Km Of Shortening Research Articles

Distributed deformation due to coupling across a subduction thrust: Mechanism of young tectonic rotation within the south Wanganui basin, New Zealand

Paleomagnetic data from Pliocene sedimentary strata in the south Wanganui basin, New Zealand, document clockwise tectonic rotation west of and increasing toward the main zone of deformation (axial tectonic belt) associated with the Australia-Pacific plate boundary zone in the North Island of New Zealand. Measured rotation with respect to the Australian plate increases from 1.4° ± 3.9° 80 km west of the axial tectonic belt to 6.5° ± 2.0° and 13.2° ± 3.1° at 45 km and 20 km west of the axial tectonic belt, respectively. These measurements indicate that (1) deformation associated with plate convergence occurs across a wide region up to 270 km west of the Hikurangi Trench, and (2) in the past 2.5 m.y. at least 30 km of transcurrent motion and 10 km of shortening at the plate boundary have been accommodated within the south Wanganui basin by distributed shear west of the plate boundary zone proper. The data support a model suggesting that the Wanganui basin is a lithospheric flexure basin driven by high friction at the plate interface. This strain in the south Wanganui basin and inferred high friction across the plate interface at the basin9s eastern margin have occurred since 2.5 Ma.

Structural evolution of a modern arc‐continent collision in Papua New Guinea

The Finisterre Mountains and western Solomon Sea of northern Papua New Guinea are the site of an active, oblique arc‐continent collision. Comparison of structures along the length of the collision zone provides a history of its structural evolution. Here we compare the results of structural mapping in the Finisterre Mountains, where collision is in an advanced state, with side scan sonar images and potential field data from the western Solomon Sea, which illuminate the structure of the juvenile collision zone. An accretionary wedge complex, composed of a southwestward younging stack of imbricate thrust sheets, developed along a westward extension of the modern New Britain Trench prior to collision. This accretionary wedge crops out in the Finisterre Mountains as the Erap Structural Complex. Consequences of collision were doubling of the crustal thickness to 50–52 km, rapid up‐lift of the Finisterre terrane, and initiation of a major out‐of‐sequence thrust that displaced the Finisterre Volcanics over younger sediments of the accretionary wedge. This out‐of‐sequence thrust is mapped in the Finisterre Mountains as the Wongat Thrust, and it is recognized in the western Solomon Sea on side scan sonar images. A second generation out‐of‐sequence thrust formed after substantial uplift had subaerially exposed the collision zone. This thrust crosscuts the Wongat Thrust, forming a window of accretionary wedge sediments bounded to both the NE and SW by Finisterre Volcanics. This younger out‐of‐sequence thrust is the Gain Thrust of the Finisterre Mountains. At least 15 km of crustal shortening has occurred across the Wongat Thrust at 146°30′E longitude, and the Gain Thrust has accommodated up to 7.4 km of shortening in the same area. It is probable that the well‐lithified Finisterre Volcanics serve as the backstop for the accretionary wedge/collision complex that is forming in the collision zone of the western Solomon Sea. Potential field models indicate that this backstop is arcward dipping. Thrusting of the back‐stop material over the accretionary wedge, as we observe along the Wongat Thrust, is expected for wedges with arcward dipping backstops. A series of NE–SW trending cross faults, perpendicular to the dominant structural trend, exist in the collision zone. These faults have components of left‐lateral strike‐slip and SE down dip‐slip motion. It is likely that the Finisterre terrane is being segmented along these faults, with the segmentation caused by differential resistance to convergence between the downgoing continental crust of the collision zone and the oceanic crust of the subduction zone to the east. The origin of the faults is uncertain. Previous authors believed that they formed during Paleogene volcanic activity in the Finisterre terrane. The colinearity of several of these cross faults with major mapped faults on the downgoing plate raises the possibility that the Finisterre Range cross faults are upper plate manifestations of lower plate structures and, as such, reveal strong interplate coupling in the collision zone.

Late Cretaceous to Palaeogene thin-skinned tectonics of the Palmyrides belt (Syria)

