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The Continental Borderland adjacent to southern California and northern Baja California is an exceptionally wide (240 km) region of ridges, islands, and bathymetric basins as deep as 2 km. This continental margin area includes the Nicolas terrane, a relatively intact outer terrane to the west characterized by the presence of Cretaceous and Paleogene forearc sedimentary rocks, and the adjacent Catalina terrane, a highly extended inner terrane to the east characterized by metamorphic basement rocks exhumed during early Miocene oblique rifting. To better understand this continental rifting process, we used regional grids of multichannel seismic reflection data, and stratigraphic information from industry wells and seafloor samples to investigate the nature and tectonic development of the boundary between the Outer Borderland forearc Nicolas terrane and the Inner Borderland denuded, exhumed Catalina terrane. These data show that this terrane boundary is largely defined by an angular unconformity that overlies and helps form an eastward termination or wedge-out of Outer Borderland forearc strata of earliest Miocene through Cretaceous age. These forearc sedimentary rocks were removed along the boundary primarily as a result of early-to-middle Miocene uplift and erosional truncation associated with exhumation of the Catalina basement. The boundary is not predominantly fault controlled. Along the northern part of the boundary, the East Santa Cruz Basin fault system, previously postulated to control the Nicolas–Catalina terrane boundary, is a predominantly east-dipping, Miocene oblique-normal fault system that has been reactivated with blind to partly blind oblique-reverse displacement. It does not align with the terrane boundary, and given its geometry, slip history, and the presence of Nicolas forearc sedimentary rocks on both sides of this fault system, it is unlikely to have had a major influence on terrane boundary development. South of San Nicolas Basin, there is no simple, through-going fault system, and where faults are present, they are often discontinuous segments that strike oblique to the boundary. This implies that major displacements associated with translation of the Outer Borderland were not localized to the eastern Outer Borderland boundary itself, but rather were likely distributed farther east within the evolving Inner Borderland rift.

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