James A. Wright

Deep seismic reflection profile across the northern Appalachians

A marine seismic reflection line across the northeast extremity of the Canadian Appalachians in Newfoundland indicates a collisional suture(?) in the lower crust beneath the central ophiolitic Dunnage terrane. The thrust and fold belt (miogeocline) above the Grenville basement, and the Dunnage and Gander tectonostratigraphic terranes all appear to be allochthonous with respect to lower crustal basement. The Gander-Avalon terrane boundary to the east is a near-vertical feature that penetrates the crust. The data also suggest that the ancient passive margin of North America extends eastward under the Dunnage terrane for about 70 km. The Newfoundland deep seismic experiment indicates major tectonic differences compared to results for the southern Appalachians.

Lithoprobe east onshore-offshore seismic refraction survey: constraints on interpretation of reflection data in the Newfoundland Appalachians

Abstract Combined onshore-offshore seismic refraction/ wide-angle reflection data have been acquired across Newfoundland, eastern Canada, to investigate the structural architecture of the northern Appalachians, particularly of distinct crustal zones recognized from earlier LITHOPROBE vertical incidence studies. A western crustal unit, correlated with the Grenville province of the Laurentian plate margin thins from 44 to 40 km and a portion of the lower crust becomes highly reflective with velocities of 7.2 km/s. In central Newfoundland, beneath the central mobile belt, the crust thins to 35 km or less and is marked by average continental velocities, not exceeding 7.0 km/s in the lower crust. Further east, in a crustal unit underlying the Avalon zone and associated with the Gondwanan plate margin, the crust is 40 km thick, and has velocities of 6.8 km/s in the lower crust. Explanations for the thin crust beneath the central mobile belt include (1) post-orogenic isostatic readjustment associated with a density in the mantle which is lower beneath this part of the orogen than beneath the margin, (2) mechanical thinning at the base of the crust during orogenic collapse perhaps caused by delamination, and (3) transformation by phase change of a gabbroic lower crust to eclogite which seismologically would be difficult to distinguish from mantle. Except for a single profile in western Newfoundland, velocities in the crust are of typical continental affinity with lower-crustal velocities less than 7.0 km/s. This indicates that there was no significant magmatic underplating under the Newfoundland Appalachians during Mesozoic rifting of the Atlantic Ocean as proposed elsewhere for the New England Appalachians. A mid-crustal velocity discontinuity observed in the Newfoundland region does not coincide with any consistent reflection pattern on vertical incidence profiles. However, we suggest that localized velocity heterogeneities at mid-crustal depths correspond to organized vertical incidence reflections.

A vibroseis reflection seismic survey at the Buchans Mine in central Newfoundland

We present results from the first major vibroseis seismic reflection survey at a mine site in North America. It is demonstrated that conventional high-stack fold reflection seismic techniques can image fault structures associated with volcanogenic massive sulfide bodies, despite the fact that these structures are locally steeply dipping and produce records with low signal-to-noise ratios.The new lines were recorded at the locations of two earlier experimental explosive surveys that failed to image many strong reflectors. The principal reasons for the success of the vibroseis experiment were the proper choice of sweep frequencies for maximum signal-to-noise ratio, the use of high-stack fold, and the careful analysis of velocities and statics during processing.A comparison of the new seismic sections with borehole and other geophysical data indicates that the origin of reflections at Buchans can be attributed to various mechanisms including contrasts in lithologies and rock competence and intrusion of diabase sills into pre-existing fault zones. The best reflections emanate from shallow-dipping brittle-ductile thrust fault zones characterized by fault gouge within broader fractured zones. The two seismic lines presented have proven to be a useful and cost-effective supplement to existing borehole and geophysical data and have provided enough new information on the nature of thrust faulting at the mine to significantly influence current exploration plans.

Deep Seismic profiling in the Nosop Basin, Botswana: cratons, mobile belts and sedimentary basins

Abstract Deep Seismic reflection data from the Kalahari of western Botswana has been interpreted in terms of the tectonics of southern Africa. The 600 km of 12 fold data exhibit good quality reflections in the range 5–15 s TWT. The data outline the Nosop Basin, a depositional centre with greater than 10 km of Palaeozoic and younger sediments. Five prominent reflecting horizons are traced in the basement underlying the sediments. On the basis of the Seismic data, the Nosop is now interpreted as a single basin. The previous multi-basin interpretation of the aeromagnetic data -esults from features within the magnetic basement. The uppermost two basement units are interpreted as Proterozoic sediments that rest on Archean crust. The Seismic data show that the crust appears to thicken to the southeast as the Kaapvaal Craton is approached. At the same time, the sedimentary cover progressively thins suggesting that the western edge of the Kaapvaal is an extended margin. Interpreted deep crustal shear zones indicate a major terrane boundary separating the Proterozoic from the Archean. Depths to basement inferred from aeromagnetic data are consistent with the depth to the Seismic boundary associated with the top of the Archean. Some of the aeromagnetic expression may be due to Karoo volcanic intrusives in the basement. A thrust belt in the northwest of the area now buried in the mid crust is correlated with the nearby Ghanzi-Chobe belt and allows the mid-crustal layering to be dated as late Proterozoic.

Deep structure and tectonic setting of sedimentary basins in Western Botswana

Deep seismic reflection data (15-20s TWT) collected from the Passarge basin in central Botswana and the Nosop basin to the southwest have been used to study the tectonic setting and evolution of these two sedimentary basins. The reflection character throughout both of these basins is remarkably clear and consistent. The data (>5 s TWT) reveal approximately I5 km of basin fill. These basins are located on the margins of the Kaapvaal craton and the tectonic setting is strongly influencedbyProterozoicthrustingending in the Damara erogenic event. Sediment thickness decreases uniformly as the outcrop of the Kaapvaal is approached, suggesting that the basins are situated on a thinned margin of the ancient Archean craton. Both basins have been depocentres throughout much of geological time from mid Proterozoic to Recent times. There is no simple, single mechanism for the subsidence that is consistent with the deep seismic data.