摘要:

Metamorphic core complexes in the D'Entrecasteaux Islands, Papua New Guinea, formed as the result of active extension at the western end of the propagating Woodlark Basin spreading center. Domes of high‐grade metamorphic rocks (i.e., amphibolites, eclogites, and migmatites), intruded by large granodiorite bodies, comprise the lower plate of the D'Entrecasteaux metamorphic core complexes. The domes are transected by kilometer‐scale shear zones. A thermochronologic study of the D'Entrecasteaux Islands utilizing K/Ar,40Ar/39Ar, and fission track techniques has documented the unroofing history of these active metamorphic core complexes. Gneisses in the cores of the domes cooled rapidly (≥100°C/m.y.) as indicated by hornblende and biotite40Ar/39Ar apparent ages of ∼2.7 to 3.0 Ma and ∼1.6 to 1.7 Ma, respectively, and apatite fission track ages of ∼0.4 to 0.9 Ma.40Ar/39Ar apparent ages on white mica, biotite, and potassium feldspar and fission track ages on apatites from shear zone gneisses indicate extremely rapid cooling (in some cases>500°C/m.y.) and suggest shear zones were active from 4.0 to 3.5 Ma and 1.9 to 1.4 Ma. In general,40Ar/39Ar mineral ages for retrogressed core zone gneisses, shear zone gneisses, and granodiorites are 2.0 to 3.0 Ma (amphiboles), 1.5 to 1.7 Ma (muscovites), and 1.4 to 1.8 Ma (biotites) and 1.0 to 2.0 Ma (K‐feldspars). Apatite fission track ages from core zone gneisses, shear zone gneisses and granodiorites range from 0.4 to 1.0 Ma. Thermochronologic results indicate that emplacement of granodiorites closely coincided with retrogression of the metamorphic basement and movement on the outer shear zones bounding the gneiss domes. The granodiorite bodies associated with the D'Entrecasteaux Islands domes represent syn‐kinematically emplaced granitoids intruded into an area of active cont

ISSN:0278-7407    

DOI:10.1029/93TC00235

年代:1993

数据来源: WILEY

摘要:

Retreating subduction boundaries, formed where the rate of subduction exceeds the rate of overall plate convergence, appear to be commonly developed features within regions of early or incomplete continent‐continent collision. They are characterized by regional extension within the overriding plate and, at their leading edge, by thin‐skinned arcuate thrust belts that are concave towards the overriding plate. As is illustrated by examples from the Mediterranean region, the formation of retreating subduction boundaries is intimately related to the process of continental collision. During the early stages of collision, retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary. Retreating plate boundaries can also form before the main collision, and the associated thrust belts emplaced as precollisional accretionary assemblages. Because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab (slab pull), subduction usually ceases when, and only when, thick buoyant continental crust enters the subduction zone. Thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted. In some cases, retreating subduction boundaries may “escape” into the open ocean, where they form nearly isolated, local tectonic systems. In these systems the rate of subduction is approximately compensated by the rate of upper plate extension, and migration of the system across the oceanic region may be very rapid. For example, the Horseshoe Seamounts, located about 800 km offshore in the eastern North Atlantic, may be the active expression of an east dipping, westwardly migrating retreating subduction boundary that has evolved from the Betic Cordillera‐Rif system active in Miocene time and may now be progressing across the Atlantic at approximately 50 mm/yr. An analogous situation may be represented by the Scotia Arc system, a westward dipping retreating subduction system located between the South American and Antarctic plates, which may have “escaped” into the South Atlantic ocean from a zone of crustal shortening in the Andes and is now progressing across the Atlantic at a rate of a

ISSN:0278-7407    

DOI:10.1029/92TC02641

年代:1993

数据来源: WILEY

摘要:

We present the results of a dynamical model of lithospheric rifting and rupture which show that a wide range of crustal thinning patterns across rifted passive margins can be produced by varying the steady state geotherm, lithospheric composition (dry versus wet materials), and strain rate. The basic mechanism of continental rupture is assumed to be passive rifting and necking. We use a numerical thermomechanical model of lithosphere extension based on a finite element approach. When plasticity is significant (i.e., at lower temperatures or for “harder” materials) deformation is unstable and thinning takes place abruptly, over a narrow area. Conversely, a progressive thinning across the margin is observed when creep is dominant (i.e., in warm or ductile conditions). Cooling and associated hardening of the thinned area can occur during extension and cause the locus of extension to migrate laterally. In these circumstances, rupture is likely to take place asymmetrically along one edge of the thinned area, producing a narrow margin and a very wide conjugate. The eastern margins of Canada and their conjugates across the North Atlantic provide examples which cover this range of theoretical profiles. The crustal thinning patterns, inferred from deep seismic data, and the duration of rifting compare well with model results. We discuss also the constraints that these geodynamical models provide on such current issues as the seismic reflectivity of the lower crust, or the location of the ocean‐continent boundary in wide areas supposedly underlain by 5‐km thin continenta

