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Item Application of extensional models to the northern Viking Graben, North Sea(1987-05) Giltner, John Patrick, 1962-; Not availableSeveral 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.Item Chronostratigraphy, depositional rates, continental margin progradation, and growth-fault dynamics within the tertiary wedge, San Marcos arch, northwest Gulf of Mexico(1988) Travis, Deborah Sue, 1963-; Galloway, William E.Sedimentation, growth fault slip, and shelf-margin progradation rates were determined for a transect of Tertiary sediments along the axis of the San Marcos arch in the northwestern Gulf of Mexico basin. This low relief area was the site of shorezone depositional systems throughout most of the Tertiary and has not been affected by salt mobilization or the sedimentological overprint of large delta systems. Consequently, rates calculated in the area represent average rather than extreme values. Detailed correlation of 142 well logs and the utilization of paleontologic data from 17 wells made possible the construction of a chronostratigraphic cross section which divided the Tertiary section into 1 or 2 million year increments. A decompaction computer program, which utilizes published compaction curves and distinguishes between normally pressured shale, underconsolidated overpressured shale, and sandstone, calculated original sediment thickness and sedimentation rates for each of the chronostratigraphic layers. Regional sedimentation rates varied almost two orders of magnitude during the Tertiary. These findings seriously challenge the common assumption of relatively uniform sediment supply over geological time spans. Within individual chronostratigraphic layers, sedimentation rates ranged from 4 cm/1000 yrs in the fluvial and transgressive facies to 130 cm/1000 yrs in the lower Miocene outer shelf and slope facies. Decompaction of the section also allowed the calculation of growth fault slip rates and of the percent increase in accommodation on the downthrown side of the faults relative to their upthrown equivalents. Slip rates during times of maximum movement ranged from 250-600 m/my, agreeing with rates calculated for salt dome growth. Percent increase in accommodation on the downthrown side of the faults ranged from 20-83%. Shelf-margin progradation rates were calculated and ranged from -1.5 mi/my (2.4 km/my) during brief times of temporary retrogradation (Queen City and Yegua Formations) to 17.5 mi/my (28.2 km/my) during the Early MioceneItem Controls on the development of clastic wedges and growth strata in foreland basins : examples from Cretaceous Cordilleran foreland basin strata, USA(2008-05) Aschoff, Jennifer L., 1978-; Steel, R. J.Tectonic signatures such as growth strata, clastic progradation, detrital composition, thickness trends, paleoflow shifts, lithofacies distribution, and vertical stratigraphic stacking patterns provide the basis for a range of tectonic/structural interpretations. Complete understanding of the application and limitations of tectonic signatures is important to maintain consistency and reduce uncertainty of interpretations that use them. This study provides insight into the external controls on two frequently used tectonic signatures in foreland basins: (1) growth strata, and (2) clastic wedge progradation. First, two syntectonic unconformity types are recognized in non-marine, Cenomanian growth strata adjacent to the Sevier thrust-belt in southeastern Nevada, USA. Unconformities with larger angular discordance (>10°, “Traditional Type”) developed when uplift outpaced sediment accumulation. More subtle unconformities with less discordance (2-10°, “Subtle Type”) developed when sediment accumulation nearly kept pace with uplift. Increasing sediment supply with positive net accommodation, allows syntectonic deposits to aggrade above a growing structure, with no change in uplift rate. Hence, sediment supply and regional accommodation impart an important control over growth strata geometries that are often interpreted on the basis of tectonics alone. Identification of unconformity types in growth strata can therefore document additional phases of uplift, particularly for intervals where sediments aggraded above an active structure due to higher sediment supply during regional subsidence, or sea level rise. Second, an anomalous, Campanian clastic wedge is identified in Cordilleran Foreland basin fill, Utah and Colorado. The complex internal architecture, tide-dominated facies and characteristic flat-to-falling shoreline stacking patterns of the wedge reflect rapid progradation of wide (60-80 km), embayed, tide-influenced shorelines; these characteristics distinguish the anomalous wedge from the underlying and overlying clastic wedges in the basin. A high-resolution regional correlation and isopach maps for the anomalous wedge provide evidence that extensive clastic progradation was coeval with