Detailed mapping in the Palmyrides belt (Syria), shows that its structure resulted from thin-skinned tectonics and includes duplexes, anticlinal stacks and imbricate thrusts. In the Frontal Palmyrides the main décollement level is formed by Triassic evaporites but a Cenomanian marl interval locally forms a subsidiary décollement. Ramps linking the two décollement levels generate duplexes formed by thin horses of Jurassic and Lower Cretaceous material. To the north, in the Central Palmyrides, the décollement ramps down from a Triassic to an Eocambrian level, in which it continues as a main basal décollement. The ramp connection between the Cambrian and Triassic décollement levels is responsible for the anticlinal structure of the Central Palmyrides (Bilas block). Accurate mapping, balanced cross-sections and forward-deformation modelling allowed calculation of the amount of shortening of the sedimentary cover: the minimum estimate is 20 km. This independent, field-based result is compatible with previous estimates from a general seismic study of the Cenozoic-Mesozoic series. Due to the presence of folded Neogene deposits, the deformation of the Palmyrides belt was thought to be Miocene-Pliocene in age. However, Eocene carbonates unconformably overlie various thrust units in the Frontal Palmyrides. Reconstruction indicates 15 km of shortening during the late Maastrichtian to upper Eocene and 5 km of shortening since the Late Miocene. Thus, the main shortening event of the Palmyrides occurred during the Palaeocene. Seismic, well and gravity data are used to integrate the shallow structure in a crustal-scale deformation model. Gravity modelling confirms previous interpretations of a relatively flat Moho beneath the Palmyrides belt and suggests that deformation of the belt resulted from a movement of the sedimentary cover of northern Syria 20 km to the south, along a 200 km long, flat décollement within the Eocambrian. This Syrian décollement between the sedimentary cover and the basement is supposed to root down north of Syria, within the North Arabian plate boundary. Such a décollement implies four times more shortening in the sedimentary cover than in the Palmyrides basement.

The Kapuskasing uplift: a geological and geophysical synthesis

Over the past decade, the Kapuskasing uplift has been the subject of intense geological and geophysical investigation as Lithoprobe's window on the deep-crustal structure of the Archean Superior Province. Enigmatic since its recognition as a positive gravity anomaly in 1950, the structure has been variably interpreted as a suture, rift, transcurrent shear zone, or intracratonic thrust. Diverse studies, including geochronology, geothermobarometry, and various geophysical probes, provide a comprehensive three-dimensional image of Archean (2.75–2.50 Ga) crustal evolution and Proterozoic (2.5–1.1 Ga) cooling and uplift. The data favour an interpretation of the structure as an intracratonic uplift related to Hudsonian collision.Eastward across the southern Kapuskasing uplift, erosion levels increase from < 10 km in the Michipicoten greenstone belt, through the Wawa gneiss domain (10–20 km), into granulites (20–30 km) of the Kapuskasing structural zone, juxtaposed against the low-grade Swayze greenstone belt along the Ivanhoe Lake fault zone. Most volcanic rocks in the greenstone belts erupted in the interval 2750–2700 Ma and were thrust, folded, and cut by late plutons and transcurrent faults before 2670 Ma. Wawa gneisses include major 2750–2660 and minor 2920 Ma tonalitic components, deformed in several events including prominent late subhorizontal extensional shear zones prior to 2645 Ma. Supracrustal rocks of the Kapuskasing zone have model Nd ages of 2750–2700 Ma, metamorphic zircon ages of 2696–2584 Ma, and titanite ages of 2600–2493 Ma, reflecting deposition, intrusion, complex deformation, recrystallization, and cooling during prolonged deep-crustal residence. Postorogenic unroofing rapidly cooled shallow (10–20 km) parts of the Superior Province, but metamorphism and local deformation continued in the ductile deep crust, overlapping the time of late gold deposition in shear zones in the shallow brittle regime.Elevation of granulites, expressed geophysically as positive gravity anomalies and a west-dipping zone of high refraction velocities, dates from a major episode of transpressive faulting. Analysis of deformation effects in Matachewan (2454 Ma), Biscotasing (2167 Ma), and Kapuskasing (2040 Ma) dykes, as well as the brittle nature of fault rocks and cooling patterns of granulites, constrains the time of uplift to ca, 1.9 Ga. Approximately 27 km of shortening was accommodated through brittle upper crustal thrusting and ductile growth of an 8 km thick root in the lower crust that has been maintained by relatively cool, strong mantle lithosphere. The present configuration of the uplift results from overall dextral displacement in which the block was broken and deformed by dextral, normal, and sinistral faults, and modified by later isostatic adjustment. Seismic reflection profiles display prominent northwest-dipping reflectors believed to image lithological contacts and ductile strain zones of Archean age; the indistinct reflection character of the Ivanhoe Lake fault is probably related to its brittle nature formed through brecciation and cataclasis at temperatures < 300 °C. The style and orientation of Proterozoic structures may have been influenced by the Archean crustal configuration.

A balanced model of the southern Central Andes

To explain the actual crustal thickness of the southern Central Andes a balanced cross‐section at 21°S latitude was modeled. A forward modeling procedure was used on a crustal scale to explain the gross crustal structure obtained by seismic refraction investigations. In the model, 320 km of shortening since late Cretaceous, based on balanced geological cross sections, are assumed for the whole Andes from the trench to the foreland, and the initial crustal thickness varies between 30 and 40 km. It results that about 20% of the actual crustal volume must be explained by other sources than crustal shortening. Structures of high‐velocity material in the upper crust of the Eastern Cordillera can be explained by detached and overthrust material from the crustal base thus producing a crustal doubling in the back arc. Underplating of continental material that was removed by subduction erosion and magmatic addition of derivates of the subducted Nazca plate may contribute to the lower crust of the forearc and arc regions. Crustal thickening of the Eastern Cordillera and parts of the Altiplano was undoubtedly produced by tectonic thickening, whereas the sources of thickening in the Western Cordillera and forearc region are uncertain.

