Deep Crustal Seismic Reflection Profiling Research Articles

Micro-seismicity in the Gulf of Cadiz: Is there a link between micro-seismicity, high magnitude earthquakes and active faults?

The Gulf of Cadiz seismicity is characterized by persistent low to intermediate magnitude earthquakes, occasionally punctuated by high magnitude events such as the M~8.7 1755 Great Lisbon earthquake and the M=7.9 event of February 28th, 1969. Micro-seismicity was recorded during 11months by a temporary network of 25 ocean bottom seismometers (OBSs) in an area of high seismic activity, encompassing the potential source areas of the mentioned large magnitude earthquakes. We combined micro-seismicity analysis with processing and interpretation of deep crustal seismic reflection profiles and available refraction data to investigate the possible tectonic control of the seismicity in the Gulf of Cadiz area. Three controlling mechanisms are explored: i) active tectonic structures, ii) transitions between different lithospheric domains and inherited Mesozoic structures, and iii) fault weakening mechanisms. Our results show that micro-seismicity is mostly located in the upper mantle and is associated with tectonic inversion of extensional rift structures and to the transition between different lithospheric/rheological domains. Even though the crustal structure is well imaged in the seismic profiles and in the bathymetry, crustal faults show low to negligible seismic activity. A possible explanation for this is that the crustal thrusts are thin-skinned structures rooting in relatively shallow sub-horizontal décollements associated with (aseismic) serpentinization levels at the top of the lithospheric mantle. Therefore, co-seismic slip along crustal thrusts may only occur during large magnitude events, while for most of the inter-seismic cycle these thrusts remain locked, or slip aseismically. We further speculate that high magnitude earthquake's ruptures may only nucleate in the lithospheric mantle and then propagate into the crust across the serpentinized layers.

The Millungera Basin—new geoscience supporting exploration

An unknown sedimentary sequence was first recorded during a Geoscience Australia/ Geological Survey of Queensland/ pmd*CRC deep seismic reflection survey in the Mount Isa Inlier and adjacent undercover terrains, during 2006/07. The sequence occurs unconformably underneath the Carpentaria Basin succession in the Julia Creek area, east of Cloncurry in north Queensland, and is named the Millungera Basin. A section through the basin is recorded along seismic line 07GA–IG1, recorded between north of Cloncurry to east of Croydon. In this section three internal sequences are noted—with two strongly reflective units separated by a poorly reflective unit. As well as deep crustal seismic reflection profiles, magnetotelluric profiles were collected along the same traverse. These data show a moderately conductive Millungera Basin underlying the strongly conductive Carpentaria Basin. Zones of limited reflectors beneath the basin in the seismic sections have been interpreted as granites, raising the possibility of raised geothermal gradients. The Millungera Basin may comprise a potential geothermal target. The Millungera Basin sequence is interpreted to overlie granites. Adjacent Proterozoic granites of the Williams Batholith are known to be high heat producing granites, containing high levels of potassium thorium and uranium. The hydrocarbon potential of the basin is similarly uncertain. Strong reflectors in the seismic sections may be coal beds. Although the depth of the basin in the seismic section is insufficient to have reached the oil window, interpretation of gravity profiles by Geoscience Australia suggest the basin deepens to the south, possibly reaching 4,000 m. If fertile beds have reached the oil window, the structurally more complex eastern side of the basin may contain petroleum traps. The age of the rocks in the Millungera Basin is not known. Constraints from the seismic suggest between the early Mesoproterozoic and the Middle Jurassic. Investigations into the nature of the basin are continuing. A more detailed magnetotellurc survey is being undertaken to better define the shape of the basin. In order to reliably describe the basins components, a deep drilling program is required.

