摘要:

The Zagros mountain belt region, a Neogene continental collisional zone between Arabia and Iran that is characterized by a suture, a folded belt, and a foreland basin, is unusual among active mountain belts in that its 6–12 km thick sedimentary cover is shortening predominantly by folding while the basement is apparently thickening along numerous high‐angle reverse faults. We have used over 9000 gravity measurements in the region in conjunction with limited seismological observations and geological cross sections of the uppermost 10 km of crust to infer the deeper crustal structure beneath the region. Crustal models constrained by the Bouguer anomalies and other seismological and geological data are characterized by a Moho that dips about 1° to the northeast beneath the folded belt and increases in dip to about 5° near the Main Zagros Thrust (MZT); the Moho depth increases from 40 km beneath the leading edge of the foreland basin (the Mesopotamian foredeep and Persian Gulf) to as much as 65 km beneath the MZT. Alternative models that incorporate a subducted oceanic crust that is attached to the underthrusting Arabian crust deviate little in their Moho configurations from the above simpler models. Negative isostatic residual anomalies are interpreted to indicate local overcompensation beneath the foreland basin and near the folded belt and beneath the MZT. We show that underthrust sedimentary rocks of the converging margins along the suture zone may account for the negative isostatic residual anomalies near the MZT. However, to explain the rest of the isostatic anomalies requires other forces to be acting on this collisional plate boundary. For example, elastic flexure models required a combination of topography and subsurface loads or downward force (the origin of which is not clear) to approximate the Bouguer or isostatic residual anomalies associated with the foreland basin and the folded belt. The topography load of the Zagros mountain belt is insufficient to cause the required deflection of the underthrusting Arabian plate. A combination of isostatic, elastic flexure, and horizontal compression forces acting on the edge of the Arabian craton and the transitional lithosphere of central Iran appears to best model the crust of the Zagros region. We propose a model in which the lower crust of the converging Arabian plate located beneath the Zagros is being shortened and thickened plastically between the seismogenic, rigid upper crust and a rigid uppermost mantle lid. The mantle lid, probably decoupled from the lowermost crust, un der thrusts the Iranian crustal blocks. Thus confined from below, the horizontally compressed, plastic lower crust hydraulically depresses the Moho and raises the faulted and folded, brittle upper crust in isostatic equili

ISSN:0278-7407    

DOI:10.1029/TC005i003p00361

年代:1986

数据来源: WILEY

摘要:

The earliest (Ll) extension lineation of the polyphase deformation sequence in the Combin and Zermatt‐Saas Fee Units of the Piemont Zone of the upper Pennine nappes trends roughly NW‐SE and the geochronological and stratigraphical constraints indicate that the nappe transport and the associated stretching lineation occurred between about 100 and 70 Ma. This accords in a general way with the relative motion of the African Plate to the European Plate during the late Cretaceous as determined from reconstructions of the opening of the Atlantic Ocean and suggests that late Cretaceous thrusting in the Swiss Alps was driven fairly directly by the Africa/Europe plate mot

ISSN:0278-7407    

DOI:10.1029/TC005i003p00375

年代:1986

数据来源: WILEY

摘要:

