Fracture style in the Monterey Formation of central California varies from one mechanical unit to the next, with joints and opening-mode veins common in dolostones, limestones, and diagenetic opal-CT beds, and with faults most abundant in beds rich in biogenic opal A and weak minerals. This dependence of failure mode on lithology was consistent throughout the deformation history of the Monterey Formation, and it reflects the regional Miocene transtensional regime followed by Pliocene–Holocene compression. Furthermore, least principal stresses derived from faults in mudstone correspond closely to opening-mode fractures in adjacent dolostones and limestones, implying that partitioning of failure mode among different beds occurred in response to the same applied tectonic stress conditions. Quantitative bulk mineralogy analyses for samples that failed during the Pliocene–Holocene tectonic phase show that beds containing <9% weak minerals invariably failed in opening mode, whereas beds containing >22% weak minerals deformed by brittle faulting. Results from rocks that failed in the Miocene are more scattered but show the same general trends, thereby establishing a link between mineralogical composition and failure mode in the Monterey Formation.

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