摘要:

Active deformation in the eastern Pamir of Central Asia is concentrated on the margins of the orogen with minor deformation within the high terrain. Along the Trans‐Alai mountain front at the northern perimeter of the orogen, Quaternary thrusting is documented by uplifted pediments, now at>500 m above the piedmont, Holocene fault scarps, and large earthquakes with N to NW oriented P axes. Seismicity in the interior of the orogen outlines a N–S belt that includes normal faulting events with E–W oriented T axes. N–S striking, active normal faults in the interior Lake Karakul region are compatible with these earthquakes; they define an asymmetric graben with a master fault at the western basin margin. In the southern Pamirs, dextral strike‐slip faults root in the dextral Karakorum Fault, which bounds the Pamirs to the east. A mixture of dextral and reverse offsets totalling 135 m in Pleistocene terraces and 8 m in late Pleistocene/Holocene deposits demonstrates contemporary transpression, indicating average displacement rates of<1 mm/yr. The concentration of young thrusts along the Trans‐Alai, the northward migration of thrusting, and the scarcity of other large‐scale shortening features within the eastern Pamirs suggest that this part of the orogen moves northward en bloc and causes the progressive annihilation of the intermontane Alai Valley. Widespread dextral shear in the eastern Pamirs, both to the south and north of the extensional Karakul depression, and combined dextral strike‐slip and normal faulting in the Muji‐Tashgorgan graben of the Chinese Pamirs are interpreted as localized space accommodation phenomena, formed during progressive transfer of compressional deformation along a dextral strike‐slip deformation zone with ex

ISSN:0278-7407    

DOI:10.1029/95TC00927

年代:1995

数据来源: WILEY

摘要:

The Hawaiian‐Emperor seamount chain in the North Pacific Ocean is commonly considered to have been produced by motion of the Pacific plate over a hotspot. If the hotspot is assumed to have remained fixed in a mantle frame of reference, the 60° change in trend between the Hawaiian and Emperor portions of the chain results from a change in motion direction of the Pacific plate relative to the mantle at 43 Ma, during Middle Eocene time. Such a large and abrupt direction change should result in significant plate reorganizations and tectonic events in the Pacific and surrounding plates, but no evidence for these events can be found. In addition, in this paper it is shown that there were no changes in relative plate motions between Pacific‐area plates that can be associated with the Hawaiian‐Emperor bend. This is done by calculating motion of the Pacific, Farallon, and Kula plates relative to Asia and North America using the plate circuit Farallon‐Pacific‐Antarctica‐Africa‐North America‐Asia. With no major regional tectonic event at the time of the bend, it is suggested that there was no plate reorganization at 43 Ma and that the Emperor portion of the seamount chain was formed by a nonst

ISSN:0278-7407    

DOI:10.1029/95TC01256

年代:1995

数据来源: WILEY

摘要:

The April 25, 1992, Petrolia earthquake (M s7.1) occurred at the southern tip of the Cascadia subduction zone. This is the largest thrust earthquake ever recorded instrumentally in the Cascadia subduction zone. The earthquake was followed by two large strike‐slip aftershocks (bothMs6.6). Moment release of each of the earthquakes is as follows: 4.0 × 1019Nm in the first 10 s for the mainshock, 0.7 × 1019Nm in the first 8 s for the first aftershock, and 0.9 × 1019Nm in the first 2 s for the second aftershock. These indicate that the mainshock and each of the aftershocks may have different tectonic backgrounds. The best depth estimates of the mainshock and the two aftershocks are 14 km, 18 km, and 24 km, respectively. The slip direction of the mainshock is between N75°E and N80°E. This slip direction is not consistent with either the relative motion of the North American and Juan de Fuca plates (N60°E) or between the North American plate and the Gorda deformation zone (N40°E). It has been suggested that the North American‐Pacific plate motion is accommodated by right‐lateral slip on both the San Andreas and Maacama‐Rodgers Creek‐Hayward fault systems; the intervening block is the Humboldt plate. If we modify the relative motion of the southernmost Gorda deformation zone to conform with the seismicity trends and allow the Humboldt‐Pacific plate motion to be about half the total North American‐Pacific motion, then the Gorda deformation zone‐Humboldt relative motion matches the direction of the Petrolia slip vector. Also, the mixture of focal mechanisms in the two distinct aftershock clusters can be explained by motion between the Gorda deformation zone and Pacific plate and the Humboldt and North American plates. The Gorda deformation zone is subducting beneath the Humboldt plate in the Cape Mendocino area, and the Petrolia earthquake ruptured the entire subduction segment between the Gorda deformation z