both Sevier- and Laramide-style deformation. Stratigraphic relations suggest that development of the anomalous character of Wedge B was due to uplift of a Laramide structure within the foredeep, and possibly enhanced by reduced dynamic subsidence.Item Depositional systems and structural controls of Hackberry sandstone reservoirs in southeast Texas(University of Texas at Austin. Bureau of Economic Geology, 1984) Ewing, Thomas E.; Reed, Roneé S.Deep-water sandstones of the Oligocene-age Hackberry unit of the Frio Formation contain significant quantities of oil and gas and remain potentially one of the most productive exploration targets in southeast Texas. The Hackberry is a wedge of sandstone and shale containing bathyal fauna that separates upper Frio barrier-bar - strandplain sandstones from lower Frio neritic shale and sand. Major Hackberry sandstones lie atop a channeled unconformity that forms the base of the unit. Sandstones in a typical sand-rich channel at Port Arthur field grade upward from a basal, confined channel-fill sandstone to more widespread, broad, fan-channel deposits, Topmost are proximal to medial fan deposits and overbank turbidite deposits. The sequence suggests that Hackberry sandstones were laid down by an onlapping submarine canyon-fan complex deposited in canyons that eroded headward into the contemporaneous Frio barrier system. Regional maps and seismic interpretations outline a network of sand-filled channels extending from the barrier toward the southeast. The earliest structural activity of the Port Arthur area is lower Oligocene (Vicksburg) faulting associated with continental-slope sedimentation. Small growth faults of late Oligocene (Frio) age displace the Hackberry section less than 500 ft and extend upward into Miocene strata. Isopach and isolith maps indicate that the Orange, Port Neches, and Fannett salt domes were active uplifts during Frio and Anahuac (Lower Miocene) deposition. Near Spindletop dome, however, only a north-south-trending salt-cored ridge is present. The Hackberry channels are in part located in salt-withdrawal basins, but major channel axes extend across the uplifts. Time versus depth plots of water depth and sediment thickness indicate that most of the Hackberry Embayment in Texas could have been formed by normal subsidence during the later Oligocene if the embayment were cut off from its supply of muddy sediment. Thick, sandy, lower Hackberry deposits filled deep canyons eroded into the retreating shelf margin. The Hackberry contains two hydrocarbon plays. The updip play is relatively shallow and oil-rich and lies near the updip limit of deep-water deposition. Some of the fields in this play produce from barrier-bar - strandplain Frio sandstones erroneously correlated with the Hackberry. The downdip play is gas-rich and generally geopressured. The reservoirs lie either within or on the flanks of the major channel systems and are commonly bounded updip by small growth faults, Understanding the component depositional environments represented by the discontinuous and complex lithofacies of these sandstones will improve hydrocarbon exploration and production.Item Field and petrographic analysis of mylonitic fabrics : implications for tectonic corrugation development, Tanque Verde Ridge, Arizona, USA(2005-12-24) Perry, Ethan R.; Mosher, Sharon, 1951-Tanque Verde Ridge (TVR) in Saguaro National Park, Arizona, USA, represents a large-scale lineation-parallel ridge (tectonic corrugation) within the Santa Catalina-Rincon Core Complex. Field and petrographic features record a history of lineation-parallel extension and boudinage occurring across a ductile to brittle deformational continuum during core complex exhumation. Lineation-perpendicular ductile to brittle features suggest the importance of extension orthogonal to lineations during exhumation and need to be factored into the debate on tectonic corrugation evolution. TVR is a foliation-defined antiformal flexure, concordantly bounded at the toe by a brittle detachment fault system. The footwall mylonite is strongly foliated (suggestive of flattening, rather than constriction) and lineated (WSW-trending). Ductile, micro- (thin section) to macro- (hundreds of meters) scale features are oriented both lineation-parallel (e.g., S-C' fabrics, asymmetric porphyroclasts, shear zones) and perpendicular (e.g., shear zones, asymmetric quartz fabrics). These features record dominantly WSW, and secondarily NNW and SSE, directed non-coaxial shear, respectively. Across the corrugation, foliation flexures exhibit both lineation-parallel and perpendicular fold axes. Distended necks of micro- to meso-scale boudins contain NNW-SSE striking zones of intense brittle fracturing, brecciation and cataclasis. The ductile, mylonitic foliation is passively affected across these boudin neck zones as foliation dip varies from sub-horizontal to >45°. This suggests a temporal link between mylonite