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COCORP deep seismic reflection profiles from the New England Appalachians display some fundamental changes in crustal geometry from exterior to more interior parts of the mountain belt. Relatively shallow reflections from beneath the Taconic allochthon and Green Mountain anticlinorium are suggested to mark a zone of detachment that carries Precambrian Grenville basement of the Green Mountains in its upper plate. The data strongly suggest that tectonic thickening of the shelf sedimentary and synorogenic flysch sequence beneath the Taconic allochthon has occurred, and that major thrust-fault systems (Champlain thrust of western Vermont), which imbricate shelf lithologies, project downplunge into the seismic profile.
Eastward-dipping reflections beneath the east flank of the Green Mountains appear to project in the subsurface across the Connecticut River valley into New Hampshire. Because some of these reflections can be projected into an exposed zone of probable thrust-imbricated Precambrian Grenville basement and younger metasediments, there is a strong implication that similar rock types exist in the deeper crust beneath New Hampshire. The gross geometry of the eastward-dipping zone is one of a large thrust ramp that facilitated faulting of continental basement in the Green Mountains to higher structural levels, thus suggesting the anticlinorium is a large hanging-wall structure or ramp anticlinorium. Highly deformed satellitic massifs of Grenville basement, such as the Chester dome, and sub-Si urian and Silurian-Devonian rocks of the Bronson Hill anticlinorium and Merrimack synclinorium are probably transported along intracrustal detachments as well.
Available geologic and geophysical data collectively suggest the possibility of a tectonically buried continental margin transition zone, much like that interpreted for the southern Appalachians. This zone would have been preserved from prior rifting of the proto-Atlantic ocean and its inherited geometry could have controlled subsequent convergent deformation during Appalachian orogenesis. Similar interpretations have been made for the Ouachita Mountains. Although regional deep seismic reflection profiling has not been carried out across the Alps, existing geologic and geophysical data imply that one might expect lateral changes in crustal reflection character similar to those observed in the Appalachians. Thus a large south-dipping step or ramp may be present at depth beneath the Pen ine Alps north of the Insubric line, perhaps inherited from early continental margin structure of the European plate, and the root zone for the Pennine nappes might then have acquired its steep geometry as a result of thrust sheets riding over this ramp. The growing body of deep seismic reflection data from convergent mountain belts raises the possibility of such preserved transition zones beneath transported rocks, and provides an alternative interpretation to lithosphere-penetrating "sutures" in explaining lateral geologic and geophysical contrasts in the crust.
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