Mesozoic Thrust Research Articles

Crustal evolution of southeastern China: Nd and Sr isotopic evidence

We present a synthesis of crustal evolution in SE China based on extensive Nd and Sr isotopic data compiled from the literature for intrusive granitoids, volcanic, sedimentary and metamorphic rocks from three major tectonic units of SE China: Dabie, Yangtze and Cathaysia. Overall, igneous rocks of Phanerozoic ages possess initial ε Nd(T) values from −21 to +5 and depleted-mantle model ages ( T DM) from 2.3 to 0.5 Ga. Sedimentary and metamorphic rocks have a T DM range of 3.3 to 1.1 Ga. The model age data indicate that the most important crustal formation took place in the Proterozoic, possibly with a very minor proportion produced in the late Archaean. This finding clearly suggests that the crustal evolution in SE China, including the Yangtze craton and Cathaysia, is distinctly different from the Archaean-dominated North China Block. The Dabie terrane, due to its lack of clear indication of Archaean crustal signal, might belong more logically to the Yangtze Block. The large database shows an oceanward younging of T DM for Phanerozoic igneous rocks in SE China, with the oldest ages found in Cathaysia Interior and the youngest in coastal Fujian and Taiwan. ε Nd(T) values show parallel systematic variations. Two NE-striking low- T DM zones characterized by high-REE granites in the middle of SE China are identified, but their tectonic significance is not yet clear. Within SE China, timing of the Yangtze-Cathaysia collision (Proterozoic vs. Mesozoic) has been controversial for many years. While the Mesozoic thrust model may be supported by a recent 40Ar/ 39Ar age study that reveals Triassic to Early Jurassic ages for mylonitic rocks from the same fracture zone, a Proterozoic collision model seems to be favoured because (1) abundant Neoproterozoic ages have been obtained recently for a variety of rocks occurring along the Jiangshan-Shaoxing fracture zone and for ophiolite suites of the Banxi Group, and (2) many lines of evidence have been found to argue against the Banxi Group as a melange complex and a long displaced thrust sheet. This issue is not yet resolved by geochronological means, but the present synthesis of Nd isotopic data and model ages and recent palaeomagnetic studies of Lower to Middle Triassic rocks appear to refute the hypothesis of Mesozoic collision.

Structural Geology and Tectonic Implications of a Part of the Northern Stillwater Range, Nevada: ABSTRACT

The east flank of the Stillwater Range adjacent to the Dixie Valley geothermal area near Fallon, Nevada, hosts one of the best exposures of Mesozoic thrust faults in the Basin and Range province. The rangefront comprises four imbricate lithologic packages. The Triassic Star Peak Group sits structurally lowest beneath Triassic phyllite of the Fencemaker-B allochthon. Bedded quartz arenite of the Jurassic Boyer Ranch Formation lies above the phyllite along the Boyer thrust. Rocks of the Humboldt Igneous Complex sit structurally highest in brittle fault contact with both the arenite and phyllite. The Fenoemaker thrust is a major Jurassic structure in west-central Nevada which places Triassic basinal strata northeastward over shelf carbonates of the Star Peak Group, which depositionally overlie the Golconda allochthon. Locally, the Fencemaker thrust lies within a high strain zone characterized by mylonitic marble, phlogopite-bearing calcareous argillite schist, boudinaged siliciclastics, and phyllonite. Consistently southeast-dipping penetrative foliations and down-dip stretching lineations in these Triassic metasedimentary rocks are, however, inconsistent with northeast directed thrusting. This suggests that northwest vergent thrusting also occurred here, possibly along the Willow Creek thrust. In contrast to the Fencemaker thrust, the Boyer thrust is characterized by close folds in the hanging wall, a narrow zonemore » of fault gouge, and crenulation of footwall foliations, indicating a less ductile regime. East dipping Tertiary extensional faults expose these thrusts along the Stillwater rangefront and displace the thrust sheets downdip beneath Dixie Valley. The geometry of these thrust sheets in the subsurface is critical to the production of geothermal wells in the area.« less

