摘要:

The Rocky Mountain trench is one of the youngest, most prominent, and most enigmatic structures of the Canadian Cordillera. Approximately 650 km of seismic‐reflection data, providing regional three‐dimensional coverage over an area of 10,000 km², include six crossings of the Rocky Mountain trench between 49°N and 50°15′N. Prominent reflections from mid‐Proterozoic Moyie sills outline thrust‐and‐fold structures of a Late Jurassic to Early Cretaceous fault system that was cut by the Rocky Mountain trench fault in the Tertiary. The near‐basement reflections outline a 10 km high west facing basement ramp, the hinge line of which spatially coincides with the Rocky Mountain trench in this area. This ramp is part of a mid‐Proterozoic margin upon which the Belt‐Purcell supergroup was deposited and is preserved beneath the trench. During Mesozoic contraction, the basal detachment of the Foreland belt closely followed the craton‐cover contact across the basement ramp. Thrusting ceased and extension was initiated when a culmination of thick basinal strata was juxtaposed with the basement ramp. In the Eocene‐Miocene, the basement ramp and the culmination above it focused stress, reactivating the basal detachment and causing extensional faulting in the southern Rocky Mountain trench. The Rocky Mountain trench fault may be linked via the basal detachment to the Flathead fault on the east and the Eocene extensional faults that flank the

ISSN:0278-7407    

DOI:10.1029/95TC03288

年代:1996

数据来源: WILEY

摘要:

A major thrust system of mid‐Cretaceous age is present along much of the Coast Belt of northwestern‐North America. Thrusting was concurrent, and spatially coincided, with emplacement of a great volume of arc intrusives and minor local strike‐slip faulting. In the southern Coast Belt (52° to 47°N), thrusting was followed by major dextral‐slip faulting, which resulted in significant translational shuffling of the thrust system. In this paper, we restore the displacements on major dextral‐slip faults of the southern Coast Belt and then analyze the mid‐Cretaceous thrust system. Two reconstructions were made that use dextral faulting on the Yalakom fault (115 km), Castle Pass and Ross Lake faults (10 km), and Fraser fault (100 km). The reconstructions differ in the amount of dextral offset on the Straight Creek fault (160 and 100 km) and how much the NE part of the Cascades crystalline core expanded (30 km and 0 km) during Eocene extension. Reconstruction A produces the best match of lithotectonic units and thrust systems. Our synthesis shows that the southern Coast Belt thrust system was ≥250 – 180 km wide after thrusting. The thrust system was mainly southwest vergent but had a belt of northeast vergent back thrusts on the northeast side associated with the Tyaughton‐Methow basin, which may indicate large‐scale tectonic wedging. Thrust faults are commonly low to moderate angle, but high angle faults also occur, especially as late stage, out‐of‐sequence, structures involving plutons. The amount of thrust displacement across the system is unknown but must be at least 100 km and may be many hundreds of kilometers. Most thrusting occurred from ∼100 to ∼80 Ma and did not migrate systematically until after ∼90 Ma, when thrusting and magmatism shifted to the northeast for a few million years. Widespread thrusting occurred both near plutons and where there are no (or small) plutons, which strongly suggests that thrust faulting was caused by regional‐ to plate‐scale forces such as rapid plate converg

ISSN:0278-7407    

DOI:10.1029/95TC03498

年代:1996

数据来源: WILEY

摘要:

