摘要:

The Hercynian outcrop areas in the Schwarzwald of SW Germany and Vosges of easternmost France consist from north to south of three major lithotectonic complexes: very low‐ to medium grade metamorphics of the Saxothuringian Belt, the polymetamorphic Central Gneiss Complex, and the tectonic melange of the Southern Complex. Juxtaposition of these three complexes occurred by oblique convergent motions in early Carboniferous time. The Saxothuringian Belt is part of a major SE dipping accretionary wedge that consists of lower Paleozoic meta‐sedimentary and metaigneous rocks and superimposed upper Devonian to lower Carboniferous synorogenic volcanics and elastics. The Central Gneiss Complex consists of polymetamorphic paragneisses, orthogneisses, metabasites, and meta‐ultramafics dipping to the NW. The Southern Complex consists of an imbricated melange of north to NW dipping synorogenic elastics, volcanics, and blocks of ultramafics and gneissic basement. The Central Gneiss Complex (CGC) experienced pervasive Hercynian low‐pressure/high‐temperature metamorphisrn before it was emplaced against and thrust over the sedimentary and volcanic fill of the adjacent synorogenic basins. This convergence (340–330 Ma) produced mylonitic‐cataclastic shear zones in the CGC. The most significant shear zones are the SE verging Todtnau fault zone between the CGC and the Southern Complex and the NW verging Lubine fault zone between the Central Gneiss Complex and the rocks of the Saxothuringian Belt. These two fault zones enclose a bivergent “pop‐up” of the polymetamorphic basement. The Todtnau fault zone probably extends for more than 100 km along strike and can be traced to a depth of 10 km by deep seismic reflection studies. Detachment probably occurred along a mid crustal zone of anatexis expressed by a domain of low seismic reflectivity between about 10 and 15 km. Crustal thickening during early Carboniferous convergence led to an extensional phase of crustal reordering (between about 330 and 280 Ma) that was accompanied by the massive rise of “posttectonic” granitic plutons from middle to high crustal levels. Simultaneous creation of a structural “lamination” in the refractory lower crust neglected the main convergent tectonic boundaries; it produced a basin‐and‐range type lower crust and mantle‐crust transition in early Permian time and involve

ISSN:0278-7407    

DOI:10.1029/TC008i001p00001

年代:1989

数据来源: WILEY

摘要:

The 250‐km‐long deep seismic survey across the Pyrenees from the Aquitaine basin to the Ebro basin was sponsored by French and Spanish organizations, and was carried out in 1985 and 1986. This first seismic survey of an entire orogenic belt crosses the deformed boundary between the Iberian and European plates, the geometry of which has been greatly debated during a recent past. The main results are summarized as follows: (1) The entire crust shows well‐defined reflectors with a general fan shape geometry. (2) The Iberian crust seems to be thicker than the European one. Both are limited by a well‐defined deep layered material above the Moho discontinuity. (3) Only the Iberian crust was significantly thickened by a southward overstacking of slabs. (4) Near the surface in the external domains, reflectors define accurately the geometry of major thrusts and structures affecting the Mesozoic and Cenozoic cover of the Pyrenees. Using ECORS data (Etude Continentale et Océanique par Réflexion et Réfraction Sismique), the structure of the studied section of the belt can be modeled considering the nature and continuity of reflectors beneath the surface trace of the North Pyrenean fault considered as the initial vertical boundary between Iberian and Europ

ISSN:0278-7407    

DOI:10.1029/TC008i001p00023

年代:1989

数据来源: WILEY

摘要:

The ECORS deep seismic profile and additional geological and geophysical data are used to constrain the balancing of a structural section crossing the Pyrenees. To minimize the effects of the mid‐Cretaceous strike‐slip motion along the North Pyrenean fault, we have chosen to restore the geometry to the period after the Albian‐Cenomanian strike‐slip faulting and before the Late Cretaceous compressional tectonics. At least 100 km of shortening must be accounted for in the deep crust in order to balance the cross section. The estimated length of the top of the Iberian Paleozoic basement is 40 km shorter than the length of the layered Iberian lower crust as measured on the ECORS seismic line. A variety of restorations are thus discussed to accommodate this discrepancy. The first solution considers that the discrepancy is due to an initial absence of lower crust underneath part of the Iberian margin. This solution implies a simple shear model involving low‐angle detachment faulting during the opening stage of Albian basins. The favored solution considers that the missing Iberian deep crust is currently stacked within the axial zone; it implies that Albian basins formed as pull‐apart basins along the North Pyrenean f

ISSN:0278-7407    

DOI:10.1029/TC008i001p00041

年代:1989

数据来源: WILEY

摘要:

Within the Mediterranean region, Cenozoic deformation of the Western Alps and the West to East Carpathians has resulted in two different styles of foreland fold and thrust belt. The most prominent difference between the two belts is the presence (Carpathians) or absence (Western Alps) of contemporaneous back‐arc extension, but other important differences in structure, topography and metamorphism also exist. These differences in thrust belt style developed mainly during the final stages of thrust belt evolution and appear to reflect fundamental differences in the tectonic settings of the Western Alps and the Carpathians in middle and late Cenozoic time. In particular, they appear to be the result of convergence that is in the first case driven primarily by major plate motions and in the second case only by local motions of small lithospheric flakes or fragments. We suggest that the structural styles developed in these two mountain belts may be useful in identifying mountain belts that have evolved in similar tectonic settings elsewhere in the world. In this respect, the Western Alps and the Carpathians can be regarded as typical examples of two different styles of foreland fold and thrust belt (or more properly as end‐member examples within a broad spectrum of foreland fold and thrust belt styles). We propose that continental subduction zones and orogenic belts can be loosely divided into segments that show no major back‐arc extensional deformation adjacent to the belt (the Western Alps) and segments that exhibit back‐arc extension contemporaneously with thrusting (the Carpathians). The former are found in areas where the rate of overall plate convergence exceeds the rate of subduction, and are commonly typified by extensive involvement of crystalline basement in thrusting, exposure of high grade metamorphic rocks at the surface, high topographic elevation, and large amounts of erosion (tens of kilometers). The latter are found in areas where the rate of subduction exceeds the rate of overall plate convergence and are commonly typified by thrust belts with little to no involvement of crystalline basement in thrusting, low grade to no metamorphism, low topographic elevation, little erosion and, in some instances, an anomalously deep foredeep basin

ISSN:0278-7407    

DOI:10.1029/TC008i001p00051

年代:1989

数据来源: WILEY

摘要:

On the basis of geological, geophysical, and geochronological data, the Grenville Province has been divided into three first‐order longitudinal belts, the Parautochthonous Belt (PB), Allochthonous Polycyclic Belt (APB), and Allochthonous Monocyclic Belt (AMB). These are set apart by three first‐order tectonic boundaries, the Grenville Front (GF), Allochthon Boundary Thrust (ABT), and Monocyclic Belt Boundary Zone (MBBZ). The belts are subdivided into terranes based on internal lithological character. The GF separates the Archean to Proterozoic foreland northwest of the orogen from reworked equivalents to the southeast. Continuous at the scale of the orogen, its main characteristic is that of a crustal‐scale contraction fault. The PB, although less clearly identified along the length of the orogen, in most places represents upgraded and tectonically reworked rocks of the adjacent foreland. The boundary between the PB and the APB to the southeast, the ABT, is most clearly delineated in the eastern half of the province. It is the locus of major crustal delamination along which high‐grade, mostly middle Proterozoic, polycyclic terranes were tectonically transported northwest toward and onto the PB. The AMB comprises two separate areas underlain by the Wakeham Supergroup and what is currently known as the Grenville Supergroup, respectively; its basal contact, the MBBZ, is a décollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks. This first‐order zonation implies a tectonic polarity to the Grenville Province, superimposed on which are second‐order features evident from contrasting tectonic styles and radiometric ages. These characteristics are consistent with a diachronous or oblique collisional model for the Gren