Numerical simulation of the detachment dynamics in North China Basin

Refering to geological evidences and recent Lincheng-Julu deep seismic reflection profile in the west-central part of North China Basin, it is concluded preliminarily that a low angle detachment structure may exist in the central part of North China depression. Numerical method is used to simulate the influence of hot mantle intrusive bodies to Cenozoic basin tectonic movements. Numerical simulations show that, (1) The intrusion of hot mantle material has led to an extensional stress state in the upper crust in central North China depression. As time increasing, the extensional stress state changed slightly in the upper crust and was in keeping with the normal faulting tectonics in the upper crust in depression area. (2) In Cenozoic era, under the effects of magmatic intrusion and the resistance of Taihang Mountain, the weak zone produced by the Mesozoic thrust faulting would become a detachment structure. (3) With the elapse of time, the horizontal compressive stress gradually concentrated in the median crust, and the concentration of stress may generate strike-slip earthquakes in the median crust above the intrusive body.

>Mesozoic and Cenozoic extension recorded by metamorphic rocks in the Funeral Mountains, California

The Funeral Mountains metamorphic core complex, like so many of these complexes in the North American Cordillera, contains early ductile structures related to Mesozoic compressional tectonics that have been overprinted by structures related to later extension. The core-bounding detachment system formed in late Miocene time, and ductile extensional fabrics in its footwall have been viewed as evidence of a higher-temperature stage in the same event. Integrated structural analysis and geochronology, however, indicate that ductile fabrics with a transport direction similar to the northwest direction of movement on the detachment are part of an older extensional event of Late Cretaceous age. This event produced mylonitic fabrics, a dominant foliation, and well-developed mineral and stretching lineations. Mesoscopic folds and a widespread crenulation lineation that developed during a late stage in this event also are consistent with northwest-directed shear. Late Cretaceous mylonitic fabrics are related to extensional shear zones within the core complex that have small stratigraphic offsets but may represent significant structural and metamorphic discontinuities. Rocks in the footwall of the shear zones experienced mid-crustal depths that cannot be explained by the overburden available if the Funeral Mountains core rocks represent an intact block of Mesozoic crust. Instead, it appears that a significant section of mid-crustal material may have been excised in Late Cretaceous time by the combined Monarch Canyon and Chloride Cliff shear zones as the result of gravitational instability of an overthickened crust. In our interpretation, this early extension was made possible by a waning of compression within the Mesozoic thrust belt combined with a high regional heat flow due to the Sierran arc. This early extension took place 50 m.y. before development of the core-bounding detachment system, movement on which took place at low enough temperatures that deformation was predominantly brittle with some calc-mylonite fabric development. The youngest structures in the core are normal faults that cut the detachment and indicate a southwest transport direction orthogonal to those of both the earlier extensional events.

Ramp-flat detachment faulting and low-angle normal reactivation of the Tule Springs thrust, southern Nevada

