Regional-Scale Mid-Jurassic to Late Cretaceous Unroofing from the Adirondack Mountains through Central New England Based on Apatite Fission-Track and (U-Th)/He Thermochronology

摘要

Apatite fission-track (AFT) ages of 178 to 70 Ma for 108 samples from the Adirondack Mountains and eastern New York State,Vermont,western Massachusetts and Connecticut,and western New Hampshire indicate that widespread unroofing occurred throughout the region during the Middle Jurassic to Late Cretaceous. Additionally,(U-Th)/He ages of 167 to 85 Ma were determined on nine of those samples and support cooling rates of 1.8~°-2.2 deg C/m.yr. from 100~° to 65 deg C during the Middle Jurassic to Late Cretaceous. Assuming a geothermal gradient of 25 deg C/km,unroofing rates for the region were estimated to be,on average,approx 0.07-0.08 km/m.yr. The AFT age discontinuities between the High Peaks region and the fault dissected southeastern Adirondack Mountains and along the Ammonoosuc fault in the Connecticut River valley between Vermont and New Hampshire suggest differential unroofing that may have been accommodated by fault reactivation syn-/post-Late Cretaceous time after the youngest AFT ages,82 and 70 Ma,respectively. A preliminary trend of increasing AFT age from west to east across New Hampshire is consistent with previously published AFT age data from the Bronson Hill terrane south of the study area in Massachusetts and Connecticut,and it may indicate a similar down-to-the-east rotation in the region. Areas of Early to Late Cretaceous AFT and (U-Th)/He ages determined in this study correspond to areas of currently thin lithosphere overlying low-velocity mantle that may be a remnant of lithospheric heating associated with the Great Meteor hotspot,which was active during that time. The regional unroofing observed during the Early to Late Cretaceous is inferred to have occurred during a time of change in the local stress field from one of extension pre-/syn-hotspot heating to one of horizontal compression as shown by mid-Cretaceous fault activity.