Industry Seismic Reflection Data Research Articles

Late Pleistocene Slip Rate and 3D Geometry of the Mid-Channel Blind Thrust, Santa Barbara Channel, California, and Their Implications for Earthquake Hazards

ABSTRACTThe Mid-Channel fault is an active blind thrust overlain by a south-vergent anticline in the Santa Barbara channel of southern California. We use high-quality industry seismic reflection data to characterize the 3D geometry of the Mid-Channel fault, and define its lateral extent and interactions with other structures in the region. We use a detailed chronostratigraphic sequence of nine Pleistocene horizons ranging in age from 120 ka to 1.05 Ma to interpret the geometry and deformational history of the Mid-Channel anticline and blind thrust. These precisely dated horizons provide unique insights on the Pleistocene activity of the Mid-Channel fault because of the nearly complete preservation of sediment on the backlimb of the fold. Using a new method of relating structural relief of these horizons to slip on the underlying Mid-Channel fault, we calculate the faults late Pleistocene slip rate. Results indicate that the structure became active between 790 and 710 ka in the eastern part of the Channel and subsequently propagated west along strike. In the east, where the overlying anticline preserves a complete section of syntectonic growth strata, the Mid-Channel fault has a dip-slip rate of 1.8 mm/yr with a range of 1.6–2.1 mm/yr. Based on a map of total displacement, we infer that the maximum dip-slip rate on the Mid-Channel fault along strike is 2.1 ± 0.2 mm/yr, suggesting that it may accommodate about one-third of the observed geodetic contraction across the basin. We explore alternative ways that the fault may extend to depth and interact with surrounding structures, and consider the seismic hazard implications of these scenarios, including estimates of potential earthquake rupture areas, magnitudes, and average recurrence intervals.

Early Cretaceous evolution of the Tromsø Basin, SW Barents Sea, Norway

This study presents an example of the extensional basin, namely Tromsø, which developed along the Southwestern Barents Sea (SWBS) transform margin. Three previous models have been proposed to explain the tectonic evolution and architecture of the basin, but there is no consensus on development of individual structures. In this study, we use 2D industry seismic reflection data, potential field and well data, as well as previously published information to understand the Early Cretaceous structural evolution of the Tromsø Basin in the context of the geodynamic processes in the SWBS. Mapping of the main seismic sequences and faults, time thickness map were integrated with modeled gravity anomalies along a composite 2D regional seismic section to facilitate the interpretation of crustal structures, which then were structurally restored. We propose a revised Early Cretaceous structural model for the Tromsø Basin, which involves oblique extension and formation of an intra-basinal transpressional transfer zone. This explains formation of the compressional faulting during overall extension in the SW Barents Sea. Basement heterogeneity most likely played an important role in localizing strain. The 2D sequential restoration of the composite regional seismic section yields an estimate of ca. 35 km of crustal extension in the SW Barents Sea margin, from the earliest Cretaceous until the present. Crustal thickness along the gravity modeled 2D regional section displayed a thinner crust below the Tromsø Basin as compared to the Sørvestnaget and Hammerfest basins favoring the interpretation of the Tromsø Basin as oblique rifting. This study illustrates the importance of detailed and regionally integrated analysis of rifted basins for reconstructing their evolution, as analysis of oblique rifted basins using two-dimensional plane strain can lead to erroneous assessment of faulting style and deformation.

