摘要:

The young continental margin of the western Arabian Peninsula is uplifted 3.5 to 4 km and is well exposed. Rift‐related extensional deformation is confined to a zone 150 km wide inland of the present coastline at 17 to 18° N and its intensity increases gradually from east to west. Extension is negligible near the crest of the Arabian escarpment, but it reaches a value of 8 to 10% in the western Asir, a highly dissected mountainous region west of the escarpment. There is an abrupt increase in extensional deformation in the foothills and pediment west of the Asir (about 40 km inland of the shoreline) where rocks in the upper plate of a system of low‐angle normal faults with west dips are extended by 60 to 110%. The faults were active 23 to 29 Ma ago and the uplift occurred after 25 Ma ago. Tertiary mafic dike swarms and plutons of gabbro and granophyre 20 to 23 Ma old are concentrated in the foothills and pediment as well. The chemistry of the dikes suggests (1) fractionation at 10 to 20 kbar, (2) a rapid rise through the upper mantle and lower crust, and (3) differentiation and cooling at 1 Atm to 5 kbar. Structural relations between dikes, faults and dipping beds indicate that the mechanical extension and intrusional expansion were partly coeval, but that most of the extension preceded the expansion. A tectonic reconstruction of pre‐Red Sea Afro/Arabia suggests that the early rift was narrow with intense extension confined to an axial belt 20 to 40 km wide. Steep Moho slopes probably developed during rift formation as indicated by published gravity data, two published seismic interpretations and the surface g

ISSN:0278-7407    

DOI:10.1029/TC005i004p00477

年代:1986

数据来源: WILEY

摘要:

Geological and geophysical evidence (in particular thermal and gravity anomalies) suggests the presence of hot mantle material below the zones of continental rifting. Using a thermomechanical numerical model we quantify the lithospheric thinning caused by such a deep thermal anomaly. The mantle is assumed to be an incompressible non‐Newtonian fluid with temperature‐ and pressure‐dependent viscosity. The index power law is 3 in the mantle and 7 in the crust. This yields a viscosity minimum at the base of the lithosphere and therefore favors rapid convective thinning. The corresponding mass displacements disturb the lithosphere. The stresses at the bottom of the crust are used to quantify the vertical movements and the tectonic regime within the continental crust. The heat flow anomalies are also computed. The present work indicates that these deep geodynamic processes can induce well‐localized extensive tectonic stresses and the thinning of the lithosphere on a short time scale. This analysis also suggests that the subsidence in the rift valley is mainly due to crustal thinning while the uplift of the shoulders of the rift is a consequence of the advection of hot material from the asthenospheric channel. A regional extensional stress enhances the convective phenomena and favors the crustal thinning but is not necessary. A decrease in this far‐field force drastically increases the uplift of the shoulders and reduces the width of the rift valley. This case may be representative of the Afr

ISSN:0278-7407    

DOI:10.1029/TC005i004p00501

年代:1986

数据来源: WILEY

摘要:

High‐resolution surface ship and deep‐tow data from the Middle America Trench off Guatemala demonstrate that structures at the base of the landward slope are most simply interpreted as resulting from the offscraping and accretion of the uppermost trench sediments. There is a 250 to 300‐m‐wide ridge elevated 40–60 m above the trench floor at the toe of the trench slope. Uplifted trench sediments are resolved on the ridge in one deep‐tow profiler record. Trench strata beneath the ridge are imaged on migrated seismic reflection profiles, which show evidence for folding of the trench strata. Vergence of the structures is consistent with folding above a landward dipping thrust fault. We therefore interpret the ridge at the base of the slope as the surface expression of folded trench sediments that are presently being offscraped and accreted to the toe of the trench slope. Only the upper hundred or so meters of trench strata are offscraped; the remainder of the trench strata and the underlying plate deposits are subducted beneath the toe of the slope. Trench sediment fill buries much of the horst topography, which is then passively subducted without eroding the base of the slope. Our results indicate that (1) non‐steady state accretion has occurred at this margin, even though Deep Sea Drilling Project drilling suggested otherwise, and (2) the lower‐most slope has been affected by compressional deformation rather than extens

ISSN:0278-7407    

DOI:10.1029/TC005i004p00513

年代:1986

数据来源: WILEY

摘要:

On Mindoro, two tectonostratigraphic terranes, the North Palawan block and the Mindoro block, are separated by a steeply dipping shear zone. The North Palawan block was rifted off southern China in the Oligocene, and comprises most of the shallow southern end of the South China Sea. The eastern part of this terrane, exposed on Mindoro, includes Jurassic and Paleogene strata, probable rift basalts of Oligocene age, and Oligo‐Miocene graben fill deposits. In the late Oligocene and early Miocene, the eastern North Palawan block underwent crustal stretching, with grabens oriented at a high angle to synchronous spreading anomalies in the South China Basin. We hypothesize that grabens formed in an overall transtensional environment near a transform boundary at the eastern margin of the opening South China Basin. The Mindoro block consists of a pre‐Upper Cretaceous low‐grade metamorphic basement, overlain by Upper Cretaceous and upper Eocene strata. The Mindoro block records several episodes of intense deformation of Mesozoic and Tertiary age, and contrasts with the North Palawan block both in terms of stratigraphy and structural evolution. The Mindoro Suture, which separates the Mindoro and North Palawan blocks, is a complex structural boundary comprising slices of diverse lithologies separated by steeply dipping, anastomosing faults. Deformed tectonic slices in the suture zone include serpentinized ultramafic rock, amphibolite, and rocks derived from both of the bounding terranes. We interpret this suture as a zone of mostly transcurrent faulting that was active through much of mid‐Tertiary time. Paleogeographic considerations for early and middle Miocene time support the premise that the west side of the central Philippine archipelago was a left‐lateral transform boundary. The Mindoro and North Palawan blocks were initially juxtaposed along the transcurrent Mindoro Suture by late Miocene time. Subsequent deformation on Mindoro includes crustal shortening associated with the southern end of the Manila Trench, which projects onshore on southwest Mindoro. The onset of shortening here probably reflects a late Miocene change in Philippine‐Eurasian plate motion. The Mindoro Suture Zone is now locked, and it acts as a rigid buttress against which strata of the North Palawan block have been structurally stacked. The Mindoro Suture Zone is similar to other terrane boundaries in the northern Philippines. The history of terrane transport in this region was apparently dominated by strike‐parallel shuffling of terranes along major transcurrent boundaries within the Philippine arc. Transcurrent motion has taken place in extensional and compressional regimes, and it is ongoing today. This may be a dominant process by which complex island arc systems are disrupted, prior to accretion onto a conti

ISSN:0278-7407    

DOI:10.1029/TC005i004p00525

年代:1986

数据来源: WILEY

摘要:

The northeast trending Kapuskasing uplift transects the east‐west belts of the central Superior Province over a distance of some 500 km. Granulite to upper amphibolite facies rocks of the uplift form three distinct geological‐geophysical entities: from south to north, the Chapleau, Groundhog River, and Fraserdale‐Moosonee blocks. Uplift of the granulites along a moderately northwest dipping crustal‐scale thrust fault is attributed to an early Proterozoic compressional event. Major northeast‐striking faults that bound the Kapuskasing zone on the west were examined by modelling of geophysical anomalies to determine dip and by geobarometry of garnet‐orthopyroxene‐plagioclase‐quartz assemblages to determine vertical displacement. Granulites in the Kapuskasing zone have 7‐ to 9‐kbar signatures whereas those in the Quetico belt to the west indicate metamorphic pressure of 4–6 kbar. Individual calibrations of the barometer yield consistent pressure differences of 2–3 kbar, suggesting 7–10 km of west‐side‐down movement on the faults. Modelling of gravity and aeromagnetic gradients indicates westerly dips of 60°–65°, with west‐side‐down offset of up to 14 km. These major normal faults probably formed as collapse structures in response to crustal thickening which occurred during the preceding compressional uplift stage. Differences in the configuration of individual blocks of the Kapuskasing zone can be related to variable fault slip and intersection angles between normal and reverse faults. Thus the Groundhog River and southern Fraserdale‐Moosonee blocks are perched thrust tips analogous to the Sangre de Cristo Range of the Laramide uplift province, whereas the southern Chapleau block is a tilted slab with similar configuration to the Laramide Wind River Range. Pop‐up geometry deduced for the northern Fraserdale‐Moosonee block resembles the structure of the Laramide Uinta Mountains. A normal fault crosses the surface trace of the basal thrust fault between the Groundhog River and Fraserdale‐Moosonee blocks and causes a 65‐km “gap” without granu

ISSN:0278-7407    

DOI:10.1029/TC005i004p00553

年代:1986

数据来源: WILEY

摘要:

In‐plane lithospheric stress, though known to exist, has generally been ignored in the quantitative modeling of basin stratigraphy. However, low levels of intraplate lateral stress can induce observable plate deformations (10–100 m of vertical motion) if the lithosphere contains a preexisting deformation (such as a sedimentary basin). The importance of in‐plane stress in modifying the stratigraphy of a sedimentary basin has been assessed using an elastic plate model for the lithosphere. A compressive in‐plane stress generally induces basin subsidence with peripheral uplift (shoreline regression), while a tensile in‐plane stress induces basin uplift and peripheral subsidence (shoreline transgression). These simple results are complicated by variations in crustal thickness, as now two interfaces are involved, that is, the sediment/basement interface of the sedimentary basin and the Moho topography; the isostatic state of a sedimentary basin therefore ultimately controls the resultant deformation induced by in‐plane stress. The implications of in‐plane stress modification of basin stratigraphy are profound: regionally correlatable transgressions and regressions of basin interiors (e.g. cyclothems) and passive continental margins (the third‐order variations of the Vail et al. [1977a] coastal onlap curve), may be tectonically produced by the interaction of stress‐induced base level changes and a basin tectonic driving subsidence. In‐plane stress, its generation, magnitude, and variation, is considered a consequence of plate boundary reconfigurations during continental collisions, rifting, and s

ISSN:0278-7407    

DOI:10.1029/TC005i004p00573

年代:1986

数据来源: WILEY

摘要:

A structural analysis of the Devonian sedimentary Gramscatho formations, South Cornwall, allows us to specify the Variscan strain pattern at the Lizard front. This deformation is consistent with a NNW verging tangential shear which first induces a synmetamorphic folding phase (F1S1) and whose noncoaxial character is emphasized, on different scales, by the existence of a positive deformation gradient toward narrow shear zones, the development of syncleavage noncylindrical folds whose axes tend to be reoriented, in the S1slaty cleavage plane, toward a pervasive stretching lineation (L1) which trends N160°E and by the existence of NNW verging sheath folds. This folding phase evolves finally to tangential structures which induce, over the parautochthonous Gramscatho formations, subhorizontal shear planes bearing strong slickenside lineations trending parallel to L1, lineations which thus represent, on a regional scale, the “tectonic transport” trend of allachthonous units migrating northwards. The existence of such allochthonous units in the South Cornubian area is now well established by the discovery, at the Lizard front, of a Paleozoic (Carboniferous ?) limestone lens wedged under the Lizard hornblendeschist. The structural evolution of the South Cornubian domain is consistent with the “scheme” of a continental platform progressively implicated in a tangential deformation propagating northward. This compression leads, in the internal southern zones, to an earlier (Mid‐Upper Devonian) crustal tangential tectonic which later (Carboniferous) reaches the external domain of England, and it expresses in the Devono‐Carboniferous cover by a tegumentary tectonic. This northward verging tangential pattern is now testified by SWAT (South West Approaches Traverses) profile in

ISSN:0278-7407    

DOI:10.1029/TC005i004p00589

年代:1986

数据来源: WILEY

摘要:

The surface geology of the North Pyrenees, southern France, is markedly different east and west of longitude 0°25′W (a line just west of Lourdes). East of this longitude the North Pyrenean fault (NPF), the major structure in the North Pyrenees, is clearly identifiable, and this fault and other faults in the North Pyrenean zone are subvertical. West of longitude 0°25′W the NPF is not identifiable at the earth's surface, suggesting to some that the NPF is not continuous and is not a regionally significant feature and that low‐angle as well as high‐angle faults are present in the fault zone. While east of Lourdes there is both a relatively continuous belt of Mesozoic metamorphic rock and a geophysically detected 15‐km step in depth to the Moho, neither is clearly recognized west of Lourdes, again bringing into question the continuity of the NPF. However, in order to accommodate opening of the Bay of Biscay, as much as 400 km of translation along a continuous NPF or North Pyrenean fault zone (NPFZ) is required. We have compiled a map covering a 100‐km‐long area in the western Pyrenees. The compilaation is based on our mapping and published maps by others. The compilation and our field work provide the data to delineate four new tectonostratigraphic units. These units are coherent stratigraphic sequences characterized by marked differences in stratigraphic thickness and depositional history. Each unit is bounded by regional faults that delineate four tectonic sheets. The principal stratigraphic differences between the four tectonostratigraphic units were produced during deposition in Cretaceous basins along the northern Pyrenees. The regional bounding faults are reverse and were formed during the main phase of the Pyrenean orogeny, that is, during Late Cretaceous and Tertiary times. Some are reactivated basin‐boundary faults and some of these are reactivated faults of Cretaceous or older age. In the western Pyrenees the NPF, which was an active left‐slip fault during the Late Jurassic and Early Cretaceous, is cut by these faults and is covered by tectonic sheets. These sheets, formed during the Paleogene, account for the apparent termination of the NPF near longitude 0°25′W and obscur

