South-vergent Folds Research Articles

Evolution of antivergent folds on a Paleozoic accretionary prism, Arkansas: An alternative view

Rocks around the western plunge of the Benton uplift in the Ouachita Mountains of western Arkansas show multiple periods of deformation during the Ouachita orogeny. Seismic-reflection interpretations and surface geology are consistent with a thick section of highly deformed Paleozoic rocks that are separated as thrust sheets by north-vergent regional-scale thrust faults. North-vergent folds develop in such a setting; however, south-vergent folds with the axial planes dipping opposite to the direction of underthrusting are also observed on the Benton uplift. Development of such folds has been explained by models such as mechanical decoupling along zones of low shear strength in trenches, backthrusting, and backfolding, but none explains the south-vergent folds of the Benton uplift, mostly because of lack of adequate field data. Geometrical analyses show that reactivation of thrust faults during a secondary phase of deformation tightened and reoriented open folds of an initial phase and, as a result, developed the macroscopic and mesoscopic antivergent folds in the Benton uplift. Curvilinear map traces of the thrust faults and broad open folds that refold earlier structures indicate that there was continuous deformation after the development of antivergent folds.

Petrologic and age constraints on the origin of a low‐pressure/high‐temperature metamorphic complex, southern Alaska

The interrelationships between metamorphism, deformation, magma intrusion, and40Ar/39Ar geochronology were determined for a low‐pressure/high‐temperature metamorphic complex which formed from an accretionary prism in the Chugach Mountains, southern Alaska. Compressional deformation, which first produced south verging folds and associated thrusts, was followed by magma intrusion and development of north verging folds. Synmetamorphic southward directed thrusting of metamorphosed flysch over flysch produced increased load in the footwall, as documented by the distribution of mineral assemblages and by pressure‐temperature modeling of garnet growth. The initial heating to the greenschist facies may have been accomplished by a combination of advective heating from aqueous fluids and of conductive heating from subducted young oceanic crust. Regionally developed amphibolite facies metamorphism followed intrusion of felsic sills. The peak metamorphic conditions derived from geothermobarometry, mineral assemblages, and fluid inclusions ranged from 400° to 600°C at a depth of ∼10 km. The increased heat from associated synmetamorphic concordant felsic sills raised the ambient temperatures to produce a regional distribution of andalusite and cordierite with a core zone of sillimanite‐bearing migmatites. Subsequent cooling was initially rapid (≈55°C/Ma) to ∼350°C based on40Ar/39Ar dates of 53 Ma for hornblende and 50 Ma for biotite and may have slowed to ∼11°C/Ma until 200°C based on an40Ar/39Ar date of 35 Ma for plagioclase. Intrusions of felsic sills and at least one pluton were along the initially north dipping foliations associated with south verging folds. This suggests the source region for the melts may have been downdip in the subduction zone rather than from directly below within the accretionary prism.

Timing of structural events in the Himalayan foothills of northwestern Pakistan

Research Article| August 01, 1987 Timing of structural events in the Himalayan foothills of northwestern Pakistan ROBERT S. YEATS; ROBERT S. YEATS 1Department of Geology, Oregon State University, Corvallis, Oregon 97331-5506 Search for other works by this author on: GSW Google Scholar AHMAD HUSSAIN AHMAD HUSSAIN 2Geological Survey of Pakistan, Peshawar, Pakistan Search for other works by this author on: GSW Google Scholar Author and Article Information ROBERT S. YEATS 1Department of Geology, Oregon State University, Corvallis, Oregon 97331-5506 AHMAD HUSSAIN 2Geological Survey of Pakistan, Peshawar, Pakistan Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1987) 99 (2): 161–176. https://doi.org/10.1130/0016-7606(1987)99<161:TOSEIT>2.0.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 ROBERT S. YEATS, AHMAD HUSSAIN; Timing of structural events in the Himalayan foothills of northwestern Pakistan. GSA Bulletin 1987;; 99 (2): 161–176. doi: https://doi.org/10.1130/0016-7606(1987)99<161:TOSEIT>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 SocietyGSA Bulletin Search Advanced Search Abstract The Attock-Cherat Range forms the southern boundary of the Peshawar basin and includes rocks transitional between metasediments of the Lesser Himalaya and foreland-basin strata to the south. The Attock-Cherat Range comprises three fault-bounded structural blocks which are, from north to south, (1) Precambrian metaclastic strata overlain by unfossiliferous limestone which is itself apparently overlain by Paleozoic strata with the contact not exposed; (2) unfossiliferous flysch of Precambrian(?) age overlain by Cretaceous and Paleogene marine strata and Murree red beds at least in part of early Miocene age; and (3) unfossiliferous limestone, argillite, and quartzite correlated in part to Paleozoic strata in the Peshawar basin, overlain by a Tertiary sequence generally similar to that in block 2. Farther south, in the Kala Chitta Range, strata of Triassic to Eocene age occur in south-verging folds and thin thrust sheets. The similarity of the Tertiary sequences in the Kala Chitta Range and in blocks 2 and 3 demonstrates that the pre-Tertiary sequences were juxtaposed by faults prior to deposition of the Paleocene Lockhart Limestone. This may coincide with initial contact of the west-northwest–facing passive margin of India with Eurasia or nearby microplates. Major late Tertiary imbricate thrusting and folding took place prior to uplift of the Attock-Cherat Range and to deposition of Peshawar intermontane basin fill of Pliocene-Pleistocene age. The Peshawar basin formed as the Kala Chitta Range was faulted south on the Main Boundary thrust (MBT), forcing Siwalik foreland basins still farther south. Late Quaternary deformation in the southern Peshawar basin occurred along a seismically active zone of en echelon, stepped-left faulted pressure ridges that may reflect a subsurface ramp on the older MBT. 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.