formation, extension-related deformation and the development of the corrugated surface. Oriented thin sections and field observations from across TVR and across outcrop-scale corrugation-like features constrain the conditions during mylonite development to lower-amphibolite facies and the spatial relationship of micro- to meso-structural features across the corrugation. On the basis of field and petrographic evidence, a new model for the evolution of tectonic corrugations is proposed, whereby original anastamozing shear zones within the deep-seated, ductile shear zone are amplified in response to bidirectional extension (including boudinage, and spatially related brittle fracturing). Shear zone modification is progressive and is synchronous with non-coaxial simple shear. Overall flattening of the shear zone is predicted in response to this extension. Notably, mylonitic fabrics at TVR do not record a history of large-scale constrictional deformation. Tectonic corrugations can initiate at depth as a result of original shear zone morphologies, be modified by boudinage during bidirectional extension, and be translated up-dip during progressive core complex exhumation. Corrugations are manifestations of deep-seated, multi-scale processes that influence shear zone dynamics during large-scale crustal extension.Item Fracture-size scaling and stratigraphic controls on fracture intensity(2002) Ortega Pérez, Orlando José; Marrett, Randall A.Scaling techniques offer an opportunity to solve subsurface fracturesampling problems by extrapolating fracture properties from sub-millimeterscales to scales important for economic applications. Although extrapolation of fracture length and aperture distributions across observation scales is fraught with potential errors, sampling of opening-mode kinematic apertures along scanlines using new fracture-aperture measuring tools produces consistent power-law aperture distributions from the micron-scale to outcrop-scale. One-dimensional sampling avoids fracture connectivity issues inherent to traditional twodimensional length sampling methods. Sampling artifacts and mechanical layer effects can be diagnosed and accounted for, and extrapolation of power-law fracture intensities from the sub-millimeter scale up to the length scale of mechanical layers is feasible. Tests were performed in turbidite beds of the Ozona Sandstone, Texas, eolian Weber Formation sandstones, Colorado, and Lower Cretaceous carbonates of the Sierra Madre Oriental (SMO), Mexico. Outcrop studies in Weber Formation sandstones provided an opportunity to characterize well-exposed macrofracture systems as potential analogues for subsurface fractured reservoirs at Rangely Field. However, differences in stratigraphy and diagenetic history between surface and subsurface do not allow the direct extrapolation of these results to subsurface, reinforcing the idea that local data are necessary for fracture system characterization even in cases where long geologic time has passed between the time of sedimentation and the time of deformation that brought potential outcrop analogs to the surface. Another way to predict fracture properties in the subsurface is to analyze the relationships between fracture attributes and the geologic parameters of the rock volume that govern fracturing. Fracture-fill prediction using relative volumes of cement phases precipitated during and after fracture timing shows an empirical relationship with sedimentary facies in Weber Formation sandstones. Multivariate analysis of unbiased fracture intensity in SMO carbonates suggests that degree of dolomitization and position of a bed at the top of a stratigraphic cycle are the most important controls on fracture intensity in these rocks. Mud content has only a modest control on fracture intensity and bed thickness has the least control on fracture intensity, suggesting that published work concluding that fracture intensity is strongly governed by bed thickness may be biased by scale or sampling effects.Item Geologic evolution of the Sierra Madre Oriental between Linares, Concepción del Oro, Saltillo, and Monterrey, Mexico(1982) Padilla y Sánchez, Ricardo José; Muehlberger, William R.The Sierra Madre Oriental between Saltillo, Monterrey, and Linares shows a bend in structures that strike from approximately N 35° E to about N 35° W. Most of the rocks involved in the Curvature of Monterrey are Mesozoic in age and range from Late Triassic to Late Cretaceous. Large amplitude folds and thrust faults contribute to the structural complexity of this region. The structural trends present in northeast Mexico are the result of the Late Paleocene-Early Eocene Laramide Orogeny, and their different styles of folding are intimately related to the fundamental landforms of Early Mesozoic paleogeography. Relatively mild deformation is shown in the Mesozoic sedimentary cover that overlies the stable paleocontinental basement highs of the Coahuila, La Mula and