Research Article| August 01, 1993 Ramp-flat detachment faulting and low-angle normal reactivation of the Tule Springs thrust, southern Nevada GARY J. AXEN GARY J. AXEN 1Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Baja California, Mexico Search for other works by this author on: GSW Google Scholar GSA Bulletin (1993) 105 (8): 1076–1090. https://doi.org/10.1130/0016-7606(1993)105<1076:RFDFAL>2.3.CO;2 Article history first online: 01 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 GARY J. AXEN; Ramp-flat detachment faulting and low-angle normal reactivation of the Tule Springs thrust, southern Nevada. GSA Bulletin 1993;; 105 (8): 1076–1090. doi: https://doi.org/10.1130/0016-7606(1993)105<1076:RFDFAL>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 Detailed geological mapping of the southern and central Tule Springs Hills and northern East Mormon Mountains, Nevada, reveals details of the interaction between a Mesozoic thrust system and Tertiary detachment faults. The main structure in the area is the subhorizontal Tule Springs detachment, along which a highly deformed hanging wall of Paleozoic and sparse Tertiary strata overlies a much less deformed footwall of Permian to Jurassic beds. The hanging wall was emplaced in Mesozoic time along a décollement-style thrust with a ramp-flat footwall geometry, then extended in Neogene time synchronous with normal reactivation of the flat and upper part of the ramp. Below that point the detachment diverged from the thrust ramp, cutting more steeply into the thrust footwall, forming a similar ramp-flat detachment geometry. The Middle Member of the Jurassic Kayenta Formation forms the flat, for ∼14 km in the Tertiary transport direction. This part of the detachment dipped 3°-15° toward the transport direction and was shallower than 5 km at the onset of normal slip. Much slip may have occurred in weak subjacent siltstone. The detachment ramp is also now subhorizontal, having been rotated from an initial moderate west dip, probably due to isostatic rebound during tectonic unloading along the ramp. Permian to Jurassic strata beneath the detachment ramp in the southwestern Tule Springs Hills now dip moderately east-northeast, having been tilted from subhorizontal while the detachment ramp was flattened. Those beds now form the west limb of a range-scale asymmetric syncline, and footwall beds below the detachment flat form the east limb. In the northern East Mormon Mountains, the detachment is represented by the Toquop Chaos fault zone, which cuts downsection through lower Paleozoic strata that also probably belong in the west limb of the syncline, beneath the detachment ramp. The basal nonconformity there dips east also, so the syncline deformed the entire stratified sequence and the underlying crystalline basement.A large klippe of the detachment (∼185 km2) is characterized by north- trending ridges and valleys ∼2 km wide. These are bounded by moderately dipping or listric normal faults, many of which demonstrably end at, or merge with, the detachment. The topography of the klippe mimics the linear basins and ranges of the Great Basin at a smaller scale. If the structural style is also analogous, then many Great Basin range-valley pairs may be underlain by detachments at relatively shallow depths, rather than being bounded by deeply penetrating faults.Large north-striking faults that cut the Tule Springs detachment control range- scale topography, are each several kilometers long, and cut all other structures in the map area. They postdate detachment slip. One of them cuts Pliocene(?) calcrete-cemented alluvium that is younger than the detachment, and another has ∼8 km of left-lateral offset. 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.

Seismic reflection profiling across Tertiary extensional structures in the eastern Amargosa Desert, southern Nevada, Basin and Range province

Outcrops, shallow well control, and coincident geophysical surveys are used to interpret a seismic reflection profile in the Amargosa Desert, within the Basin and Range province, of southern Nevada. The east-west-trending, 27-km-long seismic line crosses all or parts of three Tertiary subbasins, revealing that basin growth occurred by progressive shifts of basin-bounding faults. The reflection line images Tertiary strata that is rotated by steeply dipping listric faults and that noses into normal faults. A shallow (less than 100 to 200 m deep), laterally continuous, flat-lying, low-frequency reflector, interpreted as a Tertiary basalt flow, suggests that little vertical deformation has occurred within the easternmost of the small Tertiary basins since the eruption of the flow about 10 million years ago. Moderately dipping reflections within the pre-Tertiary bedrock may image Mesozoic thrust faults. The reflection data indicate that, whereas the top of the reflective lower crust shallows to the west, possibly in the direction of increasing crustal extension, the Moho is relatively flat between 30 and 33 km deep. Apparent bright-spot reflections from the lower crust are interpreted as evidence for ductile shearing of the lower crust, not for active magma chambers. Doming of the lower crust resembles that observed elsewhere in the Basin and Range province and is consistent with ductile flow in the lower crust.

Mesozoic deformation in the Nevada Test Site and vicinity: Implications for the structural framework of the Cordilleran Fold and thrust belt and tertiary extension north of Las Vegas Valley

Detailed studies in the CP Hills and Mine Mountain area of the Nevada Test Site (NTS), together with analysis of published maps and cross sections and a reconnaissance of regional structural relations, indicate that the CP thrust of Barnes and Poole [1968] actually comprises two separate, oppositely verging Mesozoic thrust systems: (1) the west vergent CP thrust, which is well exposed in the CP Hills and at Mine Mountain, and (2) the east vergent Belted Range thrust located northwest of Yucca Flat. Regional structural relations indicate that the CP thrust forms part of a narrow sigmoidal belt of west vergent folding and thrusting traceable for over 180 km along strike. The Belted Range thrust represents earlier Mesozoic deformation that was probably related to the Last Chance thrust system in southeastern California, as suggested by earlier workers. A reconstruction of the pre‐Tertiary geometry of the Cordilleran fold and thrust belt in the region between the NTS and the Las Vegas Range bears a close resemblance to other regions of the Cordillera and suggests that west vergent deformation developed in the hinterland of a part of the Sevier fold and thrust belt characterized by substantial structural relief. Reconstruction of the fold and thrust belt also suggests that previous estimates of upper crustal Tertiary extension north of the Las Vegas Valley shear zone (e.g., 80% [Guth, 1981]) are 2 or 3 times too large.

