Vergent Thrust Research Articles

Structural divergence and transpression in the Teslin tectonic zone, southern Yukon Territory

The structural and tectonic evolution of the Teslin tectonic zone, a complex belt of ductilely deformed rocks forming the southern extension of the Yukon‐Tanana terrane in the northern Canadian Cordillera, is elucidated with new field data and by the reinterpretation of existing data. The zone includes greenschist to amphibolite facies metasedimentary and metaplutonic rocks of the Nisutlin and Anvil assemblages characterized by S and L‐S tectonite fabrics. Primary contact relationships and ages show that most rocks in the zone were contiguous by Mississippian time. Mapping identified a number of strain domains which preserve S fabrics dipping northeasterly or southwesterly, variable mineral lineation orientations, and a variety of shear directions including easterly and westerly vergent thrust shear, down‐to‐the‐east and down‐to‐the‐west normal shear, and dextral strike‐slip shear. Regional constraints and microstructures suggest that latest ductile deformation was Late Triassic to Early Jurassic in age. Structural characteristics are explained most effectively by convergent‐dominated transpression in combination with tectonic wedging and associated back thrusting above a low‐angle west dipping detachment. Oblique eastward and upward movement of the rocks over the detachment, which may have coincided with the top of thinned North American continental crust, produced easterly vergent shear, and localized or widespread tectonic wedging and back thrusting produced westerly vergent shear. The transpressive deformation postdates formation of the Nisutlin assemblage by Mississippian time and subduction of the Slide Mountain Ocean beneath the Nisutlin assemblage in Permian to Early Triassic time. The western margin of the Teslin tectonic zone was truncated by Cretaceous strike‐slip faults and translated northward. Rocks of the peraluminous orthogneiss assemblage of the Yukon‐Tanana terrane in central Yukon acted as the tectonic wedge and were overthrust by rocks of the Nisutlin and Anvil assemblages in Early Jurassic time before unroofing by local extension in Cretaceous time.

Uplift of the western border of the Altiplano on a west-vergent thrust system, Northern Chile

A high angle, west-vergent thrust system (WTS) located along the western border of the Altiplano in Northern Chile caused the westward translation of the metamorphic pre-Cambrian basement and Mesozoic rocks over late Tertiary deposits. This WTS, which was essential to the build-up of the western Altiplano, developed principally between 15 and 4.8 Ma. The chronological sequence of faults indicate a back-thrusting. The WTS forms an overstep thrust sequence with vertical throws increasing from West to East and exposes in this direction increasingly older units over younger ones. Taking the paleolake deposits of the Chucal Formation (25-19 Ma) and the ignimbrites of Oxaya Formation (19 Ma) as two regional reference levels, it is possible to calculate a 4,000 ± 200 m uplift associated to the WTS and a 392 ± 20 m/Ma uplift rate. The WTS on the west side of de Altiplano and the east vergent thrust and fold belt on the east border, indicate that this plateau is essentially a compressive asymmetric structure, formed by two thrust “belts” with opposing vergencies.

U‐Pb and 40Ar/39Ar geochronology of the Symvolon granodiorite: Implications for the thermal and structural evolution of the Rhodope metamorphic core complex, northeastern Greece

North Aegean continental lithosphere was thickened by southwest vergent thrusting and continental subduction within the Alpine collisional orogen but has subsequently been greatly extended on a northeast‐southwest axis in the back arc of the Hellenic subduction zone. Crosscutting relationships with two granodiorite bodies emplaced at ∼31–33 Ma, the Xanthi and eastern Vrondou plutons, constrain a pre‐mid‐Oligocene origin of Alpine convergent structures in northeastern Greece. Post‐Alpine thinning of the north Aegean nappe pile began in earliest Miocene time and has been accommodated by a succession of distinct structural systems. The earliest of these, the “Symvolon shear zone”, appears to represent a midcrustal, coaxial rupture of the Falakron marble series, a carbonate platform >5000 m thick that was subducted northeastward beneath high‐grade rocks of the Rhodope metamorphic province in late Alpine time. Zircon and titanite U‐Pb dates and hornblende 40Ar/39Ar dates obtained in this study constrain the intrusion and incipient mylonitization of the Symvolon or “Kavala” granodiorite within the Symvolon shear zone at ∼21–22 Ma. Following its emplacement, the Symvolon body resided at temperatures between 300°C and 500°C for 5–7 m.y., during which coaxial deformation may have continued within a widening Symvolon rupture. The Strymon Valley detachment, a regionally south‐west dipping low‐angle normal fault, succeeded the Symvolon shear zone in middle Miocene time. Southwestward displacement of the Serbo‐Macedonian gneiss complex by as much as 80 km in the hanging wall of this detachment facilitated the unroofing of the Rhodope metamorphic core complex, including the Falakron marble series and several Tertiary plutons, in its footwall. Biotite and K‐feldspar 40Ar/39Ar dates from 11.1 ± 0.2 Ma to 15.5 ± 0.3 Ma yielded by Symvolon granodiorite samples document the cooling of the Rhodope core complex below 150°C–300°C during its southwestward‐progressive exhumation in the footwall of the Strymon Valley detachment.

