摘要:

The strong Tertiary deformation of the northern margin of the Indian plate and of the Tsangpo suture zone is not matched by comparable deformation within the Lhasa block, yet the southern margin of the block was the site of large‐scale intrusive activity (the Gangdese, or Transhimalayan, batholith) throughout the late Cretaceous and was, therefore, presumably weaker than its surroundings. This poses a problem for any approach that interprets the deformation of Tibet in terms of diffuse crustal thickening. However, there is evidence that the Lhasa block was, in elevation as well as in magma type, an Andean margin by the end of the Cretaceous, and we suggest that the buoyancy force associated with the elevated crust was sufficient to inhibit compressional deformation within the southern Lhasa block. This suggestion is tested quantitatively by treating the continental lithosphere as a thin viscous sheet containing an isostatically compensated elevation contrast. When subjected to boundary conditions representing the collision between India and Asia, the sheet accommodates convergence by diffuse deformation over a region comparable in size with the Tibetan plateau; in the absence of an elevation contrast this deformation consists of approximately 100% thickening strain over the region. A precollision elevation contrast of 3000 m in the Lhasa block would have resulted in its experiencing roughly 40%, rather than 100%, postcollisional thickening; smaller elevation contrasts would have resulted in greater thickening, and vice versa. These calculations have implications for the mode of crustal thickening in northern Tibet: As the available evidence suggests that the topographic expression of the earlier Mesozoic tectonic activity in southern Asia was erased before the India‐Asia collision, we expect that there has been widespread and relatively homogeneous crustal shortening migrating northward from the Lhasa block during the Terti

ISSN:0278-7407    

DOI:10.1029/TC005i001p00001

年代:1986

数据来源: WILEY

摘要:

The late Cenozoic episode of crustal shortening in the South Island, New Zealand, which has caused present day uplift rates of greater than 10 mm/yr and has formed the Southern Alps, may have also caused clockwise rotation of the Alpine Fault by up to 20°. In contrast to the northern end of the Alpine Fault, which appears to have remained fixed relative to the Australian plate, increasing westward movement of the Fault trace has occurred southward along the fault. This westward movement may exceed 100 km where the Fault crosses the coastline in northern Fiordland. Most shortening adjacent to the southern end of the Alpine Fault has occurred within Australian plate continental crust which has been thrust beneath Pacific plate continental crust. This zone of continental underthrusting merges with the Fiordland subduction zone further south, where Australian plate oceanic crust is being subducted. Gravity modelling of the continental collision zone beneath the Southern Alps indicates that the leading edge of underthrust Australian plate crust may be close to a relatively sharp 10 km change in Moho depth. Inward dipping thrust faults on both plates mark the outer limits of the collision zone, which is over 300 km wide near the southern end of the Alpine Fault. The probable increase in crustal thickness due to the late Cenozoic shortening suggests that the amount of crustal thickening is similar in size to the amount of eroded crust

ISSN:0278-7407    

DOI:10.1029/TC005i001p00015

年代:1986

数据来源: WILEY

摘要:

We propose a kinematic model for southern California based on late Quaternary slip rates and orientations of major faults in the region. Internally consistent motions are determined assuming that these faults bound rigid blocks. Relative to North America, most of California west of the San Andreas fault is moving parallel to the San Andreas fault through the Transverse Ranges and not parallel to the motion of the Pacific plate. This is accomplished by counterclockwise rotation of California south of the San Andreas fault and by the westward movement of central California north of the Gar lock fault. The velocities of the blocks are calculated along several paths in southern California that begin in the Mojave Desert and end off the California coast. A path that crosses the western Transverse Ranges accumulates the accepted relative North America‐Pacific plate velocity, whereas paths to the north and south result in a significant missing component of motion. This implies the existence of a zone of active deformation in southern California that is interpreted to include the western Transverse Ranges and northwest trending, predominately strike‐slip faults close to the coast both north and south of the Transverse Ranges. Strain on this system accounts for about a third of the total North America‐Pacific plate m

ISSN:0278-7407    

DOI:10.1029/TC005i001p00033

年代:1986

数据来源: WILEY

摘要:

Cretaceous and early Tertiary arc and forearc rocks found along the coast in southern California and Baja California have been shown by paleomagnetic measurements to have originated many hundreds, or even several thousands, of kilometers south of their current locations. Northward transport also is found in Cretaceous batholithic rocks near the edge of the continent in Washington and British Columbia. The consistency of this pattern suggests that slices of arc and forearc rock originating on the western edge of North America have been translated along the coast by strike‐slip faulting. Strike‐slip faults are shown to be an expected response to north‐oblique subduction of the Farallon and/or Kula plates. Faulting and subduction probably were concurrent activities; i.e., the arc and forearc grew and were displaced simultaneously. Because displacement was dominantly parallel to subduction‐related lithic belts, arc and forearc rocks may have traveled large distances without producing either significant hiatuses in the rock record or conspicuous disruptions in the distribution of lithic belts. Subduction‐related strike‐slip faulting is shown to be favored by high angles of oblique convergence, shallow dip of the subducting slab, relatively easy slip on potential faults within the arc or forearc, and the existence of a place for the moving sliver to go. It is suggested that these conditions are not difficult to satisfy and that subduction‐related strike‐slip faulting may thus be a common feature of convergent orogens. The location of the San Andreas fault inboard from the continental margin may be attributable to localization of the Pacific‐North American transform by a system of precollision subducti