Lower Jurassic unconformity (J-0) from the Colorado Plateau to the eastern Mojave Desert: Evidence of a major tectonic event at the close of the Triassic

From southern Nevada to the eastern Mojave Desert, the Lower Jurassic basal unconformity (J-0) cuts down section from the Upper Triassic Chinle Formation to deformed Paleozoic carbonate rocks. The Chinle Formation is not present southwest of the central Spring Mountains. Stratigraphic relations suggest correlation of J-0 with unconformities in western Arizona and the Inyo Mountains. Close proximity of J-0 in time and space to (1) first appearance of western -derived volcanic clasts in lower Mesozoic cratonal stratigraphy, (2) early Mesozoic thrust faults, and (3) the J-1 cusp in the apparent polar wander path imply a major change in tectonic setting of the southwestern Cordillera at the Triassic/Jurassic boundary.

Petrologic constraints on the unroofing history of the Funeral Mountain Metamorphic Core Complex, California

The northern Funeral Mountains, southeastern California, comprise a metamorphic core complex that lies within a region of extreme Neogene extension but did not experience a significant late Cenozoic thermal event. This circumstance, unusual among core complexes of the North American Cordillera, permits direct examination of the preextensional thermal evolution of a portion of the Cordilleran hinterland. For pelitic schists from the highest‐grade portions of the Funeral metamorphic core, thermobarometry and thermodynamic modeling of garnet zonation define a P‐T trajectory showing: (1) attainment of “peak” metamorphic conditions at 800–850 K and 800–1000 MPa (30–37 km depths); followed by (2) 400 to 600 MPa of decompression (15–22 km of exhumation) with no substantial change in temperature. Available geochronologic data indicate that peak metamorphism occurred in Early Cretaceous time and that the decompression path developed over the Early to Late Cretaceous interval. The maximum pressures indicated by the petrologic data require substantial Late Jurassic(?)‐Early Cretaceous tectonic burial even though most recognized thrust faults at this latitude that have large stratigraphie throws are assumed or demonstrated to have early Mesozoic ages. We postulate that post‐Early Cretaceous extensional faults may have excised the necessary middle Mesozoic thrust structures. While the majority of extensional structures responsible for this excisement is likely to be of Neogene age, associated with Basin and Range extension, the Cretaceous decompression path described in this paper is similar to the theoretical P‐T paths derived from numerical modeling of extensional unroofing. Given recent evidence for the development of extensional structures in compressional regimes like the Himalayan and Alpine orogens, it seems prudent to search for evidence of Mesozoic extensional structures in future studies of the hinterland of the North American Cordillera.