The India‐Eurasia collision caused extensive deformation in the Eurasia Continent since late Eocene time. We propose that back‐arc spreading in the Okhotsk and Japan Seas was related to the movement of microplates relevant to the India‐Eurasia collision. The Kuril Basin of the Okhotsk Sea and the basins of the Japan Sea were formed as back‐arc basins simultaneously. Their late Oligocene to middle Miocene age is constrained by sediment stratigraphy, basement depth, and heat flow data. The movements of two microplates, the Okhotsk and Amuria Plates, are significant for the tectonics of the Okhotsk and Japan Seas. Simultaneous retreat of both microplates from trench hinge lines caused back‐arc spreading in the Kuril Basin and the Japan Sea. The Amuria Plate moved north‐northeastward due to the India‐Eurasia collision. This caused pull‐apart opening of the Baikal Rift along a transform boundary between the Siberia and Amuria Plates and a collision along the Stanovoy Range at its northern margin. This movement triggered a clockwise rotation of the Okhotsk Plate with a component of dextral collision between the Okhotsk and Amuria Plates, which is well observed along the central zone of the Sakhalin‐Hokkaido islands between the two back‐arc plates. The clockwise rotation of the Okhotsk Plate is suggested by the southwestward fan‐shaped opening of the Kuril Basin. The Kuril Basin narrows to the northeast and terminates south of the Kamchatka Peninsula, where a relative rotation pole between the Okhotsk and Kuril forearc plates is located. The Kamchatka Peninsula was a zone of collision on the other side of the clockwise rotating Okhotsk Plate during the opening of the Kuril Basin. The Sredinny Range in the Kamchatka Peninsula is one of the results of the collision between the Okhotsk Plate and the forearc plate of the Kuril Arc. The bending of the Japanese islands occurred in association with the back‐arc spreading of the Yamato

ISSN:0278-7407    

DOI:10.1029/TC005i003p00389

年代:1986

数据来源: WILEY

摘要:

Structural geology and geophysical data from the Kodiak Shelf suggest that the Mesozoic rocks exposed on the shelf are structurally underlain (at about 12 km depth) by several kms of Eocene age strata. Kinematic data from the Late Cretaceous to Paleocene Ghost Rocks Formation indicate that this formation and probably all of the Kodiak Islands, were uplifted vertically to nearly their present elevations. Landward tilting and imbrication are not indicated. The age of uplift is indicated by a regional, angular unconformity of Early Eocene to Early Oligocene age that separates deep‐sea rocks from shallow water to non‐marine rocks. The uplift of the accretionary prism is believed to have been caused by underplating of an Eocene sedimentary sequence because (1) a band of seismic reflections that occur 12 to 20 km beneath the shelf is interpreted as the top of the underplated material and (2) an obductively offscraped sequence of Eocene deep‐sea rocks crops out on the seaward side of the Kodiak Shelf, suggesting that a thick trench‐fill sequence may have been present prior to uplift of the prism. The underplated material is interpreted to be part of either a previously unrecognized turbidite fan of Early Eocene age or a proximal equivalent of the Zodiac fan of Late Eocene to Early Oligocene age. Other possible on‐land remnants of the underplated material may be present in Prince William Sound (the Montague belt), the Gulf of Alaska (lower sections of the Yakutat block) and in the Coast Ranges of Oregon and Washington. The large volume of underplated material beneath the Kodiak shelf suggests that underplating may be the dominant process in the growth of convergen

ISSN:0278-7407    

DOI:10.1029/TC005i003p00403

年代:1986

数据来源: WILEY

摘要:

Additional paleornagnetic data from the north‐central Rio Grande rift support previous suggestions for counterclockwise rotation of a large crustal block. Thirteen sites from intrusive and extrusive rocks associated with the Oligocene Espinaso Formation in the area around the Ortiz Mountains, central New Mexico, yield magnetic directions of I = 42.1° and D = 337.8° (alpha‐95 = 10.3°). Comparison with the expected direction for the Oligocene indicates 17.8° ± 11.4° of counterclockwise rotation. This agrees with previous studies of Miocene to Pliocene rocks north of the study area that also show about 15° of counterclockwise rotation and corroborates the timing of this motion as post 5 Ma. The extent of the rotated block now includes areas in and adjacent to the Espanola basin east of the Pajarito fault zone, from the San Luis basin in the north to the Albuquerque basin in the south. Extensive published geologic and structural information indicates that counterclockwise block rotation in the Rio Grande rift probably results from left slip along the rift. The rotated block is diamond shaped and bounded by major fault zones (Embudo fault to the north, Pajarito fault zone to the west, Tijeras‐Canoncito fault zone to the southeast, and Picuris‐Pecos fault to the east). Structural consequences of the counterclockwise block rotation are uplift at the acute ends (Sandia and Picuris uplifts) and subsidence at the obtuse ends (Velarde graben and Santa