Control of Stratigraphy on Thrust Vergence and Formation of Triangle Zones Within Thrustbelts: ABSTRACT

Control of Stratigraphy on Thrust Vergence and Formation of Triangle Zones Within Thrustbelts: ABSTRACT

History of the Sevier orogenic wedge in terms of critical taper models, northeast Utah and southwest Wyoming

Problems with applying critical taper models to ancient orogenic wedges are overcome in the Late Cretaceous–late Paleocene Sevier orogenic wedge in Utah and Wyoming by a symptomatic approach in which wedge behavior is inferred from the time-space distribution of thrust faulting, erosion, and synorogenic sediment accumulation in association with the orogenic wedge. In turn, the causes of wedge behavior are interpreted in terms of features that are known about the Sevier wedge, such as the locations of major decollements and the lithologic constituents of major thrust sheets. From Coniacian through late Paleocene time (∼35 m.y.), basement and cover rocks in the Sevier wedge were shortened by ∼100 km in three major and one minor events. An overall eastward progression of thrusting was punctuated by several episodes of out-of-sequence and hinterland vergent thrusting. Over the long term, wedge taper was controlled by internal wedge strength, initial taper, and changes in the durability of the upper surface of the wedge and its internal lithostratigraphy. Critical taper was maintained by the offsetting effects of basement duplexing in the rear of the wedge and forward imbrication at the front of the wedge. While the basal decollement was mainly in basement rocks, the wedge was highly tapered and thrust spacing was relatively close (∼15 km). Beginning ca. 75 Ma, the basal decollement ramped upward and eastward into weak Cambrian shale and Jurassic salt, thrust spacing increased to 25–45 km, and wedge taper decreased drastically. The highly tapered rear part of the wedge carried strong basement rocks above moderately weak mylonitic and cataclastic decollements, whereas the frontal, lower-taper part of the wedge carried sedimentary rocks above extremely weak (salt and shale) decollements. Through time, internal strength of the wedge decreased as it incorporated an increasing proportion of sedimentary rocks; the thrust belt was able to advance eastward in spite of its decreasing internal strength and decreasing taper because the basal decollement of the wedge also became weaker through time, both by strain softening (beneath the rear of the wedge) and by propagating along extremely weak rocks (beneath the front of the wedge). Provenance data from associated synorogenic conglomerates indicate that the durability of the upper surface of the wedge probably increased over the long-term history of the Sevier wedge, as an increasing proportion of the erosional surface became occupied by resistant Precambrian quartzite and basement rocks. This would have had the effect of decreasing the rate of erosion through time and maintaining a steeper surface slope, which would have helped the wedge to remain at critical to supercritical taper. The Sevier wedge also exhibits several shorter-term ( 10 km, and (3) was subjected to regional erosion (marked by major unconformities) that ultimately caught up with tectonic thickening and caused the wedge to stall. This cyclicity is attributed to the need for wedges to become supercritical before they can propagate forward over large distances and a lag-time between wedge thickening (and uplift) and wedge erosion. Erosion eventually catches up to the rate of uplift, the wedge stalls, and the locus of deformation shifts to the rear of the wedge in order to rebuild taper. This analysis suggests that orogenic wedges continually deform and “find a way” to maintain taper sufficient to allow forward propagation as long as a sufficient push from the rear exists. The actual taper value may change substantially through time as lithostratigraphy, internal strength, upper surface durability, and presence/absence of weak potential stress guides conspire to maintain a critical taper.