THE CHAIDAM BASIN (NW CHINA): FORMATION AND HYDROCARBON POTENTIAL

The Chaidam Basin forms one of the deepest sedimentary basins in the world, with over 14kms of sediment deposited since the Oligocene. Basement rocks consist of: (i) pre‐Sinian gneisses; (ii) Sinian and Lower Palaeozoic sediments folded by the Silurian ‐ Devonian orogeny; (iii) Upper Palaeozoic molasse in a half‐graben produced by the collapse of the Silurian mountain belt; (iv) Carboniferous terrestrial to shallow‐marine facies sediments deposited on the margins of Palaeotethys; and (v) Permo‐Triassic sediments deposited in a back‐arc basin N of the Palaeotethyan subduction zone.Triassic back‐are basin development was followed by thermal subsidence during the Jurassic to Eocene, with a low sedimentation rate, modified by flexural subsidence on the margins of the thickened and uplifted S Qilian Mountains. Gentle folds and SW‐propagating thrusts were developed in Jurassic strata. The region was uplifted as a whole during Early Jurassic and Middle Cretaceous times.The main period of subsidence occurred from the Oligocene to the Recent, producing a deep hinterland basin in western and central Chaidam. Uplift along the Kunlun and the Aerjin Fault Zone provided a ready sediment supply. Basin subsidence throughout the Neogene probably reflects lithospheric flexure due to the loading of the Tibetan lithosphere, although the rapid increase of Sedimentation during the latest Tertiary and Quaternary must reflect rapid Recent uplift of the Tibetan Plateau. During the Pliocene and Pleistocene, there was inversion and shortening of the Basin, associated with reworking of the Mesozoic strike‐slip and thrust faults. Thus, the Aerjin Fault Zone was reactivated to control the shortening direction across the Basin. Mesozoic thrusts were reactivated along the NE margin of the Basin during the Pliocene, but by Pleistocene times, WNW‐trending folds developed throughout the western part of the Basin. There was differential uplift; the zone nearest the Aerjin Fault was uplifted most, causing tilting and variations in sediment thickness and facies from NW and SE.There are numerous source rocks formed by lacustrine facies sediments of Jurassic and Neogene age. The burial history and the timing and location of structural and stratigraphic traps has controlled the locations of the major hydrocarbon occurrences in western and NW Chaidam. An evaluation of hydrocarbon prospectivity is given in the light of the tectonic ‐ stratigraphic framework and basin analysis.

Timing and kinematics of deformation in the Cronese Hills, California, and implications for Mesozoic structure of the southwestern Cordillera

A southeast-vergent ductile shear zone is exposed in the Cronese Hills in the central Mojave Desert. This shear zone formed under greenschist facies conditions and placed mylonitic metaplutonic and metavolcanic rocks upon folded Triassic( ) metasedimentary rocks. U-Pb zircon ages of prekinematic and postkinematic plutons constrain the age of deformation to between 169 and 154 Ma (late Middle or early Late Jurassic). The authors believe that the Cronese shear zone forms part of the southern continuation of the east Sierran thrust system. Mesozoic thrust faults may have controlled preservation of Jurassic arc-related rocks and may be responsible for structural stacking of Paleozoic eugeoclinal and miogeoclinal strata in the central Mojave Desert.

Uniqueness of geological correlations: An example from the Death Valley extended terrain

The northern Death Valley-Furnace Creek fault zone is a major northwest-trending fault system in the Death Valley region, but the magnitude of offset and its significance to extensional tectonism in the region are controversial. Stratigraphic data have provided first-order constraints on offset across the fault zone but offer limited resolution. Previous correlations of Mesozoic thrust faults across the fault zone have relied on regional stratigraphic trends to fingerprint structurally similar thrust plates. We show that a northeast-trending Mesozoic backfold within the predominantly east-directed fold and thrust belt was probably continuous from the southern Panamint Range, California, to the Belted Range, Nevada. The relative position of this antithetic structure allows uniquely compatible correlations of three Mesozoic structures between two range blocks across the northern Death Valley-Furnace Creek fault zone independent of stratigraphic arguments. The offset structures have geometric properties of size, vergence, order, and spacing that indicate a probability less than 10 -4 of the Mesozoic orogen identically repeating this structural suite in two unrelated places. Reconstruction of these correlative structures indicates 68 ± 4 km N48°W ± 6°W of apparent dextral offset between the Cottonwood and Funeral Mountains, in general agreement with previous estimates based on stratigraphic observations but of substantially higher precision. This reconstruction, and the restoration of a 30-km separation between the Funeral and Grapevine Mountains indicated by offset segments of the backfold, suggest a simple pre-extensional geometry for the Mesozoic orogen in the Death Valley region consistent with observations from unextended blocks and analogous to that of the Cordilleran orogen at the latitude of Calgary, Alberta.