Late Cretaceous-Paleocene synorogenic sedimentation and kinematic history of the Sevier thrust belt, northeast Utah and southwest Wyoming

Integration of cross-cutting structural relationships, overlapping sedimentary units, new conglomerate provenance data, and radiometric and palynological dates provides a basis for reinterpretation of the distribution and timing of Late Cretaceous through Paleocene thrust faulting in the northeast Utah-southwest Wyoming part of the Sevier thrust belt. These data indicate a general eastward progression of deformation that was punctuated by local out-of-sequence and hinterlandward-verging events. Provenance data delimit a sequential restoration of a regional cross section. The principal thrust systems in northeast Utah and southwest Wyoming are the Willard, Ogden, Crawford, Absaroka, and Hogsback thrusts. The Willard is the westernmost, structurally highest, and oldest of the thrusts; it carries a unique section of thick Proterozoic sedimentary rocks, as well as Paleozoic rocks, and was folded during displacement on younger thrust systems. The next youngest thrust system is the Ogden, which comprises several basement-rooted imbricate thrusts that together form a large antiformal stack with a structural culmination in the Wasatch Range. Growth of the Wasatch culmination took place during Coniacian through Paleocene time, contemporaneous with sequential displacement on the frontal Crawford, Absaroka, and Hogsback thrusts. Total shortening in this part of the thrust belt was ∼100 km, and structural relief of ∼25 km developed in the area of the Wasatch culmination. During Coniacian through Paleocene time, thrusting followed an overall eastward progression that was interrupted by local out-of-sequence and hinterlandward-verging events. Several episodes of synchronous displacement on two or more thrusts can be demonstrated. Shortening occurred in three main episodes. The first episode (∼89-84 Ma) involved ∼33 km of shortening on the Crawford thrust and its footwall imbricates. Approximately 19 km of structural relief on the basement-cover contact developed in the area of the Wasatch culmination. The second episode (∼84-62 Ma, with a break between ∼75-69 Ma) involved ∼30 km of shortening, mainly on the Absaroka thrust, and development of an additional ∼6 km of structural relief in the culmination. The third episode of shortening (∼56-50 Ma) took place on the Hogsback thrust, involved ∼21 km of horizontal shortening, and produced no significant increase in structural relief in the culmination. Long-term rates of shortening ranged between 3.0 mm/yr and 6.6 mm/yr. These three episodes of shortening produced three large accumulations of synorogenic conglomerate, totaling ∼3 km in thickness. The Henefer Formation and Echo Canyon and Weber Canyon Conglomerates were deposited during Crawford thrusting. The Evanston Formation was deposited during and after Absaroka thrusting, and the lower conglomeratic part of the Wasatch Formation was deposited during and after Hogsback thrusting. Most of the sediment in these synorogenic units, however, was derived from repeated uplift of the Willard thrust sheet and from the eastern flank of the Wasatch culmination in the rear part of the thrust belt. Only local, minor accumulations were derived from the frontal ramp anticlines. Sediment accumulation and structural deformation were generally out-of-phase. Periods of regional shortening and uplift were marked by development of unconformities and sediment bypassing to distal parts of the foreland basin. Periods of structural inactivity were marked by accumulation of aerially widespread, braided-river conglomerate on top of the thrust belt. One exception to this pattern is the Henefer-Echo Canyon-Weber Canyon conglomerate deposit, which contains evidence of progressive deformation in close proximity to the tip of the Crawford thrust. Comparison of the sequential restoration with the Late Cretaceous subsidence history and isopach patterns in the distal foreland basin of western Wyoming demonstrates that the principal controls on regional subsidence and sediment supply were the growth and erosion of the Wasatch culmination. Growth of the duplex beneath the culmination may have been a means of maintaining critical taper in the thrust wedge.

Seismic Data, Geometry, Evolution, and Shortening in the Active Sulaiman Fold-and-Thrust Belt of Pakistan, Southwest of the Himalayas