ISSN:0278-7407    

DOI:10.1029/TC005i003p00423

年代:1986

数据来源: WILEY

摘要:

The Witwatersrand Basin of South Africa is the best‐known of Archean sedimentary basins and contains some of the largest gold reserves in the world. Sediments in the basin include a lower flysch‐type sequence and an upper molassic facies, both of which contain abundant silicic volcanic detritus. The strata are thicker and more proximal on the northwestern side of the basin which is, at least locally, bound by thrust faults. These features indicate that the Witwatersrand strata may have been deposited in a foreland basin and a regional geologic synthesis suggests that this basin developed initially on the cratonward side of an Andean‐type arc. Remarkably similar Phanerozoic basins may be found in the southern Andes. We suggest that the continental collision between the Kaapvaal and Zimbabwe Cratons at about 2.7 Ga caused further subsidence and deposition in the Witwatersrand Basin. Regional uplift during this later phase of development placed the basin on the cratonward edge of a collision‐related plateau, now represented by the Limpopo Province. Similarities are seen between this phase of Witwatersrand Basin evolution and that of active basins north of the Tibetan Plateau (e.g., the Tarim and Tsaidam Basins). The geologic evidence does not appear so compatible with earlier suggestions that the Witwatersrand strata were deposited in a rift or half

ISSN:0278-7407    

DOI:10.1029/TC005i003p00439

年代:1986

数据来源: WILEY

摘要:

A part of the accretionary wedge associated with the Lesser Antilles arc system is exposed in an area of approximately 50 km² in northeastern Barbados. Zircon separates from sandstones collected from the Scotland District, Barbados, W. I., were analyzed using the fission track technique in order to assess the spectrum of source areas from which these terrigenous sediments were derived and to better constrain the timing of deposition. Results yielded a mixture of ages with strong groupings from 25–80 Ma, 200–350 Ma, and greater than 500 Ma. The youngest population indicates that some of the Scotland beds, previously dated by paleontologic methods as Eocene, may actually be as young as late Oligocene. Possible source areas include the Lesser Antilles arc, the Netherlands‐Venezuelan Antilles arc, and the Caribbean Mountains of Venezuela. The 200–350 Ma population may reflect partially annealed cratonic material, an Andean component, and/or material associated with a Triassic rifting event. The oldest zircons (>500 Ma) and metamict zircons were likely derived from the South American craton. Detrital feldspars separated from sandstone at the base of Chalky Mount were analyzed by the40Ar/39Ar age spectrum technique. Results yield what is interpreted to be a slow cooling gradient from 1350 Ma to 925 Ma and provides additional evidence of a cratonic source for the accreted sediments. Based on results from this study, and paleogeographical constraints, it is proposed that source areas for the Scotland sandstones of Barbados may have included the Guayana shield, Central Cordillera, uplifted areas of coastal South America, and the Lesser Antilles volcanic arc. We speculate that in Late Oligocene time an immense “proto‐Orinoco” river system flowed in a northeasterly direction from the shield through the northern Venezuelan mountain system (Cordillera de la Costa and Araya‐Paria) finally emerging into the Caribbean Sea in the area now occupied by Unare depression. Sediments comprising this deltaic complex and deep sea fan were either deposited directly into the trench or onto the sea floor where they were soon after caught up in the accretionary wedge of the Lesser Antilles arc system. This article contains supple

ISSN:0278-7407    

DOI:10.1029/TC005i003p00457

年代:1986

数据来源: WILEY

ISSN:0278-7407    

DOI:10.1029/TC005i003p00469

年代:1986

数据来源: WILEY

ISSN:0278-7407    

DOI:10.1029/TC005i003p00473

年代:1986

数据来源: WILEY