Compressional System Research Articles

Exploring for Hydrocarbons Under Thrust Belts--A Challenging New Frontier in the Carpathians and Elsewhere

New significant reserves of hydrocarbons may occur in subthrust autochthonous and parautochthonous series buried below the frontal zones of thin-skinned thrust belts. The subthrust plays have been tested in several orogenic belts of the world, the Carpathians being one of the best examples. The arcuate thin-skinned Carpathian orogenic belt, which evolved during the Mesozoic and Cenozoic, is thrust tens of kilometers over its Neogene foredeeps and the underlying European plate. Various structural and stratigraphic settings and potential hydrocarbon plays have been recognized within the buried margins of the European plate, including a late Paleozoic Hercynian compressional system, Mesozoic rifted margins of the Tethys, and a Cenozoic synorogenic foreland-type fault system. Possibly, deeper parautochthonous structures, documented on examples from the southern Apennines, may also be present below the thin-skinned frontal zone of the Carpathian thrust belt. In addition to these structural settings, large Paleogene valleys/submarine canyons have been found within the margins f the European plate. These structural and morphologic features, if combined with source rocks, reservoirs, and proper burial history, represent potential hydrocarbon plays. Generation of hydrocarbons from sources within the subthrust plate was greatly enhanced by emplacement of the wedge-shaped thrust belt, which may also provide a regional seal; therefore, the combination of the long and complex geological history of the European plate with the impact of the Alpine thrusting and foreland deformation created unique conditions for generation, entrapment, and preservation of hydrocarbons in subthrust settings.

Tectonic processes in offshore northern Israel and the evolution of the Carmel structure

The Carmel structure in northern Israel is an elevated tilted block that extends from the land into the continental shelf and divides the Levant continental margin into two distinct provinces. Its axis trends at a high angle to the Levant margin and deflects Mesozoic and Cenozoic sedimentary and basement rocks. The Carmel structure is probably older than the Levant margin itself and has been through several phases of activity. Most of the multichannel seismic profiles in the offshore of northern Israel were reprocessed through time migration and then interpreted. The resultant maps, combined with gravimetric, magnetic and bathymetric maps, show a complex tectonic picture of the Carmel structure and its vicinity. Four evolutionary stages can be recognized in this area: (a) Early Mesozoic rifting along the present Levant margin. We speculate that the Carmel was at that time a triple junction. A thick Mesozoic volcanic and carbonatic section of more than 6500 m indicates rapid subsidence, probably accompanied by normal faulting. (b) Folding along the NNE axis. This is probably part of the Syrian Arc compressional system, which was active from the late Cretaceous until the Eocene. Normal faults of the previous stage were probably reactivated, becoming reverse faults. (c) Left-lateral strike-slips faulting. This formed a north-west fault system which resulted in a clockwise rotation of segments of the fold axis and was active from the late Miocene to early Pliocene. (d) A tensional tectonic regime prevails since the Pliocene and is probably still active at present. The well pronounced Kishon graben, which extends from land to the continental shelf, was formed during this period. The first two stages are of regional extent, whereas the latter stages are the reslt of local modification of the regional stress fields.

Tectonic model for crustal seismic reflectivity patterns in compressional orogens

A pattern of crustal seismic reflectivity tentatively dated as Ordovician- Silurian recurs along strike in the Newfoundland Appalachians. Comparable fault patterns are produced in sandbox models of asymmetric compressional systems where oppositely dipping step-up shear zones are localized above a discontinuity in basal velocity. The reflectivity pattern may be produced by a mechanically analogous process in which a stress discontinuity results from detachment and underthrusting of mantle and possibly lower crustal lithosphere during con- tinent-continent collision. Similar seismic reflectivity patterns exist elsewhere in the Appalachian-Caledonide mobile belt as well as in older and younger compressional orogens, suggesting that detachment and underthrusting leading to asymmetric crustal deformation may be a common feature of compressional orogens.

Ordovician rocky shoreline deposits–the basal Trenton Group around Quebec City, Canada

AbstractAround Quebec City the lowest Trenton Group sediments rest directly upon a highly irregular Precambrian surface and represent the deposits of a high‐energy, irregular, rocky shoreline which developed as the Ordovician sea onlapped a major positive topographic feature known as Montmorency Promontory. The complicated lithofacies mosaic developed along this shoreline consists of: (1) Arkosic sandstones, representing reworked regolith and marginal marine elastics; (2) Solenopora gravels, which comprise partly reworked colonies from the bare rock substrate and partly reworked algal maerls; (3) Fine‐grained limestones, which were deposited in low energy environments between and in the lee of numerous islands and shoals; (4) Skeletal limestones, representing island fringing shoals and ephemeral beaches, with extensive thickets of the ramose bryozoan Hallopora; (5) Bryozoan liméstones, which represent low‐energy, subtidal deposits with abundant delicate bryozoans; and (6) Coquina limestone lenticles, which accumulated in channels cut into the subtidal Bryozoan limestones. The absence of older Ordovician and Cambrian sediments in the vicinity of Quebec City indicates that Montmorency Promontory at the time of deposition of these Trenton deposits was probably manifested as a rugged archipelago. This represented the remnant of large‐scale topographic highlands which had developed at the continental margin during the late Precambrian in association with extensional rifting, transform faulting, and the inception of the Iapetus Ocean. Gradual subsidence of this topographic high and the development of the rocky shoreline relates to the subsequent development of a compressional system and initiation of ocean closure in the early Middle Ordovician.