ISSN:0278-7407    

DOI:10.1029/TC005i001p00049

年代:1986

数据来源: WILEY

摘要:

Mesozoic metavolcanic rocks forming part of the continental volcanic arc along the eastern Sierra Nevada near Mt. Goddard and in the Ritter Range show a complex history related to extensional tectonics. The rocks comprise a thick section of tuffs, breccias, lava flows, sills, and ash‐flow tuffs deposited in a subaerial to subaqueous environment, with some subvolcanic sill‐like plutons. Pb/U ages of the rocks in the Mt, Goddard area range from ca. 130–160 Ma, while rocks in the Ritter Range have a somewhat wider age range as reported previously. Repetition of the section occurs by faulting, and with the exception of parts of the mid‐Cretaceous Minarets Caldera, all the volcanic rocks show a regional slaty cleavage which was subsequently crenulated and/or folded locally. The first cleavage has well‐developed stretching lineations, and does not appear to have been associated with significant folding. Finite strain measurements show considerable variation both in magnitude and symmetry. The Mt. Goddard rocks, however, tend to show slightly higher overall strain magnitude and greater constrictional component than the Ritter Range for rocks of comparable age. Calculations based on the strain data suggest the Mt. Goddard section has been thinned by about 50% normal to bedding, much as that documented previously for rocks in the Ritter Range. Deformation within this part of the continental arc was originally thought to have formed by regional compression during the late Jurassic (Nevadan) orogeny. However, our study indicates that (1) parts of the deformed volcanic section are younger than late Jurassic, (2) Nevadan‐age breaks in deposition are not present, (3) large‐scale folds expected during a regional compression event are not common, and (4) the beds were tilted to a high dip prior to internal deformation. An extensional model is proposed in which beds were rotated to high tilts early in the deformation as a result of listric normal faulting. This normal faulting is thought to have occurred above a regional tumescence related to voluminous magmatism at depth, with preservation of the steeply tilted Goddard and Ritter sections being facilitated by their downward transport along the margins of rising plutons. Flattening and steeply plunging constrictional fabrics superimposed on the tilted sections are related to strain induced by high‐level inflation of magma chambers and downward return flow of the keellike pendants. The main tectonic fabric shown by the continental volcanic arc rocks in the eastern Sierra Nevada is largely of Cretaceous age, rather than Jurassic (Nevadan) as originally supposed. In addition, the deformation, both rotation of beds and subsequent tectonite fabric, appears to be genetically related to the dynamic evolution of the magmatic arc, and not the result of an externally imposed

ISSN:0278-7407    

DOI:10.1029/TC005i001p00065

年代:1986

数据来源: WILEY

摘要:

Along the Grenoble‐Pelvoux‐Viso transect in the Western Alps, we schematize the geometrical relationships between (1) ductile, marly‐calcareous, sedimentary covers and (2) their more rigid substratum. This substratum may be the pre‐Alpine granitic‐gneissic basement, the Triassic dolomites, or even the Jurassic ophiolites. Basement mullions, a few kilometer size, are characteristic of the external part of the Western Alps arc (Dauphiné zone). They strongly contrast with the dolomitic or ophiolitic boudins, also a few kilometer size, in the inner part of the arc (Piedmont zone). The Dauphiné megamullions formed and then became accentuated, during the Oligocene and then the Neogene tectonic phases, which correspond to the last major Alpine contractions, at relatively shallow structural levels. In the sedimentary cover, upright or inclined folds were associated with slaty or strain‐slip cleavage development and low grade metamorphism. The corresponding stretching directions, locally subvertical, trend normal to the regional fold and mega‐mullions axes, and to the mountain belt itself. The Piedmont megaboudinage took place during the earlier, major Eocene, Alpine phase (which, but for some exeptions, did not affect the Dauphiné zone). In the very ductile “Schistes lustrés” that surround the dolomitic and ophiolitic megaboudins, generalized intrafolial recumbent folding was associated with blueschist metamorphism. The deformation, characteristic of a deep structural level, was a strong flattening within a subhorizontal plane and a preferential elongation parallel to the fold axes. Owing to regional horizontal festooning, those axes trend normal, oblique, or even parallel to the Piedmont zone, i.e., to the incipient mountain belt (restored initial trends). Coeval megaboudinage was probably omnidirectional (chocolate tablet pattern). The Eocene Piedmont structure has not been really obliterated during the later Alpine deformations leading to megamullions development in the Dauphiné zone. We summarize the very numerous stratigraphical, sedimentological, and tectonic field observations which lead us to regard each Dauphiné megamullion as directly inherited from a previous pre‐Alpine faulted block. Each Piedmont megaboudin has a similar origin. The corresponding paleofaults are in fact normal and/or strike‐slip faults of truly Tethyan age: Their synsedimentary offset occurred either during the Liassic‐Middle Jurassic passive margin rifting evolution of the southern edge of the European continent or during the Late Jurassic opening up of the Ligurian oceanic Tethys. We analyze how those Tethyan faults partly induced the style of the Alpine collisional structures, according to the regional characters of the successive synmetamorphic deformations, i.e., to their localization in t

