摘要:

Compositionally layered metamorphic rocks of the D'Entrecasteaux Islands, Papua New Guinea, are folded into domes and antiforms bounded by faults parallel to metamorphic layering and foliation. The structures are broadly similar to the metamorphic “core complexes” of western North America. Lenses of ultramafic rock lie on the bounding faults, and the same faults have served as loci for Quaternary andesitic volcanic activity. Metamorphic grade in the northern islands (Goodenough and Fergusson) is amphibolite facies, with pockets of eclogite (Fergusson Island only) and granulite, and is greenschist facies in the southern island (Normanby). In all three islands there is a characteristic tectonostratigraphic sequence (FMU sequence) from felsic metamorphic rocks at base, or internally, through mafic metamorphic rocks to ultramafic rocks at top, or externally. The association of metamorphic and ultramafic rocks apparently developed in a north dipping Paleogene subduction system and was exhumed to upper crustal level in the Oligocene‐‐Early Miocene, possibly by reversal of movement on faults in the former subduction system. Vigorous uplift and development of domes and antiforms in the Pliocene was triggered by westward propagation of the Woodlark Basin spreading ridge and was accompanied by rifting, rift‐related magmatism, rapid erosion, and deposition of coarse sediment in the adjacent Trobri

ISSN:0278-7407    

DOI:10.1029/TC007i001p00001

年代:1988

数据来源: WILEY

摘要:

A Moho root beneath the Bolivian Andes, 40 km deep, is consistent with 230 km of overlap of Neogene age on a single, trenchward dipping transcrustal thrust fault with 10° of finite ramp cutoff (Main Andean Thrust (MAT)). Only 10% of the Andean crustal volume is ascribable to magmatic addition. The MAT is located within South American crust of full thickness. It intersects the basement top 450 km inland from the Neogene crustal margin. It is not a collision suture as shown by persistent pre‐Neogene facies continuity. Thrusting is not accompanied by terrane accretion. The present bilaterally symmetrical thrust belt responds to elastic line loading and to Coulomb rheology. In the hanging wall of the MAT, a deep high‐stress wedge base builds a steep critical slope. In the footwall, the foredeep response is fast subcritical growth by progradation and blind thrusting on a low‐stress decollement. Interaction is maintained by out‐of‐sequence renewal of movement

ISSN:0278-7407    

DOI:10.1029/TC007i001p00023

年代:1988

数据来源: WILEY

摘要:

We infer from the mechanisms and depths of two large earthquakes that the Hindu Kush is actively thrusting northwest over the Tadjik basin and that the basin is closing rather than being displaced to the west. Teleseismic body waves were used to determine focal mechanisms and depths for the two largest shallow earthquakes on the southern edge of the basin. The two earthquakes, on June 24, 1972 (mb=6.0), and December 16, 1982 (mb=6.2), have seismic moments of 2 × 1018N‐m and 6 × 1018N‐m, respectively. Focal mechanisms of both events indicate almost pure thrust faulting with nodal planes striking northeast‐southwest. The inferred fault planes dip southeast, at 20° for the first event and 50° for the second. The P axes for both events are oblique to the direction of relative motion between India and Asia, suggesting that the Pamir is overthrusting the basin to the west. Depths for both earthquakes are between 20 and 25 km and place them well below the Tadjik basin sediments. The depths and steep fault planes suggest that these earthquakes represent a downdip extension within the basement of shallow folding and thrusting seen in the sediments northwest of the events. Thus convergence in Afghanistan between India and Eurasia is taken up along southeast dipping thrust faults north of the Hindu Kush as well as by northward subduction under the southern part of

ISSN:0278-7407    

DOI:10.1029/TC007i001p00041

年代:1988

数据来源: WILEY

摘要:

The Salt Range and Potwar Plateau are part of the active foreland fold‐and‐thrust belt of the Himalaya in northern Pakistan. In this region the distance from the Main Boundary Thrust (MBT) to the front of the fold‐and‐thrust belt is very wide (100–150 km) because a thick evaporite sequence forms the zone of décollement. Recent studies have combined seismic reflection profiles, petroleum exploration wells, Bouguer gravity anomalies, and surface geology to construct cross sections in the eastern, central, and western Salt Range‐Potwar Plateau areas. In this study the sections are compared with a model that considers the mechanics of a fold‐and‐thrust belt to be analogous to that of a wedge of snow or soil pushed in front of a bulldozer (Chapple, 1978; Davis et al., 1983; Dahlen et al., 1984; Dahlen, 1984). Models which include the effects of evaporites at the base (Chapple, 1978; Davis and Engelder, 1985) suggest that these thrust belts will have (1) narrow (<1°) cross‐sectional tapers, (2) larger widths than areas not underlain by evaporites, (3) symmetrical structures, and (4) changes in deformational style at the edge of the evaporite basin. The section across the eastern Potwar Plateau most closely resembles this latter model, having (1) a taper of 0.8° ± 0.1°, (2) a width of 100–150 km, (3) thrust faults that verge both to the north and south, and (4) structures rotated 30° counterclockwise with respect to the Salt Range. From the observed taper and pore fluid pressures of the eastern Potwar Plateau, estimates of the values for the yield strength of the evaporites (τo) and the coefficient of internal friction of the overlying wedge (μ) are calculated as τo= 1.33–1.50 MPa and μ = 0.95–1.04, which are then applied to the other cross sections. In the central and western sections a basement uplift, the Sargodha High, interferes with the front of the fold‐and‐thrust belt. This feature causes the ramping of the Salt Range Thrust and produces a relatively steep basement slope (2°–4°) beneath the Potwar Plateau. This dip, in the presence of the weak evaporite décollement, is sufficient to provide critical taper; no topographic slope is necessary, and the thrust wedge of the southern Potwar Plateau is pushed over the décollement without significant internal deformation. The northern Potwar Plateau is strongly folded and faulted, yet the topographic slope remains flat. Although the deformation suggests that evaporites are not present there, the observed taper in the northern Potwar Plateau is best fitted by the model with evaporites at the décollement. Combining this with published paleomagnetic and geologic constraints, a model for the evolution of the northern Potwar Plateau suggests that the area deformed as a steeply tapered (3.5°–5.5°) thrust wedge until approximately 2 million years ago, when the southward propagating décollement encountered the evaporites. Between 2 Ma and the present, the northern Potwar Plateau has been pushed along the salt décollement without deformation, and erosion has reduced its original

ISSN:0278-7407    

DOI:10.1029/TC007i001p00057

年代:1988

数据来源: WILEY

摘要:

The regional traces of folds, faults, and foliations found in many fold‐thrust belts are bent in plan view. It proves valuable to distinguish between two types of bent orogens based on their kinematic evolution: oroclines (or rotational arcs) are bent orogens in which segments of the orogen change strike during the evolution of the bend, and nonrotational arcs are bent orogens in which segments of the orogen do not change strike during development of the bend. The kinematic evolution of a bend that forms in a thin‐skinned orogen can be described in terms of three parameters: the displacement path trajectories followed by points along the strike of the orogen, the magnitude and distribution of tangential extension along the strike of the orogen, and the change in position of the endpoints of the orogen with respect to a reference line. Only slight differences in the displacement path trajectory pattern determine whether an orogen evolves as a nonrotational arc or as an orocline; thus the distinction between these two types of bends is not always of major tectonic significance. Compressional deformation along irregular continental margins or the impact of indentors on continental margins during collisional orogenies more likely leads to formation of nonrotational arcs (as can be simulated with a sand wedge model). Interaction between a fold‐thrust wedge with obstacles in the foreland or with a wrench fault more likely leads to orocline formation. Orocline formation is also associated with noncoaxial reactivation of thrust faults. Tangential extension is a necessary consequence of certain displacement path trajectory patterns and can accompany development of either nonrotational arcs or oroclines. Two examples, one from the Umbrian Apennines and one from the Makran Range, demonstrate how movement on cross‐strike fault arrays can accommodate tangential extension in thin‐skin

ISSN:0278-7407    

DOI:10.1029/TC007i001p00073

年代:1988

数据来源: WILEY

摘要:

The contact between the western border of the Tauern Window and the overlying Austroalpine nappes (Austria and Italy) is marked by a structural, petrologic, and geochronologic discontinuity that developed in the early stages of unroofing of the window. Within the window, pressure‐temperature‐time paths (J. Selverstone, 1985) indicate that significant ductile thinning occurred shortly after the cessation of thrusting (Paleocene ‐ Eocene). Sense‐of‐shear indicators (rotated porphyroblasts, asymmetric augen and pressure shadows, S‐C fabrics) associated with a west plunging stretching lineation indicate that this thinning occurred in response to top‐to‐the‐west low‐angle normal shear during metamorphism. The same sense of normal shear is present in the Mesozoic members of the Austroalpine sequence west of the window. In these rocks, however, west directed shear occurred at low temperatures (<300°C) after the thermal peak of metamorphism. Structural data and the regional geometry imply that the western end of the Tauern Window is a low‐angle normal fault (Brenner Line) that juxtaposed brittlely deformed rocks of the Austroalpine nappes against more ductilely deformed rocks from the window. Biotite K/Ar and Rb/Sr data suggest a Miocene age for final movement on the Brenner Line. These data indicate a prolonged history of east‐west extension in the Eastern Alps that affected all crustal levels (early ductile thinning of lower crust followed by mid‐to‐upper‐crustal low‐angle normal faulting). A model is presented that relates the young east‐west extension to displacement transfer from dextral movement on the Periadriatic Lineament. In this model, the Brenner Line and, by analogy, the Simplon Line can be thought of as detachment faults that tectonically unroofed the Tauern Window and Lep

ISSN:0278-7407    

DOI:10.1029/TC007i001p00087

年代:1988

数据来源: WILEY

摘要:

Seismic reflection data were collected and processed to 20 s two‐way travel time along four lines which cross the rifted continent‐ocean boundary off the Grand Banks region of eastern Canada specifically to examine the origin, age, and nature of this fundamental boundary. This represents the first regional study of its kind. The most important result is the presence of landward dipping reflectors near the foot of the continental slope. These occur where oceanic crust appears to terminate against the continent. We suggest that the dipping reflectors mark the continent‐ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary. Sedimentary basins lie just landward of the continent‐ocean boundary. Their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting, and this may be an important stage leading to continental breakup. Rift basins formed further landward on the Grand Banks do not exhibit this thinning. Other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent‐ocean boundary. Also, the oceanic crust exhibits a complexity of reflections, some of which may be due to compositional zonation during magmatic crystallization. Finally, our results may have important consequences for continental predrift reconstruction, as oceanic crust appears to extend farther landward in the Newfoundland Basin than some recent studies have

ISSN:0278-7407    

DOI:10.1029/TC007i001p00107

年代:1988

数据来源: WILEY

摘要:

Epeirogenic motions can be quantified if paleodatum planes can be recognized. These motions have been determined for the time interval mid‐Cretaceous (Cenomanian) to recent in continental Africa using the vertical displacement of ancient shoreline sediments and other nearshore or low‐elevation deposits that are inferred to represent a Cenomanian sea level datum plane. The Cretaceous Trans‐Saharan seaway brought shoreline sediments deep into the African continental interior, and additional data suggest that most of the remaining terrestrial regions were also low‐lying during the Cenomanian. The present elevations of the sea level sediments indicate that large portions of Africa have undergone epeirogenic uplift since the Cenomanian. The character of the motions in north Africa can be described as small‐scale high‐amplitude (3 km) doming separated by relatively flat‐lying areas of deposition. The largest single epeirogenic feature is the uplift associated with Red Sea and East African Rift development. The timing of post‐Cenomanian epeirogenic motions can be further constrained using paleontologic, K‐Ar, faulting, paleodrainage and sedimentary data. The most likely causal mechanism for African epeirogeny may be hot spot activity reflective of the mantle convection regime underlying the Af

ISSN:0278-7407    

DOI:10.1029/TC007i001p00125

年代:1988

数据来源: WILEY