Triassic and Jurassic rocks in the Black Rock Desert of northwest Nevada comprise part of the Mesozoic volcanic arc successions found throughout the westernmost U.S. Cordillera. Detailed structural studies in the Pine Forest Range, located in the western Black Rock Desert, provide new insight into the Jurassic structural history of this part of the Mesozoic arc system. Regional Jurassic deformation and metamorphism in the Pine Forest Range affected a thick (∼10.5 km) section of lower(?) Paleozoic through Triassic strata that were tilted after deformation and are now exposed in approximately cross‐sectional view. At high stratigraphic levels, deformation produced a slaty cleavage and relatively open folds, both of which are concentrated in rheologically incompetent rock types, and was accompanied by metamorphism at low grade. At deeper stratigraphic levels, the regional foliation intensifies and becomes more pervasive, folds tighten, and metamorphic grade increases through greenschist into amphibolite grade. At the deepest stratigraphic levels, the rocks are amphibolite grade L‐S tectonites with a pronounced mylonitic fabric. The ∼2 km‐thick‐zone of mylonites is interpreted as a ductile shear zone that formed when the stratigraphic section was still upright, whereas the less deformed, younger rocks are interpreted to occupy the upper plate of the shear zone and to record a strain gradient away from the shear zone. A variety of structural and regional relations indicate that the shear zone is a ductile thrust fault along which younger rocks moved to the northwest (in present‐day coordinates) with respect to older rocks. The timing of regional Jurassic deformation and metamorphism is constrained by relations in and around two plutons dated by the U‐Pb zircon method. The 201±1 Ma Big Creek pluton was intruded as a sill‐like body along the upper levels of the shear zone and is associated with a variety of features indicating syntectonic intrusion, including internal textural features in plutonic rocks and metamorphic changes in wall rocks with proximity to the pluton. The 185±1 Ma Theodore pluton which intrudes across part of the shear zone has a thermal aureole which statically overprints the shear zone fabric. Deformation was thus ongoing at 201 Ma and over by 185 Ma. The Early Jurassic age of regional shortening deformation in the Pine Forest Range contrasts with the Middle to Late Jurassic (∼170–150 Ma) age of regional shortening documented in many other parts of the Mesozoic arc system, including the Klamath Mountains, Sierra Nevada, and Mojave Desert regions of California. It also contrasts with the record of Early Jurassic extensional tectonism documented in the Sierra Nevada, Mojave Desert, and western Arizona segments of the Mesozoic arc system. These conflicting relations indicate a complex tectonic evolution for the Jurassic U.S. Cordillera plate margin and lend support to the idea that disparate parts of this plate margin were juxtaposed along you

ISSN:0278-7407    

DOI:10.1029/95TC03693

年代:1996

数据来源: WILEY

摘要:

We use two‐dimensional (2‐D) P‐T‐t modeling to constrain the thermal and rheological aspects of different scenarios for the late Mesozoic and Cenozoic tectonic evolution of the Eastern Alps, inferred from excellent data sets from the Tauern Window (TW). Models invoking subduction of the South Penninic (SP) oceanic lithosphere during thrusting and subsequent erosion of the Austro‐Alpine (AA) upper plate nappe stack are inconsistent with the observed thermal evolution within the AA and Penninic units. In these models, predictions for the AA peak thermal conditions are lower than observed. After exhumation and cooling to midcrustal levels and subduction of the continental Middle Penninic (MP) block, the AA undergoes a phase of renewed heating to almost the previous peak temperatures. Simultaneously, the Penninic units experience a phase of heating upon subduction, followed by cooling after onset of subduction of the North Penninic (NP) basin. The model predictions are inconsistent with the observed nearly isothermal uplift path of the SP after subduction and cannot explain observed inverted metamorphic peak conditions in the deeper AA (amphibolite facies) down to the higher Penninic unit (greenschist facies). A model with the beginning of subduction of the SP occurring after crustal thickening of the AA and subsequent return to normal crustal thicknesses is compatible with the P‐T‐t data. In this model, peak temperature conditions are higher in the AA, followed by a phase of strong cooling in the AA upper plate with the onset of underthrusting. This model also explains successfully nearly isothermal exhumation of the MP and inverted metamorphic peak conditions in the deeper AA. Material accreted to the hanging wall from the oceanic crust (SP) experiences a phase of cooling during ongoing subduction of oceanic lithosphere and begins to heat up to its thermal climax after the subduction of trailing continental lithosphere. The subsequent PT path of the Penninic units strongly depends on the timing and rates of underthrusting by the foreland. Observed PT paths in the MP within the TW require continuous subduction of the NP and the trailing European foreland under the exhuming MP block. Documented rapid cooling in the final uplift phase of the Penninic units in the TW requires exhumation rates up to approximately 4 mm/yr. Predictions of slightly elevated present‐day geothermal gradients in the TW area are consistent with available heat flow data. As a result of Mesozoic rifting followed by late Mesozoic crustal thickening of the AA, paleorheological reconstructions are characterized by a contrast between relatively strong oceanic lithosphere and adjacent weak continental lithosphere. Predicted decoupling of weak continental and oceanic lithosphere during subsequent subduction of the Penninic units can explain observed Late Cretaceous crustal extension in the AA units in terms of gravitational spreading. Ongoing subduction leads to an overall strength increase due to underthrusting of cool oceanic lithosphere, whereas subduction of continental lithosphere causes a strength decrease in the upper levels of the lithosphere. Continuous crustal thickening and relaxation of the depressed isotherms reduce the strength of the lower lithospheric mantle beneath the central orogen, further enhanced by rapid late‐stage uplift. Predictions for the present‐day rheological structure of the Eastern Alps support the existence of a strong upper crustal layer, two wedge‐shaped strong upper mantle layers to the north and the south of the orogen, and a weak upper mantle underlyi