ISSN:0278-7407    

DOI:10.1029/TC008i001p00063

年代:1989

数据来源: WILEY

摘要:

The 350‐km‐long, east‐west trending Wetar back arc thrust belt in eastern Indonesia is a result of the collision of Australia with the Indonesian island arc. Along the northwest margin of Wetar Island a short (50 km) section of the thrust belt trends northeast, coincident with the offset of the Indonesian arc by the newly discovered left‐lateral Wetar‐Atauro fault, which runs along the shelf region and trends northeast. The Wetar‐Atauro fault may be viewed as a large‐scale lateral ramp or wrench fault separating the eastern Wetar thrust belt from the western Wetar thrust belt. The interaction of strike‐slip faulting and back arc thrust faulting creates several arc “blocks” whose geometry strongly affects the structure of the deformed wedge of sediment accreting in the Wetar back arc thrust belt. The varying orientations of the arc “backstop” make the Wetar back arc thrust belt a perfect laboratory for the study of oblique convergence. Along most of its length the Wetar thrust belt parallels the arc slope. However, along the northeast trending offset in the thrust belt, SeaMARC II side‐scan sonar plus seismic reflection data show that the trend of the thrust belt is not parallel to the trend of the arc slope; rather, it is intermediate between the trend of the arc slope and the perpendicular to the expected convergence direction. The side‐scan images allow us to map the geometry of four main thrust faults: the Wetar, Liran, Atauro, and Alor faults. The Wetar, Liran, and Atauro faults trend northeast, parallel to the thrust front, and do not have an en echelon relationship. The westernmost fault in the survey area, the Alor fault, trends almost east‐west. The orientation of the Wetar‐Atauro fault is consistent with a maximum principal stress within the arc oriented between 8° and 23° west of north, depending on the internal coefficient of friction. This agrees well with the P‐axes of nearby earthquake focal mechanisms which trend consistently west of north, suggesting that the regional principal tectonic compression is oriented west of north. Because the thrust belt is oblique to the arc slope, we infer the structural directions are influenced by both the arc backstop and by the regional tectonic stress. The tectonic compression due to arc‐continent collision in this region may be modified by the arc geometry in several ways: (1) the stress necessary to support the arc topography may be significant close to the arc, (2) the change in thickness of the elastic part of the lithosphere may cause a concentration of tectonic stress, and (3) the difference in material properties between the relatively rigid arc “bulldozer” and the weak basin sedimentary fill may be an important factor in producing a thrust belt that conforms roughly to the shape of the arc. The structure of the Wetar back arc thrust zone demonstrates that the development of small rigid blocks within a major collision zone may produce complex structural patterns and local directions of shortening that are highly oblique t

ISSN:0278-7407    

DOI:10.1029/TC008i001p00085

年代:1989

数据来源: WILEY

摘要:

Late Miocene (12–5 Ma) extension around the edges of the Gulf of California has been alternatively attributed to “Basin and Range” extension, back arc extension, or development of the Pacific‐North America plate boundary. This extension was ENE directed and similar in structural style to extension in the Basin and Range province. Timing constraints permit nearly synchronous onset of this deformation in a belt extending SSE from northernmost Baja California to the mouth of the gulf. Where this extensional faulting continued through Pliocene time to the present, synchronous with motion on the modern transform plate boundary in the Gulf of California, no change in direction of extension can be resolved. Revised constraints on Pacific‐North America plate motion support the development of this late Miocene extension as a component of Pacific‐North America displacement that could not be accommodated by strike‐slip displacement along the existing plate boundary west of the Baja California peninsula. This scenario implies that transfer of Baja California from the North America plate to the Pacific plate was a gradual process, beginning about 12–10 Ma, when motion of the Pacific plate relative to North America was partitioned into separate regimes of strike‐slip and dip‐slip displacement on opposite sides

