摘要:

A simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the Basin and Range Province. However this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation (3–5 km) above core complex structures. Because topographic lows of this magnitude are not observed above core complexes today, it requires mountain ranges 3–5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat Moho as the core complexes developed. An alternate mode of isostatic compensation, whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust, provides an equally good fit to the observed detachment fault geometries, while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat Moho beneath these zones of upper crustal doming. If this interpretation is correct, then the effective elastic plate thickness below the Basin and Range Province probably cannot exceed a maximum of about 4 km, with a best guess estimate of about 0.5 to 1 km. Computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the Moho of about 100°C follows doming by several million years, perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks. Subsequent cooling of the mid to lower crust beneath the core complex could result in “freezing” of the initially ductile lower crust within the core of the dome, effectively locking the domal shape into the crust even after the detachment surface is cut by younge

ISSN:0278-7407    

DOI:10.1029/TC009i004p00557

年代:1990

数据来源: WILEY

摘要:

Cores from Cascadia deep‐sea channel contain sequences of turbidites that can be correlated and dated by the first occurrence of volcanic glass from the Mount Mazama eruption (6845±50 radiocarbon yrBP). Turbidity currents from the tributaries appear to have occurred synchronously to form single deposits in the main channel, there being only 13 turbidite deposits in the lower main channel since the Mazama eruption, instead of the twice as many expected if the tributaries had behaved independently. In addition to the Cascadia Channel, 13 post‐Mazama turbidites have been deposited in the Astoria Canyon and at two sites off Cape Blanco, sample locations that span 580 km of the Oregon‐Washington margin. Pelagic intervals deposited between the turbidites suggest that in each place the turbidity currents occurred fairly regularly, every 590±170 years on average. The best explanation of the spatial and temporal extent of the data is that the turbidity currents were triggered by 13 great earthquakes on the Cascadia subduction zone. The variability of turbidite timing is similar to that for great earthquake cycles. The thickness of the topmost pelagic layer suggests the last event was 300±60 years ago (from three places along the margin), but this number may be a biased underestimate. It is, however, consistent with the youngest sudden‐subsidence event on the Washington coast. The turbidite data demonstrate that the near‐term hazard of a great earthquake on the Cascadia subduction zone is of the order of 2–10% in the

ISSN:0278-7407    

DOI:10.1029/TC009i004p00569

年代:1990

数据来源: WILEY

摘要:

The Border Ranges fault system (BRFS) bounds the inboard edge of the subduction‐accretion complex of southern Alaska. In Eocene time a central segment of this fault system was reactivated as a zone of dextral wrench‐ and oblique‐slip faulting having a cumulative strike‐slip offset of at least several tens of kilometers, but probably less than 100 km. Early wrench folds are upright, trend at less than 45° to the strike of adjacent faults and developed with fold axes oriented subparallel to the axis of maximum incremental stretch λ1. These en echelon folds rotated and tightened with progressive deformation and then were overprinted by younger wrench folds that trend at about 60° to adjacent throughgoing faults. The latter folds are interpreted as forming during a late increment of distributed wrench deformation within the BRFS that included a component of extension (divergence) orthogonal to the mean strike of the fault system. A sharp releasing bend in exposures of a strike‐slip fault originally at>4 km depth today coincides with a narrow pull‐apart graben bounded by oblique‐normal faults that dip toward the basin. Widening of this pull‐apart graben by brittle faulting and dike intrusion accommodated less than 2 km of strike‐slip and was a late‐stage phenomenon, possibly occurring at supracrustal levels. Prior to formation of this graben during a period of predominantly ductile deformation at deeper structural levels, wrench‐folded rocks on one side of the nonplanar fault were translated around the releasing bend without significant faulting or loss of coherence. Kinematically, the earlier deformation was accomplished by fault‐bend folding and rotation of a relatively deformable block as it passed through a system of upright megakinks. Such a ductile mechanism of fault block translation around a strike‐slip bend may be typical of intermediate levels of the crust beneath pull‐apart grabens and may be transitional downward into heterogeneous laminar flow occuring along curved segments of ductile shear zones. Some degree of fault‐bend folding of strike‐slip fault blocks around releasing bends may be one reason why the amount of extension measured across natural pull‐apart basins is commonly observed to be less than the amount of s

