The Diablo Range, one of the southern Coast Ranges of California, is a faulted anticlinorium which extends south-southeast for about 200 mi and is 25-30 mi wide. At the crest and core of the range, eugeosynclinal rocks of the Franciscan Formation, of Late Jurassic age, and the serpentine and diabase which intrude that formation form a series of diapiric structures which have penetrated and caused local overturning of younger stratified rocks.
Flanking formations of the range consist of a maximum thickness of 30,000 ft of Upper Jurassic to Upper Cretaceous marine sandstone and shale of miogeosynclinal aspect, and more than 9,000 ft of Tertiary shelf-facies marine sandstone and shale.
Numerous unconformities within the thick Cretaceous-Cenozoic section indicate a history of intermittent deformation and uplift. Orogenic movements, first active at the end of the Jurassic, were repeatedly mildly in middle and Late Cretaceous times. Orogeny increased in intensity throughout the Cenozoic and culminated in great uplift, folding, overturning, and faulting which eventually emplaced the diapirs, principally in late Pliocene to middle Pleistocene time.
Geological, geochemical, and geophysical evidence bearing on the origin, form, and characteristics of the serpentine bodies and rocks of the Franciscan Formation suggests the following sequence of events.
Franciscan sediments and volcanic material were deposited to great thickness on a simatic basement consisting perhaps of ultramafic rock of the upper mantle overlain by an oceanic layer of serpentine.
As an initial step in the orogenic cycle, the simatic materials of the sea floor and Franciscan sediments and volcanic material of the continental rise were thrust under the margin of the continent.
The relatively low density of the water-saturated eugeosynclinal sediments assisted the initial upward pulse; intrusions of ultrabasic magma took place, and fragments of ultramafic mantle and serpentine were caught up and incorporated in the early folds while serpentinization of ultramafic rocks continued.
The low bulk density of serpentine and the plasticity, lubricating qualities, and extreme mobility of sheared serpentine were major factors in emplacement of the piercements.
Succeeding orogenic spasms remobilized the serpentine and its included fragments, and final upward movements were aided by erosion of the younger overlying formation.
Culmination of piercement took place in late Pliocene time, and piercement has continued to the present.
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“Diapir” and “diapirism” come from the Greek diapeirein, which means “to pierce.” Diapirism sensu lato is a process by which earth materials from deeper levels have pierced, or appear to have pierced, shallower materials; it is divided into magmatic intrusion and diapirism sensu stricto on the basis of the temperature at which piercement occurs. Diapirs s.s. are composed of evaporites, argillaceous sediments, coal, peat, ice, serpentine, or other earth materials which have the critical characteristics of low equivalent viscosity and low density. These materials range in age from Precambrian to Recent. Diapirs are found in all parts of the world except the shield areas. They have many forms, ranging from smoothly rounded pillows to complexly injected laminae, are either connected with or disconnected from the “mother” bed, and are present either at the surface, where they form distinctive features, or at considerable depth. Diapirs have well-developed internal structures indicative of an origin by flow. Strata around a diapir may be strongly affected structurally and/or stratigraphically by the diapir, or they may be unaffected. Field and model studies indicate that diapirs have developed as a result of horizontal compression, gravitational instability, or both. Diapiric structures of various types contain large quantities of oil and gas, sulfur, salt, and potash and are important for underground storage and nuclear testing.