ISSN:0278-7407    

DOI:10.1029/TC008i001p00099

年代:1989

数据来源: WILEY

摘要:

Three representative basins in the Western rift system of East Africa are bordered along one side by high‐angle normal faults with 2‐ to 5‐km throws (border faults). In plan view ∼100‐km‐long systems of linear border faults form curvilinear border fault segments bounding the East Kivu, West Kivu, and Rusizi basins. The opposite sides of these asymmetric basins are bounded by lower relief faulted monoclines or en echelon ramps. The largely unfaulted rift flanks have been uplifted 2 km above the 1.3‐km‐high East African plateau, with uplift narrowing basins during Quaternary time. Maximum estimates of ∼E‐W crustal extension within basins are less than 25% (<16 km), and planar border faults may penetrate the crust. The East Kivu and West Kivu basins are linked across the rift valley by a horst that serves as a hinge for subsidence in both basins. The westward tilted East Kivu and eastward tilted Rusizi border fault segments are linked along the rift by oblique‐slip transfer faults that also accommodate along‐axis differences in elevation. Upper Miocene‐Recent eruptive volcanic centers within the comparatively high‐strain interbasinal region (accommodation zone) generally coincide with the tips of border fault segments and transfer faults. The orientations of Miocene‐Recent dip‐slip and oblique‐slip faults show little correlation with Precambrian shear zones or foliations in metamorphic basement. Differences between the East Kivu, West Kivu, and Rusizi basins in the age of initial faulting, subsidence, and age/composition of volcanic products suggest that border fault segments developed diachronously and propagated along the length of the rift. This along‐axis border fault propagation and the crosscutting geometry of transfer faults contribute to the segm

ISSN:0278-7407    

DOI:10.1029/TC008i001p00117

年代:1989

数据来源: WILEY

摘要:

Large northeast striking normal faults in the southern Klamath Mountains may indicate that substantial crustal extension occurred during Tertiary time. Some of these faults form grabens in the Jurassic and older bedrock of the province. The grabens contain continental Oligocene or Miocene deposits (Weaverville Formation), and in two of them the Oligocene or Miocene is underlain by Lower Cretaceous marine formations (Great Valley sequence). At the La Grange gold placer mine the Oligocene or Miocene strata dip northwest into the gently southeast dipping mylonitic footwall surface of the La Grange fault. The large normal displacement required by the relations at the La Grange mine is also suggested by omission of several kilometers of structural thickness of bedrock units across the northeast continuation of the La Grange fault, as well as by significant changes in bedrock across some northeast striking faults elsewhere in the Central Metamorphic and Eastern Klamath belts. The Trinity ultramafic sheet crops out in the Eastern Klamath terrane as part of a broad northeast trending arch that may be structurally analogous to the domed lower plate of metamorphic core complexes found in eastern parts of the Cordillera. The northeast continuation of the La Grange fault bounds the southeastern side of the Trinity arch in the Eastern Klamath terrane and locally cuts out substantial lower parts of adjacent Paleozoic strata of the Redding section. Faults bounding the northwestern side of the Trinity arch generally trend northeast and juxtapose stacked thrust sheets of lower Paleozoic strata of the Yreka terrane against the Trinity ultramafic sheet. Geometric relations suggest that the Tertiary extension of the southern Klamath Mountains was in NW‐SE directions and that the Redding section and the southern part of the Central Metamorphic terrane may be a large Tertiary allochthon detached from the Trinity ultramafic sheet. Paleomagnetic data indicate a lack of rotation about a vertical axis during the extension. We propose that the Trinity ultramafic sheet is structurally analogous to a metamorphic core complex; if so, it is the first core complex to be described that involves ultramafic rocks. We infer that Mesozoic terrane accretion produced a large gravitational instability in the crust that spread laterally during Tertiary extensio

ISSN:0278-7407    

DOI:10.1029/TC008i001p00135

年代:1989

数据来源: WILEY