Abstract

INTRODUCTION

Most of the estimates of the pressure and temperature for Adirondack metamorphism fall in the range of 650 to 800 °C and 7 to 9 kbar (Whitney and McLelland, 1973; Essene and others, 1977; Boone, 1978; Valley and Bohlen, 1979; Whitney and others, 1979), suggesting an elevated geothermal gradient comparable to oceanic gradients (Mercier and Carter, 1975), and burial to as much as 25 km during the orogenesis that occurred about 1,000 m.y. ago. The structural style of the southern Highlands has recently been elucidated by McLelland (1977) and McLelland and Isachsen (1979), who emphasized the existence of complexly refolded nappe structures perhaps 70 km in amplitude. Insofar as these structures are outlined in supracrustal rock types, they furnish an important sample of structures evolved, if not also initiated, during the process of abnormal continental crustal thickening.

The most recent mapping in the extreme southeastern Adirondacks is that of Walton (1955), Berry (1965), and Hills (1965). In outlining the basic distribution of rock types, these workers recognized meta-igneous (plutonic) and meta-supracrustal rock assemblages. These supracrustal rocks are similar to those in the southern Highlands, containing metasediments such as pelites, quartzites, and calcareous rocks, and probably volcanic or at least near surface igneous rocks. For convenience, we will refer to them collectively as metasedimentary rocks. These previous workers in the southeast collected little detailed information on minor structures and consequently based their interpretations on outcrop patterns. Unfortunately, post-Grenville faulting along the Camplain Valley fault system has affected the outcrop patterns. Subsequent roadbuilding, notably along U.S. Highway 22, has created useful outcrops with well-exposed minor structures. We have used the microfabrics of some of the tectonites from these new exposures to infer some of the structural history of the metasedimentary rocks. In combining our information with the most salient points of their mapping, some constraints on kinematic modeling emerge that may have more widespread application in the Adirondack Highlands.

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