A regional Alpine graphite décollement level beneath the NW Pannonian Basin

The so-called Transdanubian Conductivity Anomaly (TCA) of the Hungarian part of the NW Pannonian Basin has been well known for more than five decades. The exceptionally low resistivity (i.e. 1–2 Ωm) zone has a very large areal extent (on the order a few thousand km2) and it is an entirely subsurface anomaly occurring at depth between circa 3–15 km, with no corresponding outcrops. Various geological explanations of this enigmatic crustal-scale geophysical anomaly range from invoking sub-horizontal Alpine nappe contacts to sub-vertical dikes with graphite and/or saline fluid content. Only one possible analogue outcrop area was considered for the high conductivity anomaly so far, namely the Drauzug/Gailtal area of the Eastern Alps in Austria, some 300 km to the West from the TCA area. Previous attempts to find correspondence between the TCA and prominent seismic reflectors seen on 2D seismic reflection profiles were based on data acquired by research institutions. This study systematically correlates, for the first time, the TCA with 2D industry seismic reflection data in the same area. Our new results show a very strong correlation between the subsurface extent and location of the TCA with various sub-horizontally oriented Cretaceous Alpine nappe surfaces. In addition, we draw on the latest structural correlation of the Alpine nappe stack of the Transdanubian Range with its proper tectonic counterpart in the Eastern Alps.At the southern edge of the Upper Austroalpine units in northern Styria, in the Veitsch Nappe of the Greywacke Zone, numerous graphite localities are known historically. These laterally extensive graphite units in NW Styria formed as the result of greenschist-grade metamorphism of a Carboniferous coal sequence during the Cretaceous. For the first time, we describe here one well penetration of possibly age-equivalent graphitic units in NW Hungary. Correlation of the magnetotelluric anomaly with the distinct reflection seismic signature suggests that the same Palaeozoic graphitebearing Upper Austroalpine units should be present at 3–15 km depth in our study area.Therefore we propose that the best explanation for the observed extent and geometry of the TCA is the presence of graphite in subhorizontal, tectonically thinned detachment surfaces at the base of the Upper Austroalpine nappe edifice of NW Hungary

Cordilleran front range structural features in northwest Montana interpreted from vintage seismic reflection data

Industry seismic reflection data spanning the Rocky Mountain Cordillera front ranges of northwest Montana were reprocessed and interpreted in this study. Five seismic profiles represent 160 km of deep reflection data collected in 1983 that span the eastern Purcell anticlinorium, Rocky Mountain Trench (RMT), Rocky Mountain Basal Décollement (RMBD), and Lewis thrust. The data were reprocessed using modern techniques including refraction statics, pre-stack time migration (PSTM), and pre- and post-stack depth migration. Results indicate the RMBD is 8–13 km below the Earth's surface and dip 3–10° west. Evidence for the autochthonous Mesoproterozoic Belt and basal Cambrian rocks beneath the RMBD is present in all of the profiles and appears to extend east of the RMT. The Lewis thrust was identified in the seismic profiles and appears to sole into the RMBD east of the RMT. The RMT fault system has a dip displacement of 3–4 km and forms a half graben filled with 1 km of unconsolidated Tertiary sedimentary deposits. The RMT and adjacent Flathead fault systems are interpreted to be structurally linked and may represent a synthetic, en echelon fault system.

Structure of the San Fernando Valley region, California: Implications for seismic hazard and tectonic history

Industry seismic reflection data, oil test well data, interpretation of gravity and magnetic data, and seismic refraction deep-crustal profiles provide new perspectives on the subsurface geology of San Fernando Valley, home of two of the most recent damaging earthquakes in southern California. Seismic reflection data provide depths to Miocene–Quaternary horizons; beneath the base of the Late Miocene Modelo Formation are largely nonreflective rocks of the Middle Miocene Topanga and older formations. Gravity and seismic reflection data reveal the North Leadwell fault zone, a set of down-to-the-north faults that does not offset the top of the Modelo Formation; the zone strikes northwest across the valley, and may be part of the Oak Ridge fault system to the west. In the southeast part of the valley, the fault zone bounds a concealed basement high that influenced deposition of the Late Miocene Tarzana fan and may have localized damage from the 1994 Northridge earthquake. Gravity and seismic refraction data indicate that the basin underlying San Fernando Valley is asymmetric, the north part of the basin (Sylmar subbasin) reaching depths of 5–8 km. Magnetic data suggest a major boundary at or near the Verdugo fault, which likely started as a Miocene transtensional fault, and show a change in the dip sense of the fault along strike. The northwest projection of the Verdugo fault separates the Sylmar subbasin from the main San Fernando Valley and coincides with the abrupt change in structural style from the Santa Susana fault to the Sierra Madre fault. The Simi Hills bound the basin on the west and, as defined by gravity data, the boundary is linear and strikes ∼N45°E. That northeast-trending gravity gradient follows both the part of the 1971 San Fernando aftershock distribution called the Chatsworth trend and the aftershock trends of the 1994 Northridge earthquake. These data suggest that the 1971 San Fernando and 1994 Northridge earthquakes reactivated portions of Miocene normal faults.