Surface and subsurface data from the Sulaiman fold-and-thrust belt are integrated to analyze the deep structure, tectonic shortening, and kinematics of the Sulaiman fold-and-thrust belt at the western margin of the Indian subcontinent. Seismic reflection data show that nearly all the 10-km-thick sequence of dominantly platform (>7 km) and molasse strata is detached at the deformation front. These strata thicken tectonically to about 20 km in the hinterland without significant thrust faults in the foreland. A balanced structural cross-section suggests that structural uplift in the Sulaiman fold-and-thrust belt is a result of a thin-skinned, passive-roof duplex style of deformation. The duplex sequence consists of Jurassic and older strata. It is separated from the roof equence by a passive-roof thrust in thick Cretaceous shales that has a backthrust sense of displacement over foreland-propagating duplexes. The roof sequence with incipient out-of- sequence thrusting is intact for about 150 km northwards, where it emerges along a passive backthrust. Sequential restoration of the balanced section reveals a series of structural and geometrical features including: (1) development of low-amplitude, broad concentric (detachment) folds (Sui and Loti) at the tip of the decollement; (2) increase in amplitude of a detachment fold to a critical level for development of ramp and duplex structures; and (3) out-of-sequence thrusting to create required critical taper for an outward translation of the foreland fold-and-thrust belt. A balanced structural cross-section 349 km long from the Sulaiman fold-and-thrust belt restores to an original length of 727 km, suggesting a maximum of 378 km of shortening since 21 Ma in the cover strata of the Indian subcontinent. Calculation of displacement rates over the Sulaiman fold-and-thrust belt (18 mm/yr) added to the resolved rate of the Chaman fault vector for the component parallel to the plate convergence direction (15 mm/yr) are close to the current India-Asia plate convergence rate (37 mm/yr).

Sedimentary basin inversion caused by igneous underplating: Northwest European continental shelf

A considerable body of evidence indicates that many of the extensional sedimentary basins in the vicinity of the British Isles underwent permanent exhumation during the Tertiary. The most dramatic indicator of this process is the present-day absence of as much as 4 km of anticipated postrift thermal subsidence in basins just north and west of Scotland. Any explanation of this observation must take into account the fact that the entire region has very small,-long-wavelength, free-air gravity anomalies. This important constraint implies either that the crust has been thickened or that low-density material has been added to or formed from the lithosphere and rules out models that invoke flexural effects arising from the opening of the North Atlantic. Tertiary epeirogeny is often attributed to compression that is assumed to be related in a general sense to Alpine mountain building. However, to remove ∼3 km of sedi- mentary rock from a basin ∼100 km wide requires >15 km of shortening. Minor Tertiary compression is observed all over the continental shelf, but nowhere is it sufficient to account for the required amount of uplift and erosion. In addition, exhumation dramatically increases from south to north, whereas the observed compression decreases markedly in the same direction. At the beginning of the Tertiary, rifting associated with the initiation of the Iceland plume generated substantial volumes of melt. Inversion of rare-earth- element concentrations of MgOrich igneous rocks suggests that a minimum of ∼5 km of melt was produced beneath at least part of the continental shelf. We infer that much of this melt remains trapped within the lithosphere, presumably close to the Moho, which acted as a density filter. Such underplating will generate rapid uplift.

The configuration of Asia prior to the collision of India: Cretaceous paleomagnetic constraints

Paleomagnetic data from Central Asia show that 1700±610 km of shortening of southern Asia since Cretaceous time have been absorbed by distributed deformation between southern Tibet and the Siberia craton. This result is based on a compilation of Cretaceous poles from the Junggar, Tarim, Tibet, Indochina, South China, North China, and Mongolia blocks, complementing the recent compilation of Enkin et al. (1992a). We propose a paleogeographic reconstruction of Asia in the Cretaceous, in which the position of Siberia is derived from the synthetic apparent polar wander path of Besse and Courtillot (1991). The resulting map, which likely represents Asia as it remained throughout the Cretaceous until the collision with India began, features an “unbent” Tibet, with an east‐west trending Andean margin at tropical latitudes and a rather continuous belt of continental red bed basins extending from Sichuan to Tarim through Tibet. The map allows one to estimate continental shortening and rotations between the blocks, which are attributed to the collision. Despite large uncertainties, these have amounts and senses which are in all cases compatible with some recent kinematic models such as that of Avouac (1991).Appendix is available with entire paper on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W.,Washington, DC 20009. Document B93‐007; $2.50. Payment must accompany order.

The origin of the West Spitsbergen Fold Belt from geological constraints and plate kinematics: Implications for the Arctic

The West Spitsbergen Fold Belt, which extends for 300 km along the western margin of Svalbard and is up to 80 km wide, has much in common with foreland fold and thrust belts. The near-foreland segment of the fold belt exhibits ramp-flat thrust trajectories whilst structurally higher nappes are typified by more listric thrusts. Higher nappes to the west contain imbricated Carboniferous and basement rocks showing that the latter were actively involved in the fold belt deformation and a minimum of 80 km total shortening perpendicular to the western margin of Svalbard is estimated. The early stages of the Eurekan deformation in North Greenland can be linked to that of the West Spitsbergen Fold Belt and the combined shortening across the two fold belts may exceed 80 km. In Ellesmere Island Eurekan structures are distributed in an arc-like belt which records between 50 to 100 km of shortening since the Late Cretaceous. Kinematic reconstructions suggest that before the opening of the Eurasian Basin and Norwegian-Greenland Sea (Chron 25), Svalbard was linked to North America. In the Late Cretaceous-Palaeocene interval the motion across the Greenland-Svalbard margin, was mainly convergent giving rise to the West Spitsbergen Fold Belt and the Eurekan structures of North Greenland. The dextral separation of Greenland and Svalbard in post-Chron 24 time was accompanied by extension followed by pure extension in post-Chron 13 time.