Tectonic evolution of the Lachlan Orogen, southeast Australia: Historical review, data synthesis and modern perspectives

The Lachlan Orogen,like many other orogenic belts,has undergone paradigm shifts from geosynclinal to plate-tectonic theory of evolution over the past 40 years. Initial plate-tectonic interpretations were based on lithologic associations and recognition of key plate-tectonic elements such as andesites and palaeo-subduction complexes. Understanding and knowledge of modern plate settings led to the application of actualistic models and the development of palaeogeographical reconstructions, commonly using a non-palinspastic base. Igneous petrology and geochemistry led to characterisation of granite types into ‘I’ and ‘S’, the delineation of granite basement terranes, and to non-mobilistic tectonic scenarios involving plumes as a heat source to drive crustal melting and lithospheric deformation. More recently, measurements of isotopic tracers (Nd, Sr, Pb) and U–Pb SHRIMP age determinations on inherited zircons from granitoids and detrital zircons from sedimentary successions led to the development of multiple component mixing models to explain granite geochemistry. These have focused tectonic arguments for magma genesis again more on plate interactions. The recognition of fault zones in the turbidites, their polydeformed character and their thin-skinned nature, as well as belts of distinct tectonic vergence has led to a major reassessment of tectonic development. Other geochemical studies on Cambrian metavolcanic belts showed that the basement was partly backarc basin- and forearc basin-type oceanic crust. The application of 40Ar–39Ar geochronology and thermochronology on slates,schist and granitoids has better constrained the timing of deformation and plutonism,and illite crystallinity and bo mica spacing studies on slates have better defined the background metamorphic conditions in the low-grade parts. The Lachlan deformation pattern involves three thrust systems that constitute the western Lachlan Orogen, central Lachlan Orogen and eastern Lachlan Orogen. The faults in the western Lachlan Orogen show a generalised east-younging (450–395 Ma), which probably relates to imbrication and rock uplift of the sediment wedge, because detailed analyses show that the décollement system is as old in the east as it is in the west. Overall, deformation in the eastern Lachlan Orogen is younger (400–380 Ma), apart from the Narooma Accretionary Complex (ca 445 Ma). Preservation of extensional basins and evidence for basin inversion are largely restricted to the central and eastern parts of the Lachlan Orogen. The presence of dismembered ophiolite slivers along some major fault zones, as well as the recognition of relict blueschist metamorphism and serpentinite-matrix mélanges requires an oceanic setting involving oceanic underthrusting (subduction) for the western Lachlan Orogen and central Lachlan Orogen for parts of their history. Inhibited by deep weathering and a general lack of exposure, the recent application of geophysical techniques including gravity, aeromagnetic imaging and deep crustal seismic reflection profiling has led to greater recognition of structural elements through the subcrop, a better delineation of their lateral continuity, and a better understanding of the crustal-scale architecture of the orogen. The Lachlan Orogen clearly represents a class of orogen, distinct from the Alps, Canadian Rockies and Appalachians, and is an excellent example of a Palaeozoic accretionary orogen.

Seismic velocities in the North West Shelf Region, Australia, From Near-vertical and Wide-angle Reflection and Refraction Studies

A significant limitation of conventional seismic reflection technology is that it poorly constrains seismic velocities in the deep part of the section. These velocities are needed for seismic migration, to convert from time to depth, and to estimate the likely rock composition at depth. Seismic observations at large offsets can compensate for this limitation. The Australian Geological Survey Organisation recorded high-quality refraction and wide-angle reflection seismic data using ocean-bottom seismographs (OBS) along 5 profiles on Australia’s North West Shelf, to maximum offsets of 300 km. The OBS transects coincide with previously acquired deep-crustal seismic-reflection profiles. Consequently, velocity information can now be derived from two independent data sets: common-depth point (near-vertical) reflection data and refraction/wide-angle reflection data. Residual (difference between these two velocity data sets) velocity images can be produced and analysed. Even within the top 5 km of the section residuals between the OBS- and CDP-derived interval velocities vary from -0.5 to +3.5 km/s and increase further with depth. Consequently, the depth equivalent of 4 s two-way time will vary by up to 1.2 km or 10%. The practical implications of co-processing of seismic velocity information are: improved seismic velocity estimation, more accurate depth migration and depth conversion of seismic data and less distortion of amplitudes and shapes of geological structures after depth conversion.

