摘要:

The 1980 El Asnam earthquake should be understood within its tectonic environment. A seismotectonic map of the lower Cheliff basin (scale: 1/500,000) has been synthesized from known geologic, tectonic and seismological work on the region, and from satellite photos. The data set was integrated by a computer in order to handle different scales and projections. Several important tectonic features divide the region into 5 domains. Active faults and recent folds are consistent with a NNW‐SSE direction of compression. The Cheliff basin is limited to the south by the Relizane fault. The basin is also crossed by three broken folds, the eastern one being the one associated with the El Asnam fault. Historical and instrumental seisinicity seems to be located at both ends of Relizane fault. It is suggested that the sites corresponding to folds and reverse faulting crossing the basin are relevant to seismic risk. A simple tectonic model is propose

ISSN:0278-7407    

DOI:10.1029/TC005i006p00809

年代:1986

数据来源: WILEY

摘要:

The provenance of Paleogene sandstones on the island of Barbados is relevant to various models of the tectonic evolution of the southern Caribbean area. Modal point counts of 26 sandstones from Barbados show that the constituent grains most likely were derived from a composite craton interior and orogenic margin. Abundant detrital polycrystalline quartz grains have affinities to a spectrum of low‐ to high‐rank metamorphic/plutonic crystalline source rocks. Microprobe data suggest that feldspars were derived from metamorphic or plutonic source terrances. The complex heavy‐mineral suite of the terrigenous sandstones, including glaucophane, chloritoid, titanite, zircon, and aluminosilicates, supports derivation from orogenic and cratonal terranes. Due to the complex history of the South American‐Caribbean plate boundary and poor age constraints on timing of deposition, widely contrasting models have been proposed concerning the provenance of the sandstones. Convergence between the Aruba‐Orchila arc and continental South America during the Paleogene resulted in the formation of an E‐W trending foreland fold‐thrust belt and foredeep. Dominant sediment dispersal was to the east–northeast along tectonic strike. The present study shows that the terrigenous sandstones on Barbados probably were deposited in deep‐sea fans to the north of present‐day Araya Peninsula during and shortly after the late middle Eocene. Probable source terranes include the Guayana Shield, the Caribbean Mountain System/interior foreland fold‐thrust belt, and possibly (to a lesser extent) the Lesser Antilles magmatic a

ISSN:0278-7407    

DOI:10.1029/TC005i006p00837

年代:1986

数据来源: WILEY

摘要:

The recognition of accreted terranes and their importance in orogenesis has spurred the search for allochthonous fragments along the western and southern margins of South America. Here we present stratigraphic and petrologic data from Chile and Argentina between 29° and 33°S latitude that demonstrate the “suspect” nature of several major terranes, which we infer to have been accreted during the Paleozoic. Three lower‐middle Paleozoic terranes are described (from east to west): (1) the Pampeanas terrane, a Cambrian‐Devonian magmatic and metamorphic province built on late Precambrian basement at the margin of South America, (2) the Precordillera terrane, a Cambrian‐Devonian shelf‐slope‐oceanic basin assemblage bounded by mélanges on both sides and bearing many stratigraphic similarities to the lower‐middle Paleozoic of the Northern Appalachians, and (3) the “Chilenia” terrane, which has largely been obliterated by late Paleozoic magmatism and metamorphism. The distribution of Carboniferous continental, deltaic, and marine strata demonstrates that these three terranes were sutured together and part of South America by the end of the Devonian. Subsequent Permo‐Carboniferous plate interactions more closely resembled the modern Andean margin, with eastward subduction, accretionary prism formation, and minor terrane emplacement exposed along the present coast of Chile and eastward migrating arc magmatism from the present coast of Ch

ISSN:0278-7407    

DOI:10.1029/TC005i006p00855

年代:1986

数据来源: WILEY

摘要:

The deeply‐towed instrument package of the Scripps Institution of Oceanography was used to study for the first time the fine scale structure of an overlapping spreading center (OSC) system: the 9°03’N OSC on the East Pacific Rise (EPR). The eastern and western limbs of this OSC system which are 8 km apart and overlap by 27 km show marked differences. Away from the tips, the spreading centers are highly tectonized and differ greatly from the ridge tips. The western ridge tip exhibits a few fissures but no sign of recent volcanic activity whereas the eastern ridge tip is highly fissured and has experienced more recent volcanic episodes which are confined to a narrow zone within this highly fissured area. In both cases, the ridge tip fissures are 10°–15° oblique to the strike of the faults that occur on the flanks of the adjacent portion of the opposite spreading axis, indicating a recent change in the direction of deviatoric tension and suggesting that the eastern and western spreading centers have been propagating southward and northward respectively. The eastern ridge tip fissures are transecting the flank of the western spreading axis suggesting that the eastern spreading center is about to become the through‐going trace of the East Pacific Rise. In contrast, the western spreading center disappears into abyssal hill terrain that was presumably created at the eastern ridge axis. Some of the very few tectonic features observed in the overlap basin may reflect the existence of a short‐lived shear couple due to the interaction of the two overlapping spreading centers. Sediment cover over the whole area and especially in the overlap basin is relatively thick, supporting the idea of a time‐averaged deficit in the magmatic budget at this OSC. We suggest that the 9°03’N OSC developed where two magmatic pulses, independent in space and time and propagating along the strike of the East Pacific Rise away from the loci of melt emplacement, failed to meet. Misalignment of the magmatic conduits resulted in the propagation of the two spreading centers past each other and the development of the ensuing OSC geometry. Independent lines of evidence derived from the three‐dimensional inversion of the magnetic field at the 9°03’N OSC (Sempere et al., 1984), from numerical modelling of the growth of two en echelon elastic cracks in a tensile stress field (Sempere and Macdonald, 1986) and from geochemical data (Langmuir et al., 1986) support our interpretation of the Deep‐Tow data. We suggest that deviations from axial linearity of the spreading centers (DEVALs; Langmuir et al., 1986) and small nonoverlapping offsets (SNOOs; Batiza and Margolis, 1986) are simply local lows or saddle points along the axial depth profile that arise when two magmatic pulses propagating toward one another meet head on. OSCs and some saddle points (i.e., DEVALs, SNOOs) are all small, rapidly evolving ridge axis discontinuities which may represent the surficial expression of the distal ends of small scale longitudinal convection cells benea

ISSN:0278-7407    

DOI:10.1029/TC005i006p00881

年代:1986

数据来源: WILEY

摘要:

Recently published age and structural data allow the reconciliation of previously conflicting models for Late Jurassic genesis of the Josephine, Smartville and Coast Range ophiolites, and the Nevadan orogeny in the Klamath Mountains and Sierra Nevada. The resulting model is consistent with the mode of initiation, location and geometry of the Great Valley forearc basin, and with the lack of a significant forearc basin west of the Klamath Mountains. The Coast Range ophiolite formed by backarc spreading west of an east‐facing intraoceanic arc. Soon thereafter, a remnant arc was calved off the west side of this arc, and the Smartville ophiolite formed by backarc (interarc) spreading. During this time, the Sierran phase of the Nevadan orogeny began as the intraoceanic arc encountered the west‐facing continental‐margin arc of North America. An east‐west‐trending calcalkaline dike swarm in the Sierra Nevada foothills may mark the trajectory of the colliding arcs at the initiation of the collision. Simultaneously, a new subduction zone was initiated west of the collision (suture) zone, and this new trench propagated southward, thus trapping the Coast Range ophiolite in the new forearc area south of the Klamath area. Intense deformation in the Sierran region resulted from this collision, and both magmatic arcs became inactive as the last remnant of intervening oceanic crust was subducted. Continued westward relative movement of the North American arc was permitted north of the Sierra Nevada owing to the lack of a colliding intraoceanic arc. The result was the westward rifting of the continental‐margin arc by intraarc spreading, which formed the Josephine ophiolite in the Klamath area. The Klamath phase of the Nevadan orogeny resulted from contraction of the west‐facing intraoceanic arc and Josephine backarc basin beneath the continental margin. Basal sediments of the Great Valley forearc basin were derived primarily from the sutured arc/ophiolite terranes, and were deposited on top of the Coast Range ophiolite, the southern edge of the Klamaths, and the western side of the Sierra Nevada. A new (late Mesozoic) magmatic arc was superposed across the previously accreted terranes, and formed the primary sediment source for the Cretaceous

ISSN:0278-7407    

DOI:10.1029/TC005i006p00901

年代:1986

数据来源: WILEY

摘要:

The thermal consequences of stacking‐up thrust slices in collision zones are investigated using simple one dimensional thermal models. The metamorphic evolution (geotherms and (P,T, time) paths) of a given tectonic unit belonging to a crustal stacking wedge made of more than two units is governed by three effects: cooling and pressure decrease associated with erosion, cooling by the lowest units (screen effect), heating by the upper units (cover effect). It is shown that the efficiency of these effects are dependent on the tectonic evolution of the collision zone. The metamorphic evolutions are very sensitive to the following tectonic parameters: the number of thrusted units involved in thickening, the time delay between each thrust and finally the mode of stacking of the different units (over and understacking). It appears that, for a given depth of burial, the temperature increase during uplift is less important in a crust thickened by three or four units than in the case of two units. The screen effect during understacking is more efficient for a short time delay (∼ 10 Ma) between each thrust.Overstacking leads to higher temperatures before uplift when compared to understacking. It is also shown that the thermal perturbation induced in an intermediate unit of the pile is more efficiently recorded when its thickness is rather small (∼ 10 km). It is shown that, for an erosion‐controlled uplift, the shape of the (P,T,t) path depends on the position of the rocks within a given unit and on the unit position within the pile. Finally, the general metamorphic evolutions during crustal thickening (HP‐LT metamorphism and subsequent overprint) are discussed in terms of the previously mentioned parameters. In the case of the Western Alps, it appears that the more or less efficient greenschist overprint of the units involved in thickening can be explained by different time intervals between the thru

ISSN:0278-7407    

DOI:10.1029/TC005i006p00913

年代:1986

数据来源: WILEY

摘要:

A combined K‐Ar and40Ar/39Ar geochronological study of the Ruby Mountains‐East Humboldt Range, Nevada, a Cordilleran metamorphic core complex, has revealed a complex tectonothermal history. The three structural subdivisions of the core complex (migmatitic core, mylonitic zone, and cover) share magmatic and deformational histories in part, but also display important contrasts in structural style, metamorphic paragenesis, and thermal history. During the Late Jurassic (ca 160 Ma), miogeoclinal rocks were polyphase‐deformed, metamorphosed under amphibolite facies conditions, and pervasively intruded by peraluminous granitic magmas, thereby forming an igneous and metamorphic complex. Late Jurassic muscovite granite porphyry plugs also intruded unmetamorphosed cover rocks. Late Cretaceous cooling of the igneous and metamorphic complex is locally suggested by poorly resolved Cretaceous incremental‐release40Ar/39Ar hornblende spectra and by U‐Pb monazite ages from pelitic schist exposed in the northern Ruby Mountains. Between ca 45 Ma and 20 Ma, the igneous and metamorphic rocks experienced an episode of complex extensional tectonism which involved the development of low‐angle, normal‐sense, simple shear zones. As a result, rocks metamorphosed at depth in the Mesozoic were translated to higher crustal levels which resulted in cooling through temperatures required for the intracrystalline retention of argon within minerals. Near concordance of hornblende and biotite plateau ages from the migmatitic core of the complex suggest relatively rapid exhumation. However, it is not certain if this cooling followed a prolonged maintenance at elevated temperatures after the ca 160 Ma metamorphism or if distinct, locally superposed late Mesozoic and/or middle Tertiary thermal overprints had affected the cooling history. A major extensional simple shear zone is marked by an approximately 1‐kilometer‐thick mylonite zone that experienced late kinematic, superposed brittle deformation. A geometric consequence of the development of low‐angle extensional fault zones is the eventual translation of colder rocks over hotter rocks. Such a history is supported by the40Ar/39Ar data. For example, mylonitic mafic orthogneisses from a major allochthon record40Ar/39Ar hornblende plateau ages between 44 and 48 Ma and biotite plateau ages between 32 and 33 Ma, and are structurally positioned above mylonitic rocks of the northern Ruby Mountains which record biotite cooling ages of ca 22–24 Ma. Such relations suggest either multiple episodes of Tertiary mylonitization or a protracted, multiphase history of mylonitization. However, after ca 20 Ma, all of the presently exposed mylonitic rocks were maintained below 100°C and have only experienced upper crustal brittle deformation. This article contains

ISSN:0278-7407    

DOI:10.1029/TC005i006p00931

年代:1986

数据来源: WILEY