The Relief Canyon gold deposit, Nevada; a mineralized solution breccia

The Relief Canyon gold deposit in the Humboldt Range of western Nevada is a low-grade, high-tonnage orebody of Tertiary or younger age. The host rocks include limestones of the Triassic Cane Spring Formation, which are overlain by shales of the Triassic Grass Valley Formation. The rocks were folded and metamorphosed to greenschist grade during Jurassic and Cretaceous regional tectonic activity. Mesozoic thrusting may have occurred along the shale-limestone contact, but evidence has been obscured by later hydrothermal activity. The sedimentary rocks were nominally offset along several Late Tertiary normal faults related to uplift of the range.The upper part of the Cane Spring Formation is composed of a poorly sorted breccia composed of limestone clasts with a clay matrix. Irregular pockets within this zone are filled with clay- to pebble-sized fragments derived from the Grass Valley shale. The enclosing limestone beds were tilted moderately to the southwest during Mesozoic deformation, whereas bedding within these pockets is generally horizontal, indicating post-tilting deposition of the sediments. The sediments show graded bedding and other sedimentary features that indicate deposition from flowing water. Thermally mature carbon derived from the limestone is also concentrated in small pockets in the matrix. The breccia unit is likely the product of low-temperature solution brecciation. Ground water dissolved much of the limestone directly beneath the shales, progressively creating irregular cavities and the breccia. Sediments derived from the overlying Grass Valley shale were fiuvially deposited as a matrix to the developing solution breccia.Episodic pulses of hydrothermal fluids were introduced along faults and possibly mixed with the ground water in the breccia zone. Initially, jasperoids formed along the faults, but later hydrothermal pulses introduced gold, silica, and fluorine into both the early jasperoids and the unconsolidated cave-fill sediments to form the orebody. Continued solution-related brecciation chaotically disrupted the gold deposit.Gold, fluorite, pyrite, silver, calcite, and fine-grained silica are the principal hydrothermal minerals in the deposit. Gold was deposited as micron-sized flakes of native gold and rarely as electrum during a relatively late stage of silicification of the jasperoids, the carbon-rich zones, and the clay-rich matrix of the breccia. Fluorite was deposited with and later than the gold in the jasperoids, and it in part replaced the clay-rich breccia matrix. Antimony, arsenic, mercury, and thallium are directly associated with gold in the orebody.The deposit formed at a relatively shallow depth. On the basis of fluid inclusion data, late-stage hydrothermal fluids related to gold and fluorite deposition were extremely dilute and had temperatures near 200 degrees C. The fluid inclusions in fluorite show no evidence for boiling, but porous crackle breccias in the jasperoids suggest that hydrobrecciation took place.

Structural Constraints for Proposed Fort Hancock Low-Level Radioactive Waste Disposal Site (NTP-S34), Southern Hudspeth County, Texas: ABSTRACT

Structural complexities reduce the homogeneity necessary for a site characterization model to an unacceptable level for performance assessment for radioactive waste disposal sites. The proposed site lies between the northern, stable Diablo platform and the southern, mobile Mesozoic Chihuahua tectonic belt. Structural movement along this interface has been active for the past 14,000 years. In addition, the area lies along the northern margin of the Permian Marfa basin and the northeastern margin of the deeply faulted Hueco bolson segment of the late Cenozoic Rio Grande rift system. Recent seismic activity with extensive surface rupture in Quitman Canyon (30 mi southeast of the site) is also documented from the 1931 Valentine, Texas, earthquake (6.4 Richter scale). The site is underlain by either a thrust fault or the complex terminus of a Mesozoic thrust fault. This fault is a segment of the continuous thrust sheet extending from exposures in the Sierra Blanc area, 30 mi east (Devil Ridge fault), to the El Paso area west (Rio Grande fault). This segment of the Devil Ridge-Rio Grande thrust is documented by the Haymond Krupp No. 1 Thaxton wildcat drilled at Campogrande Mountain immediately south of the site. The recent rift fault scarp (Campo Grande)more » immediately south of the Thaxton well has a 17-mi surface trace and is, no doubt, related to the subsurface Clint fault to the west in the El Paso area. An additional complexity is the presence of a monoclinal flexure with a minimum of 900 ft of surface relief (2 mi northeast of NTP-S34). A 4.5-mi, east-west, down-to-the-south normal fault occurs near the top of the monocline with a small associated graben. These complexities seriously compromise the proposed Fort Hancock site.« less

Detailed three-dimensional structure of the deep crust based on COCORP data in the Cordilleran interior, north-central Washington

