摘要:

The WINCH marine deep seismic reflection profile crosses the Hebridean shelf, the Proterozoic foreland to the Caledonian orogen, west of Scotland. The data quality is very good. The upper crust is largely devoid of coherent seismic reflections, although this may in part be due to acquisition techniques being inappropriate for this problem. In contrast, the middle and lower crust (10–25 km depth) exhibits good reflections; the mid crust contains reflectors which may be relics of early Palaeozoic, Caledonian (or earlier Grenvillian) eastward‐dipping thrust zones, which pass into an acoustically strongly layered lower crust. The Outer Isles Thrust is mapped from the surface to the mid crust, and tied into its land outcrop on north Lewis. Reactivation of this thrust offshore, in a normal sense, in Mesozoic times caused the formation of the Minch and North Lewis Basins. The Mono is defined by a strong band of reflections at about 27 km depth, which correlates well with results from the HMSE explosion seismic survey. Moho depth is apparently rather uniform in the area of the foreland crossed by WINCH. WINCH and HMSE results together suggest a mean crustal velocity for the Hebridean shelf of 6.4+0.1 km s−1. The eastward‐dipping Flannan Thrust can be mapped into the upper mantle on three lines from about 15 to 45 km depth, well into the upper mantle. Neither the Flannan Thrust nor the Outer Isles Thrust appear to pass straight through the reflective lower crust, suggesting that the lower crust is a region of high strain. The Outer Hebrides is a positive block probably formed as an isostatic response to Mesozoic normal faulting which reactivated the Outer Isles

ISSN:0278-7407    

DOI:10.1029/TC005i002p00171

年代:1986

数据来源: WILEY

摘要:

Imbrication of the north‐west margin of the Baltic Shield during the mid‐Silurian continental collision caused continental “subduction” as recorded by the eclogites of the Western Gneiss Region of western Norway (Griffin et al., 1986). During early Devonian times bouyancy forces resulting from the extreme crustal thickening initiated a period of extension which led to the development of a major westerly dipping extensional fault affecting the whole thickness of the imbricated margin. As the footwall of this fault was uplifted the erosional products were deposited on the hanging wall to form a series of basins containing thick sequences of coarse clastic continental sediments. Similarities between this extensional regime and that of the Basin and Range province of the western United States are di

ISSN:0278-7407    

DOI:10.1029/TC005i002p00195

年代:1986

数据来源: WILEY

摘要:

A critical review of Devonian and Carboniferous paleomagnetic results from the Hercynian basement of Western and Central Europe reveals large rotations. Their geographic distribution suggests a tectonic origin for large scale curved structures, such as the Ibero‐Armorican arc or the Variscan V, together with a coherence of the Hercynian domain with time. Geodynamic models which can have produced the observed rotation pattern are discussed. It appears that lateral compression, due to the accommodation of continental collision as in the India‐Asia convergence, is a plausible mechanism although Variscan and Himalayan mountain belts have different characterist

ISSN:0278-7407    

DOI:10.1029/TC005i002p00205

年代:1986

数据来源: WILEY

摘要:

We attempt to reconstruct the kinematics (deformation and motion) of the Western Alpine Arc between the Mercantour and Aar Massifs. First, we compile a map showing principal extension (X) direction within three major structural units. In the external zone, X directions are broadly radial to the Alpine Arc; in the internal zone, they are uniform or smoothly varying in some domains; whereas in the intermediate zone they often show much scatter. Next, we use geochronological data and P‐T conditions, estimated from metamorphic mineral reactions, to time the deformation. On each of a series of interval maps, we show X‐directions for strain accumulated during a chosen time interval. We argue that these incremental X‐directions are approximately equivalent to segments of particle paths, showing displacements relative to stable Europe. For 120–100 Ma, the displacement direction was about N150 and coeval with eclogite facies metamorphism; it is interpreted as a direction of overthrusting by the African Plate. Around 40 Ma, the displacement in the south part of the arc became westerly. Finally, between 25 and 15 Ma, displacement directions were generally radial to

ISSN:0278-7407    

DOI:10.1029/TC005i002p00215

年代:1986

数据来源: WILEY

摘要:

Previously proposed models for the evolution of the Tyrrhenian basin‐Apenninic arc system do not seem to satisfactorily explain the dynamic relationship between extension in the Tyrrhenian and compression in the Apennines. The most important regional plate kinematic constraints that any model has to satisfy in this case are: (1) the timing of extension in the Tyrrhenian and compression in the Apennines, (2) the amount of shortening in the Apennines, (3) the amount of extension in the Tyrrhenian, and (4) Africa‐Europe relative motion. The estimated contemporaneous (post‐middle Miocene) amounts of extension in the Tyrrhenian and of shortening in the Apennines appear to be very similar. The extension in the Tyrrhenian Sea is mostly accomplished in an E‐W direction, and cannot be straightforwardly related to the calculated N‐S Africa‐Europe convergence. A model of outward arc migration fits all these constraints. In a subducting system, the subduction zone is expected to migrate outward due to the sinking of the underthrusting plate into the mantle. The formation of a back‐arc or internal basin, i.e. of a basin internal to the surrounding belt of compression, (in this case the Tyrrhenian Sea) is then expected to take place if the motion of the overriding plate does not compensate for the retreat of the subduction zone. The sediment cover will be stripped from the underthrusting plate by the outward migrating arc of the overriding plate, and will accumulate to form an accretionary wedge. This accretionary body will grow outward in time, and will eventually become an orogenic belt, (in this case the present Apennines) when the migrating arc collides with the stable continental foreland on the subducting plate. An arc migration model satisfactorily accounts for the basic features of the Tyrrhenian‐Apennine system and for its evolution from 17 Ma to the present, and appears to be analogous to the tectonic evolution of other back‐arc settings both inside and outside the Mediterranean region. An interesting implication of the proposed accretionary origin of the Apennines is that the problematic “Argille Scagliose” (scaly clays) melange units might have been emplaced as overpressured mud diapirs, as observed in other accretionary prisms, and not by gravity slides fr

ISSN:0278-7407    

DOI:10.1029/TC005i002p00227

年代:1986

数据来源: WILEY

摘要:

The most useful criteria used for the determination of the sense of shear can be found in the huge Himalayan ductile shear zone known as M.C.T. They allow the determination of the sense of shear in relation with an unequivocal southward‐directed intracontinental thrust. The various criteria analyzed at different scales of observation show the permanence of the shear flow regime throughout the metamorphic evolution. However, most of the observed shear‐criteria are due to late nappe emplacement postdating the climax of the metamorphism. The most striking feature is the stretching lineation which is roughly normal to the thrust trace all along the belt. It parallels everywhere the shear direction and thus indicates the transport direction of the nappes. The observed radial pattern of stretch trajectories implies that the local motion does not correspond to the average plate converge

ISSN:0278-7407    

DOI:10.1029/TC005i002p00247

年代:1986

数据来源: WILEY

摘要:

The Bir Zreir Graben, a rhomb‐shaped depression located in eastern Sinai is well exposed in three dimensions. A nearly horizontal nonconformity between Precambrian crystalline basement and overlying Phanerozoic sediments, contacts of basement plutons and a system of early Miocene basaltic dikes permit very precise mapping of the local structures and the determination of vertical and horizontal displacements along faults. Applying the standard “pull‐apart model” to the Bir Zreir rhomb‐shaped graben predicts greater vertical separations, 3 to 6 times larger, than those observed. An alternative model, based on a rotational movement of the Bir Zreir block, agrees more closely with field evidence. It is concluded that the shape and size of the graben was determined by the original fault pattern, and that its final shape was changed relatively little by later movements. The rotation of the Bir Zreir block resulted from a combination of sinistral displacement along N‐S faults, a change of their trend to NNE, and a regional E‐W extension. The model proposed here does not require the presence of large, deep gaps along the diagonal faults as in

ISSN:0278-7407    

DOI:10.1029/TC005i002p00267

年代:1986

数据来源: WILEY

摘要:

The Transantarctic rift, an extensional continental rift valley, formed between East and West Antarctica during latest Early and Middle Jurassic time and is represented today by the high Transantarctic Mountains, which contain large volumes of continental flood basalt, diabase, and gabbro. Transantarctic rifting marked the beginning of the breakup of Gondwanaland; it was contiguous and synchronous with continental rifting between East Antarctica‐India and Africa as represented by the continental basalt and diabase of Queen Maud Land and the Karroo of southern Africa. During Late Jurassic time, about 150 Ma or slightly earlier, East and West Gondwanaland separated and new oceanic crust of the earliest Indian Ocean formed between East Antarctica‐India and Africa. If, as assumed, West Antarctica and South America remained fixed through a tip‐to‐tip join between the Antarctic Peninsula and Tierra del Fuego, then this seafloor spreading required major right‐lateral transform faulting of 500 to 1000 km on the Transantarctic rift system between East and West Antarctica. The Transantarctic Mountains were elevated at about the same time in Late Jurassic; such uplifts are characteristic of active rift margins worldwide. During Cenozoic time, extensional block faulting, independent of the Jurassic rifting, further disrupted large areas of West Antarctica. During the same time, the Transantarctic Mountains were further

ISSN:0278-7407    

DOI:10.1029/TC005i002p00279

年代:1986

数据来源: WILEY

摘要:

COCORP seismic reflection profiling in the western and northern Mojave Desert of southern California has revealed the presence of numerous major low‐angle reflecting horizons within the crust. These complex, though laterally continuous, horizons are interpreted to represent major southwesterly dipping crustal fault zones, and as such they place important constraints on the tectonic evolution of the region. The upper‐most horizon is interpreted to be the Rand thrust, which, where exposed, places Precambrian and Late Cretaceous crystalline rocks over possibly younger Pelona‐Rand‐Orocopia Schist. This reflecting horizon extends for 25 km southwest of the Rand Mountains where it appears to be truncated at a depth of about 7.4 km by another horizon, which may be a later low‐angle normal fault. The other reflecting horizons are not traceable to the surface, and so greater ambiguity remains in their interpretation. The most prominent of these horizons occurs at midcrustal depths (15±6 km), exhibits a “ramp and flat” geometry, and extends over the northern area of the Mojave survey into the Basin and Range Province. A lower horizon, at depths of 20–30 km in the northern part of the survey area, is antiformal and appears to terminate above a flat and relatively continuous Moho‐depth horizon. The crustmantle transition appears to be represented by a continuous series of reflections which occur at about 10 s (33 km) in the north of the survey and at about 8–9 s (26–29 km) in the south. These reflections are offset in the vicinity of the town of Mojave. The deep intracrustal fault zones inferred from the COCORP survey may represent (1) the deep crustal continuation of the system of Mesozoic thrusts which crops out in southern Nevada and southeastern California, (2) Late Cretaceous to Early Cenozoic, northeast‐vergent thrusts related to the uplift of the Pelona‐Orocopia‐Rand Schist, or (3) low‐angle normal faults related to Early Miocene, northeast‐southwest directed crustal extension. The COCORP survey also traversed the major strike‐slip faults that bound the Mojave block. The San Andreas fault zone appears to truncate reflectors at depths of 6, 8 and 20 km within the Mojave basement, suggesting that it is a major vertical feature which extends to at least 20 km depth. Conversely, the Garlock fault does not offset an underlying reflecting horizon which occurs at 9 km depth and therefore appears to be a

ISSN:0278-7407    

DOI:10.1029/TC005i002p00293

年代:1986

数据来源: WILEY

摘要:

A COCORP seismic reflection profile across the northern Sierra Nevada in California shows several east‐dipping zones of discontinuous reflections. Correlation with surface geology suggests that these zones probably originate from faults of the Foothills fault system. In particular, the Melones fault, which coincides with the “Mother Lode” of the central and southern Sierra foothills, appears to be marked by prominent reflections in the midcrust. Migration of the COCORP data suggests that these faults are approximately planar, have moderately steep east dips (35°–47°), and penetrate at least to midcrustal depths (>20 km). At present it is unclear whether these faults are primary Nevadan thrusts, “late” Nevadan backthrusts (retrocharriage), or younger Cretaceous or Cenozoic faults, also known to occur in the region. Other more problematic features imaged on the profile include a prominent west‐dipping zone of reflections in the midcrust beneath the Eastern belt, and subhorizontal reflections at 22‐ to 26‐km depth beneath the Tahoe graben. The former might represent a west‐dipping thrust analogous to the Taylorsville thrust cropping out to the north of the survey route. The latter might represent the base of the Sierra Nevada batholith, the westward extension of any one of several thrust systems cropping out in Nevada, a low‐angle extensio

ISSN:0278-7407    

DOI:10.1029/TC005i002p00321

年代:1986

数据来源: WILEY