摘要:

New constraints on the mechanics of Red Sea opening were obtained by correlating Neoproterozoic outcrops of the Arabian and Nubian Shields along two thirds of the Red Sea coastlines. Using a mosaic of 23 Landsat thematic mapper scenes (5×105km²) together with field, geochemical, and geochronological data, we identified and mapped lithologic units, mobile belts, and terranes within the Arabian and Nubian Shields. Features best align if Arabia is rotated by 6.7° around a pole at latitude 34.6°N, longitude 18.1°E. Implications of our reconstruction include (1) the amount of continental crust underlying the Red Sea is small because the restored Red Sea coasts are typically juxtaposed, (2) only a single pole is needed, implying that the Arabian and Nubian Shields were rigid plates during Red Sea rifting, (3) coastlines reorient to align with preexisting structures, suggesting the rift propagated in part along pre‐existing zones of weakness, (4) large sinistral displacements of up to 350 km along the Red Sea are not supported, (5) the pole is inconsistent with the Pliocene‐Pleistocene motion along the Dead Sea transform (pole: 32.8°N, 22.6°E +/− 0.5° [Joffe and Garfunkel, 1987]), indicating that more than one phase of motion is required to account for the Red Sea opening. However, our pole is similar to that for the total motion along the Dead Sea transform (pole: 32.7°N, 19.8°E +/− 2° [Joffe and Garfunkel, 1987]), suggesting that the motion between Arabia and Nubia was parallel to the total motion along the

ISSN:0278-7407    

DOI:10.1029/93TC00819

年代:1993

数据来源: WILEY

摘要:

Ultrahigh‐pressure metamorphic rocks with coesite and diamond form a tectonic slice over 20 km thick, called the eclogite zone, within the Dabie Shan complex in the Qinling orogen in central China. The orogen separates the Sino‐Korean block in the north from the Yangtze block in the south. The Dabie Shan Complex is a composite terrane made up of eclogite facies and amphibolite facies gneiss slices and represents fragments of the lower continental crust of the Yangtze block. The Dabie Shan Complex is bounded in the south by a Triassic foreland fold‐thrust belt and in the north by a greenschist facies metaclastic unit, the Foziling Group, which probably represents the passive continental apron deposits of the Yangtze block. Farther north is a granulite facies gneiss complex, the Qinling Group, which has ultramafic slivers and includes the remnants of an island arc with two bounding suture zones. North of the Qinling Group are early Paleozoic active margin deposits of the Sino‐Korean block. The eclogite zone in the Dabie Shan Complex is sandwiched between amphibolite facies gneiss slices. Dating by Sm‐Nd, Rb‐Sr, and Ar‐Ar of two eclogite samples from the eclogite zone gives early to middle Triassic ages (236–246 Ma); the initial εNdvalues indicate reworking of a 2.11 and 1.55 Ga continental crust. A Himalayan‐type tectonic evolution is envisaged for the Qinling orogen with the creation of a 100‐km‐thick crustal thrust wedge through continuous underplating during the subduction of the Yangtze continental lithosphere. Exhumation of the ultrahigh‐pressure metamorphic rocks was chiefly achieved by the southward propagation of the thrust planes, thereby isostatically uplifting and eroding the earlier deeply subducted parts of the orogen. A total of 680 km of southward thrusting in front of Dabie Shan is inferred, based on the abrupt termination of the Tanlu fault. Normal faulting possibly caused by gravitational collapse probably also had a role i

ISSN:0278-7407    

DOI:10.1029/93TC01544

年代:1993

数据来源: WILEY

摘要:

Orthogneiss samples taken from the Kongur antiform show ages varying from 2 Ma to 1 Ma for40Ar/39Ar ages of biotites and muscovites and fission tracks on apatites, leading to cooling rates of 150°C/m.y. Modeling of K‐feldspars highlights the effect of a range of diffusion domains with contrasting diffusion characteristics, yielding closure temperatures from 400° to 150°C. The feldspar data document the cooling history since 5 Ma and indicate a sudden change in cooling rates of the antiform at 2 Ma. At that time, cooling increases by a factor of 5, from an average of 20°C/m.y. to a minimum of 150°C/m.y. Consideration of the regional thermal history, ongoing uplift, and erosional history of the antiform during the Quaternary suggests that denudation rates have been of the order of 5–7 km/m.y. since 2 m.y. ago and could be associated with significant upward surface movement triggered by major normal faulting. The antiform is interpreted to have formed during thrusting at the Pamir front as a result of the development of thrust ramps and normal faulting at the crustal scale. Ramp stacking is an important process of mountain building, and normal faulting in this context must be regarded as a very efficient way of building hig

