摘要:

Deep seismic reflection profiling (ECORS program vertical and wide‐angle survey) coupled with field and subsurface geology are used to propose a complete NNE–SSW section of the Variscan Belt in western France from Belgium to southern Brittany. On this 800 km long profile, which probably crosses two Paleozoic sutures, the Variscan Belt appears as a broad fanlike orogen characterized by large (100 km) northward and southward overthrusts with polyphase deformation and metamorphism. The central part of the Belt (Central Armorican zone) suffered a simpler dextral shearing parallel to the strike of the belt with only green schist metamorphism. Deep reflection profiling shows that all the large thrusts visible at the surface root deeply in the lower crust or even crosscut the Moho. In the same way, all the large strike‐slip faults parallel to the belt appear at depth as narrow discontinuities which crosscut the whole crust and, in some cases, the Moho. The seismically well‐layered crust develops essentially at depth in the most deformed and metamorphic internal parts of the belt, in contrast with the relatively transparent Brabant foreland basement, which escaped the Variscan deformation. The Moho appears as a relatively flat boundary at an average depth of 35 km with different characters: It is sharp and strongly reflective both by wide angle and vertical seismics below the internal thrust zones, much less distinct in the central Armorican zone, and only defined by low frequency signals at wide angle below the Brabant foreland. All these characteristics led us to consider the layering of the lower crust and the Moho itself as initially tectonometamorphic features essentially produced by shearing and crust mantle decollement during the Variscan stacking of the crust by intracontinental lithospheric sub

ISSN:0278-7407    

DOI:10.1029/TC007i002p00141

年代:1988

数据来源: WILEY

摘要:

Nearly 270 km of crustal seismic reflection data obtained by Lithoprobe in the southern Canadian Cordillera provide a geometric link between the Rocky Mountain foreland thrust and fold belt, the Purcell anticlinorium, and the extensional regime superimposed on the crystalline core zone. Autochthonous North American basement and its overlying deformed and transported cover can be traced from the thrust and fold belt, beneath the Rocky Mountain trench, to 20 km depth beneath the central part of the Purcell anticlinorium. The Purcell anticlinorium is cored by foreshortened Proterozoic supracrustal rocks that were carried northeastward on a series of west dipping imbricate thrust faults. These faults crop out within and east of the anticlinorium and converge downward with subhorizontal detachments above the autochthonous North American basement. Beneath the western Purcell anticlinorium and Kootenay Arc, reflections associated with the Purcell stratigraphy and its underlying crystalline basement terminate at about 20 km depth and may be truncated against the east dipping Eocene Slocan Lake fault zone. The Slocan Lake fault zone is clearly imaged from the surface to about 12 km depth and can probably be followed discontinuously to about 25 km depth. A west‐dipping, high amplitude reflection from beneath the Valhalla gneiss complex outlines the domal geometry of the complex and is probably related to an east verging compressional shear zon

ISSN:0278-7407    

DOI:10.1029/TC007i002p00157

年代:1988

数据来源: WILEY

摘要:

Eocene extension contributed significantly to the present crustal architecture of the southern Omineca Belt in British Columbia and Washington. High grade gneiss complexes (Valhalla, Okanagan, Kettle‐Grand Forks, Monashee, and Priest River) preserve Cretaceous to Eocene deformation superimposed on older structures and have Eocene biotite and muscovite cooling ages. They are juxtaposed by regionally extensive, low‐ and moderate‐angle, ductile and/or brittle normal faults (Valkyr‐Slocan Lake, Okanagan Valley, Kettle River, Granby‐Greenwood, Columbia River, Standfast Creek (in part), and Newport and Purcell Trench faults) against metamorphosed rocks with a late Paleozoic to Middle Jurassic compressional tectonic history. Some upper plate rocks are overlain by Middle Eocene strata. Upper plate rocks preserve middle Cretaceous and older mica cooling dates indicating that they were less than 300°C in the Eocene, in contrast to lower plate rocks. The complexes have features in common with metamorphic core complexes of extensional origin elsewhere. U‐Pb zircon and monazite dates on mylonitic granitic rocks in the footwalls of the Okanagan and Valkyr‐Slocan Lake shear zones prove that a significant part of their ductile fabric is related to displacement on Eocene extensional faults. On the eastern side of the Monashee complex, 55 Ma U‐Pb zircon and circa 54 Ma Rb‐Sr synkinematic muscovite ages demonstrate that the ductile‐brittle Columbia River fault is a predominantly Early Eocene normal fault. Contrary to previous interpretations, the circa 162 Ma Galena Bay stock does not intrude footwall mylonites, and therefore the interpretation that at least some of the mylonites are related to Eocene extension is permissible. The distribution of Eocene cooling ages implies that part of the Standfast Creek fault on the eastern boundary of the Clachnacudainn complex is a ductile (+/−brittle) normal fault. Analogous interpretations are made for the Kettle‐Grand Forks and Priest River complexes where similar isotopic cooling age patterns prevail. Normal fault systems which bound the metamorphic complexes are fundamental crustal breaks, with displacements of 10–20 and in some cases 40 km, and probably accommodated about 30% extension across the 300 km width of the southern Omineca Belt. Most of the east dipping fault systems were active mainly between 58 and 52 Ma, in contrast to west dipping systems which are 52–45 Ma old, although both systems may have had some younger brittle displacement. Comparison of east–west cross sections with palinspastic restorations implies that the crust was more than 50 km thick prior to extension, that the high grade core complexes were not exposed to erosion prior to the Eocene, and that they were technically denuded and exhumed on Eocene normal fault systems. This extensional model is consistent with known geology, helps to explain several enigmatic geologic relationships, and has important implications for interpreting the pre‐Eocene, compres