Multiple phases of basin formation along the Stateline fault system in the Pahrump and Mesquite Valleys, Nevada and California

Two phases of deformation are needed to describe the Cenozoic tectonic evolution of the Pahrump and Mesquite basins in the southern Great Basin and eastern Mojave Desert, United States. By interpreting seismic reflection and gravity observations along with bedrock and surficial mapping, we infer an extensional phase of basin formation followed by a transtensional phase, in this area straddling the border of southern Nevada and southeastern California. We reprocessed ∼220 line km of industry seismic reflection data from the Pahrump and Mesquite Valleys to emphasize reflections in the basin fill, and combined these results with analysis of gravity data. The seismic lines portray the complex geometry of the Stateline fault system, a major Neogene dextral strike-slip system that passes through these valleys, and provide evidence for multiple ages of faulting along structures that bound the Pahrump basin. Locally thick sequences of preextensional Tertiary sedimentary rocks are cut by large-offset, relatively high-angle normal faults that record a phase of extensional basin formation that preceded transtension. The existence of preextensional basins beneath the Pahrump and Mesquite Valleys bears on tectonic reconstruction of the region and suggests that tilted ranges blocks to the west of these valleys need not restore to positions immediately adjacent to the Spring Mountains to the east. Subsequent dextral offset on the Stateline fault system resulted in the formation of steep-sided basins, local arching and tectonic inversion, and the burial of earlier-formed normal faults with coarse clastic detritus. Gravity models that are constrained to match the basin architecture observed in the seismic lines require lateral variations in basin-fill and bedrock density, and they confirm that the Paleozoic outcrop of Black Butte, a topographic high separating the Pahrump and Mesquite Valleys, is unrooted to underlying bedrock.

Earthquake‐by‐earthquake fold growth above the Puente Hills blind thrust fault, Los Angeles, California: Implications for fold kinematics and seismic hazard

Boreholes and high‐resolution seismic reflection data collected across the forelimb growth triangle above the central segment of the Puente Hills thrust fault (PHT) beneath Los Angeles, California, provide a detailed record of incremental fold growth during large earthquakes on this major blind thrust fault. These data document fold growth within a discrete kink band that narrows upward from ∼460 m at the base of the Quaternary section (200–250 m depth) to <150 m at 2.5 m depth, with most growth during the most recent folding event occurring within a zone only ∼60 m wide. These observations, coupled with evidence from petroleum industry seismic reflection data, demonstrate that most (>82% at 250 m depth) folding and uplift occur within discrete kink bands, thereby enabling us to develop a paleoseismic history of the underlying blind thrust fault. The borehole data reveal that the youngest part of the growth triangle in the uppermost 20 m comprises three stratigraphically discrete growth intervals marked by southward thickening sedimentary strata that are separated by intervals in which sediments do not change thickness across the site. We interpret the intervals of growth as occurring after the formation of now‐buried paleofold scarps during three large PHT earthquakes in the past 8 kyr. The intervening intervals of no growth record periods of structural quiescence and deposition at the regional, near‐horizontal stream gradient at the study site. Minimum uplift in each of the scarp‐forming events, which occurred at 0.2–2.2 ka (event Y), 3.0–6.3 ka (event X), and 6.6–8.1 ka (event W), ranged from ∼1.1 to ∼1.6 m, indicating minimum thrust displacements of ≥2.5 to 4.5 m. Such large displacements are consistent with the occurrence of large‐magnitude earthquakes (Mw> 7). Cumulative, minimum uplift in the past three events was 3.3 to 4.7 m, suggesting cumulative thrust displacement of ≥7 to 10.5 m. These values yield a minimum Holocene slip rate for the PHT of ≥0.9 to 1.6 mm/yr. The borehole and seismic reflection data demonstrate that dip within the kink band is acquired incrementally, such that older strata that have been deformed by more earthquakes dip more steeply than younger strata. Specifically, strata dip 0.4° at 4 m depth, 0.7° at 20 m depth, 8° at 90 m, 16° at 110 m, and 17° at 200 m. Moreover, structural restorations of the borehole data show that the locus of active folding (the anticlinal active axial surface) does not extend to the surface in exactly the same location from earthquake to earthquake. Rather, that the axial surfaces migrate from earthquake to earthquake, reflecting a component of fold growth by kink band migration. The incremental acquisition of bed dip in the growth triangle may reflect some combination of fold growth by limb rotation in addition to kink band migration, possibly through a component of trishear or shear fault bend folding. Alternatively, the component of limb rotation may result from curved hinge fault bend folding, and/or the mechanical response of loosely consolidated granular sediments in the shallow subsurface to folding at depth.

