THE MORGANTOWN INTRUSIVE COMPLEX: ITS 3D GEOMETRY AND EVOLUTION

The Morgantown intrusive complex is in the eastern Narrow Neck region between the Newark and Gettysburg rift basins of the eastern North American rift system. The intrusive complex is part of the Central Atlantic Magmatic Province (CAMP), a global-scale, short-lived event ~ 201 Ma. We have used structural data to define the 3D geometry of the intrusive body today and at the time of emplacement.

After CAMP activity, this part of the Narrow Neck was tilted ~ 20° NNW, resulting in erosion ranging from < 1 km in the north to > 7 km in the south. Reconstructions that remove tilting and restore missing section indicate that, at the time of emplacement, the Morgantown intrusive body had a complex geometry influenced by heterogeneities associated with faulting and bedding. Its base was composed of three sills (interconnected via two small ramps) that stepped up-section toward the west. Its northern side was an inclined sheet subparallel to the W-striking, S-dipping border fault zone; its northeastern side was a wide, vertical dike subparallel to the NW-striking Birdsboro fault zone; and its western side was a N-striking inclined sheet. The southern extent of the intrusive body is unknown.

We propose that a NE-striking feeder dike (striking perpendicular to the regional extension direction) reached the base of the synrift section near the border-fault zone (i.e., the deepest part of the basin), initiating the formation of the deep basal sill at ~ 8 km. Magma climbed upward toward the north subparallel to the border-fault zone and spread laterally toward the southeast just beneath and along the basin floor. As the sill inflated, roof uplift led to the formation of the NE-striking Birdsboro dike near the pre-existing Birdsboro fault zone. Toward the west, the magma climbed upward in discrete steps, intruding and flowing along bedding within the synrift section. The N-striking inclined sheet on the west side connected to a small intrusion with a separate feeder dike. The steeply-dipping sheets that bound the Morgantown intrusive complex on the north and northeast climbed > 7 km and likely breached the surface where they fed fissure eruptions of the flood basalt. The reconstructions are consistent with other sill-dike complexes and explain the observed distribution of crystal-rich pyroxene gabbros and more evolved rocks within the sheet.