摘要:

Seismotectonic characteristics of the transition area between the Zagros continental collision zone and the Makran ocean‐continent convergence zone (the ‘Oman Line’) are carefully examined. A northeast trending zone of earthquakes terminates the Zagros belt of seismicity just west of the Oman Line. This seismic zone extends northeastward beyond the Main Zagros Thrust and coincides with a surface escarpment clearly visible on LANDSAT imagery. Events with accurately determined depths along this seismic zone occurred in the upper 20 km of the crust and possibly show a shallow (<10°) northeastward dip. East of the Oman Line, the Zendan‐Minab fault system is relatively aseismic. The Oman Line region may be characterized by underthrusting of a wedge of Arabian shelf edge beneath Iranian crust or by an indentation of Arabia into the Iranian crustal block as a promontory. Available seismological and geological data cannot uniquely distinguish between these two possible tectonic settings. The area located to the northwest of the Oman Line region, now a zone of continental collision, appears to have been characterized by an episode of Neogene subduction. A well‐located, intermediate‐depth earthquake occurred in this area to the northeast of the Main Zagros Thrust on November 9, 1970. It was accurately located at 107‐km depth beneath a line of Quaternary volcanoes. Its focal mechanism may indicate downdip tension, with nodal planes that strike closely parallel to the trend of the Zagros arc. Comparison of this event and other neotectonic features in this part of Iran with those in other active convergent zones suggests that a descending oceanic lithosphere beneath the Zagros volcanic arc may still be attached to the colliding

ISSN:0278-7407    

DOI:10.1029/TC001i005p00389

年代:1982

数据来源: WILEY

摘要:

COCORP seismic reflection profiling in the Ouachita Mountains of western Arkansas indicates that (1) Carboniferous foreland basin deposits within the Arkoma Basin thicken dramatically toward the south, reaching an aggregate thickness in excess of 12 km (4.5 s two‐way travel time) beneath the southern edge of the Frontal Thrust Zone, (2) evidence for northward directed low‐angle thrusting within this clastic sequence is prominent, with a probable decollement surface lying at or near the contact with underlying Early Paleozoic shelf carbonates, (3) beneath the Benton Uplift a sequence of discontinuous events defines a broad antiform cresting at approximately 7 km (2.8 s); this structure appears to mimic the anticlinorial shape of the Benton Uplift indicated by the surface outcrop pattern, (4) beneath the Southern Ouachitas, south dipping stratified events are observed to depths in excess of 14 km (5.0 s), (5) deeper in the section a prominent, gently north dipping reflection occurs at approximately 22 km depth (7.6 s) beneath the northern Coastal Plain/Southern Ouachitas.Extrapolation of data along strike in the Ouachita belt, and consideration of large‐scale structure observed in other collisional orogenic belts, suggest that the reflection events observed beneath the Benton Uplift represent Early Paleozoic shelf carbonates correlative with those that floor the Arkoma Basin to the north. This interpretation requires that the Early Paleozoic deep‐water sediments exposed in the core of the Ouachitas be allochthonous, and also implies significant basement uplift beneath the core zone. To the south, the prominent package of layered reflections occurring beneath the Southern Ouachitas and northern Coastal Plain indicates that a significant portion of the crust in that region is composed of imbricate sedimentary and/or metasedimentary strata. At relatively ‘shallow’ levels these strata are correlative with the Carboniferous flysch cropping out in the Southern Ouachitas, and at somewhat deeper levels, they are correlative with the Early to Middle Paleozoic deep‐water sediments exposed on the

ISSN:0278-7407    

DOI:10.1029/TC001i005p00413

年代:1982

数据来源: WILEY

摘要:

Analyses of regional gravity and magnetic patterns, LANDSAT images and geological information have revealed two major lineaments crossing western Pennsylvania and parts of surrounding states. These lineaments are inferred to be expressions of fracture zones which penetrate deeply into the crust and possibly the upper mantle. The extensions of the Tyrone‐Mt. Union and the Pittsburgh‐Washington lineaments bound a distinct crustal block (Lake Erie‐Maryland block) over 100 km wide and probably more than 600 km in length. Evidence exists for the lateral displacement of this block at least 60 km northwestward during late Precambrian to Lower Ordovician time. Subsequent movements have been mainly vertical with respect to neighboring b

ISSN:0278-7407    

DOI:10.1029/TC001i005p00431

年代:1982

数据来源: WILEY

摘要:

The climactic phase of the Caledonian orogeny in Scandinavia involved collision of the Baltic and Greenland cratons in late Ordovician (?) to Silurian time. We infer that the collision resulted in large‐scale subduction of the leading edge of the Baltic craton beneath an ‘Andean‐type’ Greenland continental margin. Deep erosional levels in the Lofoten‐Rombak area, Norway‐Sweden (˜69°N) permit direct examination and evaluation of the structural evolution of a continental or A‐type subduction zone. The early stages of collision involved en bloc underthrusting of the continental margin to depths of at least 30 km. During this process, deformational effects in the lower plate were restricted to its upper few hundred meters. Ultimately, en bloc A‐type subduction should be limted by buoyancy effects. As convergence continued, the lower plate became involved in a series of imbricate thrusts with the same vergence as the intial subduction zone. The temporal and spatial relationships between these basement‐involved thrusts are similar to those observed in foreland fold and thrust terrains. Field and geophysical data bearing on lower plate behavior elsewhere in the Caledonldes and in other collisional orogens suggest that the structural features observed in the Lofoten‐Rombak area may be generally characteristic of A‐type subduction in continent‐conti

ISSN:0278-7407    

DOI:10.1029/TC001i005p00441

年代:1982

数据来源: WILEY

ISSN:0278-7407    

DOI:10.1029/TC001i005p00463

年代:1982

数据来源: WILEY

ISSN:0278-7407    

DOI:10.1029/TC001i005p00465

年代:1982

数据来源: WILEY