ISSN:0278-7407    

DOI:10.1029/93TC00767

年代:1993

数据来源: WILEY

摘要:

Mahia Peninsula is a prominent coastal landmark in eastern North Island and is the closest point of land in the North Island to the Hikurangi Trough, where the Pacific plate plunges beneath the subduction complex at the eastern margin of the Australian plate. Uplifted Holocene marine deposits of both estuarine and open beach affinities are found in many parts of the peninsula and provide the basis for Holocene tectonic characterization. Estuarine deposits record the later part of the postglacial transgression that culminated about 6500 years B.P. in New Zealand. The deposits have been differentially uplifted since that time at a rate of 2.5 ± 0.3 mm/yr in the central north coast area, decreasing to 0.7 ± 0.2 mm/yr about 6 km to the west. The coastal plain is characterised in many places by a stepped sequence of emergent shore platforms overlain by fossiliferous beach deposits. Extensive radiocarbon dating of samples from beach deposits shows that terraces in widespread parts of the peninsula are of five distinct ages. Each of the terraces is inferred to have formed in conjunction with a large prehistoric earthquake because of the stepped terrace morphology, clustering of ages on each terrace, differential uplift of terraces across the peninsula, and historic coseismic uplift events in this tectonic setting. Paleoseismic events of Mw7.5–8.0 are estimated to have occurred approximately 250, ∼1600, ∼1900, ∼3500, and ∼4500 years B.P. Uplift distribution of the Holocene and late Pleistocene marine terraces shows that the peninsula lies on the west (gentle) flank of the active Lachlan anticline, which is cored by a major west dipping reverse fault (the Lachlan fault). Holocene active deformation at Mahia Peninsula and other coastal areas of eastern North Island is a continuation of structures developed in Pleistocene time in the landward part of the subduction complex adjacent to the Hikurangi subd

ISSN:0278-7407    

DOI:10.1029/93TC01542

年代:1993

数据来源: WILEY

摘要:

A sequence of seven marine terraces of late Pleistocene age constitutes approximately 40% of the area of Mahia Peninsula in eastern North Island. The peninsula is one of the closest land areas to the Hikurangi Trough (approximately 80 km further east), where the Pacific plate is being subducted beneath the Australian plate. Terrace ages of 40, 59, 81, 106, 124, 176, and 212 ka are assigned by correlation with the marine terrace sequence of Huon Peninsula, Papua New Guinea. Paleontological data and amino acid dating of marine components of the 124 ka terrace at Mahia Peninsula provide a basis for correlation to the Huon Peninsula sequence. Loess and tephra stratigraphy constrain the ages of older and younger terraces. The marine terrace sequence is tilted to the WNW on the flank of the NNE trending Lachlan anticline. Uplift of at least 130 m of the circa 124‐kyr‐old shoreline indicates uplift rates of at least 1 mm/yr at the axis of the anticline. Younger marine terraces have been uplifted more rapidly (about 3 mm/yr), and there is a progressive increase in rate to the youngest Holocene shoreline. Structural contours normalized to the circa 124 ka terrace illustrate the tilt on the flank of the Lachlan anticline and on a secondary structure, the Aurora Point fold. Seismic profiling has confirmed the presence of a reverse fault (the Lachlan fault) at the eastern margin of the Lachlan anticline. The Lachlan fault is inferred to be responsible for growth of the anticline and deformation of the marine terraces. The relationship of the reverse fault to the subduction thrust, which separates the two crustal plates at about 20 km depth below Mahia Peninsula and dips about 6°–12° to the NW, is uncertain. However, the late Pleistocene and Holocene growth of the Lachlan anticline indicates substantial coupling across the plate interface, deforming the inner part of the frontal wedge of the Australia

ISSN:0278-7407    

DOI:10.1029/93TC01543

年代:1993

数据来源: WILEY

摘要:

