PUNCTUATED LATERAL FOLD AND FAULT GROWTH: CAMARILLO FOLD BELT, VENTURA COUNTY, CALIFORNIA

The Camarillo Fold Belt (CFB) is composed of several south-verging anticlines, which comprise the western extent of the east-trending highly segmented Simi fault zone that extends from the northern Simi Valley to the city of Camarillo. Subsurface and surface geologic relations indicate that most of the faults in the CFB are reactivated Miocene transrotational faults that now accommodate Quaternary transpressional deformation. Quaternary structural domains are delimited by steeply dipping north-striking faults that locally cut or are cross cut by east-striking reverse faults. East-trending Quaternary fold axes within discrete structural domains plunge 10° to 20° west toward north-striking faults, yet are truncated to the east. Quaternary structural relief decreases from east to west, across each successive north-striking fault, yet deformation of a late Pleistocene erosion surface suggests greater rates of deformation to the west since ~23 ka. These relations suggest that westward propagation of the fold belt and the zone of active deformation occurred by punctuated lateral growth of the Simi fault due to the presence of Miocene faults. The model of punctuated lateral growth of the Simi fault zone entails, 1) accumulation of fault displacement and fold amplification at constant length over a protracted period (40-60 k.y.); 2) increasing strain gradients near the fault and fold tip as displacement accumulates; 3) westward propagation and cross-cutting of the orthogonal fault once the strain gradient becomes critical; and 4) a return to constant length displacement and amplification. Given the polyphase deformational history of the western Transverse Ranges, this style of lateral fault and fold propagation is likely common in southern California. Punctuated lateral fault growth has important implications for seismic hazard assessment, as it suggests that transverse faults may be capable of limiting the magnitude of a particular seismic event, and that the intersection of orthogonal faults may represent future seismic sources.