sigmoid to oblique progradational style, an increase in clinoform slope angle, and an increase in siliciclastic sediment flux. The basal sequence boundary of the upper San Andres sequence can be correlated to the unconformity between the Cherry Canyon Tongue/Brushy Canyon Formation and Cutoff Formation along the Western Escarpment (southern Guadalupe Mountains). The Brushy Canyon Formation, which is restricted to the basin, is interpreted as a "lowstand fan" equivalent in time to the basal sequence boundary and perhaps to part of the lowstand/shelf margin systems tract in the Brokeoff Mountains. The basal sequence boundary correlates to the top of the lower-middle San Andres sequence on the Algerita Escarpment (central Guadalupe Mountains). This sequence boundary can also be correlated to the base of the Cherry Canyon Tongue in Last Chance Canyon (central Guadalupe Mountains), although correlation between the platform sections of the Brokeoff Mountains and Last Chance Canyon is somewhat uncertain. The upper sequence boundary correlates to the San Andres-Grayburg contact on the Algerita Escarpment and in Last Chance Canyon. The correlation of this boundary to the Western Escarpment remains uncertain. The progressive decrease in accommodation exhibited by upper San Andres late highstand strata in the Brokeoff Mountains, and the inferred correlation of lower Grayburg shelf sandstones into the upper third of the basinal Cherry Canyon Tongue, supports the hypothesis that the terminal ramp margin of the upper San Andres sequence lies 8-10 km north of the Western Escarpment near the New Mexico-Texas state line. This entails that the upper sequence boundary lies within the Cherry Canyon Tongue on the Western Escarpment.
The structural and stratigraphic evolution of the Celtic Sea Basins, offshore Ireland
(1993) Rowell, Philip, 1952-; Buffler, Richard T.; Kominz, Michelle A.
The Celtic Sea area, offshore southern Ireland, is underlain by several discrete rift basins, part of a larger set of such basins which developed marginal to the North Atlantic within the tectonic framework of Pangea break-up and episodic opening of the North Atlantic Ocean. This study investigates tectonic processes of rift basin formation by analysis of the structural and stratigraphic evolution of the Celtic Sea basins, with particular emphasis on the influence of preexisting crustal structure on that evolution. The Paleozoic Caledonian and Variscan orogenies each imprinted distinct structural grains on this area, and the intersecting pattern of these tectonic lineaments provided the structural framework for the subsequent Triassic, Late Jurassic and Early Cretaceous rifting phases. Detailed study of the largest basin in the area, the North Celtic Sea Basin, using an extensive well and seismic database, indicates that the mode of reactivation of these preexisting lines of structural weakness depended on the orientation of the principal extensional stress direction relative to these lineaments. The Triassic and Late Jurassic northwest-southeast oriented extensional stress reactivated Caledonian lineaments as half-graben boundary faults and Variscan lineaments as transfer zones. On the other hand, the Early Cretaceous extensional stress was oriented north-south, oblique to the Caledonian trend, resulting in transtensional pull-apart basin geometries dominated by Variscan lineament reactivation. Subsequent north-south compressional stresses related to the Alpine orogeny reactivated both Caledonian and Variscan lineaments in a conjugate shear pattern of strike-slip deformation. This structural evolution model demonstrates the intrinsic relationship between plate tectonic setting, preexisting crustal structure and the resulting structural style. It also provides insight into the variable influence of tectonism on depositional system development. During Late Jurassic and Early Cretaceous rifting, depositional sequences were predominantly controlled by tectonics and partly conformed to empirically derived tectono-stratigraphic models of sequence development in nonmarine rifts. Depositional environments changed from fluvial to lacustrine as each rifting episode evolved through its early to active rift stages. In addition, axial transport of sediment, accentuated by regional tectonics and climatic changes, was a dominant control on facies distribution during Early Cretaceous rifting. During periods of tectonic quiescence, changes in base level were more eustatically driven and sedimentation was less influenced by preexisting crustal structure. The identification and discrimination of these controls on depositional system development in evolving rift basins aids in determining potential stratigraphic distributions within these basins
Geological investigations at the southern tip of the Americas :the development of the Patagonian Orocline and uplift of the Cordillera Darwin metamorphic complex, southernmost Chile
(1993) Cunningham, W. D.; Dalziel, Ian W. D., 1937-
The Patagonian Orocline is the 90° bend in the southernmost Andes between 50°S and 56°S. New paleomagnetic data indicate that the orocline is, at least in part, the product of tectonic rotation. Field work in the Beagle Channel region of southernmost Chile provides evidence for widespread left-lateral strikeslip faulting in the internal zones of the mountain belt. Both arms of the Beagle Channel are interpreted to be left-lateral strike-slip faults based on detailed study of mesoscale strike-slip faults (Riedel shears) in coastal outcrops. Although much of the evidence indicates Cenozoic brittle strike-slip faulting, Late Cretaceous brittle-ductile strike-slip and oblique-slip shear zones and S1, L1 fabric trends in southern Cordillera Darwin indicate that a component of strikeslip deformation accompanied Late Cretaceous deformation. Detailed mapping of D1 and D2 structural trends in three separate areas in southern Cordillera Darwin and the identification of an uplifted upper ophiolitic sequence on Isla Gordon immediately south of Cordillera Darwin suggest that the mid-Cretaceous Andean orogeny involved the transpressional inversion of the Rocas Verdes marinal basin. Progressive transpression appears to have been the dominant deformational regime in the region for the last 120 Ma years. This conclusion is supported by quantitative analysis of southern South America-Antarctic Peninsula relative motion for 150-0 Ma that indicates a significant strike-slip component of relative motion has existed between southern South America and the Antarctic Peninsula since the Early Cretaceous opening of the South Atlantic. A new model is proposed that integrates the development of strike-slip faulting and the structural evolution and uplift of the southernmost Andes with the rotational development of the orocline. The Patagonian Orocline appears to be the product of broad interplate shearing accommodated by strike-slip faulting, block rotation and contraction and is probably continuing to evolve today. The uplift of the Cordillera Darwin metamorphic complex may have been due to hinged unroofing in a transpressional restraining bend setting.

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