Geological significance of seismic reflections in collision belts

Abstract

Geological problems raised by the interpretation of deep seismic reflections in collision belts result from the structural complexity of the crust and the lack of surface control and well-documented references. Some of these problems are illustrated by examples coming from various tectonic settings in France and adjacent areas, and tentative interpretations are proposed. From field, conventional seismic and borehole data, interpretation of shallow structures such as thin-skinned detachments is fairly well controlled down to 5 s TWT, but deeper reflections are more difficult to interpret. The shallow detachments are linked to mid-crustal dipping reflections that are regarded as ramps, which may root into deeper intracrustal detachments. Dipping reflections may sometimes develop in the whole crust and produce puzzling criss-crossing patterns. The Wind River thrust provides a well-documented analogue' of such features, although it is located in the foreland of the belt. It suggests that this kind of feature was formed in a late stage of the tectonic development. The lack of any seismic image of the external thin-skinned detachment in large parts of the Palaeozoic belts of Europe can be explained by such a late deformation which affected the belts up to the suture areas. Interwedging patterns ('crocodile') are commonly associated with shallow ramp anticlines. Though hard to establish in the deep crust, interwedging may be present in collision belts as the result of lithosphere punching (e.g. Pyrenees) and/or reactivation of former antithetic shear zones. Steep faults, unambiguously cutting across the whole crust, are scarcely observed and seem to be restricted to areas where major transcurrent displacements are in progress. Dipping reflections across the deep crust and the upper mantle may be interpreted as the ghosts of former crustal slabs now largely metamorphosed in eclogites.

The Môn–Deemster–Ribblesdale fold–thrust belt, central UK: a concealed Variscan inversion belt located on weak Caledonian crust

“…A similar zone of strong reflectivity is observed on BIRPS profiles (Fig. 11) west of Anglesey (Brewer et al 1983, Bois et al 1990, Bois 1991, Klemperer & Hobbs 1991 identified as the South Irish Sea Lineament. The latter is an inferred reactivated Caledonian thrust and terrane boundary, correlated with the Menai Strait Fault System (Gibbons 1987) onshore.…”

Deep structure of the southern Ural mountains as derived fromwide-angle seismic data

“…The Urals are a mid to late Palaeozoic orogenic belt which today forms the boundary between Europe and Asia. In contrast to other mountain belts of similar age, for example the Variscides (Bois 1991; Aichroth et al . 1992) or Appalachians (Cook et al .…”

Subduction, platform subsidence, and foreland thrust loading: The late Tertiary development of Taranaki Basin, New Zealand

“…The dipping Moho can be seen at two-way travel times of 12-14 s, but above this is a series of subhorizontal, diverging reflectors. Diverging reflection patterns have been noted elsewhere in the upper crust of collision belts [Bois, 1991;Meissner et al, 1991]. Bois speculates that these diverging crustal reflectors or "crocodiles" are related to thickening of the crust by a process of interwedging.…”

“…Bois speculates that these diverging crustal reflectors or "crocodiles" are related to thickening of the crust by a process of interwedging. Deep crustal interwedging is less common, although Bois [1991] does note that crocodiles are seen in the deep crust of the Pyrenees collision zone.…”