Crustal thickness variations in southern California from Los Angeles Region Seismic Experiment passive phase teleseismic travel times

AbstractThe goal of the 1993 Los Angeles Region Seismic Experiment (LARSE93) passive phase was to collect waveform data from local and distant earthquakes to study lower crust and upper mantle structural features in southern California, particularly under the San Gabriel Mountains and San Andreas fault. During LARSE93, approximately 88 stations were deployed in a 175-km-long, linear array across the Los Angeles basin, San Gabriel Mountains, and Mojave Desert northeast of Los Angeles. During the four weeks of continuous recording, teleseismic events recorded at each site provided a wide range of ray path backazimuths. The teleseismic events included a number of intermediate-magnitude earthquakes with epicenters in the Aleutian Island, Kamchatka, Kuril Island, mid-Atlantic Ridge, Solomon Island, Japan, Fiji Island, and Chile regions. This experiment was followed by LARSE94, which involved land refraction and deep-crustal seismic reflection profiles from offshore and onshore explosion sources. We have used the joint data sets in this study to distinguish upper crustal features from adjacent lower crustal and upper mantle structures. P-wave travel times were determined from 17 teleseisms, and upper crustal residual signatures were removed by incorporating LARSE94 upper crustal velocity model results. Within each backazimuth range, the resulting relative traveltime residuals increase from negative values (−0.5-sec average) recorded in the northern San Gabriel Valley-southern San Gabriel Mountain foothills to positive values (0.2-sec average) in the central and northern San Gabriel Mountains. The residual patterns display variations for different backazimuths and incidence angles but show almost no lateral spatial shift of maximum or minimum residual along the array, indicating that the dominant source of the residual pattern is shallow (<50 km). The patterns of residuals require a sharp lateral gradient in shallow velocities between the northern San Gabriel Valley (located in the northernmost Los Angeles basin) and the San Gabriel Mountains over a distance of less than 50 km. Most of the residual pattern can be explained by laterally varying crustal thickness and a high-velocity anomaly in the upper mantle. In our model of Moho depth variations, the northern San Gabriel Valley to the southwest of the array is underlain by thinned continental crust. Crustal thickness increases laterally by 12 km over a distance of less than 50 km into the San Gabriel Mountains. This conclusion is supported by widespread surface evidence for rift-style volcanism and crustal extension in southern California crustal rocks.

Continental extension of the Pacific Plate at the southern termination of the Hikurangi subduction zone: The North Mernoo Fault Zone, offshore New Zealand

A complex transition from subduction at the Hikurangi margin to transpressive, continental collision across the Southern Alps occurs in the NE South Island region of New Zealand. Offshore, the southern limit of oceanic subduction is constrained by the Chatham Rise, a submerged continental plateau on the Pacific Plate, which is presumably too buoyant for normal subduction. In the NW corner of the rise is the North Mernoo Fault Zone (NMFZ), a 100 by 300 km region of basement‐involved, active normal faulting of late Miocene (circa 8–6 Ma) to Recent age. The NMFZ evolved contemporaneously with the rotation and development of the adjacent plate boundary zone. Growth faulting is characteristic within the sedimentary cover, which is generally less than 2 km thick, but the distribution of faulting has varied temporally. The amount of mid Pliocene‐Recent extension is estimated to be not more than a few percent. The NMFZ consists of an array of overlapping, south dipping normal faults, which are typically 2–5 km apart and which trend roughly E–W at a high angle to the plate boundary zone. Late Quaternary surface traces are widely distributed on the mid to upper continental slope, but many surface scarps are poorly preserved due to extensive erosion of the seafloor. The western end of the fault zone crosses submarine canyons at the southern end of the Hikurangi Trough and almost abuts two fold and thrust fault systems on the NE South Island continental margin. Although few data constrain the kinematics of extension, a single MW 6.1 earth quake at the western end of the NMFZ suggests planar rotating faults with a component of left‐lateral oblique slip. Two possible models of regional extension are considered; (1) lateral buckling of the upper crust, in which faulted slivers of crust are peeled off as the Chatham Rise slides past the transpressive Marlborough fault system toward the Southern Alps and (2) flexure of the edge of continental Pacific Plate as the Chatham Rise is bent downward into the southern end of the Hikurangi subduction zone. Improved seismological data and deep crustal seismic reflection profiles may help to constrain the kinematics of deformation.

