摘要:

The main tectonometamorphic alpine events which occurred in the western part of the western Himalayan virgation at south of the main suture, in northern Pakistan, are dated by the39Ar/40Ar method. These events are as follows: (1) the blueschist metamorphism (80 ± 5 Ma) present in tectonic/ophiolititic melanges along the suture (this event occurred during an early subduction or obduction stage) (2) the alpine barrovian metamorphism (50–30 Ma) developed in the sedimentary cover and orthogneisses of the Indian Plate during the postcollisional northward underthrusting of this plate below Asia, and (3) the emplacement of postmetamorphic leucogranites (23 Ma) resulting from the melting of the Indian crust during the climax of the alpine metamorphi

ISSN:0278-7407    

DOI:10.1029/TC003i001p00001

年代:1984

数据来源: WILEY

摘要:

Part of the continuing northward convergence between India and Eurasia is probably taken up by strike slip displacement, with Tibet as a whole moving eastward away from the collision zone and undergoing east–west extension (Molnar and Tapponnier, 1975; Ni and York, 1978). On the basis of our interpretation of Landsat images we propose that strike slip faulting occurs throughout the Tibetan plateau such that its north–south cross section is being shortened, with little contemporary folding or thrust

ISSN:0278-7407    

DOI:10.1029/TC003i001p00019

年代:1984

数据来源: WILEY

摘要:

The eastern part of the Saudi Arabian craton is a platform underlain by gently tilted and horizontal Mesozoic‐Cenozoic sedimentary rocks everywhere more than 250 km distant from the Zagros deformation front. Despite its tectonic simplicity, the platform contains several laterally extensive, but subdued, structures and lineaments belonging to more than one system of fault zones and flexures. New microtectonic observations of dominant sets of extension and conjugate hybrid joints permit regionally significant extension directions to be inferred and the origin of the macrostructures to be assessed. The central Arabian arch, the E‐W crest of which coincides with an axis of Mesozoic subsidence, developed during the Late Cretaceous to Eocene when the basin inverted as a consequence of the emplacement of ophiolite‐bearing nappes on the Zagros and Oman margins. During arching, there was strike‐parallel elongation of beds. At the same time as the arch amplified, the central Arabian graben system evolved as a result of the northward displacement of an East Arabian block or sheet that became detached across an arcuate separation zone, which in the south is coincident with the crest of the arch. An important Neogene response in the Arabian platform to NE‐SW shortening in the Zagros ranges was the formation of a swarm of NE striking master joints and lineaments. These structures reflect the diffuse and peripheral expansion of the foreland beyond the Zagros deformat

ISSN:0278-7407    

DOI:10.1029/TC003i001p00027

年代:1984

数据来源: WILEY

摘要:

Bouguer gravity anomalies and aeromagnetic anomalies display a northward increase of values in the Charlotte and Carolina slate belts from Georgia to Virginia. Analysis of these data with constraints imposed by seismic reflection, seismic refraction, conductivity measurements, and surface geology imply that this increase is due to a corresponding increase in the percentage of mafic rocks in the upper crust. A regional trend in the Bouguer field is determined for an observed eastward decrease in crustal thickness based on seismic refraction measurements. A comparison of the calculated regional with surface boundaries shows that in places the crustal thinning occurs more than 50 kilometers west of the Inner Piedmont‐Charlotte belt transition. The comparison also shows that the crustal thinning may take place west of exposed Grenville basement. The evidence thus indicates that the southern Appalachian gravity gradient is not likely related to a crustal or lithospheric suture. The evidence further indicates that, because the upper crustal and deep crust/upper mantle variations are independent in terms of spatial position, the upper crust of the Charlotte/slate belts is decoupled from the lower crust, perhaps at subhorizontal boundaries seen on reflection dat

ISSN:0278-7407    

DOI:10.1029/TC003i001p00045

年代:1984

数据来源: WILEY

摘要:

Phanerozoic addition rates to the continental crust are calculated by using seismic profiles through magmatic arcs to measure the crustal volumes added during the active lifespans of the arcs. Data for 17 arcs give addition rates per kilometer of arc in the range 20 to 40 km³ km−1Ma−1. From these data we deduce a world‐wide addition rate of 1.65 km³ a−1after adding other contributions to the formation of the continental crust, e.g., from hot spot volcanism. We infer a subtraction rate, mainly by subducting sediments, of 0.6 km³ a−1and arrive at a net crustal growth rate of about 1 km³ a−1. Growth of the continental crust is necessary to maintain approximately constant freeboard, because the secular decline in the heat production of the mantle causes the ocean basins to deepen. An equation for the growth of the continents as a function of the decline in terrestrial heat flow yields approximately constant growth rate since the Archean of 0.9 km³ a−1, in good agreement with the above estimate. On the average, Archean growth rates must have been 3 to 4 times the present rate. Island arc growth rates are inadequate to explain the formation of the Arabian‐Nubian Shield and the Archean granite‐greenstone terrain of the Superior Province, and a captured island chain in Oregon. We confirm the oceanic island origin of the Oregon terrain on the basis of the large growth ra

ISSN:0278-7407    

DOI:10.1029/TC003i001p00063

年代:1984

数据来源: WILEY

摘要:

The Williston Basin is a large, circular, intracratonic basin containing over 4 km of sediments ranging in age from Cambrian to Tertiary. Data on seven formations from over 1300 wells in Montana, South Dakota, and North Dakota were used to determine the shape of the basin over time. By assuming that the lithosphere behaves like an elastic plate on a fluid substrate, the shape of the basin could be closely matched using a point load. The match was improved by adding the equation of a plane to the equation for elastic deformation before fitting the model to the data. The planar adjustment allows for the effects of regional sedimentation or erosion, or eustatic sea level changes. The mechanical modeling indicates that the elastic strength of the lithosphere in this area has generally increased from the Ordovician to the present. The geometrical center of the basin migrated less than 20 km over 450 m.y. The direction and magnitude of tilt of the plane removed from the data is related to tectonic events in western North America. The increasing deflection of the basin over time can be explained by a simple thermal model (which would explain the thickening of the lithosphere) consisting of conductive cooling of a hot region in the lithosphere. Depths to formations in a single well differ from the subsidence predicted by the elastic model with regional tilt removed because of global sedimentation events and the development, in the Mesozoic, of a foreland thrust basin of which the Williston Basin became part. The mechanical and thermal models constrain possible initiating mechanisms for this basin. The thermal modeling and the absence of volcanics in the stratigraphic column rule out a shallow (crustal) heat source. Neither wrench faulting nor crustal stretching were likely to have been significant in the formation of the Williston Basin.

ISSN:0278-7407    

DOI:10.1029/TC003i001p00079

年代:1984

数据来源: WILEY