Consortium For Continental Reflection Profiling Research Articles

Geophysical Constraints on the Crustal Architecture of the Transtensional Warm Springs Valley Fault Zone, Northern Walker Lane, Western Nevada, USA

AbstractThe Walker Lane is a zone of distributed transtension where normal faults are overprinted by strike‐slip motion. We use two newly acquired, high‐resolution seismic reflection profiles and a reprocessed Consortium for Continental Reflection Profiling (COCORP) deep crustal reflection profile to assess the subsurface geometry of the Holocene‐active, transtensional Warm Springs Valley fault zone (WSVFZ) near Reno, Nevada, USA. Our multiscale observations extend to 12 km depth and suggest that the WSVFZ is more complex in the subsurface than implied by late Pleistocene surface fault traces. Two 4‐km‐long high‐resolution profiles image to a depth of ∼2 km and reveal moderately dipping reflections and truncations, some of which project to mapped scarps formed in late Pleistocene surfaces. The shallow lines are collocated with COCORP profile NV 08 along ∼40°N latitude. Reanalysis of the COCORP data reveals previously unidentified coherent reflections to a depth of ∼12 km and a previously mapped ∼30° west‐dipping fault at 8–12 km. From these seismic profiles, the WSVFZ is not a simple, subvertical fault zone extending through the entire seismogenic crust. Instead, the reflections are consistent with a zone of steeply and moderately dipping faults that simplify and steepen with depth before intersecting a mid‐crustal, low‐angle (∼25–30°) fault. The complex fault geometry of the WSVFZ implies that crustal shear is accommodated by a mix of dipping and subvertical faults in the transtensional northern Walker Lane. If so, transtensional fault zones may present challenges to paleoseismic and geodetic studies and require careful treatment when included in seismic hazard analyses.

Reassessing the lithosphere: SeisDARE, an open-access seismic data repository

Abstract. Seismic reflection data (normal incidence and wide angle) are unique assets for solid Earth sciences as they provide critical information about the physical properties and structure of the lithosphere as well as about the shallow subsurface for exploration purposes. The resolution of these seismic data is highly appreciated; however they are logistically complex and expensive to acquire, and their geographical coverage is limited. Therefore, it is essential to make the most of the data that have already been acquired. The collation and dissemination of seismic open-access data are then key to promote accurate and innovative research and to enhance new interpretations of legacy data. This work presents the Seismic DAta REpository (SeisDARE), which is, to our knowledge, one of the first comprehensive open-access online databases that stores seismic data registered with a permanent identifier (DOI). The datasets included here are openly accessible online and guarantee the FAIR (findable, accessible, interoperable, reusable) principles of data management, granting the inclusion of each dataset in a statistics referencing database so its impact can be measured. SeisDARE includes seismic data acquired in the last 4 decades in the Iberian Peninsula and Morocco. These areas have attracted the attention of international researchers in the fields of geology and geophysics due to the exceptional outcrops of the Variscan and Alpine orogens and wide foreland basins, the crustal structure of the offshore margins that resulted from a complex plate kinematic evolution, and the vast quantities of natural resources contained within. This database has been built thanks to a network of national and international institutions, promoting a multidisciplinary research and is open for international data exchange and collaborations. As part of this international collaboration, and as a model for inclusion of other global seismic datasets, SeisDARE also hosts seismic data acquired in Hardeman County, Texas (USA), within the COCORP project (Consortium for Continental Reflection Profiling). SeisDARE aims to make easily accessible old and recently acquired seismic data and to establish a framework for future seismic data management plans. SeisDARE is freely available at https://digital.csic.es/handle/10261/101879 (a detailed list of the datasets can be found in Table 1), bringing endless research and teaching opportunities to the scientific, industrial, and educational communities.

Teleseismic Scattered‐Wave Imaging Using a Large‐N Array in the Albuquerque Basin, New Mexico