Monclova Islands, Tamaulipas Archipelago, and El Burro-Peyotes Peninsula. The tight folding observed in the Sierra Madre Oriental is the result of regional northeastward décollement blocked by the basement highs. Two prominent west-northwest-trending lineaments transect the region: herein named the "Boquillas-Sabinas" and "Sierra Mojada-China" Lineaments. Prominent paleogeographic highs lie on the outside of the area bounded by these lineaments, with the area between occupied by major basins and small "islands". Recurrent motion along these lineaments seems likely and movement along them in Early Mesozoic time blocked out the paleogeographic elements discussed in this dissertation. It is proposed in this study that the structural features of northeast Mexico are the result of a sinistral relative movement of southern United States (westward) with respect to northern Mexico (eastward) during the Laramide Orogeny, contemporaneously with a regional décollement event produced by the tilting toward the northeast of the so-called "Unnamed Occidental Continent" . Thus from the detailed study of the mapped structures and stratigraphic sequences at the Curvature of Monterrey, and from detailed interpretations of satellite photographs, the model presented here for the mechanism of deformation of northeast Mexico explains not only the bend in structures at the Curvature of Monterrey, but also most of the structural trends in northeast Mexico, including the en e[]chelon folds in the Sabinas GulfItem Lineament analysis and inference of geologic structure: examples from the Balcones Ouachita trend of Texas(University of Texas at Austin. Bureau of Economic Geology, 1982) Caran, S. Christopher; Woodruff, C. M.; Thompson, Eric J.Lineaments perceived in remotely sensed images are reliable indicators of geologic structure. Lineaments on ten Landsat multispectral scanner images (band 5; 1:250,000 scale) were mapped covering the Ouachita/Balcones-Luling-Mexia-Talco structural trend between the Rio Grande and Red river in Texas. More than 5,000 lineaments were perceived in these images. Maps depicting the lineaments (individually and in various combinations) were compared with maps of structural/ tectonic features and geothermal gradient contours, noting instances of apparent correlation among these themes. Lineaments are correlative with the individual faults and the aggregate fault patterns of the Baicones, Luling, Mexia, and Talco fault zones, Transverse lineaments, which trend almost perpendicular to these fault zones, mark the northernmost extent of the Balcones fault system and outline carbonate piatforms, such as the Belton High/Moffat Mound trend and the San Marcos arch. Transverse lineaments are coincident also with the axes of the buried Chittim and Preston anticlines and with the flanks of the Sherman and Round Rock synclines. Numerous salt domes occur at depth in the western part of the east Texas basin near the trend; many of these domes, particularly those in Henderson, Anderson, and Freestone Counties, are found along and at the intersection of major lineament zones where the concentration of individuai lineaments is greatest. Most of the buried Late Crelaceocs volcanoes of central Texas near Austin lie along northeast southwest- trending lineament zones; the altered pyroclastic rocks and associated beachrock facies at many of these volcanoes are hydrocarbon reservoirs. The orientation and spacing of geothermal gradient contour lines ("isograds") also correspond to major structures and thus, to the pattern of lineaments throughout the region. Correlation of (1)individual lineaments, zones of cortiguous or nearly parallel lineaments, and areas of homogeneous lineament density and orientation to (2) surface and subsurface structure and (3) geothermal,'isograd" patterns indicates that lineament analysis has many potential applications to regional mineral resource assessment.Item The mapping of tectonic features in the ocean basins from satellite altimetry data(1988-05) Gahagan, Lisa Marie, 1963-; Not availableSatellite 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.Item Mesozoic stratigraphy and structural history of the East Texas Basin(Stanford University, 1953) Barrow, Thomas Davies, 1924-Item Post-cretaceous structural geology near Del Norte Gap, Brewster County, Texas(1964-08) Everett, John R.; Muehlberger, William R.The-west-dipping, north-northwest-trending Black Peak fault and associated monoclines which form the western flank of the Marathon dome, are well exposed near Del Norte Gap. Field mapping shows that the dip of the Black Peak fault increases downward from zero to 80 degrees. A northeast-trending right lateral fault cuts the hanging wall of the Black Peak fault at Del Norte Gap. The Black Peak fault has greater displacement south of the gap than north of the gap. Several north and northwest-trending normal faults cut the Cochran Mountains. The folding and faulting took