Seismic reflection data across the upper trench slope off the Nicoya Peninsula, Costa Rica, reveal a wide zone of nearly trench‐parallel normal faults. Although work in the last decade has shown that normal faults are present at many convergent margins, most examples (e.g., Japan, Peru‐Chile, and Guatemala) have been associated with margins experiencing subduction erosion or non‐accretion. In contrast, extension in the Costa Rica study area apparently is coeval with frontal accretion and underplating. The normal faults across the Costa Rica forearc are striking in seismic section due to the well‐layered, 2‐km‐thick upper slope apron. Fault plane reflections and reflector terminations show that the faults extend through the sedimentary apron and apparently into the underlying accretionary prism, indicating a deep‐seated deformation process. The zone of extension is from the midslope area to within 10 km of the shelf edge, a minimum width of about 20 km; the estimated extension across the zone is at least 1.5 to 3 km. Within the apron section, spacing between the faults is generally 200–500 m, and nominal fault dip is 20°–40° and predominantly landward. Activity on the normal faults appears to have occurred over a significant period of time based on increased displacement with depth and on fault‐controlled sedimentary thickening. At least some of the faults may be presently active; shallow reflectors and possibly the seafloor are displaced by faulting. Contemporary sediment accretion is documented by the same seismic reflection profiles showing offscraping and underplating near the toe of the wedge and out‐of‐sequence thrusting primarily below the midslope area. The consistent landward normal fault dip may be influenced by structural anisotropy in the prism and possible extensional reactivation of earlier thrust faults associated with accretion processes. With the available data it is not possible to conclusively determine the cause of the stress field leading to the upper prism and apron extension. However, the three most likely causes are underplating, changes in basal shear stress, or a brief epis

ISSN:0278-7407    

DOI:10.1029/93TC01792

年代:1993

数据来源: WILEY

摘要:

Surface geology and seismic and well data from the northwestern flank of the Venezuelan Andes indicate overthrusting of Andean basement rocks toward the adjacent Maracaibo Basin along a blind thrust fault. The frontal monocline is interpreted as the forelimb of a northwestward verging fault‐related fold deformed over a crustal‐scale ramp. The Andean block has been thrust 20 km to the northwest and uplifted 10 km on a ramp that dips about 20°–30° southeastward. The thrust fault ramps up through crystalline basement rocks to a decollement horizon within the shaly units of the Cretaceous Colon‐Mito Juan formations. Backthrusts in the monocline produce a wedge geometry and reduce the amount of blind slip required on the decollement northwest of the Andes. The rigid Andean uplift was caused by northwest‐southeast compressive tectonic forces related to the convergence of the Caribbean plate, the Panama volcanic arc, and northwestern South America. The thick (up to 6 km) molasse deposits accumulated in the foredeep basin indicate that the Venezuelan Andes started to rise as early as the early Miocene. However, a late Miocene intramolasse unconformity marks the beginning of the formation of the monocline and the greatest uplift. The crustal‐scale fault‐related fold model may explain structural features seen in other areas of basement‐involved for

ISSN:0278-7407    

DOI:10.1029/93TC01893

年代:1993

数据来源: WILEY

摘要:

Models that reconcile the thicknesses of various Appalachian stratigraphic sequences in terms of subsidence associated with thrust loading are based on various assumptions about regional stratigraphy. Different assumptions, based on a different interpretation of Middle and Upper Ordovician stratigraphy, suggest that the geometry of lithospheric flexure due to Taconian thrust loading may have been different than predicted by existing models. Taconian flysch facies restricted to the eastern margin of the outcrop belt is the basis for identifying a thrust load‐induced foredeep. Coeval shallow marine facies were probably beyond the limit of thrust load influence. Stratigraphic thinning near the transition between the flysch and shallow marine facies is interpreted as a peripheral bulge and is located within the Valley and Ridge. Comparison is made with the Timor area north of Australia. Thrust load‐induced foredeeps and peripheral bulges predicted by this model are much narrower than predicted by previous mod

ISSN:0278-7407    

DOI:10.1029/93TC01791

年代:1993

数据来源: WILEY

摘要:

Similarities in geology and potential field data that have in the past been noted among the regions of southern Alaska, southern Vancouver Island, and central California are now seen to be accompanied by similarities in deep crustal structure. A number of tectonic elements have been identified in the deep structure along transects in these three regions, although not all elements are present along each transect. These elements are (A) an actively subducting oceanic plate and (B) an overriding continental plate that consists of (1) a Cenozoic accretionary prism, (2) a Mesozoic accretionary prism, (3) a backstop to the Mesozoic prism, (4) a tectonically underplated body of oceanic rocks, and (5) a crustal root. The Mesozoic prism is in some cases an underthrust body (type 2a) but in other cases forms the principal component of a landward verging tectonic wedge (type 2b). The technically underplated body of oceanic rocks extends landward from the fault contact between the Cenozoic and Mesozoic prisms to a point beneath the backstop. The crustal root lies beneath the backstop and landward of the underplated body. All of these elements are interpreted to be present along the Alaskan and Vancouver Island transects. In Alaska the underplated body is interpreted to be fragments of the Kula plate; the same may be true at Vancouver Island. These two transects appear to differ in that, in Alaska, the Mesozoic prism, in one interpretation, is the principal component of a tectonic wedge (type 2b), whereas at Vancouver Island, it is an underthrust body (type 2a). Along the central California transect, active subduction is no longer taking place, and the San Andreas fault has removed the Cenozoic prism from this region of the North American plate. On the North American plate (i.e., east of the San Andreas fault), the Mesozoic prism, interpreted as the main component of a tectonic wedge (type 2b), and the backstop to the Mesozoic prism are present. There is, however, no clear evidence of tectonically underplated oceanic rocks, and the crust is thin (no root). In both Alaska and Vancouver Island, the Mesozoic prisms above the underplated bodies experienced low‐pressure/high‐temperature metamorphism at about the time of tectonic underplating; no such metamorphism is currently exposed in California. The metamorphism may have been caused by the underplating of young, hot oceanic crust, or, alternatively, by subduction of an oceanic ridge. The presence of a tectonic wedge (type 2b) in Alaska and California and the absence of such a wedge at Vancouver Island could arise either from the fact that in the former two locations the Mesozoic prisms were more voluminous, owing to either more rapid trench sedimentation or more rapid convergence, or to the possibility that at the latter location the Mesozoic prism was juxtaposed with the backstop primarily by strike‐slip fau

ISSN:0278-7407    

DOI:10.1029/93TC01063

年代:1993

数据来源: WILEY

摘要:

With up to ∼45 km of slip, the Meade thrust fault in southeastern Idaho is one of the major thrusts in the Sevier thrust belt, yet its age of displacement has remained enigmatic because of the lack of a derivative synorogenic deposit. We propose that the synorogenic conglomerate produced by initial Meade thrusting crops out on Red Mountain, in southeastern Idaho, ∼1.5 km east of the present trace of the Meade thrust. This conglomerate historically has been considered part of the Ephraim Formation of the Lower Cretaceous Gannett Group, but we suggest that it is a conglomeratic facies of the Bechler Formation localized to the area of Red Mountain. Regional stratigraphic considerations and paleontological dates from underlying and overlying strata indicate that the Bechler conglomerate facies (BCF) is Aptian in age. The BCF is 850 m thick and consists of pebble‐ to boulder‐conglomerate, sandstone, and mudrock deposited by fluvial and mass flow processes on medial to distal parts of an alluvial fan. Paleocurrent data indicate an eastward, fan‐shaped dispersal pattern. The BCF contains clasts of micritic limestone, chert‐pebble conglomerate, and cherty litharenite that were derived from the Ephraim Formation. In addition, the conglomerate contains abundant clasts of Ordovician, Carboniferous, and lower Mesozoic rocks that crop out on both the Meade thrust sheet and the Paris thrust sheet ∼25–30 km to the west. The base of the BCF is marked by a local 27° angular unconformity on top of the Ephraim Formation. The BCF contains intraformational, progressively rotated angular unconformities, internal growth folds, and minor dip discontinuities that were produced by simultaneous folding and sediment accumulation on the proximal footwall of the Meade thrust. Provenance modeling indicates that the BCF was derived from Mesozoic strata that were exposed along the frontal part of the Meade thrust sheet and from Paleozoic strata that were coevally exposed in the hindward located Paris thrust sheet. Topography on the Paris sheet was rejuvenated as it was carried passively over a major ramp in the Meade thrust. Provenance and structural data are combined to produce an incremental, bulk‐rock retrodeformation of initial Meade displacement. Clasts of Jurassic Twin Creek Limestone with pressure‐solution cleavage in the lower part of the BCF indicate that an episode of layer‐parallel shortening occurred in the Meade hanging wall prior to Meade displacement; this may have been related to emplacement of the Paris thrust sheet. The BCF recorded ∼6 km of initial Meade‐related shortening by thrust slip and fault‐propagation folding during Aptian time. Illite crystallinity, systematic fracture sets in the Ephraim Formation and BCF, finite strain in Meade footwall and hanging wall rocks, and thermal models by previous workers indicate that the Meade thrust sheet ultimately overrode the BCF and other footwall rocks, probably during Albian‐Cenomanian time. The BCF and the Meade thrust were folded during slip on thrust faults related to the northern Crawford thrust during Coniacian time. A revised interpretation of Gannett Group provenance throughout the thrust belt suggests that the Ephraim Formation was derived from both the Paris thrust sheet and an older, hindward located thrust sheet. The Bechler was derived f

ISSN:0278-7407    

DOI:10.1029/93TC01790

年代:1993

数据来源: WILEY