LOW TEMPERATURE THERMOCHRONOLOGY ALONG THE NORUMBEGA FAULT SYSTEM IN SOUTHWESTERN AND SOUTH-CENTRAL MAINE: ACCESSING THE ROLE OF OROGEN-PARALLEL FAULTS IN POST-PALEOZOIC EXHUMATION

As a part of a regional effort to determine the extent of low-temperature thermochronological discontinuities across major orogen-parallel faults in northern New England, we have investigated a 175 km segment of the Norumbega fault system (NFS) in Maine using apatite fission track (AFT) and (U-Th)/He dating methods. For the purposes of this discussion, the portion of the fault system under consideration can be divided into 3 segments, from southwest to northeast these include: (1) a southwestern Maine segment extending from the ME-NH border to Portland, (2) a Casco Bay segment extending from Portland to Gardiner, and (3) a south-central Maine segment extending from Gardiner to Palermo.

All but one (9 total) of the AFT ages determined across previously mapped faults of the NFS along the southwestern segment range from 100 to 126 Ma and show no significant discontinuities across the faults. Previously determined AFT ages (6 total) from the Casco Bay segment reveal a significant Early Cretaceous time-temperature discontinuity with ages west of the NFS ranging from 89 to 113 Ma, while those to the east are significantly older ranging from 140 to 159 Ma. An additional 9 new AFT ages from the northeastern portion of this segment range from 117 to 159 Ma, but considering analytical uncertainties in these ages (± 10%) show no significant discontinuities. Four new apatite (U-Th)/He ages from this same region are slightly younger than co-existing AFT ages and range from 101 to 130 Ma and similarly show no discontinuities across the fault system. Finally, continuing to the northeast, 11 new AFT ages from the south-central Maine segment range from 103 to 139 Ma and show no significant discontinuities across the NFS.

Taken collectively, the new AFT and (U-Th)/He ages along the NFS record latest Jurassic to Early Cretaceous cooling through approximately 100 and 70 degrees C, respectively. The only significant time-temperature discontinuity appears to be in the immediate Casco Bay region. This discontinuity appears to “die out” to the northeast (about the latitude of Richmond) and to the southwest it may similarly “die out” or alternatively the late faults may extend offshore. While portions of the NFS appear have accommodated significant Late Cretaceous movement, the structure as a whole does not appear to have played a major role in the late exhumation history of the region.