place after the deposition of the upper Boquillas Limestone and before the deposition of Quaternary gravels. Vertical uplift of the Marathon dome during the Laramide orogeny produced the Black Peak fault and associated features. Normal faults later cut the area. The structural features near Del Norte Gap correspond well to previously described analytical and experimental configuration of features produced by differential vertical movement of basement blocks and previously described examples of vertical tectonics.Item Sedimentology and structural geology of the Housetop Mountains-Castle Mountain area, Marathon Basin, Trans-Pecos, Texas(1989) Diggs, Timothy Neighbours, 1960-; McBride, Earle F.; Muehlberger, William R.In many sedimentary sections recording extreme soft-sediment and tectonic deformation, the origin of outcrop-scale structures is commonly difficult to resolve. Numerous soft-sediment deformational features developed in the Mississippian-Pennsylvanian Tesnus Formation in the Marathon Basin in response to rapid sedimentation rates and tectonic instability. Soft-sediment structures were overprinted during Late Pennsylvanian to Early Permian compressional deformation that emplaced allochthonous and parautochthonous rocks of the Marathon Basin. Because of its sedimentary and tectonic history, the Tesnus Formation provides an ideal section for evaluating the role of soft-sediment and tectonic processes in producing outcrop-scale structures. The Tesnus Formation was rapidly deposited in deep water in an unstable, migrating foredeep. Paleocurrent data derived from sole marks on turbidite sandstone beds in the study area indicate that the overall trend of sediment dispersal during Tesnus deposition was from southeast to northwest. The most well-developed trend is in an arc between N20°W to N30°W. Extensive synsedimentary and early-post-depositional deformation, particularly sediment liquefaction and clastic intrusion, accompanied sedimentation. The most common clastic injection features in the Tesnus Formation are dikes, which are interpreted to be coeval with previously undescribed clastic intrusions including sills in excess of 2.5 m thick, cylindrical clastic "plugs", and composite structures that are partly concordant and partly discordant. Clastic injections are interpreted to have formed under conditions of essentially non-deviatoric stress, and commonly in response to bedding-parallel (horizontal) pressure gradients. Clastic injections are not systematically oriented locally or regionally, provide no information regarding paleogeography (paleoslope), and are not related to any later structural trends. "Decompaction" of ptygmatically folded dikes has commonly been used to determine the extent of post-injection compaction, but data from the Tesnus Formation indicate that the method conventionally used for determining sediment compaction and dike injection depth is unreliable. Tectonically generated structures post-date all soft-sediment deformation and are here interpreted to have formed in a uniformly-oriented, compressional stress field with a maximum compression direction oriented between approximately Nl9°W and N22°W. The Dagger Flat Anticlinorium, a major structural culmination west and southwest of the study area, acted as a structural buttress that greatly influenced structural evolution in the eastern part of the Marathon Basin. Compression and rotation of the Tesnus/Dimple/Haymond flysch package against and around the Dagger Flat Anticlinorium modified the orientation of pre-rotation structures and changed the orientation of rocks in the map area with respect to prevailing stresses. The sequence of Paleozoic tectonic events affecting rocks in the study area is interpreted as follows. 1. Generation of first-stage joints. 2. Rotation of the Tesnus/Dimple/Haymond flysch package around the Dagger Flat Anticlinorium. 3. Flexural-slip folding of the flysch package. 4. Formation of major thrust faults and related tear faults. Thrust sheet rotation around the Dagger Flat Anticlinorium. Formation of first-stage extensional microfaults with northeasterly trends in major thrust décollement zones. 5. Space constraints along the northeast margin of the Dagger Flat Anticlinorium prevent further thrust sheet rotation. 