The West Spitsbergen Fold Belt: The result of Late Cretaceous‐Palaeocene Greenland‐Svalbard convergence?

AbstractThe West Spitsbergen Fold Belt, together with the Eurekan structures of northern Greenland and Ellesmere Island, are suggested to be the result of Late Cretaceous‐Palaeocene intracontinental compressional tectonics. The Late Palaeozoic –Mesozoic rocks of western Spitsbergen are characterized by near‐foreland deformation with ramp‐flat, top‐to‐the east thrust trajectories, whereas structurally higher nappes involving Caledonian complexes are typified by more listric thrusts and mylonite zones. A minimum of 40 km of shortening is estimated for the northern part of the West Spitsbergen Fold Belt. The axial trends in the West Spitsbergen and the North Greenland Eurekan fold belts parallel the principal fault zones which accommodated the separation of Greenland and Svalbard after Chron 25/24. In northern Greenland, north directed Eurekan thrusts associated with mylonites and cleavage formation represent at least 10 km of shortening. Between 50 and 100 km of shortening is estimated for the markedly arcuate Eurekan Fold Belt of Ellesmere Island, but the principal tectonic transport is eastwards. Kinematic reconstructions suggest that Svalbard was linked to North America before the opening of the Eurasian Basin and Norwegian — Greenland Sea. In the Late Cretaceous — Palaeocene interval, the relative motion between Greenland and North America was convergent across the Greenland — Svalbard margin, giving rise to the West Spitsbergen Fold Belt and the Eurekan structures of North Greenland.

Integrated crustal structure across the south central California Margin: Santa Lucia escarpment to the San Andreas Fault

The continental margin of south central California has undergone different styles of deformation over the past 30 m.y. as the North America‐Pacific plate boundary has evolved from subduction to transform tectonics. Results presented here identify strike‐slip and compressional structures in the lower crust that probably developed during this period of tectonic transition. An integrated model of crustal structure along deep seismic line PG&E‐3 crosses the entire continental margin west of the San Andreas fault near the latitude of San Luis Obispo. A compressional wave velocity model was developed using coincident marine multi‐channel seismic and onshore/offshore wide‐angle reflection/refraction seismic data and was tested independently with gravity modeling. A high‐velocity layer (6.8–7.0 km/s, about 6 km thick) in the lower crust that extends east possibly as far as the San Andreas fault is interpreted as tectonically underplated oceanic crust. The upper and middle crust of the Patton and Sur‐Obispo Franciscan terranes extends to depths of 14 to 20 km. The velocity and density modeling indicate lower average velocities and densities for the Patton terrane, supporting earlier interpretations that it is a distinct terrane within the Franciscan complex. Abrupt crustal thickening associated with broken and downdropped oceanic crust beneath the Santa Lucia Escarpment probably results from active transform faulting that occurred along the continental slope from about 22 to 16 Ma following the subduction of the Pacific‐Arguello ridge. An imbrication within the oceanic crust centered beneath the coast is evidence for about 15 km of shortening within the oceanic crust. Convergence between the Monterey and Arguello microplates during the final stages of Farallon subduction (about 24 to 16 Ma) probably caused the imbrication.

Style and magnitude of tectonic shortening normal to the San Andreas fault across Pyramid Hills and Kettleman Hills South Dome, California