ISSN:0278-7407    

DOI:10.1029/TC005i001p00095

年代:1986

数据来源: WILEY

摘要:

The northern Appalachian, dextral fault system of late Paleozoic age is also present in the central and southern Appalachians. An example of a dextral strike‐slip fault is the 4 km wide Brookneal shear zone in the southwest Virginia Piedmont. The shear zone is, in part, superimposed on the Melrose Granite where an S‐C mylonite was produced by dynamic recrystallization of all constituent minerals. The Arvonia metasedimentary and Charlotte belt metavolcanic rocks contain spaced dextral shear bands at a consistent +24° ± 3° to the shear zone boundary. A minimum displacement estimate of 17 km was obtained from rotated foliation measurements in the Melrose Granite. The age of movement on the Brookneal shear zone has been constrained by isotopic dating to between 324 and 300 Ma. Other faults in the southern Appalachians, including the Nutbush Creek and Modoc zones show similar ages and relative offsets. Possible plate tectonic models that could account for the late Paleozoic dextral fault system throughout the Appalachians include: (1) tectonic escape resulting from the collision of a plate with North America to the north of the Canadian Appalachians, (2) postcollision interplate readjustments involving counterclockwise rotation of Africa relative to North America, and (3) oblique convergence of eastern North America with an oceanic plate movin

ISSN:0278-7407    

DOI:10.1029/TC005i001p00119

年代:1986

数据来源: WILEY

摘要:

Palaeomagnetic results from the late Middle to early Late Devonian Comerong Volcanics in the Budawang Synclinorium, Lachlan Fold Belt (LFB), New South Wales, Australia, satisfy a fold test and support the hypothesis that foliated parts of the LFB have been folded by megakinks during the terminal mid‐Carboniferous orogenic phase. The preferred pole position is at 76.9°S, 330.7°E (A95= 7.2°), allowing calibration of the Australian pole path at around 370 Ma ago. An alterative pole position at 71.3°S 283.1°E (same A95) takes into account a possible rotation of the megakinked terrane by 15° clockwise with respect to the Australian craton. Although there are still problems unravelling previously published results from Silurian rocks in the LFB, both because of unreliability and uncertainty as to where the LFB crustal units lay in relation to the Australian craton, there is no geological reason to suspect any large relative movement between the LFB and the craton since the Middle Devonian, and thus the Comerong Volcanics pole position is representative of the Late Devonian for Australia. The new pole requires reassessment of many other poles that have previously been accepted as representing the Siluro‐Devonian for Australia, and Gondwanaland. We present reasons for believing that the magnetization, or remagnetization of many rock units (e.g., Mereenie Sandstone and Mulga Downs Group, Australia, and the Gneigura Supergroup, Africa) was later than sedimentation, probably during widespread Early Carboniferous deformation. Another rock unit (Msissi Norite, Africa) could be of Early Carboniferous, rather than Late Devo

ISSN:0278-7407    

DOI:10.1029/TC005i001p00135

年代:1986

数据来源: WILEY

摘要:

Overlapping spreading centers (OSC’s) are a fundamental aspect of accretionary processes at intermediate and fast‐spreading centers and typically occur at deep points along the axial depth profile. They have a characteristic geometry consisting of two en echelon overlapping, curving ridges separated by an elongated depression. The length to width ratio of this overlap basin is typically 3∶1. We have been successful in reproducing the overlapping spreading center geometry by modelling the growth of two initially parallel elastic cracks of given length and offset in a tensile stress field at infinity. A boundary element displacement discontinuity method was used to solve this problem. Our calculated results are compared with seafloor observations in terms of the size and shape of the overlap region. For small OSC’s, there is a very good agreement between calculations and observations but, for large ones, the overlap basin tends to be longer than our predicted results indicate. This suggests that the assumptions made in the model (i.e., perfectly elastic, isotropic and homogeneous medium) are perhaps valid for the brittle lid above the magma chamber that underlies OSC’s with small offsets (<2 km) but oversimplified for OSC’s with large offsets. Our modelling shows that the initial interaction of closely spaced surface ruptures along spreading centers is to deflect away from one another as they approach. The deflection will be the greatest for small misalignments of the fracture systems, thus even minor misalignments of the spreading centers may result in the development of OSC’s. Where the misalignment is less than the width of the cracking front, the fracture systems may meet head‐on creating a saddle point along the axial depth profile. Our results support the hypothesis suggested by Macdonald et al. [1984] in which overlapping spreading centers develop where two magmatic pulses migrate toward each other along the strike of the spreading center following fracture systems and magmatic conduits which are impe

ISSN:0278-7407    

DOI:10.1029/TC005i001p00151

年代:1986

数据来源: WILEY

ISSN:0278-7407    

DOI:10.1029/TC005i001p00165

年代:1986

数据来源: WILEY