ISSN:0278-7407    

DOI:10.1029/TC007i002p00181

年代:1988

数据来源: WILEY

摘要:

Simple shear and pure shear extension of the lithosphere produce very different patterns of heat flow and topography. These differences are investigated using a numerical technique which solves for two‐dimensional conductive and advective heat transport through time. Simple shear extension of the lithosphere is modeled as occurring along a straight shear zone. Two parameters define the simple shear model: the dip of the shear zone and its width. Likewise, the pure shear model is defined by two variables: the initial width of a vertical zone of pure shear extension and the rate of change of its width. These pairs of parameters are varied between calculations, as is the overall rate of extension. Each model results in distinct patterns of crustal thinning, lithospheric thermal structure, heat flow, thermal uplift, crustal subsidence, and topography. For the simple shear model, extension results in asymmetric uplift across the rift, while the total volume of uplift is limited by the total amount of extension. The peak heat flow and thermal uplift are centered over the intersection of the shear zone with the surface. Isostatic response to simple shear extension results in successive, formerly active shear zones being rotated into listric faults which sole into a subhorizontal detachment. The pure shear results show that the surface heat flow is greater for smaller widths of the zone of extension. For the same overall extension rate, a pure shear model with a narrow zone of extension can result in pressure release melting of the mantle long before low angle simple shear models. These results are compared with topographic and heat flow data from the northern Red Sea rift, a Neogene continental rift which is close to initiating seafloor spreading. The long wavelength topographic asymmetry across the Red Sea, which has been cited as evidence for simple shear extension of the lithosphere, is not matched by any of the models. The observed high heat flow anomalies in the Red Sea require a large component of pure shear lithospheric extension centered under the region of maximum crustal extension. In contrast, at the plate separation rate of the northern Red Sea, simple shear extension of the lithosphere along a shallow (<30°) dip detachment is ineffective in reproducing the observed heat flow anomalies. Only a narrowing region of pure shear extension can satisfy the width of the rift, and the peak heat flow values and generate pressure release melti

ISSN:0278-7407    

DOI:10.1029/TC007i002p00213

年代:1988

数据来源: WILEY

摘要:

Anisotropy of magnetic susceptibility (AMS) is capable of recording finite strain in weakly magnetized rocks. AMS was measured for 228 samples from 20 sites in two mylonite zones with the same deformational history. AMS measurements were compared with finite strains determined from dike rotations and from foliation orientations. In one zone (the Santa Catalina Mountains) the orientations of susceptibility and finite strain ellipsoids are in excellent agreement, and there is a logarithmic relationship between susceptibility difference(ΔKi=[Ki−K¯]/K¯)and finite strain magnitude. In the second zone (the Pinaleno Mountains) minimum susceptibility is perpendicular to the finite flattening plane, but the maximum susceptibility does not parallel the maximum extension direction, and there is no systematic relationship between susceptibility magnitude and strain magnitude. Oriented polished thin sections indicate that magnetite in the protolith of the Santa Catalina mylonite occurs as randomly oriented, elongate grains. With subsequent deformation, the long axes are rotated into the maximum extension direction. In the Pinaleno mylonites, both equant and elongate magnetite grains are present. With deformation, the elongate magnetite grains are rotated into the maximum flattening plane but show no preferred orientation within this plane. AMS in the two mylonite zones appears to be predominantly controlled by the orientation of elongate magnetite grains with respect to the megascopic fabric. The final orientation of the elongate grains is a function of their initial orientation as well as the finite strain. Therefore, despite similar deformational histories, the two zones display different AMS patterns due to the differences in occurrence, initial orientation, and shape of ferromagnetic gr

ISSN:0278-7407    

DOI:10.1029/TC007i002p00235

年代:1988

数据来源: WILEY

摘要:

Previous investigations have suggested that the direction of least principal stress in the western United States has shifted from northerly to easterly between 50 to 30 Ma. An effort is made to more tightly constrain the time of this shift using igneous dike‐host rock systems in which the isotopic age of both host and dike is known and differs by less than 10 Ma. In such systems, the possibility that magma intruded along old fractures unrelated to the state of stress at the time of intrusion is minimized and the axis of least principal stress is considered to be normal to the strike of the subvertical dike during the time interval between the ages of the dike and host rock. These systems indicate that a northerly least principal stress orientation persisted more recently than previously believed, until probably after 30 Ma, possibly about 26 Ma, and conceivably as recently as 18 Ma in some areas. The data are insufficient and too loosely bracketed in time to decide how fast the change in state of stress occurred in any one place and if by systematic rotation or abrupt shift. However, the change could be diachronous—earlier in the southeast and later in the northwest. Major easterly trending magmatic and tectonic elements of Tertiary age in the Great Basin of Utah and Nevada are compatible with the dike data. However, numerous metamorphic core complexes and detachment fault terranes seem to indicate more or less easterly extension throughout the time period represented by the dikes. This inconsistency is not clearly resolvable, but may reflect differences between the regional plate stress field and gravitational stresses in a crust overthickened by Mesozoic compress

ISSN:0278-7407    

DOI:10.1029/TC007i002p00249

年代:1988

数据来源: WILEY

摘要:

Emplacement‐driving mechanisms for thrust systems have been briefly reviewed and compared with distinctive structural styles in thrust belts. A B‐type mode, which implies a push from the rear and an elastic‐brittle behavior of the rock mass, appears to be suitable as a general model for the emplacement of thrust sheets in thin‐skinned fold and thrust belts where stratal shortening of sedimentary covers and imbrication of upper level crustal rocks occur. In such an environment that the mechanical behavior of the crustal section involved the mode, and timing of deformation, and the geometry of thrust systems are all strongly dependent upon the coupling between induced tectonic strain and transient, close to lithostatic pore fluid pressure buildup over a critical areal extent. The model presented here does not require a weak basal horizon for the detachment of thrust sheets, and pore pressurePpdoes not need to be high at once under the entire length of the thrust wedge. Rather, the transient buildup ofPpand hence the spatial and temporal progressive weakening of the rock mass are directly coupled with defo

ISSN:0278-7407    

DOI:10.1029/TC007i002p00261

年代:1988

数据来源: WILEY

摘要:

The stratigraphic and structural features of the Hazara syntaxis are described. A special aim of this work was to integrate modern approaches to rock deformation with the regional tectonics. The region is one of overthrust and shear zone tectonics associated with the development of at least two superimposed sets of major folds and associated minor structures (microfolds, cleavage, vein systems and various types of lineations related to rock strain or intersections of planar structures). It is concluded that the syntaxis results from an early set of nappe units developed by southwestward overthrusting of previously metamorphosed (Himalayan) rocks followed by the formation of a large shear zone structure and finally by the transport of overthrust units from northwest to southeast.

ISSN:0278-7407    

DOI:10.1029/TC007i002p00273

年代:1988

数据来源: WILEY

摘要:

New39Ar−40Ar radiometric data are presented. They concern the metasedimentary series and the orthogneiss pluton of Kangmar. Ages as young as 13 Ma are found which may be related either to the metamorphism or to the resetting of the minerals during the emplacement of the numerous intrusive bodies into the North Himalaya Belt. Inherited excess argon is found in micas from the orthogneiss previously dated at 562 Ma. These bodies are constituted by leucogranitic magmas, the biotites and the muscovites of which the granites intrude (Lhagoi Kangri Massif). Same range of ages is found in the High Himalaya Belt for micas from leucogranites. Ages as young as 4 Ma are found on micas from the southern part of the cross section between Nyalam and Zham. The recent uplifts and/or the movement of recent normal faults may be related to these young age

ISSN:0278-7407    

DOI:10.1029/TC007i002p00299

年代:1988

数据来源: WILEY

摘要:

It is proposed that many summit basins along the Aleutian Arc form from the clockwise rotation of blocks of the arc massif. Summit basins are arc‐parallel grabens or half‐grabens formed within the arc massif and are commonly located near or along the axis of late Cenozoic volcanism. Geomorphically, the Aleutian Arc appears to consist of contiguous rhombic blocks of varying size, tens to hundreds of kilometers in length. The boundaries between adjacent blocks are delineated by fault‐controlled canyons that cut the southern slope of the arc transverse to its regional trend. Evidence that these blocks have rotated clockwise is provided by the triangular‐shaped summit basins bordering the blocks to the north, oblique physiographic trends, offsets in the summit platform, and broad deflections in the southern slope of the arc. We present a model for block rotation that involves translation of blocks parallel to an arc. It is suggested that block rotation, which appears to have accelerated in late Cenozoic time, is linked to (1) a shift in the Euler pole for the Pacific plate, (2) the consequential start‐up of late Cenozoic volcanism, (3) improved interplate coupling instigated by sediment flooding of the Aleutian Trench, and (4) westward subduction of northeast striking segments of the inactive Kula‐Pa

ISSN:0278-7407    

DOI:10.1029/TC007i002p00327

年代:1988

数据来源: WILEY