ISSN:0278-7407    

DOI:10.1029/95TC01975

年代:1995

数据来源: WILEY

摘要:

Along a 10‐km length of coast north of the Mendocino triple junction, a Neogene accretionary complex has been uplifted ≥2 km and tilted northward in response to the interaction of the southern Juan de Fuca (Gorda) plate with the older North American and Pacific plates. These plate interactions were accompanied by tectonic intercalation of Miocene to Pliocene deposits of the southern Cascadia forearc (Wildcat Group), with penetratively deformed Oligocene to Miocene accretionary prism deposits of the False Cape terrane (new name) and the Cretaceous to Eocene Coastal terrane of the Franciscan Complex. North of Cape Mendocino, more than 2 km of stratigraphic section of the accretionary False Cape terrane crops out at beach level in an east‐west trending anticline and recumbent syncline. The folded False Cape rocks are juxtaposed with forearc deposits to the north along the subvertical Russ shear zone. To the south, a second subvertical shear zone truncates the False Cape terrane near the mouth of the Bear River. This North Bear River shear zone interleaves rocks of the False Cape and Coastal terranes with additional Neogene forearc deposits (the Bear River beds). South of the North Bear River shear zone, the Bear River beds crop out for about 5 km in a continuous folded and imbricated section that is again truncated to the south by a third subvertical shear zone. This South Bear River shear zone interleaves rocks of the Coastal terrane, the Bear River beds, and Pliocene to Pleistocene shelf deposits. Folding, tilting, and shear zone development reflect ongoing north‐south crustal shortening that has occurred in response to interplate coupling and wedge thickening in the deforming Miocene and younger accretionary complex. The strain recorded in the accreted terranes and in the forearc overlap assemblage reflects a stress regime unlike that in the Cascadia fold‐and‐thrust belt north of the Eel River syncline. Thus the deformation pattern may be unique to the triple junction area. The False Cape terrane is one of only two localities along the Cascadia margin where the Oligocene to Miocene accretionary complex is exposed on land. The other locality is that of the Hoh melange on the southwest Olympic Peninsula. On‐land exposure of accretionary rocks in these two areas is a consequence of high uplift rates, focused compression, and triple‐junction tectonism. Framework grain composition data for sandstones in the Hoh melange and coeval continental deposits in northernmost California (the Oligocene to Miocene Weaverville Formation) suggest a provenance link, while sandstone of the False Cape terrane, presently situated directly west of the Weaverville Formation, is dissimilar. Given the northward component of Farallon (Juan de Fuca) plate translation with respect to North America for the past 20 m.y., provenance links of Hoh and Weaverville sandstones might reflect a substantial component of northward translation between the Farallon and North American plates that was accommodated within the acc

ISSN:0278-7407    

DOI:10.1029/95TC01695

年代:1995

数据来源: WILEY

摘要:

The history of motion of the Philippine Sea Plate is poorly known because it is isolated from the oceanic ridge system. Interpretation of palaeomagnetic results from the plate has been controversial because declination data have been obtained only from the eastern margin where subduction‐related tectonic processes may have caused local rather than plate‐wide rotations. New palaeomagnetic data relevant to the problem have been obtained from 34 sites north of the Sorong Fault and 29 sites within the Sorong Fault system. These sites record southward movement during the Eocene and northward movement during the Neogene. Sites within the Sorong Fault system record both counterclockwise and clockwise rotations interpreted as the result of Neogene block movements at the southern boundary of the Philippine Sea Plate. North of the Sorong Fault, all sites record clockwise declinations. Neogene rocks have small deflections consistent with rotation about the present‐day Eurasia‐Philippine Sea Plate pole. Oligocene‐middle Eocene rocks show consistent clockwise declination deflections of ∼40°. Declinations of lower Eocene rocks indicate ∼90° of clockwise rotation. We propose that the entire area north of the Sorong Fault in east Indonesia has always been part of the Philippine Sea Plate and that the whole plate has rotated clockwise in a discontinuous manner by approximately 90° since the early Eocene. The new data from north of the Sorong Fault provide a basis for determining rotation poles which satisfy all the palaeomagnetic data from the Philippine Sea Plate and permit it