Restraining segments and reactivation of the Santa Monica–Dume–Malibu Coast fault system, offshore Los Angeles, California

A regional west striking system of surface and blind faults transects northern metropolitan Los Angeles, separating the Santa Monica Mountains from two deep sedimentary basins. The surface faults include the Santa Monica, Dume, and Malibu Coast faults. The three‐dimensional (3‐D) geometries of these faults and deformed dated strata were examined in order to determine how oblique shortening is accommodated, how structural relief grows along a mountain front, and how block translation is related to block rotation. Industry seismic reflection, well, and outcrop data were used to construct digital structure‐contour maps of several of these faults and three stratigraphic horizons that intersect them. These maps, swath bathymetry, digital elevation models, and seismicity were incorporated into a 3‐D digital database. Modeling included new approaches to determining strike‐slip displacement by separate analyses of shortening and structural relief in a restraining double bend. Stratigraphic thicknesses indicate Miocene extension across the Santa Monica and Dume faults, which have been reactivated as a single or linked moderately dipping arcuate fault that accommodates one coherent block motion via left‐lateral and left‐reverse displacement. Modeling indicates 5 km (+8/−1 km) of left‐lateral displacement on part of the Santa Monica–Dume fault and ∼11° of clockwise rotation of the Santa Monica Mountains during the last ∼4 ± 1 m.y. Modern displacement rates modeled from GPS data are similar to our modeled post‐∼4 Ma rates of westward escape and clockwise rotation of the Santa Monica Mountains.

Deep seismic reflectivity beneath an intracratonic basin: Insights into the behavior of the uppermost mantle beneath the Illinois basin

Reflectivity images of the lower crust and uppermost mantle beneath the Paleozoic Illinois basin have been derived from reprocessing of several hundred kilometers of industry seismic reflection data using extended vibroseis recorrelation. The recorrelation was based on extending an originally 4-s correlated record, acquired with a 16-s sweep, from 14 to 126Hz, to the absolute limit of the full 20s (∼70km) listening traveltime. Sub-Moho reflections are recorded to 18s two-way traveltime (unmigrated) and are observed on intersecting profiles apparently dipping to the southwest and striking northwest–southeast. Occasional Moho reflections are also observed across the profiles (∼12s or ∼38km) while reflectivity in the lower crust is generally marked by intermittent horizontal packages and short, gently dipping reflections and diffraction segments. The presence of newly observed mantle reflectivity beneath the Illinois basin indicates significant uppermost mantle heterogeneity, relative to other parts of the USA studied using reflection methods. The mantle reflectivity effectively defines an area of anomalous mantle that rests beneath the depocenter of the early Paleozoic Illinois basin, which is itself superimposed over a deeper Proterozoic “basinal” depocenter. Filtering and inversion of geopotential field data reveal that this anomalous mantle also partially correlates with a positive density anomaly in the upper mantle. These correlations suggest that the anomalous mantle zone exerted an influence on the early Paleozoic subsidence of the basin, perhaps as a buried load due to eclogitization of former lower crust. The relatively isolated occurrence of sub-Moho reflections beneath the basin makes it difficult to uniquely infer their origin. However, available geologic and geophysical constraints, especially from geochemical and geochronological studies of drilled and exposed basement rocks, limit the possibilities to: (1) remnants or “scars” of sub-crustal processes associated with lithospheric extension or delamination related to the melting of the Proterozoic crust that led to the emplacement of the granite–rhyolite province that underlies much of USA Midcontinent; or (2) deformation caused by plate subduction associated with the hypothetical accretion of a juvenile island arc to the pre-1.6Ga southern margin of the Laurentian continent.

