BS in Earth Sciences
Natural Resources Management and Environmental Sciences Department
This study was conducted to determine if the orientations of observed joint sets in the late Miocene diatomite bedding of the exposed Miguelito Member in Montaña de Oro State Park relate to the San Andreas wrench fault compressional forces that caused folding of the formation. Strikes and dips of bedding planes and their corresponding first and second generation joint sets were gathered and compiled into a stereonet program for analysis. A cross section of the bedding plane data shows compressional antiform (fold axis oriented ~304/013) and synform (fold axis oriented ~300/001) features. Previous studies indicate post Miocene, tectonic shortening with a stress field oriented σ1: ~028/002 and 0.2 to 0.23 mm/yr uplift of the San Luis/Pismo structural block. Mode one intensive fractures develop ~90° dips and define a stress field with a maximum stress (σ1) parallel and a minimum stress (σ3) perpendicular to resulting joints. The average σ3 of the finite stress field of the primary joints is ~095/010. The resulting σ3 of the finite stress field of the secondary joints have a less consistent strike of ~090 to ~180. The data shows that primary joints are syntectonic hydrofractures, created by the same horizontal shortening that caused ductile bedding deformation. The conjugate secondary joint systems are created by a stress field at a more recent time and orientated σ3 ~135/000. I suggest that the predominant secondary orthogonal joint systems are created by outer arc extension and/or unroofing related to uplift and erosion. Results from this study indicate that brittle and ductile deformation of the Miguelito Member agree with the horizontal northeast to southwest tectonic shortening of the San Luis Obispo/Pismo block. Evidence for a clockwise rotating stress field from the conjugate joint set data may also support the wrench tectonic kinematic model of theCaliforniaCentralCoast.