Intra‐arc opening and closure of a marginal sea: The case of the Guerrero Terrane (SW Mexico)

Abstract The pre‐Oligocene structure of southwest Mexico, south of the trans‐Mexico volcanic axis, is investigated from Taxco (Guerrero state, abbreviation: Gro) to the Pacific coast. Three volcano‐sedimentary units are recognized; from east to west the calc‐alkaline Teloloapan, tholeiitic Arcelia and calc‐alkaline Zihuatanejo suites. Structural and stratigraphic data show that the Teloloapan volcanic arc, active during ?Late Jurassic and early Cretaceous, was built upon continental basement. The Teloloapan lavas are overlain by the Albian–Cenomanian Morelos platform carbonates and followed by the Upper Cretaceous Mexcala flysch. In contrast, the Arcelia pillow lavas are associated with sandstones and cherts of Albian‐?Cenomanian age. The Zihuatanejo arc was also installed upon continental basement and its magmatic activity was in part coeval with Arcelia magmatism. Unlike the almost undeformed Zihuatanejo volcanic rocks, all the other volcanic units are involved in east‐vergent thrusting and recumbent folding associated with ductile tectonics, as well as the Late Cretaceous Mexcala flysch overlying the Morelos platform carbonates. Contrasting with previous views, the present results do not support a major mid‐Cretaceous thrusting event in the study area. The new geodynamic interpretation proposed here considers that the Arcelia rocks were formed in a marginal basin situated east of the Zihuatanejo arc. Closure of this basin in Paleocene times is responsible for the east vergent thrust tectonics in SW Mexico.

Changing mechanical response during continental collision: Active examples from the foreland thrust belts of Pakistan

We have used data from teleseismic, seismic reflection and field geologic studies, along with both geomechanical and gravity modeling to contrast the tectonics of four active orogenic wedges in Pakistan: the Kashmir Himalaya, the Salt Range-Potwar Plateau foldbelt, the Sulaiman Range and the Makran accretionary wedge.In Makran, oceanic crust is still being subducted, and a thick pile of sediments is being accreted and underplated. Undercompaction and excess pore pressures can explain the narrow cross-sectional taper and frontal aseismicity of this wedge. Beneath the Sulaiman wedge, continental crust is just starting to be underthrust. Indirect evidence suggests that fine-grained carbonate rocks found in abundance deep in the stratigraphic section may be deforming ductilely at the base of the Sulaiman wedge and provide a zone of ductile detachment. The collision has proceeded to a much more mature stage in the Salt Range-Potwar Plateau foldbelt and the Kashmir Himalaya. Isostatic response to underthrusting of continental crust has kept the sedimentary pile quite thin in both of these wedges, so in that respect the two foldbelts are similar. However, thick Eocambrian salt beneath the Salt Range and Potwar Plateau permits that foldbelt to be much wider in map view, with a thinner cross-sectional taper and a mixture of thrust vergence directions. A major normal fault in basement causes the Salt Range to rise in front of the mildly deformed molasse basin of the southern Potwar Plateau.Much of the diversity among these mountain belts can be understood in terms of differences in the maturity of the collision process in each area, the resulting thickness of the sedimentary pile encountered at the deformation front, and the presence or absence of large contrasts in strength between the various layers of the stratigraphic section and basement relief.

Structural and tectonic evolution of the Humber Zone, western Newfoundland, 1. Implications of balanced cross sections through the Appalachian structural front, Port Au Port Peninsula

