Thermal Effects of Overthrusting, Little Mountains Thrust Belt, Eastern New York
Zintars Zadins, Gautam Mitra
Thermal models constrained by clay mineral assemblages, vitrinite reflectance, and palinspatic restoration are used to interpret the thermal history of the Appalachian thrust belt of eastern New York. The deformed Appalachian foreland of eastern New York is comprised of two adjacent north-south-trending thrust belts: (1) the Taconic Mountains, allochthons emplaced during the Late Ordovician Taconic orogeny; and (2) Little Mountains thrust belt, faulted and folded Lower Devonian carbonates exhibiting well-developed solution cleavage, deformed during the Devonian Acadian or Pennsylvanian Alleghenian orogeny. Conodont alteration index from the Little Mountains range from 4.0 to 4.5, suggesting temperatures of 190°-240°C. This thermal imprint may be attributed to bu ial heating from 8 km of post-Lower Devonian molasse in the Catskill Mountains, whose present thickness is 2.3 km. Alternatively, it may be the effect of conductive heating from an overriding thrust sheet emplaced during post-Ordovician thrusting. Elevated temperature-pressure conditions resulting from an overriding thrust sheet would facilitate formation of solution cleavage in the Little Mountains, in a manner similar to that demonstrated in the Idaho-Wyoming thrust belt.
X-ray analysis of clay minerals from Catskill shales in the undeformed foreland show: (1) there is little or no discrete smectite; (2) illite predominates, with illite peaks being asymmetric toward lower 28 suggesting a minor component of random interstratified illite-smectite; and (3) illite has decreasing crystallinity with increasing burial depth. These results indicate sedimentary overburden has had little effect on illite crystallinity, as crystallinity should systematically increase with increasing temperatures (i.e., increasing depth). Catskill illites exhibiting decreasing crystallinites may reflect that they are detrital and are derived from progressive erosion of a Taconic thrust sheet, resulting in an inverted illite stratigraphy. Middle Devonian uplift and erosion of Tacon c lithologies are also suggested by lithic fragment distribution in the lower Catskill molasse representing an inverted Taconic stratigraphic sequence. Clay residue from cleavage seams in the Little Mountains show illites exhibiting higher crystallinities than undeformed units. Increased crystallinity in cleavage seams may reflect conductive heating effects of an overriding thrust sheet.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.