Strain accommodated by brittle failure in adjacent units of the Monterey Formation, U.S.A.: scale effects and evidence for uniform displacement boundary conditions

Extensional strain accommodated by brittle deformation was measured in adjacent mudstone and dolostone units at Arroyo Burro beach, California. The dolostone failed in effective tension whereas the mudstone failed in shear, both in response to the same extensional tectonic event. In the mudstone unit initial bed-length and fault-displacement methods document extensional strains of 6.6% ± 0.3% and 6.0% ± 0.3%, respectively. Upon adjusting the displacement estimates according to theoretical fault displacement population analysis, the corrected strain becomes 9.7% ± 0.3% for the mudstone. Measurements of vein apertures in the dolostone document extensional strain that varies according to scale, with outcrop vein scanlines indicating a strain of 3.4% ± 0.1%, and thin-section scanlines yielding a strain of 5.8% ± 0.2. Applying theoretical fault displacement population analysis to vein apertures in dolostone shows that small veins below the detection limit of outcrop surveys contribute significantly to fracture-related strain within the dolostone. This difference in extensional strain may arise because veins measured in outcrop extend across the entire bed height and thus are controlled by the dolostone mechanical layer thickness, whereas microscopic veins measured in thin-section terminate without regard to a bounding layer. The corrected dolostone strain becomes 10.2% ± 1.0%, matching the revised strain calculated in the adjacent mudstone unit and indicating uniform displacement boundary conditions for the two markedly different lithologies. The ~10% extensional strain at Arroyo Burro indicates significant strike-parallel (NW-SE) extension accommodated by brittle failure during development of the western Transverse Ranges fold and thrust belt.