Research Article| April 01, 1993 Style and magnitude of tectonic shortening normal to the San Andreas fault across Pyramid Hills and Kettleman Hills South Dome, California ROGER B. BLOCH; ROGER B. BLOCH 1Department of Applied Earth Sciences, Stanford University, Stanford, California 94305-2225 Search for other works by this author on: GSW Google Scholar ROLAND VON HUENE; ROLAND VON HUENE 2U. S. Geological Survey and GEOMAR, Kiel, Germany Search for other works by this author on: GSW Google Scholar PATRICK E. HART; PATRICK E. HART 3U S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar CARL M. WENTWORTH CARL M. WENTWORTH 3U S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information ROGER B. BLOCH 1Department of Applied Earth Sciences, Stanford University, Stanford, California 94305-2225 ROLAND VON HUENE 2U. S. Geological Survey and GEOMAR, Kiel, Germany PATRICK E. HART 3U S. Geological Survey, Menlo Park, California 94025 CARL M. WENTWORTH 3U S. Geological Survey, Menlo Park, California 94025 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1993) 105 (4): 464–478. https://doi.org/10.1130/0016-7606(1993)105<0464:SAMOTS>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 ROGER B. BLOCH, ROLAND VON HUENE, PATRICK E. HART, CARL M. WENTWORTH; Style and magnitude of tectonic shortening normal to the San Andreas fault across Pyramid Hills and Kettleman Hills South Dome, California. GSA Bulletin 1993;; 105 (4): 464–478. doi: https://doi.org/10.1130/0016-7606(1993)105<0464:SAMOTS>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 A blind thrust zone and the overlying fold forming the Kettleman Hills South Dome were produced by late Cenozoic shortening normal to the San Andreas fault. West of Kettleman Hills, toward the San Andreas fault, the region of Pyramid Hills experienced structural growth as early as Late Cretaceous time, and growth accelerated in Pliocene to Holocene time. These structures were imaged to a depth of 7 km by reprocessing a seismic reflection record section. Restored structural cross sections indicate a three-stage growth history for Kettleman Hills South Dome: (1) late Miocene reverse faulting, (2) Pliocene fault-propagation-type folding, and (3) late Pliocene to Holocene tectonic wedge emplacement. Late Pliocene to Holocene structural growth at Pyramid Hills probably resulted from thrust emplacement of the Franciscan Complex. A forward model demonstrates that tectonic wedging could account for post-2.5 Ma deformation of Pyramid Hills and Kettleman Hills South Dome. Horizontal shortening across Pyramid Hills and Kettle-man Hills South Dome normal to the San Andreas fault since the Cretaceous period is 3.7 km. This figure does not include shortening within the basement; adding shortening of our tectonic wedge model yields 7.9 km of shortening since Cretaceous time. Post 2.5 Ma shortening determined by our tectonic wedge model is 6.3 km. The rate of post-2.5 Ma shortening (2.5 mm/yr), when added to that on the west side of the San Andreas fault, is consistent with the values derived from plate-motion models that describe the "San Andreas" discrepancy. 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.

Intraplate shortening in the central Indian Ocean determined from a 2100-km-long north-south deep seismic reflection profile

Although intraplate deformation is commonly observed on the continents, there are only a few places in the oceans where deformation is distributed over a broad area rather than being localized at a single plate boundary. The central Indian Ocean has long been recognized as a site of active widespread compressive deformation of the oceanic lithosphere, reflecting an exceptionally high stress level. However, the strain pattern, derived from indirect and incomplete observations, remains poorly known. A more complete data set was obtained during the Phedre cruise (1991) by means of deep seismic reflection profiling. One 2100-km-long profile runs along the 81°E meridian from lat 14°S to the coast of Sri Lanka, and provides us with the first long cross section through the region of observed crustal-level faults. Fault analysis offers new estimates of the amount of shortening, which was obtained across individual faults by measuring the maximum vertical uplift of sedimentary reflectors at the base of the sedimentary cover and converting it to horizontal throw under various assumptions of the fault geometry. The contribution of the seismically resolvable faults amounts to 22-37 km of shortening or 2.5%-4.3%. This is two to three times greater than previous estimates extrapolated from local measurements on shorter seismic reflection profiles.

Structure of the Eastern Cordillera of Colombia: Implications for Trap Styles and Regional Tectonics

The northwest margin of South America has undergone multiple phases of compressional deformation beginning in the Late Cretaceous and culminating in the Pliocene-Pleistocene. The latest phases (Miocene to Pleistocene) of this deformation created the Eastern Cordillera of Colombia. Compressional structures in the Eastern Cordillera formed traps that have petroleum exploration potential; these are generally in the form of fault-bend and fault-propagation folds containing top-sealed reservoir and mature source rocks. Trap delineation, although difficult to achieve because of the poor seismic quality obtained in the region, can be accomplished by supplementing the seismic data with geometric analysis of surficial structures. The Eastern Cordillera is asymmetrical, with the dominant tectonic transport direction towards the east. West-directed thrust faults are localized along the western flank of the Eastern Cordillera and are interpreted to be back thrusts splaying off a deep mid-crustal detachment. A regional, retrodeformable cross section across the Eastern Cordillera shows that basement-detached deformation of late Miocene to Pliocene age was followed by Pliocene to Holocene uplift of the Cordillera on high-angle, basement-involved reverse faults. These two structural styles were superimposed on preexisting Jurassic and Lower Cretaceous grabens, whose normal faults were partially inverted during the later stages of compressional deformation. A palinspastic restoration of this regional cross section indicates that approximately 150 km of shortening (40% shortening) has occurred: 105 km eastward-directed towards the Llanos basin and 45 km westward-directed towards the Middle Magdalena basin. This calculated shortening is very close to that calculated by summing microplate motions for the northwest corner of South America. Shortening in the sedimentary cover rocks exceeds that in the basement rocks by about three times. This shortening can be regionally balanced with a gently dipping, mid-crustal detachment that extends beneath the Middle Magdalena basin and Central Cordillera and which roots in the Benioff zone beneath the Western Cordillera. The presence of a mid-crustal detachment, as postulated here, provides a mechanism to transmit the shortening from the convergent plate margin in the Pacific to the Andean foreland, and is interpreted to have provided the structural link between the three ranges of the Colombian Andes. The detachment also provides a mechanism to explain the imbalance of shortening seen between Eastern Cordillera basement and the overlying strata, and implies that the Western and Central Colombian Andes have been tectonically transported eastwards a distance of at least 150 km.