ISSN:0278-7407    

DOI:10.1029/TC005i004p00607

年代:1986

数据来源: WILEY

摘要:

The pattern of seismicity in southern California indicates that much of the activity is presently occurring on secondary structures, several of which are oriented nearly orthogonal to the strikes of the major through‐going faults. Slip along these secondary transverse features is predominantly left‐lateral and is consistent with the reactivation of conjugate faults by the current regional stress field. Near the intersection of the San Jacinto and San Andreas faults, however, these active left‐lateral faults appear to define a set of small crustal blocks, which in conjunction with both normal and reverse faulting earthquakes, suggests contemporary clockwise rotation as a result of regional right‐lateral shear. Other left‐lateral faults representing additional rotating block systems are identified in adjacent areas from geologic and seismologic data. Many of these structures predate the modern San Andreas system and may control the pattern of strain accumulation in southern California. Geodetic and paleomagnetic evidence confirm that block rotation by strike‐slip faulting is nearly ubiquitous, particularly in areas where shear is distributed, and that it accommodates both short‐term elastic and long‐term nonelastic strain. A rotating block model accounts for a number of structural styles characteristic of strike‐slip deformation in California, including: variable slip rates and alternating transtensional and transpressional features observed along strike of major wrench faults; domains of evenly‐spaced antithetic faults that terminate against major fault boundaries; continued development of bends in faults with large lateral displacements; anomalous focal mechanisms; and differential uplift in areas otherwise expected to experience extension and subsidence. Since block rotation requires a detachment surface at depth to permit rotational movement, low‐angle structures like detachments, of either local or regional extent, may be involved in the contemporary strike‐slip deformation of southern California. A block nature of the crust also implies that not only will strains be inhomogeneous and likely concentrated along edge‐bounding faults, but that local stress orientations will largely be responding to local kinematic constraints of block rotation and fault interaction. This behavior, coupled with the presence of possible regional detachments, accounts for some of the precursory changes observed at considerable distances prior to large earthquakes and the triggering of seismicity or slip on nearby faults or around adjacent block edges. Although fault displacements along secondary structures associated with block rotations remain small, they may still influence the nucleation and the characteristic rupture length of large earthquakes. A more complete description of what these structures are, and how they interact, may prove critical to any fundamental understanding of the earthquake process and any realistic assessment of th

ISSN:0278-7407    

DOI:10.1029/TC005i004p00629

年代:1986

数据来源: WILEY

摘要:

Consistent displacement of its paleomagnetic pole positions suggests that the Colorado Plateau has rotated with respect to cratonic North America. Since pole positions derived from the Colorado Plateau form a significant portion (35%) of the pre‐Cretaceous population of North American paleomagnetic data, the rotation has significant impact on the continent's data base. Comparison of paleopoles from nearly coeval strata on and off the plateau shows that the plateau pole positions are always displaced clockwise from non‐plateau counterparts, in amounts varying from 9° to 14°. The time periods for which such comparisons are possible are the Late Triassic, the Early Triassic, the Late Pennsylvanian, and the Middle to Late Devonian. Other time periods contain data which appear to be consistent with a clockwise displacement of plateau pole positions, but poor age control, rapid apparent polar wander, and late Paleozoic remagnetization all hamper comparisons. Only one of the time periods examined, the Permian, does not show net rotation of the plateau. Observation of the rotation of the plateau has been obscured by the fact that a great many of the plateau poles are from times of rapid apparent polar wander (APW), and the displacement effected by rotation parallels the major APW trends. This renders the discrepancy of plateau poles relatively inconspicuous and is only readily observable between poles from the same age strata. The rotation of the plateau has particular impact on the early and middle Mesozoic paleopoles, to which the plateau had contributed heavily. In particular, for the Late Jurassic, plateau poles are the only data available for the analysis of western North America allochthonous terrane move

ISSN:0278-7407    

DOI:10.1029/TC005i004p00649

年代:1986

数据来源: WILEY