BRITTLE DEFORMATION PATTERNS AND LAMPROPHYRE INTRUSIONS IN HOLYOKE BASALT OF THE HIGBY MOUNTAIN FAULT BLOCK

The Holyoke Basalt, 200 ma, of the Higby Mountain Fault Block demonstrates a complex pattern of brittle deformation. Detailed fieldwork reveals three sets of structures with orientations of 060°, 85° SE; 030°, 88° SE; and 220°; 80° NW. Structures with the trend of 060° are most common and found throughout the fault block. Most brittle structures are joints that exhibit little to no slip, as their surfaces lack lineations or slickenlines. Linear valleys trending 030° have rubble at the base with slickensides and gouge. We interpret these valleys to represent fault zones. Camptonite lamprophyre intrusions occur as sinistral en enchelon segments with an overall trend of 024° while individual segments trend 060°. These observations are consistent with the interpretation that lamprophyres intrude during transtensional stress regimes.

The N-S trending Harford Basin contains previously mapped regional faults with NE-SW trending oblique-slip normal faults that dip to the northwest. The trend of brittle structures on the Higby Mountain fault block are consistent with regional fault trends, however, the difference in dip direction cannot be reconciled. We suggest that the jointing patterns correspond to a change from normal faulting to oblique-normal faulting over time.