The Appalachian structural front in western Newfoundland, which is principally a submarine feature that comes ashore on Port au Port Peninsula, is interpreted to be a structural triangle zone, or tectonic wedge. Rocks within the triangle zone, which are therefore transported, include the Taconian (Middle Ordovician) Humber Arm Allochthon, Taconian foreland clastic sediments, and the Cambro‐Ordovician platform succession. The transported clastic sediments and platformal rocks, as well as structurally involved Grenville crystalline basement exposed east of the peninsula, compose the Acadian (Siluro‐Devonian) Port au Port Allochthon, which carried the Humber Arm Allochthon as a high structural slice; Acadian transport of tens of kilometers westward is suggested. This interpretation contrasts markedly with the traditional and widely accepted interpretation of the Taconian and Acadian orogenies in western Newfoundland. Here we reinforce our earlier arguments for an allochthonous interpretation by presenting a series of six closely spaced, balanced cross sections across the Port au Port Peninsula. These sections illustrate our interpretation of complex and noncylindrical structures within the triangle zone, as well as the envisioned structural linkage between exposures on Port au Port Peninsula and the geometry of the triangle zone interpreted from offshore seismic data 23 km along strike to the northeast. The southeast vergent Tea Cove thrust, which formed an early upper detachment to the triangle zone, is offset and overturned by the north‐northwest vergent Round Head thrust. The Round Head thrust is interpreted to flatten into a second southeast vergent thrust, the Red Brook detachment, which formed a late upper detachment to the triangle zone. Some map‐scale structures are interpreted as fault‐bend folds above oblique ramps on the Red Brook detachment. Allochthonous crystalline basement within the triangle zone, beneath its attached platformal cover, is interpreted as a result of thrust reactivation of a preexisting basement‐cutting normal fault, which may have formed during rifting of the early Paleozoic Iapetus Ocean. This interpretation is suggested by the stratigraphy and geometry of spatially restricted coarse conglomeratic units on Port au Port Peninsula, and is supported by the restored cross sections.

Structural and Stratigraphic Analysis of the Dinaride Thrust Belt: A Frontier Exploration Province in Central-Southern Europe

The Dinarides are a 200-300-km-wide southwest vergent fold and thrust belt extending along the eastern margin of the Adriatic Sea. The complex and varied structural and stratigraphic relationships can be used to define three major tectonic units: the internal, central, and external Dinarides. In the Internal Dinarides, platform sequences deposited in the Early and Middle Triassic underwent rapid subsidence and drowning in the Late Triassic and Early Jurassic along with rifting and subsequent formation of oceanic crust. Melange and flysch were deposited during the late Jurassic through Cretaceous as the developing thrust belt encroached upon the northeastern margin of the Dinaride carbonate platform. The Dinaride carbonate platform forms the cores of the central and external Dinarides and is composed primarily of Permian-Triassic clastics and evaporites overlain by Middle Triassic through early Eocene platform carbonates. The entire sequence is overlain by late Eocene and early Oligocene synorogenic flysch. In the central Dinarides, late Jurassic and Early Cretaceous unconformities suggest structural uplift prior to the onset of thrusting. Deformation involves Paleozoic basement and includes a major decollement in the Permian Triassic clastic and evaporite unit. Thrusting in the external Dinarides occurred in the late Eocene-early Oligocene and is restricted to Middle Triassicmore » and younger units with major detachments forming near the base of the Ladinian and within a Late Jurassic-Early Cretaceous evaporite. Important oil source and seal lithofacies occur within intraplatform basins and lagoons in the Mesozoic sequences of the central and external Dinarides. Widespread dolomitized units within the Mesozoic carbonate sequence are potential reservoir zones. The presence of surface hydrocarbon seeps and of existing production on trend to the Dinarides in the thrust belt of Italy and Albania suggest the potential for hydrocarbon discoveries in this underexplored area.« less

Tectonic Evolution and Hydrocarbon Potential of the Southern Moesian Platform and Balkan-Forebalkan Regions of Northern Bulgaria

