The Arequipa Massif, between the Andes and the Pacific, is an extensive pre-Devonian metamorphic complex. The sequence of deformations, metamorphisms and magmatism in this complex has been established. Mollendo, Atico and Marcona events are distinguished by structural and metamorphic methods and dated by Rb-Sr whole-rock isochrons, at about 1918, 440 and 392 Ma respectively. The Mollendo event led to partial melting, followed by granulite-facies metamorphism, in sediments buried to about 30 km. Further NW, sillimanite-bearing migmatites and staurolite-andalusite schists are thought to represent the same event. The tectonic trend is uncertain but the structures and metamorphism suggest a collision orogeny which probably pre-dated the Pacific Ocean. The early Caledonian Atico and Marcona events are associated with coast-parallel batholiths, amphibolite- to greenschist-facies metamorphism and penetrative deformations. The Atico and Marcona events are separated by the deposition of the Marcona Formation, which is therefore thought to be Lower Palaeozoic (between about 440 and 392 Ma). The early Caledonian deformations are attributed to a subduction zone near the present Pacific margin. There is no penetrative Hercynian or Andean deformation in the Arequipa Massif. Palaeomagnetic study of Jurassic andesites and dykes suggests that there has been no latitudinal motion of the Arequipa Massif relative to the Brazilian shield during the evolution of the Andes.
Abstract Mappable surface structures control linear trends of Carlin-type gold deposits in north-central Nevada. Some of these structures probably resulted from reactivation of Palaeozoic normal faults, linked to underlying basement faults that originated during rifting of western North America during the Proterozoic. These old faults served as conduits for deep crustal hydrothermal fluids responsible for formation of Carlin-type gold deposits in the Eocene. The reactivated structures are recognized by stratigraphic and structural features. Stratigraphic features include rapid facies changes, growth fault sequences and sedimentary debris-flow breccias. Structural features resulted from inversion of the normal faults during the Late Palaeozoic Antler and subsequent orogenies. Inversion features include asymmetric hanging-wall anticlines, flower-like structures, and ‘floating island’ geometries. Inversion resulted in structural culminations that occur directly over the basement faults, providing an optimal setting for the formation of Carlin-type gold deposits.
Synopsis The Beinn an Dubhaich granite intrudes and deforms Cambro-Ordovician limestones and Tertiary dykes in west-central Strath on Skye. The limestones were intensely deformed by radial compression from a centre at the west end of the present granite outcrop, presumably due to the forceful intrusion of an early phase of the granite. Basaltic dykes associated with the Tertiary igneous centres of Skye were deformed co-axially with the earlier deformation in the limestones. Chilled boudin necks in the dykes suggest that this deformation took place before consolidation of the basalt. This was followed by passive intrusion of the main mass of the granite as a large, possibly funnel-shaped sheet.
ABSTRACT South of the Main Mantle Thrust in north Pakistan, rocks of the northern edge of the Indian plate were deformed and metamorphosed during the main southward thrusting phase of the Himalayan orogeny. In the Hazara region, between the Indus and Kaghan Valleys, metamorphic grade increases northwards from chlorite zone to sillimanite zone rocks in a typically Barrovian sequence. Metamorphism was largely synchronous with early phases of the deformation. The metamorphic rocks were subsequently imbricated by late north‐dipping thrusts, each with higher grade rocks in the hanging wall than in the footwall, such that the metamorphic profile shows an overall tectonic inversion. The rocks of the Hazara region form one of a number of internally imbricated metamorphic blocks stacked, after the metamorphic peak, on top of each other during the late thrusting. This imbrication and stacking represents an early period of post‐Himalayan uplift.
The Jiuxi (Western Jiuquan) Basin, located in the west of the Hexi Corridor, NW China, is a foreland basin which has been active since the Early Jurassic. It was formed as a consequence of the progressive northwards migration of the North Qilian thrusts in response to sinistral shearing along the 2,000‐km long Aerjin (Altun) Fault. Sedimentary deposits in the basin are controlled not only by foreland loading and thrusting, but also by the development of listric normal faults at high angles to the thrust belt. At the junctions of these two sets of faults, thick organic‐rich sediments and reservoirs have accumulated. During the Tertiary and Quaternary, thrusts propagated along the foot‐wall of the North Qilian Fault, truncating earlier‐formed oil pools and source‐rock layers and thereby causing great difficulties for petroleum exploration. A basin development model is proposed in this paper from cm integrated study of sedimentary fades, drilling and seismic data, structural analyses and cross‐section reconstructions. The average northwards movement in the frontal zone of the North Qilian Mountains since the Pliocene is estimated at about 8 mm/yr. Therefore, about one‐half of the Jurassic‐Cretaceous oil‐bearing basin could be buried beneath the Laojunmiao and North Qilian Marginal Faults, and is virtually untouched by drilling. Source rocks in the basin are black, lacustrine shales of Late Jurassic through Early Cretaceous ages, with a maximum thickness of up to 1.2 km in the Qingxi Depression. The generation of liquid hydrocarbons began in the Late Cretaceous or mid‐Oligocene; seven stratigraphical reservoirs, ranging in age from Silurian to Miocene, are described — anticlinal. fault‐ and “buried‐hill” structures are the most important traps. The petroleum potential of individual depressions is discussed, and suggestions for potential regional oil prospects are made.
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