The Ventura basin is part of an active (Pliocene-to-present) fold-and-thrust belt in which nearly 13,000 m of Cenozoic section is exposed. We examined this section in five cross-strike traverses to evaluate the possibility that penetrative deformation contributed measurably to regional shortening. We conducted structural analysis of cleavage-bedding orientations, microstructural and cathodoluminescence analysis of oriented thin sections, and two-dimensional strain analysis using the Fry technique and its variants to obtain semiquantitative estimates of shortening strain due to pressure solution. Macroscopic features indicative of pressure solution are present in rocks as young as Pliocene and include a spaced, bedding-normal cleavage and cobble interpenetrations. Microscopic deformational features include concavo-convex, sutured, or long (straight) grain boundaries, preferred grain-shape fabric, and selvages of insoluble material concentrated in cleavage domains. Upon restoration of bedding to horizontal, the mean cleavage orientation for four of the five traverses is 102°, consistent with regional Pliocene-to-present, north-south shortening. Restored cleavage orientations are more homogeneous than the raw (measured) data suggesting that cleavage formed by layer-parallel shortening prior to folding, or early in the deformational history. To obtain a first-order estimate of penetrative strain due to pressure solution, we conducted two-dimensional strain analysis on 30 samples using the normalized and enhanced normalized Fry methods. We used both Erslev's INSTRAIN fabric analysis program and a manual best-fit method to determine the shape of the strain ellipse in two dimensions (XZ). Despite a wide scatter in results from individual samples for each method, shortening values averaged 25% percent for the manual technique and 27% for INSTRAIN determinations. When the large errors calculated by INSTRAIN are taken into account, the minimum shortening strains are 17% for the INSTRAIN method and 19% for the manual best-fit method. These values of shortening strain are remarkably similar to values from other fold-and-thrust belts. Although we recognize that our reconnaissance-level strain analysis of rocks from the Ventura basin is just a first step toward characterizing penetrative strain withinthe western Transverse Ranges, we suggest that penetrative strain contributed measurably to regional shortening in the Ventura basin. Our results suggest that penetrative strain needs to be incorporated into balanced cross sections, at all structural levels, where a high-level of accuracy is desired.
|Original language||English (US)|
|Number of pages||20|
|Journal||Special Paper of the Geological Society of America|
|State||Published - 2002|
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