Seismic characterization of the structural front of southern Quebec Appalachians

Interpretation of recent industry seismic reflection data provides important insights into the structural style of the southern Quebec Appalachians structural front, south of Drummondville. Especially, the occurrence of thrusts propagating west of the mapped limit of the parautochthonous domain is widely documented. Extensive decollement planes dominate the structural style east of the Aston Fault and define the soles of tectonic slices stacked into hinterland-dipping duplexes. The Chambly-Fortierville Syncline and Logan’s Line are not imaged seismically. Growth faults were active since the deposition of the Potsdam Group and at least until the deposition of Trenton Group. Normal faults also extend into the overlying molasse and flysch units and antithetic faults are more common than previously documented. Normal faults are crosscut by Appalachian thrusts.

Upper crust beneath the central Illinois basin, United States

Newly available industry seismic reflection data provide critical information for understanding the structure and origin of the upper crust (0–12 km depth) beneath the central Illinois basin and the seismic-tectonic framework north of the New Madrid seismic zone in the central Mississippi Valley. Mapping of reflector sequences furnishes the first broad three-dimensional perspective of the structure of Precambrian basement beneath the central United States Midcontinent. The highly coherent basement reflectivity is expressed as a synformal wedge of dipping and subhorizontal reflections situated beneath the center of the Illinois basin that thickens and deepens to the northeast (e.g., 0 to ∼3 km thickness along a 123 km south to north line). The thickening trend of the wedge qualitatively mimics the northward thickening of the Late Cambrian Mt. Simon Sandstone; however, other Paleozoic units in the Illinois basin generally thicken southward into the basin center. The seismic data also reveal an anomalous subsequence defined by a spoon-shaped distribution of disrupted reflections located along the southern margin of the wedge. The boundaries of this subsequence are marked by distinct steeply dipping reflections (possible thrust faults?) that continue or project up to antiformal disruptions of lower Paleozoic marker reflectors, suggesting Paleozoic or possibly later tectonic reactivation of Precambrian structure. The areal extent of the subsequence appears to roughly correspond to an anomalous concentration of larger magnitude upper to middle crustal earthquakes. There are multiple hypotheses for the origin of the Precambrian reflectivity, including basaltic flows or sills interlayered with clastic sediments and/or emplaced within felsic igneous rocks. Such explanations are analogous to nearby Keweenawan rift-related volcanism and sedimentation, which initiated during Proterozoic rifting, and were followed eventually by reverse faulting along the rift margins caused by Grenville compression.

Active tectonics of the Seattle fault and central Puget Sound, Washington—Implications for earthquake hazards

Research Article| July 01, 1999 Active tectonics of the Seattle fault and central Puget Sound, Washington—Implications for earthquake hazards Samuel Y. Johnson; Samuel Y. Johnson 1U.S. Geological Survey, M.S. 966, Box 25046, Denver Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Shawn V. Dadisman; Shawn V. Dadisman 1U.S. Geological Survey, M.S. 966, Box 25046, Denver Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Jonathan R. Childs; Jonathan R. Childs 2U.S. Geological Survey, M.S. 999, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar William D. Stanley William D. Stanley 1U.S. Geological Survey, M.S. 966, Box 25046, Denver Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1999) 111 (7): 1042–1053. https://doi.org/10.1130/0016-7606(1999)111<1042:ATOTSF>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Samuel Y. Johnson, Shawn V. Dadisman, Jonathan R. Childs, William D. Stanley; Active tectonics of the Seattle fault and central Puget Sound, Washington—Implications for earthquake hazards. GSA Bulletin 1999;; 111 (7): 1042–1053. doi: https://doi.org/10.1130/0016-7606(1999)111<1042:ATOTSF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract We use an extensive network of marine high-resolution and conventional industry seismic-reflection data to constrain the location, shallow structure, and displacement rates of the Seattle fault zone and crosscutting high-angle faults in the Puget Lowland of western Washington. Analysis of seismic profiles extending 50 km across the Puget Lowland from Lake Washington to Hood Canal indicates that the west-trending Seattle fault comprises a broad (4–6 km) zone of three or more south-dipping reverse faults. Quaternary sediment has been folded and faulted along all faults in the zone but is clearly most pronounced along fault A, the northernmost fault, which forms the boundary between the Seattle uplift and Seattle basin. Analysis of growth strata deposited across fault A indicate minimum Quaternary slip rates of about 0.6 mm/yr. Slip rates across the entire zone are estimated to be 0.7–1.1 mm/yr.The Seattle fault is cut into two main segments by an active, north-trending, high-angle, strike-slip fault zone with cumulative dextral displacement of about 2.4 km. Faults in this zone truncate and warp reflections in Tertiary and Quaternary strata and locally coincide with bathymetric lineaments. Cumulative slip rates on these faults may exceed 0.2 mm/yr. Assuming no other crosscutting faults, this north-trending fault zone divides the Seattle fault into 30–40-km-long western and eastern segments. Although this geometry could limit the area ruptured in some Seattle fault earthquakes, a large event ca. A.D. 900 appears to have involved both segments. Regional seismic-hazard assessments must (1) incorporate new information on fault length, geometry, and displacement rates on the Seattle fault, and (2) consider the hazard presented by the previously unrecognized, north-trending fault zone. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Three-dimensional crustal structure of the Purcell anticlinorium in the Cordillera of southwestern Canada