ISSN:0278-7407    

DOI:10.1029/93TC00197

年代:1993

数据来源: WILEY

摘要:

Structural studies carried out in the Lucanian Apennines (Southern Italy) show that strike‐slip faulting was the principal mode of deformation of this area during middle‐upper Pleistocene time. W‐NW to E‐SE trending left strike‐slip fault systems dissect the entire Apennine mountain belt and affect the preexisting thrust geometry. Strike‐slip faults, activated by a roughly E‐W shortening, are characterized by different geometries representing the surface response to lateral motion occurring along deep‐seated structures. The occurrence of different structural patterns which characterize different segments of strike‐slip system is related to (1) the depth of a major decoupling surface which separates the upper tectonic multilayered horizon (Apennines thrust belt system) from the lower rigid horizon (Apulian belt) in which strike‐slip structures have originated and (2) the geometric relationships between the strike‐slip faults and the thrust belt pattern which characterize the upper horizon. The different segments of the strike‐slip system are interpreted as internal deformation developed within a crustal shear zone. This zone, which corresponds to the boundary between the Apulian block and the Apennine chain, is characterized by sinistral movement as a response to the northwesterly convergent motion of the African plate

ISSN:0278-7407    

DOI:10.1029/92TC02251

年代:1993

数据来源: WILEY

摘要:

Paleomagnetic data from the Mojave Desert around Barstow, California suggest 55° to 75° of clockwise rotation of this area in earliest Miocene time, followed by 23° of counterclockwise rotation in the Early Miocene, and no rotation after 18 Ma. Earliest Miocene clockwise rotation of the Barstow area is suggested by data from nine sites in the Oligocene‐Miocene Lane Mountain Quartz Latite and Jackhammer Formation and may be related to oroclinal bending of the southern Sierra Nevada batholith. Data from 13 sites in the Early Miocene Pickhandle Formation indicate counterclockwise rotation, which coincides with Early Miocene E‐NE‐W‐SW extension and detachment faulting in the Mojave Desert. Other parts of the Mojave Desert experienced variable senses and amounts of rotation at this time which may be related to drag along transfer zones within the upper plate, while the lower plate was not rotated. These results suggest that Miocene extension in the Mojave Desert is related to similarly oriented extension in the Colorado River extensional corridor. Early Miocene to Pliocene rocks of the Andesite and Dacite of the Calico Mountains, Barstow Formation, Andesite of Murphy's Well, and Black Mountain Basalt show no evidence for significant rotation of the Barstow area after 18 Ma. Most other paleomagnetic data from elsewhere in the Mojave are consistent with a lack of significant post‐18 Ma rotations, suggesting that post‐10 Ma right‐lateral faulting there has produced relatively little crustal rotation. The exception to this generalization is the northeast corner of the Mojave Desert which underwent substantial post‐18 Ma clockwise rotation. Paleomagnetic flattening data also indicate that post‐Oligocene N‐S translation of crustal blocks in the Mojave Desert

ISSN:0278-7407    

DOI:10.1029/92TC02813

年代:1993

数据来源: WILEY

摘要:

Thermal evolution modeling based on the geometry of the Scotian Basin, eastern Canada, and analysis of the shear resistance associated with lithospheric faulting are used to investigate the initiation of subduction at passive margins. Previous studies have demonstrated the difficulty of subduction initiation at mature passive margins. However, unique features along a passive margin may facilitate subduction initiation. Subduction may initiate along a part of the margin (1) by extensional or strike‐slip deformation rather than shortening, (2) by deformation of the weaker continental lithosphere rather than oceanic lithosphere, (3) by heating and weakening of oceanic lithosphere due to lateral heat transfer from adjacent continental lithosphere, and (4) by heating and weakening of the lithosphere due to the passage of a mantle plume beneath the margin. The values of shear resistance under these conditions are still higher than those calculated for ridge push, indicating that subduction initiation at passive margins is difficult even with these weakening mechanisms. However, pulses of compression associated with plate reorganization may be enough to trigger subduction at such passive margin

ISSN:0278-7407    

DOI:10.1029/92TC02602

年代:1993

数据来源: WILEY

摘要:

The Plio‐Pleistocene nonmarine Saugus Formation is widely exposed in the east Ventura basin and the northern San Fernando Valley, north and south of the Santa Susana fault. In the east Ventura basin, the Saugus is overlain by the Pacoima Formation. Magnetostratigraphy of the fine‐grained Saugus and Pacoima strata and the presence of the 0.76 Ma Bishop ash are used to calculate their average sedimentation rates: 0.9 km/m.y. in the east Ventura basin and 1.1 km/m.y. in the northern San Fernando Valley. Extrapolation of the Saugus and Pacoima sedimentation rates shows that they were deposited from about 2.3 to 0.5 Ma both north and south of the Santa Susana fault. The shift from remote to locally derived clasts marks the initiation of uplift of the Santa Susana Mountains. The extrapolated age of this boundary is about 0.7–0