ISSN:0278-7407    

DOI:10.1029/95TC01694

年代:1995

数据来源: WILEY

摘要:

We report paleomagnetic results from layered igneous rocks that imply substantial post mid‐Cretaceous poleward motion of the Insular superterrane (western Canadian Cordillera and southeast Alaska) relative to North America. The samples studied are from the stratiform zoned ultramafic body at Duke Island, which intruded rocks of the Alexander terrane at the south end of the southeastern Alaska archipelago at about 110 Ma. Thermal and alternating field demagnetization experiments show that the characteristic remanence of the ultramafic rocks has high coercivity and a narrow unblocking temperature range just below the Curie temperature of magnetite. This remanence is likely carried by low‐Ti titanomagnetite exsolved within clinopyroxene and perhaps other silicate hosts. The Duke Island intrusion exhibits a well‐developed gravitational layering that was deformed during initial cooling (but below 540°C) into folds that plunge moderately to the west‐southwest. The characteristic remanence clearly predates this early folding and is therefore primary; the Fisher parameter describing the concentration of the overall mean remanence direction improves from 3 to 32 when the site‐mean directions are corrected by restoring the layering to estimated paleohorizontal. All samples exhibit a magnetic anisotropy that is strong but nonuniform in orientation across the intrusion, and we show that it has no significant or systematic effect on the site‐mean directions of remanence. At least some of the anisotropy derives from secondary magnetite formed during partial serpentinization. The mean paleomagnetic inclination (56°±10°) corroborates paleomagnetic results from five coeval silicic plutons of the Canadian Coast Plutonic Complex to the south and southeast and implies 3000 km (±1300 km) of poleward transport relative to the North American craton. Between mid‐Cretaceous and middle Eocene time, the Insular superterrane and Coast Plutonic Complex shared a common paleolatitude history, with more poleward transport than coeva

ISSN:0278-7407    

DOI:10.1029/95TC01579

年代:1995

数据来源: WILEY

摘要:

The paleomagnetism of the Cretaceous Pigeon Point Formation turbidites was reexamined to determine whether the 25° of southerly paleolatitudinal offset originally observed (Champion et al., 1984) for these rocks was all, or in part, due to compaction shallowing of their paleomagnetic inclination. The study consisted of two parts: (1) A standard paleomagnetic study, including detailed thermal and alternating field demagnetization, was conducted on oriented cores collected at Pigeon Point, approximately 50 km south of San Francisco, California. The results of this study were combined with the alternating field demagnetized results for samples provided by D. Champion from the initial Pigeon Point paleomagnetic study. The combined data set has a mean direction for Pigeon Point (I=41.6°,D=346.9°) similar to that originally obtained by Champion et al. (1984). (2) Material from the Pigeon Point Formation was disaggregated, given a laboratory analog of a postdepositional remanence, and compacted to pressures as high as 0.13 MPa which caused volume losses up to 53%. The laboratory‐compacted samples were alternating field demagnetized, and their magnetic inclination and anisotropy of anhysteretic remanence were both measured. These data were used to derive correction curves, following Jackson et al. (1991), which describe the specific relationship between remanence anisotropy and inclination shallowing for the Pigeon Point Formation. Two correction curves were determined, one assuming that the magnetic particle orientation distribution experienced a prolate deformation after remanence acquisition and one assuming an oblate deformation. These two different corrections were necessary because the anisotropy of anhysteretic remanence indicates a composite fabric due to both a prelithification technically caused lineation and a burial compaction foliation. The anisotropy of anhysteretic remanence measured for each paleomagnetic sample and the correction curves determined from the laboratory compaction experiments indicate that the inclination of the Pigeon Point Formation has been shallowed by burial compaction. The compaction‐corrected Pigeon Point mean directions assuming either a prolate (I=53.1°,D=347.2°) or an oblate (I=49.8°,D=346.8°) deformation suggest only 13° to 16° of southerly paleolatitudinal offset for the Pigeon Point Formation in the Cretaceous, not the 25° originally observed (Champion et al., 1984). The resulting paleolatitude for the Pigeon Point Formation could indicate that Salinia served as a link between the cratonic Sierra Nevada arc to the north and the Peninsula Ranges/Baja‐Borderlands allochthon to the south. Alternatively, our results suggesting a 10° compaction inclination shallowing for the Pigeon Point turbidites may indicate that many of the paleomagnetic studies placing the Peninsula Ranges/Baja‐Borderlands 15° to the south of North America in the Cretaceous and Tertiary may have suffered from a similar effect and that the allochthon has been nearly in place s