The Purcell anticlinorium in southwestern Canada and the northwestern United States is situated at the transition from deformed North American rocks of the foreland thrust and fold belt to accreted rocks of the Kootenay arc. Nearly 1000 km of industry seismic reflection data were processed with extended VIBROSEIS correlation and coherency filtering and then combined with LITHOPROBE reflection data to outline the three-dimensional crustal structure of the anticlinorium. Reflection Moho is visible at ∼12.0 s (∼36 km) beneath the anticlinorium and deepens eastward to ∼15.0 s (∼45 km) beneath the Rocky Mountain thrust and fold belt. The transition from thick to thin crust coincides with westward deepening of North American basement and with the southern Rocky Mountain Trench, a basin or series of basins formed during Tertiary extension. Regionally extensive reflections in the upper crust correlate with ca. 1468 Ma gabbroic sills in the Aldridge Formation of the Mesoproterozoic Belt/Purcell Supergroup. Features outlined by the sills support an interpretation that the anticlinorium formed during Mesozoic contraction when imbricate thrust faults carried up to 15 km of Belt/Purcell and Paleozoic margin sedimentary rocks, and basement in some areas, eastward over a prominent basement ramp.

Structural interpretation of the Midcontinent Rift in eastern Lake Superior from seismic reflection and potential-field studies

Integrated interpretations of potential-field and GLIMPCE and industry seismic reflection data in eastern Lake Superior reveal the structural and stratigraphic complexity of the Midcontinent Rift in this region. Projection of the Keweenaw fault into southeastern Lake Superior suggested by early potential-field studies is confirmed by seismic reflection data. Analysis of seismic data in conjunction with aeromagnetic anomalies and regional gravity data also reveals a continuous section of basalt in the footwall of the Keweenaw fault. The lateral dimensions of this section vary along the strike of the rift from the center of the basin towards the southern flank. Spatially extensive anticlinal and synclinal features, reverse faults and related drag folds imaged by the reflection and enhanced potential-field data attest to the influence of a late-stage compressional event in this region. East-northeast trending gradients and displacements associated with observed potential-field anomalies and fault traces mapped at the surface also indicate a degree of accommodation perpendicular to the strike of the rift. These trends parallel the prevalent tectonic grain in the adjacent Archean basement rocks, perhaps suggesting that structures within the rift were in part controlled by preexisting crustal features.