ISSN:0278-7407    

DOI:10.1029/TC009i004p00585

年代:1990

数据来源: WILEY

摘要:

Geophysical data from the Levant and the eastern Mediterranean suggest that this area can be divided into distinct crustal units of different origins. For example, a marked difference in crustal structure and structural style exists in the Levant between Samaria and Judea, south of the Carmel structure, and the Galilee and Lebanon, north of it. On the other hand, several microcontinental fragments such as Cyprus, and the Eratosthenes and Anaximander Seamounts are embedded within the oceanic crust of the eastern Mediterranean. The present location of these units has resulted from complex tectonic processes which were active during the mid Paleozoic‐Mesozoic evolution of this area. During this period several crustal units were accreted to the northern margin of Africa while others were detached away from it. The crustal evolution of the eastern Mediterranean and the Levant is intimately linked with the closure of the Paleo‐Tethys, the formation of the Neo‐Tethys, and its subsequent cl

ISSN:0278-7407    

DOI:10.1029/TC009i004p00613

年代:1990

数据来源: WILEY

摘要:

A mineral assemblage comprised of chalcedony, drusy quartz, and very fine grained iron hydroxide (limonite), with or without blocky calcite (QFC), coats joint surfaces, fills fractures, and commonly occurs as pressure shadow encrustations on brittle fault surfaces in central New Hampshire but only occurs sporadically in other areas of New England. Locally, QFC includes silicified breccias of highly altered host and euhedral quartz crystal fragments suspended in a matrix of chalcedony and limonite. One hundred and four brittle faults from central New Hampshire, mineralized with QFC, can be separated into two families. Set N is composed of primarily north–northeast to northeast trending, steeply dipping normal faults (n=61), and set S is composed of north‐northwest to north‐northeast trending, steeply dipping, primarily right‐lateral strike‐ to normal oblique‐slip faults (n=43). These two sets of faults are thought to record two paleostress conditions. The extension direction for both tensors plunges gently east‐southeast, but the compression direction is near vertical for set N and trends roughly northeast, plunging 30° for set S. Fault‐slip inversion suggests that set N faults are best fit with σ1 most deviatoric (σ2 ≈ σ3). Set S is best fit with σ3 most deviatoric (σ1 ≈ σ2) and low values for fault plane coefficient of friction and/or high fluid pressure. QFC occurs in the 158 Ma Belknap Mountain Complex of the White Mountain Magma Series and is thought to have developed at relatively shallow crustal levels based on the assemblage, nature of fluid inclusions, and association of QFC as brittle fault and fracture filling mineralization. Occurrence of QFC in the 158 Ma Belknap Mountain complex, paleostress implications of QFC‐mineralized brittle faults, and range and character of fault fabrics in the Belknap units suggests that QFC developed around the time of Belknap intrusion and cooling and that Late Jurassic to Early Cretaceous is the approximate time at which a regional change in paleostress conditions occurred in New England. This age and nature of the paleostress change is in accord with a change postula

ISSN:0278-7407    

DOI:10.1029/TC009i004p00623

年代:1990

数据来源: WILEY

摘要:

New paleomagnetic data from the west Sicilian fold and thrust belt indicate large‐scale clockwise rotations during tectonic transport. For the platform and seamount paleogeographic units of the fold and thrust belt (Panormide and Trapanese units), there is a general consistency in the magnitude of rotation along strike, suggesting the existence of large coherent structural units. For the Panormide platform, clockwise rotations in the range of 90° to 140° relative to the underlying Iblean parautochthon are recorded along 125 km of outcrop strike. In the case of the Trapanese unit, clockwise rotations in the range of 47° to 70° relative to Iblei are recorded along about 90 km of outcrop strike. Intervening basinal paleogeographic units (Imerese and Sicanian units) show clockwise rotations of similar magnitude; although few reliable data are available from these units. Palinspastic reconstruction of the thrust belt, incorporating the rotations and arguments for the present orientation of the margins of the paleogeographic units, implies that the Panormide platform and Trapanese seamounts were originally elongated more or less north‐south relative to the present orientation of Iblei. The two units apparently coalesced to the south and were discontinuous to the north. They and their intervening basins were part of a passive continental margin which occupied a large part of the area that was subsequently rifted to form the Tyrrhen

ISSN:0278-7407    

DOI:10.1029/TC009i004p00641

年代:1990

数据来源: WILEY

摘要:

In the western Sicilian fold and thrust belt, large clockwise rotations of allochthons occurred during late Cenozoic contraction of part of the southern Tethyan margin. The magnitude of rotation decreases stepwise from over 120° in the upper sheets, lying on the north coast of Sicily, to no appreciable rotation in the frontal portion of the belt flanking the southern coast of the island. The allochthons are composed of imbricate thrust sheets derived primarily from individual basin and platform assemblages of the old Tethyan margin. Paleomagnetic and structural data indicate that the rotation of the allochthons was accommodated by coherent torsional displacements on relatively low‐angle detachment surfaces. Timing relations for the imbrication history of the Sicilian fold and thrust belt are derived from stratigraphic overlap and local involvement of sediments deposited in a series of foreland and piggyback basins. The locus of deposition within successive foreland basins first migrated easterly then southerly during progressive deformation in the orogen. Imbrication began in the early Miocene (Burdigalian‐Langhian) and continued at least through the early Pleistocene and appears to be continuing today. Rotation is related to thrusting and accompanies a 70° change in the tectonic transport direction from easterly to southerly. Easterly striking, right‐oblique transpressional faults and associated northeasterly trending folds postdate thrust sheet rotation in the interior of the thrust belt and were active contemporaneously with south‐directed thrusting in the foreland region. Pleistocene and possibly older (late Pliocene?) extension strongly modified the older thrust morphology along the Tyrrhenian coast of northwestern Sicily, with the development of down‐to‐the‐north listric normal faults. The extensional structures apparently are related to the opening and subsequent deformation of the Tyrrhenian S

ISSN:0278-7407    

DOI:10.1029/TC009i004p00661

年代:1990

数据来源: WILEY

摘要:

Migrated single‐channel seismic lines and Sea‐MARC II side scan and bathymetry data document an active accretionary prism along the obliquely convergent margin of south Panama, a region previously believed to be a transform margin. The eastern flank of the Coiba Ridge is being subducted in this region, creating the requisite geometry for along‐strike variation in trench sediment thickness and type. The regional east dip of the downgoing plate causes the depth of the oceanic crust along the trench to drop approximately 1800 m from west to east. In the western region of the study area the incoming sedimentary section consists of approximately 600 m of pelagic and hemipelagic sediments. A wedge of trench turbidites that overlies the incoming hemipelagic sediments thickens from less than 100 m in the western region to more than 900 m in the eastern region of the study area. The eastward increase in sediment thickness correlates with the following changes in the accretionary prism: (1) decrease in initial surface slope; (2) broadening of the inner trench slope; (3) increase in thrust spacing; (4) steepening of frontal thrusts; and (5) fold development. Each of the responses reflects, in varying degrees, the increase in volume and changing physical properties of the accreted material, as well as the changing shear strength of the décollement. The increase in overburden pressure and compaction in the deeper sediments, together with the change in lithology from oceanic to trenchfill sediments, should produce an eastward increase in sediment shear strength both within the wedge and along its base. Because the décollement will tend to form where the pore fluid pressure‐overburden ratio is at a maximum, the shear strength within the wedge should increase relative to basal shear strength. Decreased surface slopes will result from the increasing contrast in shear strength between the wedge and

