GEOMETRY OF INVERTED FAULTS AND RELATED FOLDS IN THE MONTEREY FORMATION - IMPLICATIONS FOR THE STRUCTURAL EVOLUTION OF THE SOUTHERN SANTA-MARIA BASIN, CALIFORNIA

A wide variety of mesoscopic structures observed in the Monterey Forma tion of coastal California reveal contrasting styles of deformation am ong mechanical units and provide a relative chronology of Neogene defo rmation for the southern Santa Maria basin. These structures, which in clude ptygmatically folded veins, folded beds of chert, inverted norma l faults, fault-propagation folds and axial-planar breccia zones docum ent an early extensional phase in the middle-late Miocene followed by two distinct episodes of contraction between the Pliocene and the pres ent. Miocene normal faults in interbedded carbonates and mudstones wer e inverted, resulting in geometries that include normal faults truncat ed by bedding-plane detachments, low-angle thrusts and thrust duplexes , and normal faults reactivated in reverse. Fault-block geometry, drag folds and culmination folds are characteristic features that help ide ntify inverted structures. Normal fault inversion coincides with the d evelopment of early chert folds and related structures higher in the s tratigraphic section, representing a regional phase of layer-parallel contraction. The second phase of regional contraction resulted in the development of a fold-and-thrust belt, which in the Monterey Formation is manifested by detachment and fault-related folds aligned parallel to regional fold axes. Effects of silica diagenesis contribute to the development of mesoscale structures throughout the deformation history of the Monterey Formation. (C) 1997 Elsevier Science Ltd.