摘要:

Subduction of a spreading ridge is expected to leave a geological signature upon the overriding accretionary prism distinct from that of conventional convergent plate boundaries. The late Oligocene‐early Miocene rocks of the Shimanto accretionary prism at Cape Muroto, Shikoku Island, record a tectonomagmatic fabric that is anomalous with respect to typical accretionary prisms. These accreted strata have been imprinted with an unusual late stage event that involved (1) regional cusp‐like flexing of structural trends in the prism, (2) near‐trench magmatism, and (3) pervasive faulting. This event overlaps in time with the opening of the Shikoku back arc basin (circa 26–14 Ma); the Shikoku basin spreading ridge trends perpendicular to the strike of the accretionary prism and is now located immediately offshore of Cape Muroto. The regional flexing of structural trends in the prism affects rocks as old as Cretaceous, although the flexure is most tightly developed in the late Oligocene‐early Miocene portion of the prism, where it is steeply plunging. Kinematic analysis of the flexure and the pervasive fault system indicates that the flexure most likely resulted from indentation of the prism by a rigid feature. At Cape Muroto the core of the flexure forms a locus for mafic dikes that are petrochemically equivalent to some mid‐ocean ridge basalts (MORB) from the Shikoku back arc basin. Preliminary paleomagnetic data suggest that the dikes were intruded during the later stages of flexure development. These data, combined with plate reconstructions for the area at 15 Ma, indicate that indentation of the prism most likely resulted from early Miocene orthogonal subduction of the topographically elevated Shikoku Basin spreading ridge in the Muroto area; MORB magmatic rocks in the core of the flexure appear to have emanated from the active spreading ridge during

ISSN:0278-7407    

DOI:10.1029/TC009i002p00207

年代:1990

数据来源: WILEY

摘要:

Source parameters have been determined for the earthquake (Ms5.6) that occurred offshore of Tripoli, Libya, on September 4, 1974. One nodal plane of its focal mechanism has dip 37°, strike 297°, and rake −141°, indicating oblique normal faulting. This nodal plane is subparallel to many west‐northwest striking normal faults in the epicentral area and is most likely the fault plane, indicating a component of right‐lateral strike‐slip with slip vector azimuth N84°E. Inversion of long‐period teleseismic body waves indicates 12‐km centroid depth and 0.4 × 1018N m seismic moment. A much larger earthquake (Ms7.0) on April 19, 1935, that occurred in the same zone of active oblique normal faults ∼400 km farther southeast near Sirte probably involved similar slip sense. This zone, for which the name “Tunisia‐Libya seismic zone” appears appropriate, has overall northwest‐southeast extent ∼1000 km from northern Libya to between Tunisia and Sicily. It takes up a change in motion direction relative to stable Europe from west of north inside the African plate to between N30°E and N50°E in the Ionian Sea between Sicily, southernmost peninsular Italy, southwest Greece, and Libya. This suggested motion direction of Sicily relative to stable Europe agrees with independent estimates from fault slip rates

ISSN:0278-7407    

DOI:10.1029/TC009i002p00231

年代:1990

数据来源: WILEY

摘要:

Recent study shows that Paleozoic orogeny in Inner Mongolia resulted from collisions. An early event was a collision between an island arc and the North China continent in the Late Silurian, and a subsequent event was a collision between the Sino‐Korean and Siberian platforms before the end of the Devonian. Prior to the collisions, an old Asian ocean existed. Its opening in Inner Mongolia is interpreted from the Middle Proterozoic Zartai rift zone. Late Proterozoic‐Early Paleozoic spreading and subduction are represented by a fossil trench‐island arc‐basin system, which constitutes the Xar Moron accreted fold belt along the north margin of the North China platform. Latest Silurian molasse unconformably overlies these belts, constraining the time of collision between the island arc and the North China continent. The old Asian ocean basin persisted but was finally closed by the collision between the Sino‐Korean and Siberian continental platforms. Latest Devonian‐Early Carboniferous molasse and epicontinental carbonate deposits unconformably overlie an imbricated oceanic rock suite bordering the suture zone, indicating that the collision occurred before the end of the Devonian. This collision produced only a weak orogeny with thin, local molasse and moderate deformation. The Silinhot belt marks this collision and represents the suture zone between the two great platforms. During the Carboniferous and Permian, Inner Mongolia and adjacent regions were affected by widespread development of continental rift structures with intense magmat

ISSN:0278-7407    

DOI:10.1029/TC009i002p00249

年代:1990

数据来源: WILEY

摘要:

High resolution imaging of the north (NPTB) and south Panama thrust belts, using the SeaMARC II seafloor mapping system and digital seismic reflection profiling, provides an excellent data base with which to determine the kinematics of oroclinal deformation of Panama. Remarkable continuity of structural orientations over 300 km along the NPTB argues for widespread uniformity in direction of thrusting between Panama and the Caribbean plate. Westward decrease in the width and height of the accretionary wedge indicates a westward decrease in the total amount of convergence. Two major cross faults cut the wedge in its eastern part, forming two minor blocks with different relative motions with respect to the Caribbean. Frontal thrust orientations change across these faults. Major faults have been proposed to cut the isthmus of Panama, but simple cross‐isthmus faulting alone cannot explain the orientations of the major thrusts of the NPTB. Bending of a flexible beam can explain much of the observed structure, both directions and rates, though the scale of the orocline precludes considering this as an elastic phenomenon. Plastic deformation of Panama may be a useful mechanical explanation, at least for eastern Panama, but consideration of plastic slip lines must include the northern Andes as well. Changing from more rigid, blocklike offsets on faults in western Panama to a continuum of strain in eastern Panama may be the best explanation for observations at presen

ISSN:0278-7407    

DOI:10.1029/TC009i002p00261

年代:1990

数据来源: WILEY

摘要:

The Southwestern Approaches Traverse (SWAT) seismic reflection profiles recorded through the Celtic Sea and the English Channel provide unexpected data concerning, notably, the deep structure of the Irish and south British Variscan crust. One of the most significant results is the recognition of prominent deep southerly dipping reflectors, regarded as large‐scale foreland directed Variscan thrusts which constitute, in the southern innermost zones, a crustal stacking wedge. The resulting overthickened crust may have induced the petrogenesis of the Cornubian granitic batholith by an anatectic melting process. The roots of this crustal duplex are not imaged on the SWAT lines because of their subsequent assimilation by the more recent layering of the present‐day lower crust. Upward, the deep crustal ramps are assumed to flatten out and to join a roof thrust which acts as a major midcrustal decoupling zone, not revealed by the SWAT profiles, and which underlies the thick Devonian allochthonous high‐strained units of Cornwall. Northward, this shallow northerly verging shear zone is overstepped by the Tintagel antithetic back thrusts which are closely related to deeper northward dipping basement faults considered as Caledonian features. Southward, the imbricate crustal thrust unit is overridden by the Lizard ophiolitic suture, which appears as a 8‐km‐thick gently southerly dipping sheared zone, crosscuttting the entire crust and rooting deep beneath the present‐day Moho. Its hanging wall is constituted by the Channel Cadomian block, characterized by an almost seismically featureless upper crust which is not involved in the main Variscan thrust stacking. Northward, the imbricate thrust unit is delineated by a moderately southerly dipping ramp which penetrates straight down into the upper crust without any evidence of an intervening flat‐lying decollement level. This frontal ramp emerges along a broadly 110°N trending discontinuous thrust front, located above the northern downfaulted margins of the Munster (Ireland) and Pembroke (Wales) Devonian basins. The SWAT deep seismic data relate the overall Variscan crustal shortening undergone by the south British domain to an oblique converging process involving a continental shelf pinched between the rigid allochthonous Channel block and the northern autochthonous foreland. This crustal shortening occurred during a Carboniferous intracontinental collisional context subsequent to the closure and the obduction of a small oceanic crust‐floore

ISSN:0278-7407    

DOI:10.1029/TC009i002p00283

年代:1990

数据来源: WILEY

摘要:

A 70‐km‐long seismic reflection profile in western Michigan provides new insight into the nature, distribution, and structure of the Keweenawan Supergroup volcanic and overlying sedimentary rocks and the controversial Keweenaw fault along the southern boundary of the Midcontinent Rift System in the Lake Superior basin. Interpretation of the 5‐s reflection data constrained by surface geology, magnetics, and gravity modeling shows that volcanic rocks which cropout north of the Keweenaw fault dip northerly to depths of the order of 17 km. South of the fault, volcanic rocks overlain by ∼2 km of clastic sedimentary rocks thin gradually to the south as they dip upward at a shallow angle to the outcrop in the South Range. The volcanic pile within the basin thickens rapidly to the north of the Keweenaw fault, suggesting that the volcanics were deposted in an extensional fault‐bounded basin. Clear evidence of normal faulting is not present in the seismic reflection data because of a later compressional event. The thickness of the volcanic‐filled basin implies that the upper crust was almost completely broken during the rifting event. The available evidence is interpreted to show the Keweenaw fault as a moderate‐ to high‐angle reverse fault that occurs within the volcanic pile and breaks through to the surface along the abrupt change in thickness of the volcanic sequence. There is no evidence from the seismic profiling for major faulting (except for the Keweenaw fault), intrusions, or folding of the Keweenawan Supergroup

ISSN:0278-7407    

DOI:10.1029/TC009i002p00303

年代:1990

数据来源: WILEY

摘要:

Recent tectonic models for southern California treat the entire Mojave Desert Block as the site of distributed simple shear during late Cenozoic time. These models consider that much of the region is composed of a series of narrow blocks, bounded by active NW striking, right‐slip faults that have facilitated the distortion and rotation of the region about vertical axes during translations. As much as 100 km of cumulative right slip is predicted for these faults by some of these models. These kinematic models require that the faults of the Mojave Desert Block merge with the Garlock fault, which is viewed as the intact northern boundary that served to accommodate the distortion of the Mojave Desert Block by simple shear. Map‐scale structural relations are used to test explicit and implicit features of kinematic models proposed for the region. These relationships indicate that late Cenozoic NW striking, right‐slip faults of the Mojave Desert Block possess the following characteristics: (1) the faults are discontinuous, with only the Calico‐Blackwater fault spanning the entire Mojave Desert; (2) the faults terminate before reaching the Garlock fault; (3) faults south of an irregular line extending from near Barstow eastward to Ludlow and to Soda Lake are continuous and well developed and have a cumulative net slip of>40 km, whereas faults to the north are discontinuous and display<12 km of right slip; and (4) there is a northwestward decrease in net slip along most of the faults. A new kinematic model is proposed to reconcile these new observations with existing data. We assert that integrated strain within the province since middle Miocene time is not regionally homogeneous as predicted by simple shear models but is instead partitioned into six major domains. The domains probably have deformed and rotated about vertical axes independently of each other and are separated by zones of shortening or extension or by strike‐slip faults. Strike‐slip faults and folding have likely accommodated internal deformation and rotation of some of the domains. The model predicts that the Mojave Desert has been the site of ∼65 km of right shear since middle Miocene time. The broad network of faults of the Mojave Desert Block along with similar strike‐slip faults of the Death Valley region constitute a regional zone of right shear, named here, the Eastern California shear zone. Because of its probable physical connection to the San Andreas fault system, the Eastern California shear zone may have accommodated a significant portion of Pacific‐North American transform motion. The Eastern California shear zone accounts for 9–14% of the total shear, predicted from plate tectonic reconstructions, along the Pacific‐North American transform boundary since ∼10.6 Ma. The kinematic connection of the normal faults of the Death Valley region, with the San Andreas fault system via the faults of the Mojave Desert accords with the deduction of Atwater (1970) that late Cenozoic extension in portions of the Basin and Range province is related to Pacific‐North American transform shear. Finally, the present arcuate trace of the Garlock fault is ascribed to oroclinal folding within the broad zone of distributed shear of the East

ISSN:0278-7407    

DOI:10.1029/TC009i002p00311

年代:1990

数据来源: WILEY

摘要:

With few unconstrained assumptions, a simple quantitative model for flexural isostasy between two crustal blocks of different thickness that subsequently undergoes partial relaxation of accumulated stress can explain reactivation of the Sabine Uplift in the mid‐Cretaceous. Thin salt over the Sabine Uplift indicates that it was a positive area in the Middle Jurassic but began to subside during the Late Jurassic. During the Late Jurassic and Early Cretaceous, the Sabine Uplift had no topographic expression and was the center of a large, flat bottomed basin covering most of east Texas and north Louisiana. Reactivation of the Sabine Uplift in the mid‐Cretaceous caused a minimum of 150 m of uplift and extensive erosion of Lower Cretaceous rocks. During the Late Cretaceous and Tertiary, the Sabine Uplift resumed subsiding but at a slower rate than the adjacent basins. Since the mid‐Cretaceous, the Sabine Uplift has risen more than 1 km relative to the East Texas Basin. We suggest that this structural history is consistent with the following simple quantitative model for flexural isostasy. During formation of the Gulf of Mexico in the Late Triassic‐Early Jurassic by rifting and extension of the lithosphere, brittle deformation of the crust created small‐scale wavelength of tens of kilometers, lateral variations in crustal thickness. Initially, the newly formed margin was in point‐wise Airy isostatic equilibrium. Thus synrift and/or early postrift subsidence was characterized by a series of rift valleys or half grabens separated by uplifts. However, as the margin cooled and contracted, the lithosphere became stronger, and subsequent loading was regionally compensated. Thus postrift subsidence was characterized by a broad regional downwarp. If, at some later point in time, the rigid portion of the lithosphere was weakened or relaxed, then lateral density variations would have been recompensated at shorter wavelengths. Thus areas of little or no crustal extension would have been uplifted and areas of high extension would have experienced additional subsidence. Gravity and total tectonic subsidence interpretations agree that the Sabine Uplift region is underlain by thicker crust than the adjacent East Texas and North Louisiana basins. Possible mechanisms for relaxation of continental lithosphere are thermal rejuvenation, horizontal in‐plane stress, viscous relaxation, and extensi

ISSN:0278-7407    

DOI:10.1029/TC009i002p00341

年代:1990

数据来源: WILEY