ISSN:0278-7407    

DOI:10.1029/TC009i004p00683

年代:1990

数据来源: WILEY

摘要:

Middle to Late Jurassic samples have been collected from granitic rocks in three geological subdivisions of South Korea, from north to south: Kyonggi Massif, Okchon Folded Belt and Ryongnam Massif. Triassic samples (granites and sediments) were collected in the Ryongnam Massif and its border area with the Okchon Folded Belt. The Jurassic declinations of the two massifs are northeasterly (mean D/I = 39.6°/+57.6°, N = 7 sites, k = 31, α95 = 11.1°), whereas the intervening Okchon Folded Belt reveals northwesterly Jurassic declinations (D/I = 337.5°/+47°, N = 7 sites, k = 32, α95 = 10.8°). Reversed‐polarity directions are rare, but have been observed in three localities. The widespread occurrence of similar directions in a given province argues against major tectonic tilting of the granites; two sites in Jurassic sediments provide additional structural control. We interpret the direction difference between the Okchon belt and the adjacent massifs as due to left‐lateral shear in an actively deforming continental margin during the Late Jurassic‐Early Cretaceous Daebo Orogeny. The Triassic directions from the Ryongnam Massif and its border area are east–northeasterly (D/I = 62.9°/+35.7°, N = 5, k = 27, α95 = 15°) with the sediments showing normal and reversed polarities. Comparisons with Jurassic and Triassic paleomagnetic results from adjacent blocks in Asia (Eurasia, North Korea, North and South China) are hampered by a paucity of reliable paleopoles. What seems clear, however, is that North and South China and the major part of Korea yield Jurassic and younger paleopoles that are to first approximation similar; the exception is formed by the results from the Okchon Folded Belt. The aggregate of the China and Korea Blocks yields mean paleopoles for this interval that are different from those published for Eurasia, but this may be due to imperfections in the Eurasian reference poles. For the Triassic, the directions and paleopoles of the China Blocks and the Ryongnam Massif of South Korea are dissimilar, resulting in interpretations of the area in terms of relative motions

ISSN:0278-7407    

DOI:10.1029/TC009i004p00699

年代:1990

数据来源: WILEY

摘要:

Several Ordovician volcanic terranes are preserved between the Paleozoic North American margin and the Avalon Terrane in the northern Appalachians of Maine. They have been interpreted as remnants of an island arc on the basis of their current position in the Appalachians or as ocean islands near the margin of Avalon on the basis of paleontologic data. A study of one of these terranes, the Lunksoos terrane in northern Maine, was undertaken through a combination of paleomagnetic and structural analyses in order to understand its tectonic evolution. Paleomagnetic study of the pre‐Caradoeian (pre‐Late Ordovician) Stacyville volcanics revealed three components of magnetization. The final magnetization removed (D=90°, I=36°, α95=11.8°) is interpreted to be primary and yields a paleolatitude of approximately 20°S; however, a Late Ordovician remagnetization cannot be excluded. The Stacyville volcanics now overlie a Cambrian and Lower Ordovician sequence (the Grand Pitch and Shin Brook formations) that is believed to have been deposited at the southern side of Iapetus, possibly near the margin of Avalon. During the Middle and Late Ordovieian, Avalon was at paleolatitude s higher than 40°S, while the volcanics were either formed or remagnetized at a paleolatitude of 20°S. The pre‐Caradocian Stacyville volcanics therefore could not have been deposited directly on the Grand Pitch‐Shin Brook sequence but must have been tectonically emplaced. The Rockabema Diorite which intrudes the entire Cambro‐Ordovician sequence constrains the timing of this emplacement to be pre‐late Early Silurian. These relationships indicate that the Lunksoos area is a composite terrane within the Maine Appalachians. The tectonic emplacement scenario is unnecessary if the Grand Pitch and Shin Brook formations were not located on the south side of Iapetus during the Ordovician. This alternative, however, poses conflicts with the interpretation of

ISSN:0278-7407    

DOI:10.1029/TC009i004p00719

年代:1990

数据来源: WILEY