Comparison of vibroseis and explosive source methods for deep crustal seismic reflection profiling in the Basin and Range Province

Direct comparison of low‐fold, high‐energy explosive and high‐fold, lower‐energy Vibroseis methods for acquiring deep crustal seismic reflection data in the Basin and Range Province suggests that the high‐fold common midpoint (CMP) method there does not provide the best possible image of lower crustal structure. During the recent acquisition of a Vibroseis profile in the Basin and Range Province we fired single deep shot holes to obtain a coincident single‐fold explosive section. Within the upper crust (upper 3 s) the explosive source and Vibroseis records are nearly equivalent. For record times below 3 s, however, comparison of the explosive source gathers and the coincident final 60‐fold Vibroseis section demonstrates that low‐fold explosive profiling provides a higher‐quality image of the midcrust to lower crust (3–10 s). The higher record quality of the explosive sources results primarily from the larger seismic energy levels produced by the explosives, making them less sensitive to common noise sources. Whereas deeper than 4–5 s the Vibroseis energy levels on individual source efforts fall to that of ambient noise levels, the explosions provide signal‐generated energy exceeding ambient noise levels down to 18–19 s. Although individual reflections can be correlated on explosion and Vibroseis shot gathers, reflection events on the 60‐fold Vibroseis stack do not correlate to those on the single‐fold explosion profile, suggesting that the high‐fold CMP method in our study did not maintain the integrity of the weak lower crustal reflected arrivals. Reasons why the high‐fold CMP method apparently failed include complex, even time‐varying, statics, nonhyperbolic moveout at long offsets, and the difficulty in resolving stacking velocities with data having low signal‐to‐noise ratios. Reflections on the explosion section are longer and imply a greater degree of layering than one would infer from the lower‐energy Vibroseis section.

Deep crustal seismic reflection profiling across two major tectonic zones in southern Sweden

A seismic reflection profile has been shot in Värmland, southwestern Sweden, across two major tectonic zones, the Protogine Zone and the Mylonite Zone. The crustal bedrock units separated by the tectonic zones are clearly distinguished in the seismic profile by changes in the reflection character in the upper 5–6 km. The Protogine Zone is represented by a 15–20 km wide band of dipping structures. The Mylonite Zone dips too steeply to be seen directly and a lack of clear reflectors at depth makes a continuation down through the crust hypothetical. In the uppermost 5–6 km between the two tectonic zones, stronger and more continuous reflections can be seen, and are interpreted as being connected with “hyperite” intrusions, which have been mapped at the surface. The lower crust shows generally little reflectivity, with only short and weak reflectors. An exception occurs at mid-crustal depths where a number of clearly recognizable bands of reflected energy are suggested to be major shear zones formed during Sveconorwegian-Grenvillian thrusting towards the east. A slight increase in reflectivity in the depth range 42–48 km correlates well with the Moho depth determined from refraction seismic studies. The possible relationships between data acquisition parameters and the observed scarcity and low strength of reflections from the deeper crust have been considered in some detail. It is shown that the use of 28 Hz single geophones instead of 10 Hz geophone strings along a major part of the profile did not reduce the signal-to-noise ratio for deep reflections. However, the varying source characteristics, in combination with the different quality of the recordings, made computation of the residual static corrections difficult, which may have resulted in a partly destructive stacking of seismic traces. In this way, the low reflectivity of the deeper crust in this area is further brought out in the stacked seismic sections.

The extension of Grenville Basement beneath the northern Appalachians: Results from the Quebec‐Maine seismic reflection and refraction surveys

From 1983 to 1984 the United States Geological Survey and the Geological Survey of Canada undertook a series of seismic experiments to determine the crustal structure of the Appalachian mountain belt in Maine and southeast Québec. These experiments included four deep crustal seismic reflection profiles perpendicular to the strike of regional geological trends and several high‐resolution refraction/wide‐angle reflection profiles both parallel and perpendicular to strike. Here the data recorded in Québec and the northwest part of Maine are interpreted in conjunction with existing reflection profiles from Québec. Synthetic seismogram calculations in laterally varying media are used to model the refraction/wide‐angle reflection data, while reprocessing, migration, and attribute plotting are used in the interpretation of the reflection data. A major zone of reflections extending over a distance exceeding 200 km can be traced from shallow depths beneath the St. Lawrence Lowlands southeast to about 25 km depth beneath the southeast edge of the Chain Lakes Massif. This zone of reflections is interpreted as a master décollement separating autochthonous Grenville basement from overlying allochthonous rocks of the Appalachian Orogen, including foreshortened miogeoclinal rocks, the remnants of one or more magmatic arcs, the Connecticut Valley ‐ Gaspé Synclinorium, and the Chain Lakes Massif. In the vicinity of the Chain Lakes Massif, extension has caused Grenville basement to be thinned by as much as 40%. Reflections from the décollement show strong layering and evidence for basement faulting associated with the Guadeloupe and other faults. Normal incidence and wide‐angle reflections are observed to be almost continuous beneath the Baie Verte ‐ Brompton Line and Connecticut Valley ‐ Gaspé Synclinorium, indicating that the Baie Verte ‐ Brompton Line is confined to the upper crust. The Connecticut Valley ‐ Gaspé Synclinorium is characterized by P‐wave velocities of about 5.3 km s−1 and is shown to be a shallow structure with a maximum depth of about 3 km. Rock velocities immediately below the Connecticut Valley ‐ Gaspé Synclinorium are typical of Chain Lakes material (6.1 km s−1). A new model for the tectonic development of the region is proposed in which all pre‐Silurian units between the Baie Verte ‐ Brompton Line and the southeastern edge of the Chain Lakes Massif are allochthonous and confined to the upper crust. We infer that the Chain Lakes Massif, volcanics of the Ascot‐Weedon Formation, and associated deeper crustal rocks were thrust over Grenvillian basement as a composite unit and may now underlie parts of the Connecticut Valley ‐ Gaspé Synclinorium.