AbstractThe advent of low‐cost continuously recording cable‐free autonomous seismographs, commonly referred to as nodes, enables dense spatiotemporal sampling of seismic wavefields. We create virtual source reflection profiles using P waves from five teleseismic events recorded by the Sevilleta node array experiment in the southern Albuquerque basin. The basin geology records a structurally complex history, including multiple Phanerozoic orogenies, Rio Grande rift extension, and ongoing uplift from a midcrustal magma body. The Sevilleta experiment densified the long term, regionally sparse seismograph network with 801 single channel vertical‐component 10 Hz geophone nodes deployed at ∼300 m spacing for 14 days in February 2015. Results show sediment‐basement reflections at <5 km depth and numerous sub‐basin structures. Comparisons to legacy crustal‐scale reflection images from the Consortium for Continental Reflection Profiling show agreement with structural geometries in the rift basin and upper crust. Comparisons of the teleseismic virtual reflection profiles to synthetic tests using 2D finite‐difference elastic wave propagation show strong P‐to‐Rayleigh scattering from steep basin edges. These high‐amplitude conversions dominate the record sections near the western rift margin and originate at the Loma Pelada fault, which acts as the primary contact between rift‐bounding basement‐cored fault blocks and rift basin sediments. At near offsets, these signals may interfere with interpretation of upper crustal structure, but their relatively slow moveout compared to teleseismic P‐wave multiples provides clear temporal separation from sediment‐basement reflections across most of the array. The high‐signal‐to‐noise ratio of these converted Rayleigh‐wave signals suggests that they may be useful for constraining short‐period (∼1 Hz) dispersion with strong sensitivity in the uppermost ∼1 km of the rift basin sediments.

Reanalysis of the COCORP Utah Line 1 deep seismic reflection profile: Toward an improved understanding of the Sevier Desert detachment question

The Sevier Desert detachment (or reflection, SDR), which underlies the Sevier Desert basin along the eastern margin of the USA Basin and Range, is commonly cited as a type example of a low-angle normal fault (LANF) in continental crust. We present the results of reanalyzing the SDR on the COCORP (Consortium for Continental Reflection Profiling) deep seismic profile crossing the Sevier Desert basin (Utah Line 1). We employ a strategy of showing how shallow crustal velocity models, derived from first-break analysis of the shot records, may be used to reduce the effect of lateral velocity variations on imaging the SDR. Our results imply that the irregularities and discontinuities along the SDR are likely caused by overlying lateral velocity variations. The reprocessed versions of the section reveal a smoother, simpler, and more continuous SDR, lacking most of the apparent large offsets and structural variations on the currently available version of the profile. Seismic attribute and structural analyses of the reflection indicate significant variations along the profile, which are likely related in part to the field acquisition, but may also suggest lateral variations in the physical origin of the SDR. Such lateral variations may be consistent with previous studies that challenge the LANF interpretation of the SDR on the basis of attributing different origins to different parts of the reflector; however, a smoother and more continuous SDR points to a tectonically uniform origin and is thus interpreted to be more consistent with a LANF explanation.

Examining Three‐Dimensional Crustal Heterogeneity in Finland

The marriage of several high‐quality seismic experiments in Finland over the past 30 years has shown that the saying “something old, something new, something borrowed” can result in the cost‐efficient analysis of large‐scale, three‐dimensional (3‐D) seismic structures. Standing alone, each data set gives a partial view of complex 3‐D structures. When combined, they reveal a 3‐D block structure embedded in a layered crust and enable the analysis of dynamics involved in forming stable cratonic crust.Efforts to collect large 3‐D data sets around the globe include EarthScope (funded by the U.S. National Science Foundation (NSF)), the European Science Foundations (ESF) 4‐D Topography Evolution in Europe: Uplift, Subsidence and Sea Level Change (TOPO‐EUROPE), and the European Space Agency's Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE). Such endeavors are fundamental to modern crustal research. Huge emphasis is placed on collecting and archiving these data, but often only a fraction of data are used in initial studies. Fortunately, new data sets can be complemented with vintage ones (e.g., the NSF‐funded Consortium for Continental Reflection Profiling (COCORP) and ESFs European GeoTraverse (EGT), as well as continent‐wide science programs on continental evolution in Canada (LITHOPROBE), Europe (ESF‐funded EUROPROBE), and the Himalayas (NSF‐funded International Deep Profiling of Tibet and the Himalaya (INDEPTH)). Because fieldwork and archiving have already been completed for these vintage projects, new information can be extracted by new methods, with considerably less effort and funding.