Stratigraphy and structure of metamorphosed upper Paleozoic rocks near Mountain City, Nevada

Upper Paleozoic rocks near Mountain City, Nevada, consist of a marine sequence of Late Mississippian age unconformably overlying deformed Ordovician rocks of the Roberts Mountains allochthon. This is the oldest and thickest Mississippian Antler “overlap sequence” in Nevada. At its base, clastic rocks of the Grossman Formation are overlain by a fining-upward sequence of the Meramecian Banner Formation. Limestone at the top of the Banner Formation is overlain by basaltic lava, tuff, and minor limestone of the Nelson Formation. The Nelson Formation is, in turn, overlain by the Chesterian Chainman Shale. This Upper Mississippian sequence records subsidence and rifting or strike-slip tectonics that affected the continental margin of western North America soon after thrusting associated with the Late Devonian–Early Mississippian Antler orogeny. The occurrence of the Chainman Shale in the upper part of this Antler overlap sequence both near and to the east of Mountain City seems to record a westward migration of the Antler foredeep basin across a foundered portion of the Antler orogenic belt. The occurrence of submarine basalt flows of Meramecian age in the Nelson Formation, in correlative rocks to the west, and in exposures of the Golconda allochthon south of Mountain City suggests that Late Mississippian basaltic volcanism was widespread, affecting the continental margin of North America as well as the offshore Golconda basin. Block faulting and uplift during Pennsylvanian-Permian time caused disruption and erosion of Upper Mississippian sequences deposited elsewhere on the Antler orogenic belt. A metamorphosed, folded, and thrust-faulted assemblage of chert-argillite, basalt, gabbro, siliciclastic and limestone turbidites, fine-grained dolomitic quartz arenite, and minor siliceous tuff of late Paleozoic(?) age rests in thrust contact above the Mississippian overlap sequence. These basinal rocks, referred to as the “Reservation Hill sequence” (RHS), were probably deposited in deep water on the continental slope and rise of an ocean basin receiving chert-rich quartzose and carbonate detritus from North America. The RHS is lithologically similar to Mississippian-Permian rocks of the Golconda allochthon. Prior to its southward thrust emplacement during the Mesozoic, the RHS was probably deposited on oceanic or transitional crust in an east-west–trending re-entrant or embayment of the late Paleozoic continental margin in the area of the present Snake River Plain. During the late Mesozoic, rocks in the vicinity of Mountain City were regionally metamorphosed, folded, and cut by south-verging imbricate thrust faults. This episode of compressive deformation occurred prior to intrusion of a quartz diorite stock, which has a minimum age (K/Ar hornblende) of 134 Ma. Other similar dioritic stocks in northern Elko County that have K/Ar ages on biotite of ∼153–155 Ma also post-date thrusting and metamorphism of rocks as young as Triassic, suggesting that the age of deformation is post-Triassic and pre– (or syn–) Late Jurassic. Mesozoic folds and thrusts near Mountain City represent considerable north-south shortening and form part of a belt of south-verging folds and thrust faults trending east-west across northern Elko County. This belt may be an eastward deflection or splay of the Jurassic and Cretaceous Winnemucca, Luning, and Fencemaker fold and thrust belts recognized farther to the south and is subparallel to other major Cordilleran lithotectonic belts in northeastern Nevada, including the edge of the Precambrian sialic crust. The wholesale eastward deflection of Cordilleran trends in northeastern Nevada may reflect either the presence of a long-standing indentation in the continental margin of western North America or an oroclinal bend of post-Triassic and, possibly, post–Late Cretaceous age.

Tectonic relationships between forearc-basin strata and the accretionary complex at Bath, Barbados

The island of Barbados exposes the crestal zone of the accretionary prism of the Lesser Antilles forearc. Tertiary rocks at Bath, Barbados, include four lithic suites: (1) inter-bedded volcanogenic and mainly calcareous pelagic rocks (the Oceanic beds), (2) radiolarite, (3) siliceous hemipelagic rocks, and (4) melange. Suites 1–3 are in structurally discrete groups of fault packets; melange occurs in apparently diapiric crosscutting bodies. The Oceanic beds occupy a pre-Pleistocene nappe complex that underlies much of Barbados. Radiolarite and hemipelagic rocks belong to a thick accretionary basal complex and constitute the relative autochthon to the Oceanic nappes. Fault rocks of the sub-Oceanic fault zone developed during emplacement of the Oceanic nappes. The lower portion of the tectonic stack is deformed in a major south-verging fold couple with wavelength of ∼250 m. Lower, early emplaced Oceanic nappes are restricted to the core of the major synform and are increasingly deformed with depth. Structures suggest easterly transport of Oceanic nappes and upward younging of nappe emplacement, indicating that the deformation of Oceanic beds propagated westward. The Oceanic nappes provided a seal to rising fluids; transport of nappes may have been facilitated by elevated fluid pressure beneath their sole thrust. Pre- and post-nappe folds record north-south shortening about shallow, west-plunging axes, consistent with folded accretionary strata elsewhere on Barbados. Deformation occurred under shallow nonmetamorphic conditions. The Oceanic beds are interpreted as being trapped outer forearc-basin strata that have been tectonically shouldered by the arcward migrating prism flank or crest. The hemipelagic and radiolarite suites may be offscraped abyssal-plain and trench-wedge accumulations. Paleogene-lower Miocene Oceanic strata were deposited at depths between 2.0 and 4.8 km. Middle Miocene Oceanic beds accumulated at shallower depths (1–1.5 km); thus, early Miocene was the probable time of inception of major uplift and arcward shouldering in the crestal zone of the accretionary prism. Accretion of the basal complex of Barbados probably occurred in late Eocene time.