6. Uniform compression continues. Thrust motion is no longer influenced by the Dagger Flat Anticlinorium, but is directed parallel with maximum compression direction (Nl9°W to N22°W). Second-stage joints form. Incremental strike-slip motion along major thrusts causes formation of east-northeasterly striking contraction microfaults and second-stage extensional microfaults.Item Structural analysis of progressive deformation within a complex strike-slip fault system : southern Narragansett Basin, Rhode Island(1984) Berryhill, Alan Walter; Mosher, Sharon, 1951-Complex progressive ductile deformation on Dutch Island, Narragansett Basin, Rhode Island, is localized along a system of non-parallel strike-slip faults with opposite senses of motion. The structural intensity and complexity of this area can be attributed in part to the proximity of NE- and NNE-trending, preexisting basement faults bounding early horsts and grabens. Left-lateral faults in NNE to NE orientations correspond to R' riedel shears and right lateral faults in NE to ENE orientations correspond to right-lateral primary shears within an E-trending right-lateral transform system. Structural superposition of mesoscopic folds and incremental crenulation cleavages, formed due to episodic movement along these faults, indicates that left-lateral motion preceded right-lateral motion and that many NE-trending faults underwent first left-lateral and then right-lateral movement. As many as 8 sets of discrete superposed crenulation cleavages and 7 sets of discrete mesoscopic folds were developed at fault intersections on the island. Preservation of these structures can be attributed to the presence of a strong preexisting structural and metamorphic fabric which formed as a result of initial basin closure associated with the collision of the African and North American continents in the late Paleozoic. Study of geometry and overprinting relations of the fold and crenulation sets within adjacent shear zones served to track movement along individual faults through time. Up to 32 degrees of anticlockwise internal rotation on left lateral faults and clockwise rotation on right-lateral faults has occurred. Discrete crenulations form due to flexural slip at an angle to the shear plane and are subsequently tightened and rotated toward the principal extensional strain direction until flexural slip can no longer occur. A younger set then forms and overprints the preexisting, and presently locked, crenulation cleavage at an angle varying from 4 to 22 degrees. Structural analysis of the complex deformation on Dutch Island indicates that accurate tracking of fault motion within complex shear systems is feasible in areas in which overprinting relations between progressive deformations are preserved. In addition to the regional significance of the study, the structural styles and deformation patterns described on Dutch Island should be analogous to what might be expected in a larger block caught within a system of non-parallel, successively forming strike-slip faults.Item Structural evolution and metamorphism of mid-Proterozoic basement in the northwest Van Horn Mountains, Trans-Pecos, Texas(1987) Bristol, David Arthur, 1961-; Mosher, Sharon, 1951-Psammitic, pelitic and mafic schists in a NNW-trending horst in the NW Van Horn Mountains of west Texas show evidence of a multiphase, synmetamorphic, ductile Grenville-age deformation. Other basement exposures in the Van Horn area record a later phase of Grenville-age brittle deformation that has wholly or partly obscured the earlier-formed ductile features. Detailed mapping indicates that basement rocks exposed in the NW Van Horn Mountains underwent three phases of progressive isoclinal folding followed by two later, less intense, nearly coaxial folding episodes. The second phase of folding produced the dominant foliation and foliation intersection (S₁/S₂) lineation observed throughout the area. This deformation is inferred to have resulted from a large scale folding event, at or very near, peak metamorphic conditions, possibly associated with nappe formation. Garnet-biotite and garnet-hornblende Fe-Mg exchange thermometry indicate maximum temperatures of 640±50°C, corresponding to mid-amphibolite facies conditions, for this event. The intrusion of late syn- to post-orogenic pegmatite sills and dikes was widespread across the area. The associated pegmatitic fluids either facilitated or became the principal catalysts of a static recrystallization event. This event altered previously formed deformational fabrics and produced diagnostic strain-free textures in many minerals. The Grenville-age, polydeformational history recorded by the basement rocks of the NW Van Horn Mountains is similar to that noted in portions of the Llano Uplift of central Texas, about 650 km to the east. The strong similarity of deformational styles in these two areas suggests that large-scale folds with localized shear zones were produced in this region of the Grenville ProvinceItem The structural evolution of the Sunshine Springs thrust area, Marathon Basin, Texas(1984-08) Kraft, Jennifer Lucille; Muehlberger, William R.Detailed mapping (1:6,000) of Lower Ordovician through Lower Pennsylvanian strata, exposed in the vicinity of the Sunshine Springs thrust fault, shows that the thrust ramps up-section twice in a direction parallel or subparallel with the thrusting, and that the geometry of folds can be attributed to their proximity to the two closely spaced ramps. The lower ramp is a frontal ramp which originated as a forelimb thrust through the overturned limb of a tight anticline-syncline