The major tectonic elements of northern Bulgaria are the east-west-trending Balkan-Forebalkan fold belt and the Moesian platform. Moderate hydrocarbon exploration potential exists in trapping geometries generated during the tectonic evolution of the region coupled with reservoir/seal pairs and source rocks within Mesozoic strata. The tectonic evolution of the region includes Early Triassic to Early Jurassic intracratonic rifting followed by multiphase compression that contracted the rift basin and produced a north vergent fold and thrust belt along the southern margin of the stable Moesian platform. Compression began during the Early Cretaceous, continued during the Paleocene, and concluded during the middle Eocene. Trap types generated during the tectonic evolution include normal fault-bounded rotated blocks in the autochthonous section and elongate, asymmetric anticlines in the allochthonous section. Triassic to Upper Jurassic Marine facies were deposited in an east-west-trending rift. Sediments deposited in a shallow foredeep, which evolved during Lower cretaceous compression, overlay the rift sequence. The Early Mesozoic rift sequence provides the depositional settings for Middle Triassic and lower Middle Jurassic source rock shales and sandstone/carbonate reservoirs ranging from Middle Triassic to Lower Cretaceous. Carbonate reservoirs generally are porous dolomites with intercrystalline, moldic, and vugular pore types interbedded with nonporous limestones. Clastic reservoirsmore » are quartz-rich sandstones with pore types that are reduced intergranular, dissolution, and microporosity. These heterogeneous reservoir targets exhibit poor to good reservoir characteristics and are overlain with sealing lithologies of variable thicknesses.« less

The Overthrust Belt of Western North America

The Overthrust Belt extends for 5000 mi (8000 km) from the Brooks Range in Alaska to the Sierra Madre Oriental in Mexico. It consists of northeastward vergent thrust and fold structures involving late Precambrian to early Tertiary sedimentary section. These sediments represent deposition off the western rift margin, formed in late Precambrian time, of the North American Precambrian craton. The northeastward thrusting continued throughout the Mesozoic as a response to the convergence of the East Pacific Plate with the North American Plate. This convergence resulted in subduction beneath the North American Plate except at the northwest end (the Brooks Range) where the result was obduction. Convergence ceased when the west edge of the East Pacific Plate reached the subduction zone. The sedimentary section involved in the Thrust Belt contains good Devonian to Cretaceous hydrocarbon source rocks, and Ordovician to traps related to the thrusting (simple thrust sheets, imbricate thrust sheets, folded thrust sheets, step anticlines, footwall cutoffs, footwall anticlines, etc.). Field methods involved in exploration for hydrocarbons include field geological mapping, remote sensing (aerial photography and Landsat imagery), various seismic refraction and seismic reflection techniques (including modern detailed three dimension surveys) and potential field methods such as gravity and magneticmore » surveying. Studies of the field data include paleontology, source rock and hydrocarbon migration studies, structural and stratigraphic analyses, and the processing of geophysical data. This work has succeeded in two major areas: the Western Canadian Rocky Mountain Foothills, a major gas province producing mainly from Paleozoic reservoirs; and the Wyoming-Idaho-Utah portion of the thrust belt, also a major gas producer from Paleozoic reservoirs and, in addition, a major oil producer from the Jurassic Nugget Sandstone.« less

Anomalous structural evolution of the Shimanto Accretionary Prism at Murotomisaki, Shikoku Island, Japan

Abstract The Late Oligocene‐Early Miocene Nabae Sub‐belt of the Shimanto Accretionary Prism was created coevally (ca 25‐15 Ma) with the opening of the Shikoku back‐arc basin, located to the south of the southwest Japan convergent margin. The detailed geology of the sub‐belt has been controversial and the interaction of the Shimanto accretionary prism and the opening of the Shikoku Basin has been ambiguous. New structural analysis of the sub‐belt has led to a new perception of its structural framework and has significant bearing on the interpretation of the Neogene tectonics of southwest Japan.The sub‐belt is divided into three units: the Nabae Complex; the Shijujiyama Formation; and the Maruyama Intrusive Suite. The Nabae Complex comprises coherent units and mélange, all of which show polyphase deformation. The first phase of deformation appears to have involved landward vergent thrusting of coherent units over the mélange terrane. The second phase of deformation involved continued landward vergent shortening. The Shijujiyama Formation, composed mainly of mafic volcanics and massive sandstone, is interpreted as a slope basin deposited upon the Nabae Complex during the second phase of deformation. The youngest deformational pulse involved regional flexing and accompanying pervasive faulting. During this event, mafic rocks of the Maruyama Intrusive Suite intruded the sub‐belt. Fossil evidence in the Nabae Complex and radiometric dates on the intrusive rocks indicate that this tectonic scheme was imprinted upon the sub‐belt between ∼23 and ∼14 Ma.The timing of accretion and deformation of the sub‐belt coincides with the opening of the Shikoku Basin; hence, subduction and spreading operated simultaneously. Accretion of the Nabae Sub‐belt was anomalous, involving landward vergent thrusting, magmatism in newly accreted strata and regional flexing. It is proposed that this complex and anomalous structural history is largely related to the subduction of the active Shikoku Basin spreading ridge and associated seamounts.