Unconformity-bounded seismic reflection sequences define Grenville-age rift system and foreland basins beneath the Phanerozoic in Ohio

Interpretation of reprocessed Ohio Consortium for Continental Reflection Profiling (COCORP) OH-1 seismic reflection profiles indicates four structurally complex Precambrian unconformity-bounded stratigraphic sequences that clarify the relative timing of formation of the Fort Wayne Rift and East Continent Rift System with respect to the Grenville orogeny. Petrographic examination of sparse deep well samples in the region indicates or suggests sedimentary lithologies beneath the Paleozoic sedimentary cover. Other seismic profiles in the region, some with excellent well control, support our proposed model. A generalized model for the latter part of the Grenville orogeny suggests polyphase sedimentation and deformation with multiple episodes of crustal extension and compression. We propose the following events for Ohio and the surrounding region: (1) a major regional unconformity developed on the Eastern Granite-Rhyolite Province and accreted Grenville terranes; (2) western Ohio became the site of extensive fault-bounded rift basins, beginning with the Fort Wayne Rift and extending into west-central Ohio as the East Continent Rift System; (3) westward-advancing thrust sheets followed with deposition of sediments into newly developed basins; (4) continued Grenville thrusting created foreland basins in a westward progression; and (5) a long period of Neoproterozoic to Middle Cambrian erosion removed much of the foreland basin sedimentary sequences prior to Paleozoic deposition. Erosion in the Ohio region did not remove the large volume of rock as in Canada north of Georgian Bay. Other seismic lines in the region suggest that Grenville-age sedimentary basins are preserved beneath the Phanerozoic from Georgian Bay southward. These new findings demonstrate the importance of using fault- and unconformity-bounded seismic sequences to enhance and clarify the relative timing of Proterozoic events in regions where Paleozoic sedimentary cover exists and core samples are sparse or lacking.

Integration of the NEES T-Rex Vibrator and PASSCAL Texan Recorders for Seismic Profiling of Shallow and Deep Crustal Targets

In September 2004, Stanford University and the University of Texas (UT) at Austin collected crustal reflection and refraction data with the Network for Earthquake Engineering Simulation (NEES) tri-axial (T-Rex) vibrator in two separate surveys: (1) a 40-km “crustal-scale” profile in the Black Rock Desert and Black Rock Range, NV; and (2) a 16-km “high-resolution” basin imaging profile in Surprise Valley, CA (figure 1). Both experiments were completed at the northwestern margin of the Basin and Range Province, embedded in a larger, 300-km wide-angle refraction line recorded by an Incorporated Research Institutions for Seismology (IRIS) PASSCAL transportable array (Lerch et al. 2007). Our broad goals were to test the feasibility of the T-Rex in crustal-scale applications with existing IRIS-PASSCAL equipment and to augment our refraction data with structural information. T-Rex (figure 1, inset A), acquired by NEES-UT Austin in 2002, is a 29,000-kg buggy-mounted vibrator with a tri-axial mass, capable of generating P , SV , and SH waves (for details on T-Rex, see Stokoe et al. 2004). Given its relatively large mass, and hence its high peak force output of 267 kN for frequencies between 12 and 180 Hz, T-Rex is a potentially viable source for single-vibrator crustal profiling compared to COCORP (Consortium for Continental Reflection Profiling) profiling with five vibrators, each producing ≤ 120 kN (Allmendinger et al. 1987). Additionally, T-Rex's ability to produce shear waves may open the door to a variety of exciting research avenues, from Vs velocity modeling of basin sediments for earthquake hazard analysis to the ability to record mid- and lower-crustal shear-wave anisotropy resulting from regional tectonic deformation. The integration of T-Rex into our crustal-scale and high-res profiles revealed its strengths and limitations as well as the logistic and quality-control problems associated with recording continuous vibrator data with PASSCAL's transportable instrumentation. Our attempts …

Thermo-rheological, shear heating model for leucogranite generation, metamorphism, and deformation during the Proterozoic Trans-Hudson orogeny, Black Hills, South Dakota