Comment and Reply on “Deep crustal seismic reflection profiling, New England orogen, eastern Australia: Telescoping of the crust and a hidden, deep, layered sedimentary sequence”

Research Article| December 01, 1987 Comment and Reply on “Deep crustal seismic reflection profiling, New England orogen, eastern Australia: Telescoping of the crust and a hidden, deep, layered sedimentary sequence”: REPLY R. J. Korsch; R. J. Korsch 1Division of Continental Geology, Bureau of Mineral Resources, G.P.O. Box 378, Canberra, A.C.T. 2601, Australia Search for other works by this author on: GSW Google Scholar J. F. Lindsay; J. F. Lindsay 1Division of Continental Geology, Bureau of Mineral Resources, G.P.O. Box 378, Canberra, A.C.T. 2601, Australia Search for other works by this author on: GSW Google Scholar P. E. O'Brien; P. E. O'Brien 1Division of Continental Geology, Bureau of Mineral Resources, G.P.O. Box 378, Canberra, A.C.T. 2601, Australia Search for other works by this author on: GSW Google Scholar M. J. Sexton; M. J. Sexton 2Division of Petrology and Geochemistry, Bureau of Mineral Resources, G.P.O. Box 378, Canberra, A.C.T., Australia Search for other works by this author on: GSW Google Scholar K. D. Wake K. D. Wake 2Division of Petrology and Geochemistry, Bureau of Mineral Resources, G.P.O. Box 378, Canberra, A.C.T., Australia Search for other works by this author on: GSW Google Scholar Author and Article Information R. J. Korsch 1Division of Continental Geology, Bureau of Mineral Resources, G.P.O. Box 378, Canberra, A.C.T. 2601, Australia J. F. Lindsay 1Division of Continental Geology, Bureau of Mineral Resources, G.P.O. Box 378, Canberra, A.C.T. 2601, Australia P. E. O'Brien 1Division of Continental Geology, Bureau of Mineral Resources, G.P.O. Box 378, Canberra, A.C.T. 2601, Australia M. J. Sexton 2Division of Petrology and Geochemistry, Bureau of Mineral Resources, G.P.O. Box 378, Canberra, A.C.T., Australia K. D. Wake 2Division of Petrology and Geochemistry, Bureau of Mineral Resources, G.P.O. Box 378, Canberra, A.C.T., Australia Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1987) 15 (12): 1181–1182. https://doi.org/10.1130/0091-7613(1987)15<1181:CARODC>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. J. Korsch, J. F. Lindsay, P. E. O'Brien, M. J. Sexton, K. D. Wake; Comment and Reply on “Deep crustal seismic reflection profiling, New England orogen, eastern Australia: Telescoping of the crust and a hidden, deep, layered sedimentary sequence”: REPLY. Geology 1987;; 15 (12): 1181–1182. doi: https://doi.org/10.1130/0091-7613(1987)15<1181:CARODC>2.0.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 SocietyGeology Search Advanced Search Abstract No Abstract Available. 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.

