Neogene Basins And Transform Motion Within The Pacific Continental Margin Of Baja California
Authors: Normark William R. 
Published: October 2008

ABSTRACT
Seismic reflection profiling data together with lithologic and fossil age data from dredge samples permit a reconstruction of the Neogene tectonic history of the Pacific margin of Baja California south of the California continental borderland. Major prisms of late Miocene and Pliocene sediments were deformed during a period of transform shear between 10 and 4.5 MM years ago. During this interval, the Pacific-North American plate boundary occupied much of the continental slope and shelf west of Baja California. After 4.5 MM years ago, the plate boundary shifted to the Gulf of California. Deformation along the Pacific margin of the peninsula has continued, however, with further horizontal displacement along the northwest-trending structures that developed during the transform motion interval. This deformation of late Neogene sediments along the southwestern corner of the peninsula suggest that the trailing edge of the Pacific plate is a nonrigid area.

INTRODUCTION
The tectonic history of Baja California reflects events that include the subduction of the oceanic Farallon plate along the western margin of North America, which continued well into Cenozoic time, and subsequent right-lateral transform faulting along the later formed Pacific-North American plate boundary.1,10 The San Andreas fault system and the Gulf of California, which lie inboard of the Baja peninsula, are both products of this transform motion along a continually evolving plate boundary.1 The basin-and-ridge topography of the California continental borderland is a product of this transform plate boundary.6,11 The southern part of the borderland extends along nearly half the Pacific margin of the peninsula as a distinct physiographic province that terminates at Cedros Island.17 South of Cedros Island, the western margin of Baja California exhibits a different, varied morphology, but plate tectonic reconstructions suggest a sequence of events similar to that recorded in the borderland province.
The Gulf of California and the southern California borderland have been extensively surveyed, but the submarine geology of the intervening continental margin off-Baja California is much less well known. A general tectonic interpretation for the southern borderland as far south as Cedros Island was presented by Krause13, and additional information on the regional structure and sedimentation resulted from the reflection profiling work by Moore.17. Local studies include those by Emery,8 Doyle and Bandy7, and Normark19 in the southernmost borderland. Taylor et al.22 conducted an extensive low-elevation aeromagnetic survey over much of the borderland and adjacent deep-sea areas between the U.S.-Mexico border and Cedros Island. Fisher and Hess9 prepared a crustal cross-section of the margin near Cedros Island using marine seismic refraction data.
South of Cedros Island, little offshore work has been recorded. Chase et al.4 used magnetic anomaly patterns from the sea floor west of the Baja margin to interpret the history of plate motions prior to the opening of the Gulf of California. Normark and Curray20 mapped the submarine geology around the tip of Baja Peninsula, and my continued interest in the geology of the Baja margin has led to the present study.