This paper evaluates thermotectonic models for metamorphism and leucogranite generation during the Proterozoic Trans-Hudson orogeny, as recorded in rocks exposed in the Black Hills, SD. Intrusion of the Harney Peak Granite and associated pegmatites at ∼1715 Ma occurred at the waning stages of regional deformation and staurolite-grade regional metamorphism. Published Consortium for Continental Reflection Profiling (COCORP) results indicate that Proterozoic sedimentary rocks were thrust over the Archean Wyoming province during the Trans-Hudson collision. Isotopic compositions of the Harney Peak Granite suggest that the exposed Proterozoic and Archean metasedimentary rocks in the Black Hills represent source rocks of the granites. Numerical simulations of the regional metamorphism and Harney Peak Granite generation, assuming crustal thickening by thrusting coupled with erosion, show the following: (1) Doubling of the crust with normal distribution of radioactive elements does not yield sufficiently high temperatures to cause anatexis anywhere in the crust or growth of garnet in the now exposed part of the crust; (2) a 35-km drop-off length for internal heat production can yield sufficient temperature for garnet growth at the current erosion level; it is, however, insufficient to produce staurolite, and melting can occur only in the deepest parts of the crust; (3) temperatures in crust with stable 70 km thickness for ∼40 Ma and 35 km drop-off length for heat production could become sufficient to produce staurolite at the current erosion level, and subsequent rapid denudation of the crust could potentially trigger decompression-melting of lower crustal rocks. Although this model could potentially explain the observed temporal relationship between regional metamorphism and leucogranite generation, it is inconsistent with melting of upper crustal Proterozoic source rocks that is indicated by isotopic compositions of the granites, with lack of evidence for rapid denudation of the Trans-Hudson orogen, and with confinement of the leucogranites to the deformed Proterozoic metapelitic rocks. Production of the Harney Peak Granite and its relationship to regional metamorphism of the country rocks are best explained by shear heating at the interface between the Wyoming province and overthrusted sedimentary rocks. We suggest that with reasonable rheologic properties of metapelites and rates of plate convergence, shear heating sufficiently perturbs locally the geotherms to cause anatexis in a deep shear zone system and growth of staurolite in the overlying crust. Modeling rheology of the lithologically stratified thickened crust, with granitic basement and metapelitic upper plate shows that the currently exposed part of the crust and the granite source region were ductile through much of the orogeny, which explains regional folding of the schists and predicts ductile shear zones in the granite source region. Because of the lithologic stratification, the granitic basement is likely to become significantly weaker during crustal thickening than the upper crust dominated by schists. A weak basement under a folded upper crust is likely to contribute to the observed relatively flat topography of high plateaus over thickened orogens.

Abstract: Interpretation of Precambrian Geology along the Structurally Restored Ohio Consortium for Continental Reflection Profiling (COCORP) Seismic Lines 

ABSTRACT A structural restoration of the original 1987 Ohio COCORP seismic lines, reprocessed to 3 seconds two-way time, has clarified the configuration and chronological development of the Fort Wayne rift (an arm of the East Continent Rift Basin) and a series of sediment-filled Precambrian foreland basins east of the previously accepted Grenville Front. Grenville deformation post-dated development of the East Continent Rift Basin and associated Precambrian Middle Run Formation sedimentation. Grenville foreland basins developed in an east-to-west sequence during progressive westward thrusting. The foreland basins were filled with sediment derived from the Grenvilleage mountains to the east. Early-formed Grenville basins were then partitioned and partially overthrusted by advancing Grenville thrust sheets. East Continent Rift Basin rift assemblages were also folded and transported westward. Grenville foreland basins are defined by (1) marked impedance contrasts of seismic reflectors between essentially horizontal basin fill and underlying Grenville metamorphic rocks, and (2) abnormally low interval velocities in basin strata. Structural restoration of fault displacements at the basin-fill/Grenville metamorphic rock unconformity reveals a geometry that supports an interpretation of three large foreland basins that were later progressively partitioned as Grenville thrusting advanced from east to west. Total shortening at the top of the Grenville sequence is about 13 percent across Ohio from the West Virginia border to the western boundary of the Grenville tectonic zone in west-central Ohio. Horizontal shortening appears to culminate within the tectonic zone at about 16.5 percent. This new interpretation presents the possibility of a previously unknown regional exploration play in low-velocity foreland-basin-fill sediments and reaffirms the effects that Precambrian structures have had on Paleozoic geology. The fundamental architecture and extent of the Central Ohio Platform and the Appalachian Basin date from the time of East Continent Rift Basin development through Grenville deformation, although shallow well control in Ohio defines the Appalachian Basin only to late Ordovician time. Well control confirms (1) the absence of the Mount Simon Sandstone on basement horst blocks, and (2) presence of anomalously thick Mount Simon and older sediments in grabens, which appear to be related to Precambrian structures. Younger structures and faults of Late Ordovician to Mississippian age, are also related to reactivated Precambrian structures, and define a series of terraces, bounded by monoclines, which step eastward down into the early Appalachian Basin. These faults and structures, and, in part, controlled facies distribution and subsequent hydrocarbon accumulation during the Paleozoic. Drilling has yet to confirm or refute the presence of the Grenville foreland basins. The authors thank Tom McGovern of Lauren Geophysical, Denver, CO, for providing industry-processed COCORP data.

Seismotectonic structures along the Savannah River Corridor, South Carolina, U.S.A.

