摘要:

Six years of geological research in eastern Nepal has resulted in a new geological map of the eastern Nepal Himalaya which includes the region stretching from the Sikkhim border in the east to the Kathmandu Valley in the west, and from the summits of the Higher Himalaya in the north to the Ganges Plain in the south. This research has permitted the determination of the tectonostratigraphy and structure of one section of the central Himalayan arc. South of the Tibetan Plateau the eastern Nepal Himalaya can be divided into three distinct, thrust‐bound tectonic packages: (1) the Higher Himalayan thrust sheet composed of the Higher Himalayan Crystallines, (2) the Lesser Himalayan thrust sheet composed of the Lesser Himalayan Series, and (3) the Sub‐Himalayan imbricate zone composed of sedimentary rocks belonging to the Siwalik Group. The Higher Himalayan thrust sheet of eastern Nepal has been thrust over the Lesser Himalayan Metasediments a minimum of 140 km, and possibly as much as 175–210 km, along the Main Central Thrust (MCT). The Lesser Himalayan thrust sheet is overlain by the MCT and is underlain by the Main Detachment Fault (MDF) and the Main Boundary Thrust (MBT). Out‐of‐sequence thrust faults in the hanging wall of the MBT have breached and offset the presently inactive MCT. The Sub‐Himalayan imbricate zone is an emergent imbricate fan bounded by the MBT to the north and the Main Frontal Thrust (MFT) to the south and is underlain by the MDF which lies at a depth of between 5 km and 7 km. A balanced cross section constructed across the Higher, Lesser, and Sub‐Himalaya of eastern Nepal shows that the eastern Nepal Himalayan orogenic wedge has undergone a minimum of between 210 and 280 km of horizontal, north‐south tectonic shortening since the initiation of the MCT. The Lesser and Sub‐Himalaya have absorbed 70 km of north‐south shortening by thrusting along the basal MDF, of which the Sub‐Himalayan imbricate zone has accommodated 25 km, the Sun Kosi Thrust has accommodated about 10 km, and the MBT has accommodated the remaining 35 km of shortening. Since the initiation of the MCT between 15 Ma and 25 Ma shortening across the eastern Nepal Himalaya has occurred at an average rate of 8.4–18.6 mm per year. The structural geometry of the eastern Nepal Himalaya suggests an overall “piggyback” sequence of thrusting, with motion transferred from the MCT to the underlying MDF and its emergent splay thrust, the MBT, and with the MBT rotated to its present steep orientation by imbricate thrust

ISSN:0278-7407    

DOI:10.1029/92TC00213

年代:1992

数据来源: WILEY

摘要:

The Portable Array for Numerical Digital Analysis (PANDA) network, a digitally recorded seismic array, operated for nine months in Jujuy province of northwestern Argentina. The network was deployed along the eastern edge of the Altiplano‐Puna plateau in a major N‐S thrust belt that is transitional in style between the thin‐skinned deformation of the Bolivian foreland to the north and basement‐involved deformation of the Pampean region to the south. Teleseismic locations of crustal earthquakes in the region indicate that seismicity is associated with compressional structures found near the eastern deformation front. No crustal seismicity was detected beneath the Puna plateau to the west. Peak seismicity levels beneath the foreland occurred between 20 and 25 km depth; a sharp decrease in seismicity was observed below 25 km. An estimate of 42 km for the thickness of the Jujuy foreland crust was inferred from wide‐angle Moho reflections observed on the digital seismograms. The highest concentration of crustal seismicity was located beneath Sierra de Zapla, a broad anticline immediately east of San Salvador de Jujuy. Many of the earthquakes in the 20–25 km depth range have a shallow, west dipping nodal plane as does the focal mechanism solution for a moderately large 1973 earthquake. Inversion of focal mechanism data for the orientation of principal stresses shows that maximum compression is oriented at azimuth 74°, closely paralleling both the current Nazca‐South America convergence direction and the shortening direction derived from regional Quaternary fault slip data. We interpret the earthquakes as occurring on planes of weakness first produced during Cretaceous rifting and later reactivated by Neogene compressive stresses. Crustal seismicity patterns and fault plane solutions suggest the presence of a midcrustal detachment, along which significant late Cenozoic E‐W shorten