Comment and Reply on “Deep crustal seismic reflection profiling, New England orogen, eastern Australia: Telescoping of the crust and a hidden, deep, layered sedimentary sequence”

Comment and Reply on “Deep crustal seismic reflection profiling, New England orogen, eastern Australia: Telescoping of the crust and a hidden, deep, layered sedimentary sequence”

Deep crustal seismic reflection profiling, New England orogen, eastern Australia: Telescoping of the crust and a hidden, deep, layered sedimentary sequence

Australian Bureau of Mineral Resources (BMR) deep crustal seismic reflection profiling in the New England orogen of eastern Australia has revealed a thick, deeply buried, layered sequence that was telescoped by thrusting into a stack 20 km thick. The seismic character suggests that the sequence is probably sedimentary and comparable in style to that revealed by the Consortium for Continental Reflection Profiling (COCORP) beneath the Menimack synclinorium in New Hampshire. The Beenleigh terrene, a metamorphosed Paleozoic accretionary wedge that overlies the deep, layered sequence is thin skinned and has a maximum thickness of about 4.6 km. The boundary between the sequence and the New England orogen is a well-defined west-dipping thrust fault, which extends to at least 14 km. We suggest that telescoping of the deep, layered sequence occurred away from the New England orogen and that it and the Beenleigh terrene were later accreted to the orogcn by underthrusting, between 265 and 235 Ma.

The seismic reflection character of the continental mohorovicic discontinuity

The continental Mohorovicic discontinuity is most often interpreted as a step‐function velocity boundary. However, on deep crustal seismic reflection profiles, reflections, at depths where refraction data places the Moho, have laminated character and are laterally discontinuous. These observations on continental reflection data point to a model of the Moho that has thin layered rather than block structure. Previous workers, who have synthetically generated seismic responses from various crust‐mantle boundary models, have shown that thin layered models of alternating high and low velocity generate responses that best emulate observed Moho arrivals. Geologic interpretations of the thin layering include relatively undeformed metasediments, cumulate layering, tectonic banding, and lenses of partial melt. To obtain more direct geologic evidence, a synthetic seismogram is generated from geologic cross‐sections of the Ivrea‐Verbano Zone, an exposed section through the lower crust and upper mantle. The synthetic shows laminations similar to those observed on reflection data. Without more direct evidence, no single geologic interpretation of the Moho is reasonable. Indeed, the Moho may be a laterally variable boundary, its composition and structure dependent upon the geologic history of the overlying crust.

COCORP seismic reflection profiling across thrust faults

Summary It is very important to have good subsurface data in order to understand the nature and behaviour of thrust faults. Deep crustal seismic reflection profiling is the best technique currently available to make detailed subsurface studies of such important problems as the attitude and extent at depth of major faults, and hence deduce the mode of deformation and tectonic forces producing them. The Consortium for Continental Reflection Profiling (COCORP) is collecting large quantities of seismic reflection data from the deep crust and upper mantle in many parts of the U.S.A. Areas of major thrusting which have been profiled so far by COCORP include the Wind River Mountains in Wyoming and the Southern Appalachians of Georgia and Tennessee. Seismic profiles have been very successful in delineating a major thrust fault of moderate dip underlying the Wind River Mountains, thus demonstrating that compressional tectonics were dominant in their formation. In Georgia and Tennessee the seismic profiles demonstrate that the major tectonic feature of the Southern Appalachians is a relatively thin overthrust sheet, which may have moved at least 260 km. Deep crustal seismic reflection profiling thus appears to be an indispensable tool for the study of areas in which thrusting and nappe formation have occurred.

Nature of the Wind River thrust, Wyoming, from COCORP deep-reflection data and from gravity data

Critical data for the interpretation of Laramide structure, a major tectonic problem bearing on the formation of the Rocky Mountains, have been obtained by the Consortium for Continental Reflection Profiling (COCORP) in the form of deep crustal seismic-reflection profiles. The Wind River Mountains in Wyoming have been uplifted by the Wind River thrust, which can be traced on COCORP seismic-reflection profiles to at least 24-km depth with an average dip of 30° to 35°. This Laramide uplift is thus the result of extensive horizontal compression with a minimum horizontal displacement of 21 km and a minimum vertical displacement of 13 km. The crust appears to have deformed essentially as a rigid plate. Gravity anomalies across the uplift can be modeled by a thrust, with the same geometry as indicated by the seismic-reflection profiles.