The Savannah River Corridor is defined herein to extend from the Brevard zone in South Carolina to just northwest of Charleston, South Carolina. The length of the Corridor is 400 km, the width 300 km, and the strike is S 40°E. The Corridor includes (1) the Savannah River Site, which is the locus of the most detailed seismic reflection data set in the eastern United States, and is the keystone for the extrapolation of regional reflection seismic data away from the Site, and (2) the South Carolina–Georgia seismic zone. The principal data used for the conclusions of this study are reflection seismic data acquired by the Consortium for Continental Reflection Profiling (COCORP), the Appalachian Ultra-Deep Core Hole (ADCOH) project, and Conoco, Inc. (SRS). Relatively higher seismicity appears to be associated with a crustal scale antiform that has been imaged in the Savannah River Corridor as well as to the northeast along strike in Virginia. An increase in earthquake activity occurs in that part of the crystalline allochthon that overlies and conceals a foreland-dipping imbricate stack that has developed in early Paleozoic rocks beneath the overthrust Piedmont crystalline allochthon. To the southeast, hypocenters of two SRS earthquakes (1985 and 1988), projected along the strike of the Pen Branch northwest-bounding fault of the Dunbarton Triassic rift basin plot on a highly reflective southeastward-dipping shear zone that underlies the basin. Here, the proximity of the hypocenters to the border fault of the Dunbarton basin suggests that syntectonic structures associated with the Pen Branch border fault might be involved. Some of the seismicity is probably associated with reactivation of older Paleozoic and Mesozoic structures.

Estimation of crustal stochastic parameters from seismic exploration data

Stochastic models for the crystalline crust produce synthetic seismograms that compare well with recorded data for a variety of crustal ages and tectonic environments. In this paper, we explore the parameter space describing such stochastic models as a basis for formulating the inverse problem; that is, we wish to estimate the parameters which define a stochastic model from the recorded backscattered wave field. We base the estimation on approximate relations between the primary reflectivity series, which is the ideal wave field response of a medium, and various seismic gathers. A two‐dimensional lateral correlation method is used to investigate the sensitivity of synthetic wave fields to horizontal characteristic length. A derived empirical relationship relates the scale length to the half width of the correlation coefficient. A horizontal wavenumber‐time domain spectral analysis successfully estimates the horizontal characteristic length (ax) and the fractal dimension (D) of the stochastic medium from which the wave field was backscattered. We employ a Monte Carlo based optimization scheme to fit the observed spectral values to a one‐dimensional von Kármán function and simultaneously estimate values for ax and the Hurst number ν. The method yields consistent results when tested on synthetic data. Application of this technique to data sorted into different domains shows that zero‐offset sections yield the best estimate. We also present results of our estimates using this method on a Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) dynamite source shot gather and a vibroseis‐source Consortium for Continental Reflection Profiling (COCORP) data set recorded in the Basin and Range Province.

Project conducts seismic reflection profiling in the Ural Mountains

Geophysicists from east and west are flocking to the Uralide orogen of Russia to acquire modern, multichannel deep seismic reflection profiles over targets in the Ural Mountains. Seismic reflection profiling promises to help answer geological and geophysical questions about the structure of the mountain belt at crustal and lithospheric scales, the development and evolution of a crustal root, the drift of the continents involved in the collision and tectonic evolution of the orogen, and the recent uplift history.EUROPROBE, a European Science Foundation (ESF) scientific program that is seeking to unravel key aspects of the tectonic evolution of Europe, involving both eastern and western researchers, is leading the effort to acquire profiles. Going a step further, EUROPROBE is collaborating with Consortium for Continental Reflection Profiling (COCORP) scientists in the United States to develop new, integrated geological and geophysical studies of the Urals. Priority is being given to investigating the relationships between the upper and lower crustal structure.

Upper-mantle reflectivity beneath the Williston basin, phase-change Moho, and the origin of intracratonic basins

COCORP (Consortium for Continental Reflection Profiling) deep-reflection profiles crossing the northern Williston basin show (1) dipping reflections within the crust of the underlying Trans-Hudson orogen that extend downward into the upper mantle and (2) no clearly defined reflection Moho. In contrast, Lithoprobe (Canada9s national geoscience project) profiles crossing the Trans-Hudson orogen north of the basin show dipping reflections within the crust that terminate at a sharply defined subhorizontal reflection Moho—a reflection character typical of the interior of collapsed Phanerozoic orogens. We suggest that a crustal root produced during Hudsonian collision was incompletely removed by “normal” postorogenic extension in the region now underlain by the Williston basin. This remnant crustal keel much later underwent eclogite-facies metamorphism, which overprinted a new nonreflective Moho across the root and induced the subsidence of the Williston basin.