Coast Plutonic Complex: A mid-Cretaceous contractional orogen

Early Late Cretaceous east-vergent thrusts deform much of the eastern margin of the Coast Plutonic Complex in western British Columbia. Similarities in timing and style suggest that the faults represent a system of backthrusts to a west- vergent thrust belt on the west side of the complex. This geometry and the rarity of mid-Cretaceous strike-slip faults indicate that the Coast Plutonic Complex was a strongly contractional orogen in mid-Cretaceous time.

Opposite vergence of Nappes and crustal extension in the French‐Italian western Alps

In the Internal Pennine Zone of the French‐Italian Western Alps (Dora Maira, Monviso, Rocciavre massifs and Queyras region), the structural trends define an apparent “dome” geometry that is truncated to the east by the Pô Basin. This geometry is classically interpreted as a backthrusting system. The kinematic history of the area has been investigated using structural, metamorphic and geochronologic data, on the basis of which two main deformation episodes have been distinguished that occurred under differing metamorphic conditions: (1) The first deformation occurred under greenschist facies metamorphism. It caused a regional scale foliation and an EW trending lineation that lie parallel to ductile deformation zones and thrust contacts. Sense‐of‐shear indicators associated with the lineation indicate contradictory top‐to‐the‐east and top‐to‐the‐west senses of shear. From these, it is deduced that coeval east and west vergent thrust faulting accommodated major displacements between nappes. Phengite Ar‐Ar data yield late Eocene‐early Oligocene ages for the greenschist facies metamorphism. (2) The second deformation caused the widespread development of high‐angle shear bands and normal faults on opposite sides of the Dora Maira massif resulting in the apparent “dome” geometry of the massif. A steep (50–80°) network of west‐dipping normal faults is particularly well developed on the west side of the Dora Maira massif. Differences of 2–3 kbar and 100–150°C between juxtaposed Monviso + Rocciavre and Queyras nappes implies ≥8 km of vertical displacement along the Monviso upper tectonic contact. These normal faults formed in a more brittle regime. Only a post‐early Oligocene age can be estimated from microstructural studies for these final movements. These deformations postdate HP‐LT metamorphism and resulted in the juxtaposition of nappes of distinctly different origin, age, grade, and P‐T‐t history. A kinematic model is presented in which opposite vergence of nappes followed by large‐scale crustal extension occurred in response to uplift during the convergence between the European and South Alpine plates. This model may account for rapid uplift, rapid unroofing and cooling of high pressure‐low temperature rocks (thereby explaining the absence of amphibolite facies metamorphism overprint in this part of the Alps) that may have led to the preservation of coesite‐bearing rocks in the southern part of the Dora Maira massif.

Contrasting geothermal regimes of the Barbados Ridge Accretionary Complex

Three E‐W transects of closely spaced heat flow measurements across the Barbados Accretionary Complex are reported. One transect, about 100 km in length at 14°20′N and a shorter 15‐km line at 14°35′N show that heat flow decreases arcward roughly in proportion to the thickening of the wedge over the actively accreting zone. This decrease is due to the combined thermal effects of the underthrusting oceanic lithosphere and thickening of sediments that have been ingested into the wedge. Except for anomalously high heat flow over a mud volcano on the eastern end of the 100‐km transect, fluid expulsion appears to have only a secondary effect on the seafloor heat flow in this region. An increase of heat flow is observed at the western end of the 100‐km transect where high‐angle arcward vergent thrusting is occurring. We attribute this increase primarily to a decrease in the arcward thickening of wedge material, and heat generated by friction on the décollement. In strong contrast a short line of stations at 15°30′N (the Deep Sea Drilling Project/Ocean Drilling Project (DSDP)/(ODP) drilling transect) found anomalously high heat flow (a 40% increase over the heat flux from the lithosphere) with large variability over the most recently accreted material and also in the adjacent basin. The thermal regime in the vicinity of the DSDP/ODP transect is dominated by a strong flow of pore fluids along the décollement and upward flow along fault planes in the toe of the wedge and adjacent basin sediments. Heat transport by pore fluids also appears to dominate the subseafloor thermal regime in a zone near 13°50′N. The thermal effects of dewatering of the sediments by upward diffusive flow were examined using a simple one‐dimensional model, which showed that the resulting effects are too small to produce detectable heat flow anomalies. Consequently, the flow of fluids must be channelized in subseafloor conduits to produce the observed anomalies in heat flow.

