Abstract

Deformed marine terraces provide excellent measures of Quaternary tectonism along the Pacific coast of Baja California. Between Ensenada and El Rosario, the nature and magnitude of deformation are particularly well expressed by the present elevation of the early Pleistocene marine limit, which has been upwarped to over 340 m in two localities but depressed to near present sea level elsewhere. Multiple terraces are found below this marine limit where appreciable tectonic uplift has occurred, for example on the Punta Banda peninsula. Where subsidence has prevailed, either fewer broad terraces occur or thick alluvial and eolian deposits have accumulated. Quaternary features are further dislocated by strong strike-slip faulting, related in part to major trans-peninsula faults, and by normal faulting that has blocked out horst and graben structures that trend southwest across the coastal zone. Such faults are clearly indicated by breaks in the continuity of successive Quaternary shorelines. Both folding and faulting are measurably less significant for the lower, later Quaternary terraces than for the higher, older features. Crustal movements revealed by these terraces are readily interpreted in terms of plate tectonic concepts. Over the past 4 million years, the peninsula of Baja California has been carried northwestward away from mainland Mexico, at an average rate of 6 cm/yr, on the dislocated limb of a spreading sea floor associated with the East Pacific Rise. The coastal segment south of Ensenada borders one of several discrete slices of continental crust which, on the evidence presented by the marine terraces, has been warped and dislocated in response to tectonic forces associated with peninsular rafting.