ALLEGHANIAN SYNTECTONIC LOAD ABOVE THE MASSANUTTEN SYNCLINORIUM, NORTHERN VIRGINIA

The Massanutten synclinorium is a doubly plunging fold in the Great Valley province of northern Virginia. The fold has a steep to overturned east limb and a moderately dipping west limb. The core of the fold consists of ~1.5 km thick section of the Ordovician Martinsburg Fm. that is overlain by a ~0.7 km Siluro-Devonian section of dominantly clastic rocks. The synclinorium sits atop the North Mountain thrust sheet which has over 60 km of displacement toward the foreland, and is locally overthrusted by the out-of-sequence Blue Ridge thrust.

The rocks in the synclinorium were buried by syntectonic load during the growth of large crystalline thrust sheets in the hinterland, culminating with the emplacement of the Blue Ridge massif. By using fluid inclusion microthermometry of inclusions trapped in syn-deformational quartz veins, an estimate of this load can be determined.

CH₄±CO₂ fluid inclusions from the Martinsburg Fm. have minimum homogenization temperatures (T_hH) of -143.3 to -134.1 °C, and have ~1.0 % CO₂. Using a geothermal gradient of 18 °C km^-1, trapping depths for these inclusions range from 11.3 to 15.4 km, with a maximum temperature of ~300 °C. CH₄±CO₂ fluid inclusions from the Lower to Middle Devonian rocks in the core of the synclinorium have T_hH values of -134.5 to -131.1 °C, and have <1.0 % CO₂. Trapping depths for these inclusions range from 10.4 to 11.7 km. Aqueous inclusions in these rocks have a homogenization temperature of 228 °C, also giving a trapping depth of 11.7 km.

The original thickness of the Middle Devonian through Mississippian is unknown, but it may have been similar to the ~3.5 km thickness in the eastern Valley & Ridge. Assuming that the preserved section above the sampled interval of Martinsburg through Siluro-Devonian rocks is ~1.2 km thick, a total pre-Alleghanian load would be ~4.7 km. With a 15.4 maximum load determined by the inclusions, at least 10.7 km of post-Mississippian syntectonic load was emplaced over the synclinorium.