An unusual opportunity for special processing of selected COCORP data has provided three-dimensional information on deep crustal structure related to Proterozoic rifting, Mesozoic thrusting, and Cenozoic extension in the interior of the United States Cordillera, near the Okanogan Dome, north-central Washington. Using a set of short, intersecting seismic line segments in conjunction with longer regional east-west and north-south seismic lines, we have found dramatic variation vertically in the orientation of structure throughout the crust in this region. A lower crustal block of south-dipping structures lies above a regionally horizontal Moho found at about 34 km depth.This block is ∼13 km thick and is bounded above by a southwest-dipping structure which is discordant to structures both above and below it and is interpreted as a midcrustal thrust formed during Mesozoic to Paleocene convergence. These relationships suggest that the south- dipping lower crustal features were formed prior to, or during, Mesozoic convergence and may be associated either with Proterozoic rifting of the North American continental margin, earlier Proterozoic basement structure, or with a Mesozoic duplex structure formed along a lateral ramp. A zone of relatively few reflections occurs above the mid-crustal thrust and below a highly reflective upper crust and may represent a separate thrust slice in a basement duplex structure. The inferred duplex is cut by east-dipping reflections that project to surface exposures of mylonites and that may be an Eocene(?) normal fault traceable to mid-crustal depths. This duplex structure beneath the Okanogan Dome is analogous to the proposed crustal-scale duplex beneath the Monashee Deecollement exposed in a doubly plunging antiform 150 km to the north in the Shuswap Complex. The three-dimensional analysis, along with the southerly plunge of the antiform, indicates that a thrust at depth just south of the United States-Canadian border may be continuous with the Monashee Deecollement or with the proposed lower thrusts of the Canadian duplex. If so, then by extending the survey north across the international border, a structure deep in the crust may be traceable to the surface.

Kinematics of compressional and extensional ductile shearing deformation in a metamorphic core complex of the northeastern basin and range

Analysis of shear criteria enables the kinematics of two main ductile-shearing events (D1 and D2) to be established in the Raft River, Grouse Creek and Albion ‘metamorphic core complex’. The first event (D1) is a NNE-thrusting and corresponds to Mesozoic shortening. A well developed non-coaxial ductile deformation (D2), of Cenozoic age, is marked by the occurrence of opposing eastward (in Raft River) and westward shear criteria (in Albion-Grouse Creek). These characterize an arch structure where the shear strain increases outwards. In the axial zone of the complex, D2 seems coaxial. Cenozoic extension is considered to be related to gravitational instability induced by mesozoic overthickening of the crust (involving uplift, erosion and abnormal heating). Brittle extension occurs in the upper part of the uplifted domain. It is transformed laterally above undeformed basement towards stretched domains of the middle and lower crust through the ductile shear zones localized at the Precambrian-Paleozoic interface. This extension of the middle and lower crust occurred in the vicinity of the root zones of the former Mesozoic thrusts, which may thus have been reactivated as ductile normal faults during Cenozoic extension.

Reevaluation of Late Mesozoic Thrusting in East-Central Nevada: ABSTRACT

Reevaluation of Late Mesozoic Thrusting in East-Central Nevada: ABSTRACT

Depositional Environments and Hydrocarbon Occurrence of Mississippian Antler Basin, Nevada and Utah: ABSTRACT

New field and paleontological data suggest that much of the Utah/Nevada Mississippian Antler basin siliciclastic sediments shed eastward off the Antler Mountains were deposited in marginal marine to nonmarine environments. Tongues of siliciclastic sediments thicken and coarsen westward from central Utah to central Nevada. Terrestrial plants found in place throughout the basin offer the best evidence of nonmarine deposition. Other depositional indicators include fluvial sequences, coal beds, palynomorph assemblages, bars parallel to depositional strike, and regional depositional patterns. A correct understanding of depositional environments helps explain patterns of Mississippian source and reservoir rock in the basin. The main source rocks of the eastern Great Basin are Mississippian black shales. Three large lobes of higher organic richness in eastern Nevada coincide with three large delta lobes. Generally, source rocks become more coaly and herbaceous westward and more amorphous eastward. Locally, excellent reservoir rocks can be encased by these source rocks. The best reservoir rocks are clean, reworked sandstones associated with river mouth bars and shoreline sandstones. Upland facies are abruptly truncated to the west by a Mesozoic thrust belt. Subthrust Mississippian sediments are probably the main source for Tertiary, fault-trap, upper plate fields such as Grant Canyon, Trap Spring, and Blackburnmore » fields. All 16 million barrels of oil produced in Nevada are produced along the leading edge of the thrust belt. A great possibility exists for many more millions of barrels yet to be found in new, major pools associated with the thrust belt.« less

