Hi all,

was not sure whether to post this, but here it is. We may or may not cut 40% from this (the explanations for non-specialists and/or non-geologists) and submit for a March 7 abstract deadline for Pacific Section AAPG joing with GSA Cordilleran section. I'm going anyway to present something on California on tilts and folds.

We are seeing young faults and folds now in a deep basin north of the earthquake fault the last few hours. 5 AM; time for bed.
Chris

Blind thrust-related St. Marc and Gonave Island anticlines: North and northwest of Port-au-Prince Haiti

Along the southern California margin and offshore, there are active anticlines growing above blind thrust faults. Blind thrust faults are faults that slip at depth but whose thrust component of slip is absorbed by folding above it. Partially blind faults may reach the surface or sea floor but part of the thrust slip is absorbed by folding. Earthquake on such faults might sometimes reach the surface or sea floor but need not every time. 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 (hanging-wall), with more rapid subsidence of the downthrown side (footwall). Thus, because of regional subsidence, lack of uplift of the crest of an anticline with respect to sea level does not preclude that the underlying blind fault is active.

Offshore southern California fold limbs commonly exhibit progressive tilting (progressive limb rotation/ rotation about a horizontal axis). If the rotation/tilt rate is constant across the width of the limb, if you know the vertical motion of one spot on a cross section, you know the vertical motion of all other locationss on the section. Vertical motion is continuously varying by location perpendicular to trend of an anticline with progressively tilting limbs.

One final concept is required in order to understand our hypothesis for active blind thrust faults in the bay north and northwest of Port-au-Prince Haiti. This is subsiding and uplifting coastal (marine) terraces and their relation to ice ages. Depending on rock type and exposure to wave energy, relatively smooth unconformities can be cut into the rock by waves. In the case of Haiti, these relatively flat smooth surfaces may instead be built by growth of coral. Where sediment supply is high and the outer shelf is subsiding, shelf edge deltas may grow during glacial times. Where there is progressive tilt of a regional fold limb, the offshore area may be subsiding as the onshore area uplifts. Marine terraces formed during the low sea levels of glacial time tend to form in the subsiding areas and are there preserved. Those formed during high sea interglacial sea levels tend to be preserved in uplifting areas.

An anticline forms the St. Marc Peninsula north of Port-au-Prince and another one forms Gonave Island, located north of the 2010 earthquake rupture (e.g., Mann et al., 1995). Uplifted marines terraces are present on the coasts of both locations and the ~120,000 year terraces dated on coral at both locations. The uplift rate on the northwest corner of Gonave Island has been zero while it is 0.19 mm/year on St. Marc Peninsula (Mann et al., 1995). SRTM topography data (and Google Earth) image that the surface of the 120,000 year and older terraces are folded at St. Marc Peninsula, and also folded on western Gonave Island. Seismic reflection profiles made publicly available by Western Geophysical through a U.S. Geological Survey web site image blind N-dipping faults beneath both anticlines. Both limbs of both anticlines are progressively tilted. The crest of the anticline is eroded northwest of Gonave Island at a depth of 350 m, and a series of planar sea floor surfaces are as deep as 1 km, suggesting subsidence if formed at sea level. The Gonave anticline is 40 km across and about 150 km-long. The offshore St. Marc anticline folds the youngest strata of the basin at the base of its backlimb. The evidence seems relatively strong that the St. Marc anticline and underlying thrust fault are at least potentially active. The evidence for activity of Gonave Island anticline is less strong but the existence of tilt and subsidence offshore suggest that activity cannot be precluded just because one location has experienced no vertical motion in the last 120,000 years. The hanging-wall of St Marc Peninsula could be exposed to strong ground motion if the fault proves to be active. The rather enormous size of the submerged part of Gonave Island anticline suggests a tsunami hazard if active.