A Preliminary Characterization and Kinematic Analysis of Fractures within the Miocene Vaqueros and Topanga Formation, Point Mugu Rock, Ventura County, California

Abstract

A complex, transpressive kinematic history exists within the Transverse Ranges. Deformation is observed to be consistent with roughly N-S shortening. The structural trends and fracture patterns are in response to this deformation. This study analyzes the kinematic indicators, structural geometries and fracture patterns within California’s western Transverse Ranges. Fracture systems provide permeability as well as create traps in petroleum reservoirs. An understanding of the orientation and timing of fracture systems can test current tectonic models and allow for predictions of subsurface fracture orientation and permeability. This data will increase the understanding of regional fracture sets and identify fractures which may be ideal fluid pathways for the transmission of hydrocarbons. Data from minor fault stations (4) and joint stations (2) were systematically collected throughout the area within the Topanga and Vaqueros Formations to obtain average joint orientations and to calculate the ideal σ 1 orientations of minor faults. Shear sense was determined using Petit’s (1987) RO criteria. When possible, cross-cutting relationships were observed so that timing relationships between different faulting events could be assessed. Joint surfaces were discriminated from faults by lack of gouge, slicks, or cataclasite, and whether plumose structures (hackles, ribs, or arrest lines) were present. The full data set was systematically analyzed to reveal kinematic relationships. Stereonet software was used to create plots of the data split by type of measurement or calculation (i.e. joint or fault plane, slickenline, or sigma 1), and type of fracture (i.e. Mode I, II, or slip sense). Rose plots, rounded with a smoothing increment of 10 degrees, of average σ 1 trend, average slickenline trend, average joint strike, and average shearband trend were created using kinematic analysis software. The angle used for the calculation of the ideal sigma 1 for preliminary analyses was assumed to be 25 degrees in accordance with Byerlee (1978). Preliminary analyses of minor faults are underway. Joint measurements are bimodal in distribution, with average strikes of N0E and N78E. Abutting relationships suggest that the N0E-striking joints are primary (J1) and the N78E-striking joints are secondary (J2). Calcite mineralization is present along J1 joint planes suggesting fluid flow. Shearbands, fault zones and intrusive diabase dikes are found parallel to both J1 and J2.

AAPG Datapages/Search and Discovery Article #90339 ©2019 AAPG Pacific Section Convention 2019, Long Beach, California, April 1-3, 2019