ISSN:0278-7407    

DOI:10.1029/92TC00215

年代:1992

数据来源: WILEY

摘要:

Transpression is a process that thickens the crust and therefore in obliquely convergent orogens as in normally convergent orogens there is the potential to generate granitic melts. Individual transcurrent shear zones may not only control the ascent paths, siting, and emplacement mechanisms of plutons, but they may also cause the genesis of the granitoids themselves. Two contrasting situations are examined. In the Hercynian shear zones of Iberia, thickening together with hydrous fluxing along shear zones created intracrustal wet melting of fertile Gondwanan sediments to produce syntectonic granites. In the northern part of the British Caledonides, the association of compositionally expanded granitoids with a major mantle component and transcurrent shear zones may be explained by melting of continental crust at the lower limits of crustal transpressional faults detaching into the Moho.

ISSN:0278-7407    

DOI:10.1029/92TC00336

年代:1992

数据来源: WILEY

摘要:

A right‐lateral shear zone trending northerly along more than 2000 km is recognized from central Japan to northern Sakhalin. It was active mainly during the Neogene and has accommodated several hundreds of kilometers of displacement. The whole structure of Sakhalin is built on this shear zone. En échelon sigmoidal folds and thrusts, en échelon narrow Miocene basins, and a major discontinuity which is observed along more than 600 km, the Tym‐Poronaisk fault, characterize the deformation there. In Hokkaido, en échelon folds and thrusts and a ductile shear zone with high‐temperature metamorphism constitute the southern extension of this transpressional shear zone. It continues to the south as a zone of transtensional deformation along the eastern margin of Japan Sea, as en échelon basins and dextral transfer faults observed as far south as Noto peninsula and Yatsuo basin. The style of the shear zone thus evolves from transpressional in the north far from the subduction zone, to transtensional in the south in the back‐arc region. Strike‐slip motion along this shear zone was primarily responsible for the dextral pull‐apart opening of Japan Sea during the early and middle Miocene. Dextral motion is still active in the north along the Tym‐Poronaisk fault in Sakhalin as well as on the continental margin of Japan Sea (Korea and Asia mainland). Active E‐W compression replaced the dextral motion along the eastern margin of Japan Sea in late Miocene time, and incipient subduction began in t

ISSN:0278-7407    

DOI:10.1029/92TC00337

年代:1992

数据来源: WILEY

摘要:

In the southern part of the Indian Tethys Himalaya between SE Zanskar and NW Lahul, nappe tectonics with transport toward the WSW during the main deformation phase has been ascertained. If the stratigraphy of the different units considered as nappes is combined, it results in a composite section ranging from lower Cambrian to Middle Cretaceous. A metamorphism of amphibolite facies in the lowest unit is associated with the nappe stacking. The Lower Miocene cooling ages of metamorphic biotite and muscovite suggest that the nappe emplacement starts at the Oligocene, or at the Oligocene‐Eocene boundary. After the compressional phases, an extension provokes the reactivation of older main thrusts as normal fault and the formation of a steeply dipping normal fault. This interpretation is based on detailed mappin

ISSN:0278-7407    

DOI:10.1029/92TC00338

年代:1992

数据来源: WILEY

摘要:

The area between the Colorado Plateau on the north, New Mexico and Chihuahua to the east, and southern and Baja California to the southwest has undergone a complicated thermal and tectonic history climaxing in medial Cretaceous time and manifested in a sequence of events which continue to the present time, independent of plate boundary transform faulting. This sustained sequence of events was initiated by subduction‐related magmatic emplacement, largely between 120 and 50 Ma, beginning in the west and sweeping eastward across the region. The emplacement of these rocks generated a large isostatic welt which began to elevate in late medial Cretaceous time in the west central portion of the area and spread outward until in Oligocene time the west coast of southern California was emergent almost to the continental escarpment. Both erosional and tectonic unloading accompanied elevation, with the axis of the welt achieving its maximum elevation in earliest Cenozoic time when north central Sonora may have had elevations of the order of 5000 m. Body forces caused the welt to relax laterally, producing thrust and nappe structures which were westward vergent in the west and eastward vergent in the east. To the east these structures are included in what is called the Larimide Orogeny; to the west they include such features as the Santa Rosa mylonite belt and related structures. As with the magmatic emplacement, the gravitational spreading began in the west. During the Eocene, additional rebound in response to deep erosion and tectonic unroofing began to expose midcrustal rocks within the core area of the welt, and subhorizontal detachment began along the brittle‐ductile boundary. During Oligocene and early Miocene time, renewed isostatic uplift resulted from rising magma, and continued erosional and tectonic denudation spawned both brittle and ductile extension, hydrothermal alteration, and mineral deposition. From mid‐Miocene to the present time the central portion of the area has subsided, while the marginal mountain ranges have ele

ISSN:0278-7407    

DOI:10.1029/92TC00596

年代:1992

数据来源: WILEY

摘要:

The East African Rift system (EAR) is the archetypal continental rift and a widely proposed analogue for the early stages of evolution of passive continental margins. The three‐dimensional structure of parts of the EAR has been recently elucidated by a multifold seismic (MFS) survey of Lakes Tanganyika and Malawi (Project PROBE). Analysis of fault geometries displayed on the PROBE MFS data coupled with the more extensive 28‐kHz echosounder data has improved understanding of the geometry and kinematics of the linked fault systems that underlie the lakes. In particular, it has been recognized that profiles across (i.e., at high angles to) the rift elongation commonly display fault geometries that are not readily retrodeformable. There must therefore be significant displacement out of the plane of what would conventionally be regarded as “dip” lines, that is, perpendicular to rift elongation and parallel to the tectonic transport or extension direction. Careful determination of fault plane dip and the dip of synrift reflectors in the hanging wall demonstrates that the dip directions of the shallowest faults and the steepest hanging wall sediment dips are oriented either NW or SE. This defines the direction of maximum fault block rotation and therefore the direction of extension or tectonic transport as NW/SE, oblique to the trend of the regional rift axis and to the apparent strike of many of the major border faults in the survey area. The border faults and many other faults imaged on the PROBE MFS data must therefore have significant components of strike‐slip motion and the Tanganyika and Malawi rift zones have undergone extension oblique to and not perpendicular to their axes. Near dip‐slip normal faults, steeply dipping oblique‐slip faults and subvertical strike‐slip or transfer faults have all been recognized in a complex, linked fault system that accomplishes the extension. The overall orientation of the rift lakes and their internal segmentation are influenced by major, preexisting, subvertical fault zones within the basement. Complex accommodation zones act principally as transfer zones that allow switches in gross polarity of the border fault system. The detailed geometries of the accommodation zones result from the specific relations between juxtaposed half‐graben and the strike and internal geometry of the influencing basement structure. The variations in fault geometry, subsidence, water depth and basin or rift morphology can be better explained by an oblique‐slip extensional model influenced by basement structures than by a simple orthogona

ISSN:0278-7407    

DOI:10.1029/92TC00821

年代:1992

数据来源: WILEY

摘要:

This study presents a reconstruction of the Neotectonic evolution of western Nicaragua by means of structural analysis of outcrop scale joints, faults and bedding planes integrated with macroscale structures. The results indicate three clearly separable deformation phases which are closely related to specific volcanic events and account for the major tectonic features of Nicaragua. The first deformation phase was a Late Miocene to Early Pliocene shortening that produced the Pacific Plain anticlines and controlled the contemporaneous extrusion of the El Coyol Group volcanic rocks. The second phase, active between the Pliocene and the Pleistocene, resulted in extensional structures such as the 500‐km‐long and 50‐km‐wide NW‐SE trending Nicaragua Depression. This phase was accompanied by a gradual trenchward migration of the volcanic activity responsible for the formation of the Las Sierras Group. The third deformation phase is still active and is responsible for the present shallow focus‐seismic activity associated with N‐S trending volcanic vents and the N‐S Managua pull‐apart Graben. The Neotectonic development of western Nicaragua, as interpreted here, is closely related to the contemporaneous evolution of the neighboring areas of Guatemala (south of the Jocotan Fault), Honduras, El Salvador and Costa Rica (north of the Valle Central). The Valle Central of Costa Rica represents the southern boundary of this structurally homogeneous domain and may indicate the southern boundary of the Neogene Chortis Block. Changes in deformation style and the trenchward migration of volcanism which occurred during the Pliocene may indicate an increase in the subduction angle of the Cocos Plate and may also be correlated to Pliocene shortening of Jamai

ISSN:0278-7407    

DOI:10.1029/92TC00859

年代:1992

数据来源: WILEY

摘要:

Zircon fission track (ZFT) ages of 17 Precambrian samples from deep boreholes and outcrops in southern Israel and Sinai fall within the range 328–373 Ma (Late Devonian‐Early Carboniferous). Single zircon grain age distributions are unimodal with a high chi‐square probability. The age data indicate total resetting of the ZFT clocks but only partial resetting of coexisting sphenes, constraining the temperatures attained to about 225° ± 50°C, followed by relatively rapid regional cooling at the times indicated. In the study area, the lower Paleozoic section presently overlying Precambrian rocks is limited to Cambrian strata, up to 300 m thick. Further, stratigraphic evidence for sub‐Carboniferous erosion is preserved only in SW Sinai. To the east, in southern Jordan, a 2–2.5 km thick lower Paleozoic succession is reported. Despite the lack of stratigraphic evidence in the study area, the ZFT data (1) strongly suggest that an equivalent or thicker section also existed, (2) constrain the timing of the erosion to Late Devonian‐Early Carboniferous (Hercynian), and (3) indicate that thermal gradients may have reached ≥50°C/km prior to the uplift/erosion event. Regional stratigraphic evidence indicates that the Late Devonian‐Early Carboniferous event was confined to a relatively narrow belt extending from the Gulf of Suez area to the vicinity of NE

ISSN:0278-7407    

DOI:10.1029/92TC00636

年代:1992

数据来源: WILEY

摘要:

Cenozoic tectonic evolution of the Japanese rifted continental arc‐trench system is reflected in the detrital modes of sand and sandstone deposited in forearc and backarc basins sampled by the Deep Sea Drilling Project. At present, the Japan arc is divided into two segments along a complex plate boundary where the Izu‐Bonin Ridge intersects the Japan arc, creating two triple junctions, in front of and behind the ridge. Southwest of the Izu‐Bonin Ridge, quartzofeldspathic Cretaceous forearc sediments were uplifted and recycled into Tertiary forearc deposits in response to strike‐slip movement associated with backarc spreading in the Shikoku basin. Quaternary forearc sections record the first major influx of volcanic detritus to southwest Japan sites. Triple‐junction‐related deformation in central Honshu has produced sand of mixed volcanic/sedimentary provenance, which is funneled by the Suruga Canyon into the Nankai Trough along the southwest Japan margin. North of the triple junction, Oligo‐Miocene forearc sand compositions indicate Oligocene forearc uplift and volcanism, possibly related to initial backarc rifting and formation of the Japan Sea, and subsequent Miocene exposure of arc basement. In contrast, Pliocene and Quaternary forearc sand from northeast Japan is primarily volcaniclastic and documents uninterrupted arc volcanism. Within the backarc region, sand compositions vary from east to west across the Japan Sea. The Asian rifted continental margin and submerged remnants shed quartzofeldspathic sand into the western side of the basin, whereas the Japan arc sheds volcaniclastic sand into the eastern side

ISSN:0278-7407    

DOI:10.1029/91TC03183

年代:1992

数据来源: WILEY