ISSN:0278-7407    

DOI:10.1029/95TC03289

年代:1996

数据来源: WILEY

摘要:

The Yulong‐Haba Xue Shan range, in the northwestern part of Yunnan (China), is a large N‐S antiform that folds the Paleozoic series of the Yangzi platform. The upper Yangzi River (Jinsha Jiang) has cut a 3500 m‐deep valley (Hu Tiao gorge) across this antiform, thus exposing folded, bedding‐parallel, ductile shear zones (décollements), with transport toward the SSW (in the present geographical coordinates). The large finite shear strain implies tens of kilometers of transport, pointing to the regional significance of these décollements. Rb/Sr radiometric dating of phlogopites that crystallized in marbles within the foliation planes yields the age of the metamorphic and deformation event (35.9 ± 0.3 (2σ) Ma). The age derives from an internal Rb‐Sr isochron, made on different size fractions of the same mineral, which provides a novel demonstration of the feasibility of such plots. Transport on the décollement and related shortening occurred prior to, or at the onset of, extrusion of Indochina along the Ailao Shan‐Red River shear zone, ≈80 km west of the Yulong Shan. The39Ar/40Ar age spectra of K‐feldspar from the core of the Yulong Shan suggest uplift by antiformal folding around 17 Ma, as Indochina's extrusion came to an end. We infer that other large‐scale Cenozoic décollements such as that exhumed in the Yulong Shan underlie some of the vast, folded areas that surround the eastern Himalayan syntaxis. Transport on such décollements, first toward the south and then toward the east, and folding above them, might have occurred during two principal shortening phases, whose ages bracket Indochin

ISSN:0278-7407    

DOI:10.1029/95TC03749

年代:1996

数据来源: WILEY

摘要:

In North Canterbury, New Zealand, the southeastern edge of an active fold and thrust belt lies 20 km offshore beneath the continental shelf. High‐resolution marine seismic reflection profiles and a detailed Quaternary sequence stratigraphy are integrated to map and characterise the seismic potential of actively growing folds. Eleven large‐scale folds occur within the upper few hundred metres of a Pliocene to Recent succession of silty mudstone, representing the upper part of a sedimentary cover sequence up to 2 km thick. The folds are gentle, NE‐SW trending, overlapping, asymmetric structures approximately 10–32 km in length, which verge consistently to the northwest in accordance with major thrust faults and folds exposed in nearby coastal hills. The folds are inferred to overlie a system of blind, southeast dipping thrust faults that are accommodating a small component of regional NW‐SE crustal shortening in North Canterbury and to develop by coseismic uplift during thrust fault earthquakes. The critical interactions between the rates of folding, regional tilting, coastal uplift, outer shelf subsidence, and the local sedimentation response to high‐amplitude Quaternary sea‐level cycles have led to the development and preservation of a stack of folded sedimentary sequences, an essentially smooth, gently sloping seafloor throughout most of the off‐shore deformation zone, and net uplift close to zero at the deformation front. Eleven unconformity‐bounded sedimentary units of middle Pleistocene to Recent age (<0.75 Ma) constrain the rates and timing of folding and enable comparisons with onshore deformation rates, examination of the coastal tectonic gradient, and speculation about the rates of upper crustal deformation beneath the shelf. Deposition, folding, and coastal uplift have occurred contemporaneously throughout the last 0.75 m.y., and all measurable deformation can be accommodated within approximately 0.8 m.y. Fold amplitude growth rates of 0.02 m/kyr to 0.14 m/kyr (typically 0.05–0.09 m/kyr) near the deformation front are low and up to 25 times lower than some actively growing folds exposing basement rocks onshore. There is a significant decline in strain rate across the coastal zone and inner shelf, toward the off‐shore deformation front. It is inferred that blind thrust faults beneath the offshore folds have slip rates typically of the order of 0.1–0.9 mm/yr and that the probable recurrence interval of moderately large magnitude (M6.2–6.8 ± 0.3) thrust earthquakes beneath individual folds is of the order of several