Collision tectonics of the Ladakh-Zanskar Himalaya

The collision of the Indian Plate with the Karakoram-Lhasa Blocks and the closing of Neo-Tethys along the Indus Suture Zone (ISZ) is well constrained by sedimentologic, structural and palaeomagnetic data at ca. 50 Ma. Pre-collision high P— low T blueschist facies metamorphism in the ISZ is related to subduction of Tethyan oceanic crust northwards beneath the Jurassic-early Cretaceous Dras island arc. The Spontang ophiolite was obducted south westwards onto the Zanskar shelf before the Eocene closure (Dl). The youngest marine sediments on the Zanskar shelf and along the ISZ are Lower Eocene, after which continental molasse deposition occurred. After ocean closure, thrusting followed a SW-directed piggy-back sequence (D2). This has been modified by late-stage breakback thrusts, overturned thrusts and extensional normal faulting associated with culmination collapse and underplating. The ISZ and northern Zanskar shelf sequence are affected by late Tertiary redirected backthrusting (D3), which also affects the Indus molasse. A 50 km wide ‘pop-up’ zone with divergent thrust vergence was developed across the Zanskar Range. Balanced and restored cross sections indicate a minimum of 150 km of shortening across the Zanskar shelf and ISZ. Post-collision crustal thickening by thrust stacking resulted in widespread Barrovian metamorphism in the High Himalaya that reached a thermal climax during Oligocene-Miocene times. Garnet-biotite-muscovite + tourmaline granites were generated by intracrustal partial melting during the Miocene within the Central Crystalline Complex. Their emplacement on the hangingwall of localized ductile shear zones was associated with SW-directed thrusting along the Main Central Thrust (MCT) zone and concomitant culmination collapse normal faulting along the Zanskar Shear Zone (ZSZ) at the top of the slab. Metamorphic isograds have become inverted by post-metamorphic SW-verging recumbent folding and thrusting along the base of the High Himalayan slab. Along the top of the slab, isograds are the right way up but are structurally and thermally telescoped by normal faulting along the ZSZ. 1

Formation and Prediction of Structural Traps in Northwest Beartooth Mountains Near Livingston, Montana: ABSTRACT

The bounding faults on the northeastern flank of the Beartooth Mountains display a reversal of thrust vergence, with southwest-dipping thrust faults in the southeast, and northeast-dipping thrust faults in the northwest. Previous structural models of the Beartooth uplift do not explain this reversal in vergence because they do not balance by sedimentary bed length or basement block area and are not restorable to conceivable initial geometries. In addition, the models do not account for the thin-skinned deformation exposed along the northwestern corner. Restorable models for the northwest Beartooth uplift predict a blind master thrust, dipping to the southwest with wedges of basement back thrust over the main block. The basement-wedge formation is probably responsible for complex thin-skinned thrusts, duplexes, and ramp structures that fill apparent strain incompatibilities (space problems) for the southeast and northwest corners of the uplift. Estimates of 8 to 14 km (5 to 7 mi) of upper crustal shortening are consistent with published estimates from gravity modeling to the southeast. Geometric balancing techniques that conserve basement area and sediment bed length provide strong constraints on the structural geometry of foreland uplifts in the Rocky Mountain region. In the Beartooth Mountains, the complex interplay between the postulated southwest-dippingmore » master fault, northeast-dipping back thrusts, and thin-skinned deformation allow for multiple structural traps for petroleum accumulation. The surface exposures in the Beartooth Mountain uplift do not suggest the subsurface complexity predicted by the balanced cross sections.« less

Provenance of Conglomerate Clasts from Upper Cretaceous Kuskokwim Group, Southwest Alaska: ABSTRACT

