FRACTURE AND FLUID HISTORY OF THE BROADTOP SYNCLINORIUM, AN EXTENSIVE PIGGY-BACK BASIN IN THE CENTRAL APPALACHIANS

The Broadtop Synclinorium in the central Appalachians developed during the Alleghanian orogeny as a piggy-back basin in the deforming thrust wedge forelandward of the Blue Ridge massif. The Roaring Springs and Wills Mt. anticlinoria forelandward of the basin are interpreted to have been positive topographic features that ‘dammed’ syntectonic sediment dispersal from the hinterland, stabilizing this part of the thrust wedge. Natural fracture (joints and veins) orientations vary significantly from the Marcellus Fm. through the Mississippian Pocono Fm., suggesting that they formed prior to folding at different times during the development of the thrust wedge and record a regional anti-clockwise rotation of shortening.

The earliest-formed fractures are interpreted to be in the upper Chemung through Pocono Fms., and strike 332°±8°, while later-formed fractures strike ~300° and 025°±3°. The veins in the in these units have aqueous fluid inclusions with T_hA of ~100 to 138 °C, recording the development of ~3.6 to 5.2 km of syntectonic load early in the development of the thrust wedge. Inclusions that homogenize between ~150 to 188°C, are interpreted to have been trapped later, and record maximum burial under ~7 km of syntectonic load. In the Marcellus the dominant fracture set strikes 323°±5°, while in the Brallier and lower Chemung Fms. the dominant fracture set strikes 305°±3°. These veins are interpreted to have trapped fluids later during the growth of the thrust wedge, near maximum burial. Minimum CH₄±CO₂ T_hH values as low as -126.5 °C, and aqueous inclusion T_hA values of ~230 to 235 °C indicate up to 7.1 km of syntectonic load.

Fluid inclusions reveal a paleo-hydrostratigraphy where trapped aqueous fluids have salinities of ~15 wt. % in the Marcellus and Mahantango Fms., ~11 wt. % in the Brallier through Catskill Fms., and ~7.5 wt. % in the Pocono Fm.