Cocorp deep seismic reflection traverse of the interior of the North American Cordillera, Washington and Idaho: Implications for orogenic evolution

Deep seismic reflection data collected by the Consortium for Continental Reflection Profiling along a transect of the interior of the northwestern U. S. Cordillera indicate the presence of crustpenetrating faults; these faults developed during an orogenic cycle consisting of Jurassic to Paleocene accretion‐related crustal thickening followed by Eocene crustal thinning. The reflection data also suggest that the Moho is a young, dynamic feature which developed a relatively smooth, flat geometry beneath a complexly deformed crust, probably during an Eocene episode of crustal extension. The profiles reveal prominent west dipping reflection sequences (and associated diffractions) interpreted as a Mesozoic thrust system of crustal dimensions that linked accretion‐related shortening in the western Cordillera with the thrust belt in the eastern Cordillera, east dipping reflections that crosscut the west dipping reflections in the middle and lower crust and are interpreted as ductile deformation zones that accommodated Eocene crustal extension, and essentially flat Moho reflections at 33–35 km beneath complexly deformed crust. These features are consistent with a sequence of (1) late Mesozoic crustal imbrication of transitional crust beneath the easternmost accreted terrane which overrode the North American passive continental margin along a Jurassic thrust (2) propagation of crust‐penetrating thrusts to shallower levels in the east, where they probably formed the sole of the Rocky Mountain Thrust Belt (3) Eocene extension that disrupted these thrusts and led to uplift of core complexes along detachments which fed into broad deformation zones in the lower crust and (4) development of a flat Moho geometry during Cenozoic extension and magmatism. These data provide new information about the structure of the entire crust in the Cordilleran interior and offer a working model, both for evolution of Cordilleran core complexes and possibly for other collisional orogens.

The deep crustal structure of the Mojave Desert, California, from Cocorp seismic reflection data

COCORP seismic reflection profiling in the western and northern Mojave Desert of southern California has revealed the presence of numerous major low‐angle reflecting horizons within the crust. These complex, though laterally continuous, horizons are interpreted to represent major southwesterly dipping crustal fault zones, and as such they place important constraints on the tectonic evolution of the region. The upper‐most horizon is interpreted to be the Rand thrust, which, where exposed, places Precambrian and Late Cretaceous crystalline rocks over possibly younger Pelona‐Rand‐Orocopia Schist. This reflecting horizon extends for 25 km southwest of the Rand Mountains where it appears to be truncated at a depth of about 7.4 km by another horizon, which may be a later low‐angle normal fault. The other reflecting horizons are not traceable to the surface, and so greater ambiguity remains in their interpretation. The most prominent of these horizons occurs at midcrustal depths (15±6 km), exhibits a “ramp and flat” geometry, and extends over the northern area of the Mojave survey into the Basin and Range Province. A lower horizon, at depths of 20–30 km in the northern part of the survey area, is antiformal and appears to terminate above a flat and relatively continuous Moho‐depth horizon. The crustmantle transition appears to be represented by a continuous series of reflections which occur at about 10 s (33 km) in the north of the survey and at about 8–9 s (26–29 km) in the south. These reflections are offset in the vicinity of the town of Mojave. The deep intracrustal fault zones inferred from the COCORP survey may represent (1) the deep crustal continuation of the system of Mesozoic thrusts which crops out in southern Nevada and southeastern California, (2) Late Cretaceous to Early Cenozoic, northeast‐vergent thrusts related to the uplift of the Pelona‐Orocopia‐Rand Schist, or (3) low‐angle normal faults related to Early Miocene, northeast‐southwest directed crustal extension. The COCORP survey also traversed the major strike‐slip faults that bound the Mojave block. The San Andreas fault zone appears to truncate reflectors at depths of 6, 8 and 20 km within the Mojave basement, suggesting that it is a major vertical feature which extends to at least 20 km depth. Conversely, the Garlock fault does not offset an underlying reflecting horizon which occurs at 9 km depth and therefore appears to be a relatively shallow crustal feature.