ISSN:0278-7407    

DOI:10.1029/95TC01648

年代:1995

数据来源: WILEY

摘要:

The Butte Valley and Layton Well Thrusts are major structural features in two adjacent mountain ranges west of southern Death Valley. The Butte Valley Thrust in the southern Panamint Range underlies most of the range and emplaces Proterozoic rocks over strata as young as Jurassic(?) in age. The Layton Well Thrust to the southwest in the Slate Range has been interpreted to have Proterozoic rocks juxtaposed on rocks as young as Jurassic, suggesting that the Butte Valley Thrust and the Layton Well Thrust might be correlative. New information indicates that the allochthonous rocks of the Layton Well Thrust are Mesozoic in age and are not likely part of the same allochthon as that above the Butte Valley Thrust. In addition, the Butte Valley Thrust cuts sharply downward to the north and west across lower plate Paleozoic strata, suggesting that the fault roots beneath the Layton Well Thrust. The Layton Well Thrust probably belongs to the East Sierran thrust system and thus would be in the upper plate of the Butte Valley Thrust.

ISSN:0278-7407    

DOI:10.1029/95TC01932

年代:1995

数据来源: WILEY

摘要:

An approximately 180‐km‐long, deep crustal shear zone in western Fiordland, New Zealand, has been shown to have formed during continental extension between 116 and 100 Ma. The western Fiordland shear zone forms a major tectonic boundary between two sets of gneisses with unrelated deformation histories. A detailed study of the northern part of the shear zone has revealed that the shear zone formed at depths of ∼40 km (approximately 12 kbar and 680°C) and was originally very gently dipping or subhorizontal prior to subsequent upright folding. The current thickness (postfolding) of the shear zone is ∼3–4 km. Abundant asymmetric structures in the shear zone indicate noncoaxial deformation and a top‐to‐the‐NE sense of shear. Unlike ductile shear zones in metamorphic core complexes from elsewhere in the world, there is no evidence that decompression of the shear zone rocks or foot wall rocks occurred during the active life of the shear zone, suggesting that displacement along the shear zone by noncoaxial deformation must have been largely horizontal. This shear zone provides important evidence for the nature of deep crustal deformation occurring during conti

ISSN:0278-7407    

DOI:10.1029/95TC01508

年代:1995

数据来源: WILEY

摘要:

The thermal and structural evolution of a Paleozoic thrust system in central Australia, the Arltunga Nappe Complex, has been examined through field mapping, petrographic, microstructural and chemical analyses, K/Ar and40Ar/39Ar thermochronology, and multi‐diffusion‐domain (time‐temperature) modeling of K‐feldspar40Ar/39Ar data. The Arltunga Nappe Complex contains three distinct tectonic units, bounded by regional faults and shear zones, that have undergone contrasting deformational and metamorphic histories. The cooling history of the three tectonic units has been constrained by K/Ar and40Ar/39Ar isotopic age data from K‐feldspars, micas, and amphiboles. Results are consistent with a piggyback history of crustal imbrication spanning over 100 m.y., with the hottest rocks uplifted and cooled first and with imbrication acting progressively toward the foreland. Implications for the intracratonic Paleozoic tectonics of central Australia are (1) uplift within the structurally highest tectonic unit was initiated before 400 Ma; (2) continued piggyback imbrication, footwall failure, and uplift and exhumation of the crust resulted in duplexing in the lowest structural unit at approximately 325–300 Ma; (3) the contrast in thermal history between the tectonic units indicates that once widely separated crustal segments were juxtaposed during thrusting above a hinterland dipping megathrust, resulting in ∼85 km of crust

ISSN:0278-7407    

DOI:10.1029/95TC00335

年代:1995

数据来源: WILEY