Western frontal fault of the Canyon Range: Is it the breakaway zone of the Sevier Desert detachment?

Geologic evidence developed from surface exposures demonstrates that the western frontal fault of the Canyon Range is a major structure representing the eastern breakaway zone of the Sevier Desert detachment. The western frontal fault of the Canyon Range east of Delta, Utah, is a north-trending, west-dipping, low-angle (18 °–24 ° ) normal fault that juxtaposes syntectonic conglomerates and large-rock avalanches of the Miocene Oak City Formation against footwall rocks that have been ductilely, cataclastically, and brittlely deformed. The Oak City strata dip variably to the east; dips are shallow in eastern outcrops (5 ° –15 ° ), steeper in central outcrops (20 ° –60 ° ), and shallow in the western outcrops (10 ° –20 ° ). In eastern exposures, the shallow dips of the Oak City Formation (5 ° –15 ° ) and the acute angle of 20 ° –35 ° between the western frontal fault and Oak City units limit past rotation of the fault to ≤15 ° . The systematic variation in the dip of Oak City units from east to west suggests that the western frontal fault probably flattens beneath present Oak City outcrops, linking the fault to the shallow reflector seen in the COCORP (Consortium for Continental Reflection Profiling) Utah line 1 seismic line beneath the westernmost outcrops of the Oak City Formation. Upper plate movement is constrained by Oak City clast compositions and rock-avalanche lithologies, fault geometry, and lower plate reconstruction to a maximum of 6–7 km since Oak City syntectonic deposition at ∼12–13 Ma. Maximum upper plate displacement for the entire Sevier Desert detachment cannot be determined from these breakaway-zone exposures.

Moho reflectivity and seismic signal penetration

Analysis of amplitude decay and frequency change is applied to determine limits of seismic signal penetration on deep seismic reflection data collected by LITHOPROBE and the Consortium for Continental Reflection Profiling (COCORP). Changes in spectral frequency content are efficiently monitored with averaged instantaneous frequency and bandwidth. Results from Montana show pronounced increases in average frequency and slight drops in amplitude associated with a die-out of reflections at the Moho. Results from Quebec show amplitude and frequency change occurring through the Moho on lines with and without Moho reflections. These contrasting observations confirm both adequate signal penetration and that the differences in Moho reflectivity are representative of geologic differences.

Grenville foreland thrust belt hidden beneath the eastern U.S. midcontinent

Research Article| January 01, 1993 Grenville foreland thrust belt hidden beneath the eastern U.S. midcontinent Ernest C. Hauser Ernest C. Hauser 1Institute for the Study of the Continents, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar Geology (1993) 21 (1): 61–64. https://doi.org/10.1130/0091-7613(1993)021<0061:GFTBHB>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Ernest C. Hauser; Grenville foreland thrust belt hidden beneath the eastern U.S. midcontinent. Geology 1993;; 21 (1): 61–64. doi: https://doi.org/10.1130/0091-7613(1993)021<0061:GFTBHB>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 SocietyGeology Search Advanced Search Abstract Grenville foreland thrust structures are observed beneath the eastern U.S. midcontinent on COCORP (Consortium for Continental Reflection Profiling) line OH-1 and a short seismic line in southwest Ohio. These structures represent the first evidence for a significant Grenville foreland thrust belt preserved in eastern North America. On the COCORP lines, the structures include a thrust ramp that truncates layered rocks of the footwall as well as a complex hanging-wall ramp anticline and an associated asymmetric syncline. The Grenville front tectonic zone appears to truncate these foreland structures, indicating a later, second phase, expressed as a deeply penetrating, out-of-sequence thrust zone associated with the main uplift of the Grenville province on the east. A short, shallow seismic line in southwestern Ohio reveals an east-dipping sequence of prominently layered rocks that may lie above a footwall ramp to a deeper Grenville thrust fault. A drill hole into the less reflective top of this dipping sequence encountered unmetamorphosed sedimentary rocks like those increasingly reported from other drill holes in southwestern Ohio and adjacent states. Although possibly part of a late Precambrian (Keweenawan?) rift, these clastic sedimentary rocks may instead preserve evidence of a heretofore unrecognized Grenville foreland basin in eastern North America. Alternatively, these Precambrian sedimentary rocks together with an underlying, but yet undrilled, strongly layered sequence may correlate with similarly layered rocks observed on COCORP and industrial seismic lines within the Middle Proterozoic granite-rhyolite province to the west in Indiana and Illinois and indicate that unmetamorphosed sedimentary material is an important constituent of the granite-rhyolite province. The presence of a Grenville foreland thrust belt beneath the eastern U.S. midcontinent, in contrast with its absence in Canada, may suggest a deeper level of erosion of the Grenville province and its foreland in Canada or reflect important lateral differences in the geology of the foreland. 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.