A balanced cross-section through the eastern Nepal Siwalik hills, Bagmati river region, implications for the structure of the southern Himalaya

A balanced cross-section has been constructed across the eastern Nepal Siwalik Hills along the Bagmati River in order to determine the structural geometry of the Himalayan foreland fold- thrust belt and to estimate tectonic shortening. The Siwalik Hills are underlain by a basal detachment, the Main Detachment Fault, at a depth of about 6.9km to 8km. The Main Boundary Thrust, the Chaura- Marin Thrust and the Main Frontal Thrust are splay thrusts off the Main Detachment Fault which ramp directly up-section to the surface through 6.8 to 7.7km of Siwalik Sediments with no major intervening thrust flats; the Sub-Himalayan foreland fold thrust belt displays an emergent imbricate- fan geometry. Shortening across the Bagmati section of the Siwalik Hills has been 20km, or 40%, while thrusting along the Main Boundary Thrust has resulted in a minimum of another 25km of shortening.

The tectonostratigraphy and structure of the eastern Nepal Himalaya

Six years of geological research in eastern Nepal has resulted in a new geological map of the eastern Nepal Himalaya which includes the region stretching from the Sikkhim border in the east to the Kathmandu Valley in the west, and from the summits of the Higher Himalaya in the north to the Ganges Plain in the south. This research has permitted the determination of the tectonostratigraphy and structure of one section of the central Himalayan arc. South of the Tibetan Plateau the eastern Nepal Himalaya can be divided into three distinct, thrust‐bound tectonic packages: (1) the Higher Himalayan thrust sheet composed of the Higher Himalayan Crystallines, (2) the Lesser Himalayan thrust sheet composed of the Lesser Himalayan Series, and (3) the Sub‐Himalayan imbricate zone composed of sedimentary rocks belonging to the Siwalik Group. The Higher Himalayan thrust sheet of eastern Nepal has been thrust over the Lesser Himalayan Metasediments a minimum of 140 km, and possibly as much as 175–210 km, along the Main Central Thrust (MCT). The Lesser Himalayan thrust sheet is overlain by the MCT and is underlain by the Main Detachment Fault (MDF) and the Main Boundary Thrust (MBT). Out‐of‐sequence thrust faults in the hanging wall of the MBT have breached and offset the presently inactive MCT. The Sub‐Himalayan imbricate zone is an emergent imbricate fan bounded by the MBT to the north and the Main Frontal Thrust (MFT) to the south and is underlain by the MDF which lies at a depth of between 5 km and 7 km. A balanced cross section constructed across the Higher, Lesser, and Sub‐Himalaya of eastern Nepal shows that the eastern Nepal Himalayan orogenic wedge has undergone a minimum of between 210 and 280 km of horizontal, north‐south tectonic shortening since the initiation of the MCT. The Lesser and Sub‐Himalaya have absorbed 70 km of north‐south shortening by thrusting along the basal MDF, of which the Sub‐Himalayan imbricate zone has accommodated 25 km, the Sun Kosi Thrust has accommodated about 10 km, and the MBT has accommodated the remaining 35 km of shortening. Since the initiation of the MCT between 15 Ma and 25 Ma shortening across the eastern Nepal Himalaya has occurred at an average rate of 8.4–18.6 mm per year. The structural geometry of the eastern Nepal Himalaya suggests an overall “piggyback” sequence of thrusting, with motion transferred from the MCT to the underlying MDF and its emergent splay thrust, the MBT, and with the MBT rotated to its present steep orientation by imbricate thrusting within the Sub‐Himalaya.

Intracontinental subduction and hinged unroofing along the Salmon River Suture Zone, west central Idaho

