Southern
California Borderland Tectonics
Crater-Like Structures of the Inner
Borderland
Digital mosaics of
swath and conventional bathymetry data reveal large, distinct
near-circular crater structures in the inner Continental Borderland
offshore of southern California . Two have maximum crater diameters
that exceed 30 km, and a third has a crater diameter of about 12 km.
All three features exhibit the morphology of large complex
craters—raised outer rim, ring moat and central uplift—yet their 
exact origin remains a mystery. Preliminary analyses of
available seismic, gravity and magnetic data over these structures
reveal both similarities and distinct differences in geometry,
structure, and geophysical signature to known impact sites. All three
crater structures, however, occur within the Catalina terrane, a highly
extended volcanic and metamorphic province floored by Catalina Schist
basement. A likely alternative origin may thus involve explosive
volcanism, caldera collapse and resurgent magmatism, and/or possibly
plutonism and schist remobilization, associated with the Catalina
terrane. No single model for crater formation, whether impact, caldera
or pluton, fully accounts for all of the present observations regarding
the morphology, internal structure, and known geology of these
near-circular features. Timing of crater formation postdates the
initial rifting and rotation of the western Transverse Ranges , and
appears to predate major right-slip along the San Clemente and San
Diego Trough fault systems—or about 18 to 16 Ma. Regardless of their
origin, these complex craters represent some of the largest structures
of their kind in western North America and provide a unique opportunity
to better understand the development of unusual crater structures in a
submarine environment.
exact origin remains a mystery. Preliminary analyses of
available seismic, gravity and magnetic data over these structures
reveal both similarities and distinct differences in geometry,
structure, and geophysical signature to known impact sites. All three
crater structures, however, occur within the Catalina terrane, a highly
extended volcanic and metamorphic province floored by Catalina Schist
basement. A likely alternative origin may thus involve explosive
volcanism, caldera collapse and resurgent magmatism, and/or possibly
plutonism and schist remobilization, associated with the Catalina
terrane. No single model for crater formation, whether impact, caldera
or pluton, fully accounts for all of the present observations regarding
the morphology, internal structure, and known geology of these
near-circular features. Timing of crater formation postdates the
initial rifting and rotation of the western Transverse Ranges , and
appears to predate major right-slip along the San Clemente and San
Diego Trough fault systems—or about 18 to 16 Ma. Regardless of their
origin, these complex craters represent some of the largest structures
of their kind in western North America and provide a unique opportunity
to better understand the development of unusual crater structures in a
submarine environment. 
Perspective
view of Catalina "crater" from the Northwest looking toward San
Diego. Bathymetry is partial coverage multibeam data, with
sparse sounding data in the foreground. Central uplift and "moat"
are clearly visible.
Top: Manson
Crater cross section. Right. Structure section and gravity
model for the Catalina Crater. Is it an impact?
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