Trans-Hudson orogen and Williston basin in Montana and North Dakota: New COCORP deep-profiling results

COCORP (Consortium for Continental Reflection Profiling) deep reflection profiling across the Williston basin and underlying Trans-Hudson orogen shows the following. (1)There is no evidence for a precursor rift basin beneath the axis of the Williston basin. With the exception of small-scale structures (e.g., Nesson and Cedar Creek anticlines), the basement surface beneath the Williston basin is smooth and has a dish-shaped geometry. (2) Crustal penetrating reflections within the eastern Trans-Hudson orogen dip eastward, in apparent contradiction to the conventional that the Superior province underthrusts the eastern side of the orogen. (3) Basement reflections beneath the center of the orogen define a broad crustal-scale antiform, the axis of which coincides approximately with the depocenter of the overlying Williston basin. Midcrustal reflections defining the crest and western flank of this antiform also coincide generally with the location and shape of the source of the North American Central Plains conductivity anomaly. (4) Crustal penetrating reflections beneath the western side of the orogen dip westward, supporting the current view that subduction polarity was to the west beneath the Wyoming province. (5) The eastern edge of the Wyoming province is marked by east-dipping upper-crustal reflections and subhorizontal lower-crustal reflections that are truncated by the west-dipping reflectivity of the Trans-Hudson orogen.

Aspects of the crustal structure of the western Mojave Desert, California, from seismic reflection and gravity data

Seismic and gravity data taken along line 1 of the 1982 Consortium for Continental Reflection Profiling (COCORP) Mojave Desert Survey (N‐S profile, ∼30 km long) have been used to characterize the upper crust north of the San Andreas fault in the western Mojave block of southern California. Consortium for Continental Reflection Profiling seismic reflection data were reprocessed to emphasize the upper 5 seconds (two‐way travel time). The resultant common depth point (CDP) sections provided starting models for generating a refined geologic cross‐section using a combination of ray tracing (forward modeling) and gravity interpretation. The forward modeling was used to validate the existence of faults and constrain their dips. The gravity data were used to refine the overall model, particularly in poor data areas on the CDP sections. Gravity data, taken along three nearby profiles parallel to primary line of section, were also used to determine the structural trend. Results from the first two seconds indicate the presence of a series of ENE striking reverse faults beneath the late Tertiary and Quaternary sedimentary cover of the western Mojave. The faults dip northward and offset the sediment‐basement interface. The largest such feature has an apparent throw of ∼1.8 km and exhibits a subtle scarp at the Earth's surface suggesting Holocene displacement. The orientation of these faults, although not an instantaneous representation of the present‐day stress field, is consistent with NNW compression across the western Mojave block and WNW striking San Andreas fault, as determined from nearby focal mechanisms and in situ stress measurements. The faults also appear to be closing small sedimentary basins in the Mojave block, which may have formed during an earlier extensional phase, similar to what is happening on a much larger scale in the Los Angeles basin to the south of the San Andreas fault. Reflections between 2 and 5 s, coupled with the local geology and gravity modeling, are consistent with the presence of the Pelona/Rand schist in the subsurface beneath the western Mojave. The upper surface of the schist (i.e., Vincent/Rand thrust equivalent) rises southward toward the San Andreas fault where it is displaced vertically (up to the south) at least 5 km along the E‐W trending Hitchbrook fault, such that the schist crops out between the Hitchbrook and subparallel San Andreas to the south. The same structure may exist beneath the Tehachapi mountains, with the roles of the Hitchbrook and San Andreas faults played by the north and south branches of the Garlock fault, respectively. The rising or arching of the basement toward the San Andreas fault (and toward the Garlock) is not only reflected in the geology and topography local to these faults in many places but is also generally observed on seismic reflection profiles in the vicinity of these faults in the western Mojave. Furthermore, the arching is also consistent with a strong component of fault normal compression.