The Salmon River suture zone in west central Idaho juxtaposes volcanic arc rocks of the Wallowa terrane directly against cratonic North America. Detailed metamorphic studies along a 10 km traverse perpendicular to the suture indicate that the arc and two crystalline fragments thrust upon it each record different pressure‐temperature (P‐T) histories. From lowest to highest structural level: the Wallowa terrane shows only subgreenschist metamorphism, the Rapid River plate (RRP) records unroofing and cooling from ∼8 kbar and 550°C to 6 kbar and 475°–500°C, and the Pollock Mountain plate (PMP) shows evidence for polymetamorphism and records burial and heating paths to final equilibration conditions of 9–11 kbar and 600°–625° C. Ar‐Ar hornblende ages combined with the P‐T data suggest that currently exposed levels of the RRP and PMP were juxtaposed against one another at 15–20 km depth at or prior to 118 Ma, indicating that 10–20 km of uplift, and hence also the onset of collision‐related metamorphism, occurred before ∼118 Ma. Correlation of the metamorphic and age data with geometric constraints from the initial Sr 0.706 line and the dimensions of the RRP and PMP permit construction of large‐scale retrodeformable sections of the west side of the suture from Late Jurassic through Late Cretaceous time. The abrupt nature of the Sr 0.706 line implies that the arc‐continent boundary extends vertically through most of the crust, which requires sharp downwarping of the arc lithosphere in order to account for the PMP metamorphic data. Narrow zoned overgrowths on PMP garnets record this burial event and require initially rapid (≥3 km/m.y.) uplift rates in order to be preserved. We suggest that the onset of rapid uplift resulted from the separation of the negatively buoyant lithospheric root from the downwarped arc, allowing buoyant rise of fragments of thickened crust. Detachment of the root is suggested to change the environment of crustal shortening from one in which footwalls of thrusts or shear zones sink to one in which hanging walls rise. This mechanism represents an alternative to cessation of shortening or onset of tectonic denudation as an explanation for the transition from burial to uplift of high‐pressure metamorphic terrains. Subsequent uplift appears to have been slow and to have occurred in a hinged fashion such that mineral and whole rock ages decrease systematically towards the suture zone. The consumption of lithosphere during ≥40 km of shortening between two crustal blocks implies that the Salmon River suture is the trace of an intracontinental subduction zone. Burial and collision apparently began before about 130 Ma, and thus any precollision strike‐slip faulting or tectonic escape of intervening terranes was likely accomplished in Jurassic and earliest Cretaceous time.

Coeval hindward- and forward-imbricating thrusting in the south-central Pyrenees, Spain: Timing and rates of shortening and deposition

Clear demonstrations of detailed sequences of thrust development are rare, particularly with respect to hindward-imbricating successions. Similarly, well-constrained chronologies of the timing of multiple thrust motions and associated rates of shortening are also uncommon. Along the eastern oblique ramp of the South-Central Unit (SCU) of the Pyrenees, well-exposed structural geometries and crosscutting stratigraphic packages unambiguously illustrate a sequence of break-back thrusts that developed synchronously with the growth of an adjacent duplex. The break-back sequence includes both the Sierras Marginales and Montsec thrusts: structures defining two of the major east-west partitions within the SCU. Balanced, sequentially restored structural cross sections indicate that a minimum of 7.5 km of shortening occurred within the imbricate stack. An additional 9 km of contraction was accommodated by the duplex, prior to translation of the duplex and imbricate stack to the south along a detachment within upper Eocene evaporites. Magnetic polarity stratigraphies have been used to calibrate these late Eocene to Oligocene episodes of deformation. These temporal data indicate that Sierras Marginales thrusting began in the study area ∼39.8 Ma and that the Montsec thrust ceased movement ∼36.2 Ma, yielding a mean rate of shortening of 2.1 mm/yr. Coeval shortening within the duplex was ∼2.5 mm/yr, and subsequent translation across the footwall ramp occurred at ∼2.5-4 mm/yr. Despite the partitioning of shortening into distinctively different styles of deformation, the mean rate of contraction remained steady over ∼4 m.y. These new chronologies indicate that the locus of large-scale thrusting did not progress systematically toward the foreland, but shifted episodically within the SCU, such that, during the late Eocene shortening, the Sierras Marginales thrust was active both prior to and south of the Montsec thrust.

Deep crustal reflection profiling offshore southern central California

Deep crustal reflection profiling offshore southern central California shows that the offshore Santa Maria Basin and adjacent shelf and slope are underlain by oceanic crust which underthrusts the margin. On common‐midpoint (CMP) sections the top of the oceanic crust appears as a strong coherent reflection occurring at 6 s two‐way time (twt). The reflection can be traced continuously more than 50 km from the deep ocean basin eastward beneath the continental shelf. Depth conversion places the top of oceanic crust at 6–16 km depth with a dip of 8° toward the coast. Beneath the Santa Lucia High the crustal reflection breaks up into several east dipping segments. Above the oceanic crust an accretionary wedge increases in thickness from 3 km at the base of the slope to over 12 km beneath the Santa Lucia High. The Neogene sedimentary section above the accretionary prism exhibits relatively little deformation, implying that most of the accretion occurred during pre‐Neogene convergence between the North American plate and oceanic plates located to the west. The offshore Santa Maria Basin exhibits at least two periods of Neogene deformation: an early extensional phase, accompanied by strike‐slip faulting, occurred in the lower to middle Miocene followed by compression and basin inversion in the upper Miocene‐lower Pliocene. The development of structures in the basin began in the northwest and is progressively younger to the southeast. Faults identified in the offshore sedimentary column offset acoustic basement but are not imaged in the Franciscan basement material below 2–3 s twt and do not extend to, or offset, the deep reflections. We suggest that the oceanic crust served as a detachment surface above which the shallow deformation occurred. Neogene sediments in the offshore basins show only 1–3 km of shortening. This implies that shortening, due to Neogene oblique plate motion between the Pacific and North American plates, extends east of the offshore Santa Maria Basin and is distributed across the entire transform margin.