Penetrative strain at shallow crustal levels: The role of pressure solution in accommodating regional shortening strain, Ventura basin, western Transverse Ranges, California

Abstract

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 102Њ, 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 within Duebendorfer, E.M., and Meyer, K.L., 2002, Penetrative strain at shallow crustal levels: The role of pressure solution in accommodating regional shortening strain, Ventura basin, western Transverse Ranges, California, in Barth, A., ed., Contributions to Crustal Evolution of the Southwestern

Major Miocene exhumation by fault‐propagation folding within a metamorphosed, early Paleozoic thrust belt: Northwestern Argentina

“…Instead, existing steeply dipping heterogeneities were reactivated, and the fine‐grained nature of the rocks and higher stress in the thrust belt may have resulted in a greater role for penetrative strain in accommodating sub‐décollement thickening of the crust. Previous researchers have demonstrated that significant shortening was accommodated by penetrative strain in other orogens worldwide, including western North America [e.g., Mitra , 1994; Duebendorfer and Meyer , 2002; Yonkee and Weil , 2010], and southwestern China [e.g., Burchfiel et al , 2007].…”

Growth of Fault‐Cored Anticlines by Flexural Slip Folding: Analysis by Boundary Element Modeling

“…Gratier and Gamond () used the study of McEwen () on deformation of alpine molassic conglomerates to infer strain rates of order 10 −14 –10 −15 s −1 . Duebendorfer and Meyer () inferred penetrative strains in rocks above 10 km depth in the Ventura basin of southern California of 17–27% during contractional deformation over 3–5 million years. This gives strain rates of 1–3 × 10 −15 s −1 .…”

“…Veloza et al () used geomorphic and seismic data from the active Tame Anticline in the eastern Colombia Subandes to infer that ~40% of total finite shortening of the anticline occurs as distributed off‐fault deformation. It is also known that distributed strain is a significant component of shortening at the scale of a fold and thrust belt, with distributed deformation locally accounting for 10–50% of the total shortening (e.g., Duebendorfer & Meyer, ; Hogan & Dunne, ; Mitra, ; Yonkee & Weil, ). Detachment folds are thought to grow by distributed pure shear above a detachment surface, without a ramp fault coring the uplift (e.g., Gonzalez‐Mieres & Suppe, ; Poblet & McClay, ).…”