ISSN:0278-7407    

DOI:10.1029/95TC03249

年代:1996

数据来源: WILEY

摘要:

The Portuguese mainland territory is located close to the Azores‐Gibraltar plate boundary, in a tectonic setting responsible for significant neotectonic and seismic activities. However, few data concerning the present regional lithospheric stress field were available, as testified by recently published maps of stress indicators for the Europe and Mediterranean regions. One of the authors already presented a synthesis on this subject [Cabral, 1993], where geological and geophysical stress indicators were considered. In this paper we introduce new information, mainly a considerable amount of borehole breakout data. The updated data set comprises 32 reliable stress indicators showing a mean azimuth of 145° (standard deviation 21°) for the maximum horizontal stress direction (SHmax). On the average, the geological data are rotated clockwise and the focal mechanism data deviated anticlockwise to that azimuth, while the borehole elongation results are consistent with the mean SHmaxtrend. These differences in stress trend suggest a regional progressive rotation of the SHmaxdirection from NNW‐SSE to WNW‐ESE since the upper Pliocene. To estimate stress trajectories, new and published stress indicators in the adjacent Atlantic area and northern Africa were also investigated, showing a very uniform NW‐SE SHmaxtrend in west Iberia. A high level of horizontal compressive stress acting oblique to the western Portuguese continental margin is inferred and interpreted in view of a proposed regional geodynamical model, of activation of this passive margin, with the nucleation of a subduction zone in the Atlantic SW of Iberia, at the Gorringe submarine bank, which is propagating northward along the base of the continental slope, at the transition between thinned and normal continen

ISSN:0278-7407    

DOI:10.1029/95TC03683

年代:1996

数据来源: WILEY

摘要:

Aeromagnetic surveys, spaced ≤5 km, over widely separated areas of the largely ice‐ and sea‐covered West Antarctic rift system, reveal similar patterns of 100‐ to 1700‐nT, shallow‐source magnetic anomalies interpreted as evidence of extensive late Cenozoic volcanism. We use the aeromagnetic data to extend the volcanic rift interpretation over West Antarctica starting with anomalies over (1) exposures of highly magnetic, late Cenozoic volcanic rocks several kilometers thick in the McMurdo‐Ross Island area and elsewhere; continuing through (2) volcanoes and subvolcanic intrusions directly beneath the Ross Sea continental shelf defined by marine magnetic and seismic reflection data and aeromagnetic data and (3) volcanic structures interpreted beneath the Ross Ice Shelf partly controlled by seismic reflection determinations of seafloor depth to (4) an area of similar magnetic pattern over the West Antarctic Ice Sheet (400 km from the nearest exposed volcanic rock), where interpretations of late Cenozoic volcanic rocks at the base of the ice are controlled in part by radar ice sounding. North trending magnetic rift fabric in the Ross Sea‐Ross Ice Shelf and Corridor Aerogeophysics of the Southeast Ross Transect Zone (CASERTZ) areas, revealed by the aeromagnetic surveys, is probably a reactivation of older rift trends (late Mesozoic?) and is superimposed on still older crosscutting structural trends revealed by magnetic terrace maps calculated from horizontal gradient of pseudogravity. Long‐wavelength (∼ 100‐km wide) magnetic terraces from sources within the subvolcanic basement cross the detailed survey areas. One of these extends across the Ross Sea survey from the front of the Transantarctic Mountains with an east‐southeast trend crossing the north trending rift fabric. The Ross Sea‐Ross Ice Shelf survey area is characterized by highly magnetic northern and southern zones which are separated by magnetically defined faults from a more moderately magnetic central zone. Aeromagnetic data in the south delineate the Ross fault of unknown age. The extension of the southern Central Basin south of the Ross fault is associated with an 825‐nT magnetic anomaly over the Ross Ice Shelf requiring inferred late Cenozoic volcanic rock essentially at the seafloor at its south end, as shown by magnetic models. Models show that the thickness of magnetic volcanic rocks beneath Hut Point Peninsula at McMurdo Station is probably106km³) of late Cenozoic magmatic rock remaining at volcanic centers beneath the continental shelf, Ross Ice Shelf and West Antarctic Ice Sheet. We suggest as an alternative or supplemental explanation to the previously proposed mantle plume hypothesis for the late Cenozoic volcanism significantly greater lower lithosphere (mantle) stretching resulting in greater decompression melting than the limited Cenozoic crustal extension allows. However, this implies a space problem that is not obviously resolved, because the Antarctic Plate is essentially sur