ISSN:0278-7407    

DOI:10.1029/93TC00133

年代:1993

数据来源: WILEY

摘要:

Late Cambrian stratigraphic relations are used to reconstruct aspects of the structural architecture buried between the St. Francois Mountains and the Mississippi Embayment. Such reconstructions are possible because characteristics of units deposited during early thermal subsidence help document late Proterozoic‐Early Cambrian Catoctin extension in southeast Missouri. New observations of the Lamotte Sandstone and Bonneterre Formation (Dresbachian) show that a mismatched architecture is present and was buried by late Bonneterre Formation (early Franconian) sediments prior to the deposition of the Davis Formation (early‐middle Franconian). A structural map of the region was developed by integrating such new observations with published relations. Analysis of stratigraphic and structural relations reveal that two periods of brittle thinning of the crust resulted from Catoctin extension in southeast Missouri. Although structural architecture is partially masked, these distinct periods of thinning produced (1) a compartmentalized, tilt block terrane characterized by down‐to‐the‐southeast breakaways and (2) a number of linked, convergent halfgrabens. Late Cambrian stratigraphic relations provide evidence that the linked half‐grabens were superimposed asymmetrically on the tilt block terrane. A four‐stage tectonic sequence consisting of early mafic volcanism, two periods of brittle thinning, and late recurrent ultra‐mafic volcanism is interpreted with the application of models of tectonism controlled by preexisting zones of weakness. An interplay between offset, preexisting zones of weakness helps explain the timing of associated magmatism, faulting, and graben development. Changes in structural style resulting from such an interplay suggest that Catoctin extension in southeast Missouri was a protracted episode of polyph

ISSN:0278-7407    

DOI:10.1029/93TC00109

年代:1993

数据来源: WILEY

摘要:

This paper summarizes studies of the structural geology in and around the island of Äspö before the excavation of an underground rock laboratory there to explore the problems of storing spent nuclear fuel in Swedish bedrock. Each of the major reorganizations of the plate boundaries around the Baltic shield are recognizable as distinctive fabrics or structures in the 1.7 Ga Proterozoic cratonic basement exposed at Äspö. This rock mass segmented along ductile shear zones, while in the lower crust, fragmented along much the same zones as it rose into the upper crust and has been jostled mainly by repeated reactivation of the same brittle fracture zones ever since. The first strains were ductile and penetrative and shared by all the rocks of the region. Subsequent strains became increasingly localized to the boundaries of relatively unstrained blocks. Ductile gneiss zones that segmented the crust at amphibolite facies narrowed to mylonites and then faults as the rock mass passed through the ductile‐brittle transition in greenschist facies at ≈1.4 Ga. Since they have been in the cool upper crust, many of the same shear zones have widened to brittle fracture zones. Fracture infills of decreasing age consist of different combinations of quartz, epidote, Fe‐oxihydroxides, chlorite, prehnite, and calcite. Fragmentation (generation of new fractures) generally gave way to jostling (reactivation of preexisting fractures) in successive episodes of brittle intracratonic strain at Äspö. The regional fracture pattern on all scales consists of different combinations of the same five fracture sets. By the end of Palaeozoic times, the Äspö bedrock had accumulated fracture zones with a sufficiently wide range of orientations that subsequent strains involved more jostling than fragmentation. Plate tectonic and glacial strains of Sweden anticipated in the next few million years are also likely to occur along long‐established zones of weakness that are already avai

ISSN:0278-7407    

DOI:10.1029/92TC02722

年代:1993

数据来源: WILEY

摘要:

A structurally simple, 35‐km‐thick, north facing stratigraphic succession of Late Archean to Middle Proterozoic rocks is exposed near the Montreal River, which forms the border between northern Wisconsin and Michigan. This structure, the Montreal River monocline, is composed of steeply dipping to vertical sedimentary rocks and flood basalts of the Keweenawan Supergroup (Middle Proterozoic) along the south limb of the Midcontinent rift, and disconformably underlying sedimentary rocks of the Marquette Range Supergroup (Early Proterozoic). These rocks lie on an Archean granite‐greenstone complex, about 10 km of which is included in the monocline. This remarkable thickness of rocks appears to be essentially structurally intact and lacks evidence of tectonic thickening or repetition.Tilting to form the monocline resulted from southward thrusting on listric faults of crustal dimension. The faults responsible for the monocline are newly recognized components of a well‐known regional fault system that partly closed and inverted the Midcontinent rift system. Resetting of biotite ages on the upper plate of the faults indicates that faulting and uplift occurred at about 1060 +/−20 Ma and followed very shortly after extension that formed the Midcontinent ri

ISSN:0278-7407    

DOI:10.1029/93TC00204

年代:1993

数据来源: WILEY