Stratigraphy and structure of eastern Syria across the Euphrates depression

A N-S crustal-scale geotransect across the northern Arabian platform in eastern Syria reveals an alternating series of basement uplifts and basins separated by predominantly transpressional fault zones above an effectively uniform crust. Four major tectonic provinces are crossed along a 325 × 100 km corridor that extends from the Iraqi border in the south to the Turkish border in the north: the Rutbah uplift, the Euphrates depression, the Abd el Aziz structural zone, and the Qamichli uplift. These features are the manifestations of reactivated pre-Cenozoic structures that responded to forces acting along nearby Arabian plate boundaries, particularly Cenozoic convergence and collision along the margins of the northern Arabian platform i.e., the Bitlis suture and the East Anatolian fault in southern Turkey and the Zagros suture in Iran and Iraq. The database for this study consists of 3000 km of industry seismic reflection data, 28 exploratory wells, and geologic and Bouguer gravity maps. The deep crustal structure and, in part, the basement geometry along this transect are inferred from two-dimensional modeling of Bouguer gravity, whereas the shallow (about 8 km) structure is constrained primarily by well and seismic data. Features of the geotransect reveal: 1. (1) A relatively uniform crustal column approximately 37 km thick with only minor crustal thinning beneath the Euphrates. Crustal thinning may be slightly more pronounced beneath the Euphrates (about 35 km) to the southeast of the transect where the Bouguer gravity anomaly is slightly higher. 2. (2) Along the Euphrates depression, ongoing subsidence, which began during the Late Cretaceous, resulted in the deposition of at least 3 km of Late Cretaceous and Cenozoic rocks. The structural complexity of the Paleozoic and most of the Mesozoic sedimentary sections along the transect contrasts markedly with a relatively simple, flat-lying Cenozoic section along most of the transect. A notable exception is the Abd el Aziz uplift, where Cenozoic rocks are strongly deformed. 3. (3) While Euphrates subsidence continued throughout the Cenozoic, the inversion of the E-W-trending Abd el Aziz structure into a fault-bounded tilted block began in the Miocene, perhaps as a response to the last episode of intense Miocene collision along the nearby Bitlis and Zagros suture zones.

The Cordilleran Foreland Thrust Belt in Northwestern Montana and Northern Idaho From COCORP and Industry Seismic Reflection Data (1)

COCORP and petroleum industry seismic reflection profiles in northwestern Montana reveal the structure of the Cordilleran foreland thrust belt. The Front Ranges consist of thick thrust sheets containing Precambrian Belt Supergroup and Paleozoic miogeoclinal shelf rocks above a thin remnant of Paleozoic rocks and gently westward-dipping North American basement. Interpretation of the seismic data and results from a recent petroleum exploration well suggest that 15-22 km of Precambrian Belt Supergroup sedimentary rocks are present in several thrust plates beneath the eastern Purcell anticlinorium. Previous hypotheses of a large mass of Paleozoic miogeoclinal sedimentary rocks or slices of crystalline basement located beneath the eastern Purcell anticlinorium do not appear to be supported by the data. The easternmost occurrence of allochthonous basement is interpreted to be in the western part of the anticlinorium near the Montana-Idaho border. Comparison of the Cordilleran foreland thrust belt in northwestern Montana and southern Canada suggests that a change in the deep structure of the Purcell anticlinorium occurs along strike. The anticlinorium in southern Canada has been interpreted as a hanging-wall anticline that was thrust over the western edge of thick Proterozoic North American basement, whereas in northwestern Montana the anticlinorium appears to consist of a complex series of thrust sheets above highly attenuated North American basement.

Foreland shortening and crustal balancing in the Andes at 30°S latitude

Excellent surface exposures, known Benioff zone geometry, a dynamic morphology, and the availability of industry seismic reflection data all make the Andes at 30°S an excellent transect for investigating crustal‐scale balanced sections. 150–170 km of horizontal shortening has occurred in three major belts located between the trench and the foreland. The thin‐skinned, east‐verging Precordillera of western Argentina accounts for 60–75% of the total shortening and formed mostly since major volcanism ceased at ∼10 Ma. Industry seismic reflection data show that the décollement of the Precordillera belt is located anomalously deep at ∼15 km. The belt is dominated by fault propagation folds and contains several prominent out‐of‐sequence thrust faults. Seismic stratigraphie analysis shows that Miocene strata in the Iglesia Valley, located between the Precordillera and the crest of the Andes, accumulated in a piggy‐back basin. Onlap relations on the western side indicate that the High Cordillera was uplifted as a major fault bend fold over a buried ramp. Thrusting in the two western belts, both in the High Cordillera of Chile, formed during the waning stages of arc volcanism, 11–16 Ma. and account for 25–40% of the shortening. The observed shortening is probably greater than can be accounted for with reasonable crustal thicknesses, indicating the possibility of continental truncation or erosion along the plate margin or an anomalously thick root held down by the nearly flat subducted Nazca Plate. Our preferred crustal geometry puts the ramp between upper and lower crustal deformation west of the high topography, requiring crustal scale tectonic wedging to thicken the crust beneath the crest of the Andes. This non‐unique model provides a simple explanation of the first order morphology of the Andes at this latitude.