DO THERMAL HISTORIES REVEAL TIMING OF POST-OROGENIC CRATONIZATION?: LESSONS FROM TWO PALEOPROTEROZOIC OROGENS WITHIN THE NORTH AMERICAN CRATON

Thermal history studies of the coeval 1900-1800 Ma Penokean (southern Lake Superior) and Trans-Hudson (THO; Sask-Manitoba) orogens are being carried out to reveal their post-orogenic evolution (e.g., Holm et al., 1998, Geology; Wortman et al., 1999, GSAA; Heizler et al., 1999, GSAA). Earliest collision-related metamorphism preserved in greenschist grade rocks is dated at 1860-1830 Ma. Metamorphic monazite ages from higher-grade rocks of the THO range from ca. 1820-1800 Ma, and metamorphic titanite from 1800-1765 Ma, interpreted as initial, moderate mid-crustal cooling. Metamorphic age determinations from the Penokean are currently in progress. Both orogens reveal a relatively distinct mid-to-upper crustal cooling event beginning at ca. 1760-1750 Ma (uniform Ar/Ar hornblende ages) and locally lasting until ca. 1700 Ma (Ar/Ar mica ages). This younger event has been interpreted as reflecting crustal stabilization and is supported by the presence of mature Paleoproterozoic quartzites overlying these orogenic rocks. The time of crustal stabilization (return to normal thickness crust with its top surface awash at sea-level; Nelson, 1991, GJI) revealed by the thermochronologic results does not necessarily correspond with the time of cratonization (development of a strong, rigid lithosphere). For instance, after it stabilized at ca. 1750 Ma much of the Penokean crust was strongly deformed at 1650 Ma during Mazatzal accretion from the south (Holm et al., 1998). This suggests that cratonization (i.e., via thermal re-equilibation) had not come to completion even 100 m.y. after initial stabilization. In contrast, the THO has apparently been "armored" by surrounding Archean cratons and therefore has not been structurally modified since it stabilized. Subsequent deformation would provide a means by which to evaluate the strength of the lithosphere and thus the timing of cratonization. We conclude that thermal history studies alone are inadequate for revealing the time of cratonization of ancient orogens which have not been subsequently modified since stabilization.