GEOMORPHIC EVIDENCE FOR LATE PLEISTOCENE-HOLOCENE DEXTRAL TRANSTENSION IN THE CENTRAL CHAMPLAIN VALLEY OF VERMONT

The Champlain Valley contains many geomorphic anomalies that have been largely ignored, including oriented vertical cliffs, enclosed rectangular basins, and excessively deep enclosed valleys (generally filled with Pleistocene pro-glacial sediments). The shapes of the basins and valleys, both filled and unfilled, and their areal arrangement indicates that they were formed by north-south trending, dextral shear, coupled with lesser east-west extension. Repetition of the geomorphic features across the Champlain Valley indicates that the dextral transtensional motion is distributed across the entire width of the Champlain Valley, and affects the easternmost Adirondacks and western Green Mountains. Post-glacial displacement within individual small basins reaches as much as 600m, suggesting that this is not a glacial rebound phenomenon. The smaller basins and associated geomorphic features are generally very sharply defined and are found primarily just east of major thrust traces, suggesting that they are formed by pull-apart of the Paleozoic thrust sheets by oblique down-dip motion on the thrust surfaces. The larger basins commonly show evidence of synthetic and antithetic block displacements suggesting that these are more traditional half-grabens developed above listric normal faults. Some of the larger faults appear to be reactivated Mesozoic normal faults. All of the larger basins are at least partially filled with pro-glacial lake and marine deposits from the late Pleistocene, but many have greater-than-expected depths and associated surface topographies that defy explanation as passive pro-glacial basins. Many of the smaller basins are mostly or completely devoid of Pleistocene subglacial or lacustrine/marine deposits, indicating that they opened after deglaciation was complete.