A study of two mid-crustal bright spots from southeast Georgia (USA)

SUMMARY Two high-amplitude reflections, or ‘bright spots’, occur on COCORP (Consortium for Continental Reflection Profiling) line Georgia 16 at a record time of 5.9s, corresponding to nearly 16 km depth. They are significant because bright spots often mark great contrasts in subsurface physical properties. The first is the ‘Surrency Bright Spot’ (SBS), an exceptionally bright antiformal reflection, and the second is the ‘Reedy Creek Reflection’ (RCR), which lies 5 km to the south and resembles the south half of a diffraction. The RCR is also imaged on a crossline, Georgia 18, as a relatively symmetric antiformal reflection. These reflections coincide with the Brunswick magnetic anomaly, thought to mark the late Palaeozoic Alleghanian suture. The SBS and the RCR are reflections from bodies that lie within the plane of Georgia 16. Migration and 2-D seismic modelling show the aniformal SBS is from a synformal reflector with a buried focus while the RCR is from a relatively small body. The inferred geometry of the SBS reflector, with its flat portion corresponding to the deepest part of the reflector, argues that it does not represent fluids. These results, combined with recent evidence that the SBS is a reflection from a high-velocity thin layer, suggest that the SBS and the RCR might be reflections from ultramafic slices emplaced within the Alleghanian suture zone during collision or from mafic sheets intruded into the suture zone during subsequent rifting. The brightness of the SBS is attributed to a large impedance contrast, constructive interference, good data quality, and a relatively large reflector size with respect to the Fresnel zone and neighbouring reflectors.

Constraints on the structure and tectonic development of the Early Mesozoic South Georgia Rift, southeastern United States; Seismic reflection data processing and interpretation

Seismic reflection data collected by the Consortium for Continental Reflection Profiling (COCORP) over the buried Triassic‐Jurassic South Georgia rift, reprocessed in order to improve the upper 5 s of reflection time, provide a window into the regional deep structure of the rift and delineate variations in basin structure and depositional styles. Located at the southernmost extent of the North American east coast early Mesozoic rift system, the structure of the South Georgia rift basin has previously been known only from scattered drilling and potential field interpretation, leaving the internal and boundary, as well as the deep, structure unknown. The reflection data reveal that the South Georgia basin consists of a broad, complex terrane of isolated synrift grabens with intervening structural highs, in contrast with earlier concepts of a single narrow trough or an amorphous “sag” basin. Two main subbasin depocenters are observed in west and west central Georgia, the largest of which is more than 100 km in width, and the deepest at least 7 km at its inferred base. Analysis of the reflection data indicates that the distinct structural and seismic‐stratigraphic character of the reflection packages is analogous to other, better studied extensional provinces (e.g., North American Basin and Range). The reprocessing of the reflection data provides a view of contrasting styles of faulting and subsidence within the rift. The structural style of the subbasins varies drastically along strike, changing from (1) a narrow asymmetric graben bordered by a planar and more deeply penetrating normal fault to (2) a broad and roughly symmetric graben bordered by a shallower‐dipping, possibly listric fault, along which extension may have been accommodated at depth by a low‐angle detachment or by a more complex system of high‐angle faults. Only limited crustal extension is suggested by the narrower graben and planar fault (1), whereas much greater extension would be expected from the wider graben bordered by a listric/low‐angle fault (2). Such a dual style of basin formation is predicted by segmented plate models of variable extension along continental margins and intracratonic rifts, and may imply a significant change in crustal rheology along the developing rift. Integration of the interpretation of the regional structure of the South Georgia basin with that of the deeper crustal structure, as inferred from the 16‐s (reflection time) COCORP data, indicates that the synrift basins developed mainly over the “upper plate” of the Alleghanian (late Paleozoic) suture between North America and west Africa. The major subbasin border faults dip northward in an antithetic relation to the predominantly northward vergence of the suture zone. The border faults thus do not appear to have reactivated antecedant structure. Although the development of the South Georgia rift appears to have approximately coincided with early Mesozoic thinning of the crust in the southeastern United States, no horizontally “layered” or “laminated” lower crust is observed as has been associated with other, more fully rifted terranes (e.g., North American Basin and Range; continental shelf around British Isles). However, the Moho discontinuity appears to have formed as a dynamic feature superimposed on late Paleozoic compressional structure probably associated with early Mesozoic rifting. These findings thus support the interpretation that the South Georgia rift developed as a major early Mesozoic depocenter and possible incipient spreading center associated with the initial formation of the central Atlantic Ocean.