fold couplet. The upper ramp cuts up-section through a thin, upper Paleozoic flysch sequence where the Sunshine Springs thrust becomes imbricated. Directly above the lower ramp, in the upper plate, is a broad symmetrical anticline which has a geometry similar to a fault-bend fold. Forward of the lower ramp is a large wavelength, flat-bottomed syncline, and behind the lower ramp is a series of tight to isoclinal overturned folds. As a result of fault-bend folding and continued shortening of the ramp region, upper plate folds characteristically have a larger amplitude than folds of the lower plate. Just forward of the lower ramp in the footwall is the tightly folded and truncated syncline of the syncline-anticline fold couplet. The rest of the lower plate section is only mildly deformed. A composite, down-structure cross section drawn parallel with the direction of thrusting shows that the Peña Colorada synclinorium has been transported along the Sunshine Springs thrust approximately 3.8 km. Shortening, as deduced from folding in this study alone, is 20 percent, and when the shortening by the thrust is also considered, the total amount of shortening equals 52 percent. A major left-lateral strike-slip system, trending WNW, approximately parallel with the thrusting direction, offsets the Sunshine Springs thrust fault. Strike-slip and dip-slip displacements can be calculated from a displaced fold axis of the lower plate syncline, and are 335 m and 90 m, respectively. In the vicinity of this strike-slip system, the axial traces of folds change from a dominantly southwesterly trend to a more southerly trend. The regional extent of the fault system within the Marathon Basin, and its correspondence with the change in major fold axes orientations suggests that the fault zone is a regional tear which formed in response to the impingement of the Marathon thrust front against the Diablo Platform during the Pennsylvanian Period.Item Structural evolution of the Warwick Hills, Marathon Basin, West Texas(1987-12) Coley, Katharine Lancaster, 1956-; Muehlberger, William R.A detailed structural analysis was conducted of the Warwick Hills at the northeast tip of the doubly-plunging Dagger Flat anticlinorium, Marathon Basin, west Texas. Field work delineated a folded duplex structure composed of three horses. Thrust transport was towards the northwest and resulted in a hinterland-dipping duplex. Initial thrusting In the Warwick Hills shortened the area by 2.2:1 (54%). Post-thrusting, the duplex underwent nearly isoclinal folding creating two anticlines and a syncline, second-order folds to the Dagger Flat anticlinoium. Folding combined with thrusting brought the total shortening of the rock package to 6.5:1 (85%). Earlier estimates gave a shortening for the Warwick Hills of 3:1. Finally, the folded duplex was extended by oblique tear faulting that offset the folded thrusts accommodating extension of the major folds in a northeast direction. These tear faults occurred post-plunging of the folds and were the last deformational movements that affected the Warwick Hills. The Ordovician Maravillas and Devonian Caballos Formations acted in the Warwick Hills as a structurally competent couplet. Addition or subtraction of this couplet, or units in this couplet, controlled the location of the major and minor thrusts, the style and shape of folds, and the location of the fold hinges. Bounding the couplet are incompetent shales of the Ordovician Woods Hollow and the Mississippian Tesnus Formations. Thrusts in the Warwick Hills duplex have a basal décollement in the Woods Hollow shale and ramp up through the Maravillas/Caballos couplet with an upper décollement in the Tesnus shale. The entire duplex was primarily folded by flexural slip (i.e. concentric folds) as evidenced by slickensides oriented parallel to bedding and perpendicular to fold axes, the constant thickness of the competent layers and the change in fold shape with depth. Fold wavelength, as determined from the couplet in the lowest thrust sheet, averages ~1,300 m and the average fold axis for the Warwick Hills, as determined stereographically, plunges ~54° N90°E. Shale in the Woods Hollow and Tesnus Formations bounding the couplet, flowed passively during folding into the cavities that were created by the bending of the more competent units. Lower and upper boundaries of disharmonic folding developed in the Woods Hollow and Tesnus Formations respectively. Unique to this area when compared to the rest of the anticlinorium are the presence of tightly folded thrusts and steep east-trending fold axes. The anticlinorium plunges in the Warwick Hills because it drapes off a down-to-the-northeast basement fault. Folds were "dragged" or diverted to the east during thrusting of the duplex over this transversely-oriented paleotopographic fault scarp, or were diverted subsequent to thrusting of the duplex by strike-slip movements at depth along the basement fault.