THRUST-REACTIVATED NORMAL FAULTS AND THRUST-FOLDING CALIFORNIA BORDERLAND AND HAITI

Along the southern California margin and offshore, there are active anticlines growing above blind thrust faults. These faults generally reactivate Miocene extensional faults. Some such California faults, including large parts of the Holocene-active Oak Ridge fault beneath Santa Barbara Channel, are characterized by slow subsidence of the upthrown side of the fault, with more rapid subsidence of the downthrown side. Thus regional subsidence can mask the growth an anticline with respect to sea level. Offshore southern California fold limbs also commonly exhibit progressive tilting (rotation about a horizontal axis). If the tilt rate is constant across the width of the limb and vertical motion is known at one spot, then varying vertical motion across dip can be determined. In Haiti, an anticline forms the WNW-ESE St. Marc Peninsula north of Port-au-Prince. Another 40 km by 150 km-long anticline that includes Gonave Island is located between the northern and southern peninsulas of Haiti. Uplifted marines terraces are present on the coasts of both anticlines with the ~120,000 year terrace dated using corals at both locations. This terrace is folded and uplifted at the St. Marc Peninsula. Offshore industry seismic reflection profiles image highly extended rocks beneath an unconformity. These profiles also image blind N-dipping faults beneath both anticlines, with telltale signs that they are reactivated normal faults. The unconformity separating the extension and contraction may correlate to an onshore unconformity between Eocene and Miocene rocks at both St. Marc Peninsula and Gonave Island. Both limbs of both anticlines are also progressively tilted. They, along with intervening secondary anticlines, appear to be part of a SSW-verging fold and thrust complex. Both anticlines exhibit evidence for subsidence of their offshore plunge, mainly as unconformities and other planar surfaces now in deep water. The offshore sections of the St. Marc anticline image folding of the youngest strata of the basin at the base of its backlimb. Thus, the parallelism of this transpressional system to southern California is striking. The hanging-wall St. Marc Peninsula could be exposed to strong ground motion if the fault proves to be active. The large size of the submerged part of Gonave Island anticline suggests a tsunami hazard if active.