ISSN:0278-7407    

DOI:10.1029/95TC03500

年代:1996

数据来源: WILEY

摘要:

The inversion of crustal‐scale basement grabens is studied here through laboratory experiments on small‐scale models and available oil industry seismic lines from the southern North Sea. Two basic configurations are considered. First, both the basement and the sedimentary cover are brittle, and inversion does not involve any potential décollement between them. Second, the basement and sedimentary cover are separated by a weak ductile layer (e.g., salt), which can allow décollement of the cover during both extension and later compression and inversion. The second configuration is more complicated and can lead to a large variety of geological structures. Laboratory experiments were carried out on brittle‐ductile models built with sand to represent brittle layers (basement and sedimentary cover) and silicone putty to simulate the décollement layer between basement and cover. A mechanically based classification of inversion structures is proposed. The effects of some crucial parameters are investigated, including obliquity between the direction of shortening and normal faults, as well as strength profiles, and the presence or absence of salt diapirs. The experimental investigation leads to the following conclusions: (1) the inversion of the graben by reactivation of normal faults implies that the angle between the direction of compression and the graben is less than 45°, (2) if there is a superficial décollement (e.g., basement‐cover interface), inversion initiates low dipping thrust faults in the cover, localized at graben borders, (3) salt diapirs or salt walls localized along the graben borders in the cover are preferential sites for the development of thrust faults, and (4) when the cover is decoupled from the basement by a décollement layer, inversion induced deformation in the cover which is partitioned between thrust faults along the graben borders and strike‐slip faults within the graben trending oblique to the graben borders. Experimental results are compared with field examples, in particular from the sou

ISSN:0278-7407    

DOI:10.1029/95TC03853

年代:1996

数据来源: WILEY

摘要:

The Precambrian Rio Paraíba do Sul Shear Belt comprises a 200‐km‐wide anastomosing network of NE‐SW trending ductile shear zones extending over 1000 km of the southeastern coast of Brazil. Granulitic, gneissic‐migmatitic, and granitoid terrains as well as low‐ to medium‐grade metavolcano‐sedimentary sequences are included within it. These rocks were affected by strong contractional, tangential tectonics, due to west‐northwestward oblique convergence of continental blocks. Subsequent transpressional tectonics accomodated large dextral, orogen‐parallel movements and shortening. The plutonic Socorro Complex is one of many deformed granites with a foliation subparallel to that of the shear belt and exposes crosscutting relationships between its tectonic, magmatic, and metamorphic structures. These relationships point to a continuous magmatic evolution related to regional thrusts and strike slip, ductile shear zones. The tectonic and magmatic structural features of the Serra do Lopo Granite provide a model of emplacement by sheeting along shear zones during coeval strike‐slip and cross shortening of country rocks. Geochronological data indicate that the main igneous activity of Socorro Complex spanned at least 55 million years, from the late stage of the northwestward ductile thrusting (650 Ma), through right‐lateral strike slip (595 Ma) deformation. The country rocks yield discordant age data, which reflect a strong imprint of the Transamazonian tectono‐metamorphic event (1.9 to 2.0 Ma). We propose a model for the origin of calc‐alkaline granites of the Ribeira Belt by partial melting of the lower crust with small contributions of the lithospheric mantle during transpressional thickening of plate margins, which were bounded by deep shear zones. The transpressional regime also seems to have focused granite migration from deeper into higher crustal leve

ISSN:0278-7407    

DOI:10.1029/95TC03247

年代:1996

数据来源: WILEY