The predominantly Upper Cretaceous (Albian to Coniacian) Kuskokwim Group consists of marine turbidites and subordinate fluvial and shallow marine strata, deposited in an elongate southwest-trending basin covering over 70,000 km2 in southwestern Alaska. In the Sparrevohn and Cairn Mountain areas of the Lime Hills A-7 and A-8 quadrangles, fluvial, inner fan, middle fan, and outer fan facies are stacked with distal facies over proximal facies by northwest vergent thrust faults. Inner fan pebble-to-cobble conglomerate and pebbly sandstone were deposited as submarine grain flows up to 10 m thick containing reversely graded bases and normally graded tops. Clasts from these conglomeratic deposits are predominantly sedimentary rock fragments--particularly sandstone, si tstone, and argillite--originally thought to have been derived from the nearby Jurassic and Lower Cretaceous flysch (Kihiltna terrane). Detailed examination of these clasts, however, indicate that they contain as minor constituents Paleozoic coral, oolitic limestone, algal boundstone, radiolarian chert, and mafic, intermediate, and felsic volcanic rocks, most likely derived from the adjacent Nixon Fork, Dillinger, and Mystic terranes. Sandstone clasts are arkosic (Q25-F37-L38, n = 7) and contain subequal amounts of K-feldspar and plagioclase. Sand grains within these clasts are moderately sorted, subrounded, and have presolved contacts. Similar arkosic rocks have been described from the Dillinger terrane of the McGrath quadrangle and are the most likely source for the pebbles and cobbles within the Kuskokwim Group. End_of_Article - Last_Page 660------------

Structural Style--Brooks Range Mountain Front, Alaska: ABSTRACT

The Brooks Range mountain front between the Sagavanirktok River and Kurupa Lake is characterized by thrust sheets of Lisburne rocks, which dominantly have stratigraphic tops to the north and either dip northward or are overturned to the south. The early (Jurassic-Neocomian) thrust belt strikes obliquely (N70°W) into the mountain front and can be traced on seismic sections into the foothills. Individual thrust structures die out westward into overturned folds and ultimately into plunging noses as the thrust belt plunges to the west and successively higher structural levels are exposed. The total thrust displacement remains essentially constant because of transfer of motion to higher thrusts. Most of the west occurs along narrow zones of instant plunge with essentially zero occurring along trend. The present east-west mountain front is oblique to the original thrust vergence (N20°E) of Jurassic-Neocomian age and is due to later (Albian and younger) Brooks Range core uplift, folding, and comparatively minor thrusting. The Lisburne folds at the mountain front respond almost plastically to the late core uplift and gravitationally slide downward on rotated north-dipping thrust faults to form cascading folds with easiest relief upward and northward. Core uplift has rotated original thrust sheets 90° in some instances so that geologic map patterns are plunge-projection cross sections of thrust plates. The pattern at Eskimo Creek (Table Mountain quadrangle, eastern Brooks Range) illustrates how a view can reveal the true structure of the thrust belt. Examples of mountain-front structures are given at Killik River, Kurupa Lake, Kaikshak Hill, Akmagolik Creek, Atigun River, and Ivishak River. End_of_Article - Last_Page 660------------

Concepts of the evolution of the Austroalpine basement complex (Eastern Alps) during the Caledonian-Variscan cycle

The Austroalpine basement complex has a complicated pre-Alpidic history which begins with the Caledonian era. In the late Precambrian (?) and early Paleozoic a magmatic-sedimentary rock sequence is formed presumably in an island-arc or active continental margin environment. Subduction with eclogite formation is followed by collision, high-grade metamorphism and anatexis in the Ordovician. This Caledonian basement is preserved in parts of the Austroalpine crystalline mass. The post-Caledonian deposits are mainly shelf type sediments with intercalated volcanics, although there is evidence for an oceanic basin to the south. The Variscan facies zones are arranged in SW-NE direction, oblique to the Alpidic trend. In a first stage of Variscan orogeny in the Carboniferous, south(east)-vergent decollement nappes, syntectonic flysch deposits, and granitoids are formed along with regional metamorphism. This is followed by a second stage in the Permian with north(west)-vergent thrusting, renewed granite formation, and metamorphism. The Variscan nappe pile is today exposed in a deeper level in the west or northwest than in the east or southeast.