Miocene Hypabyssal Magmatic Stoping in the Santa Monica Mountains of Southern California

An exposure of the Conejo Volcanics in the western Santa Monica Mountains of southern California reveals a Miocene volcano that was extruded about 17 Ma as a result of decompression melting during rotation of the western Transverse Ranges, then tilted northward during Pliocene deformation. Erosion provides a cross-sectional surface exposure of the volcano and the strata that underlie and overlie it. Also exposed are internal features of the volcano, such as feeder dikes, volcanic necks, and small magma chambers. Opposing walls of feeder dikes are identical in pattern and show offset of the intruded strata, the result of simultaneous extensional fracturing, intrusion of magma, and relative displacement of the dike walls as they settled into the soft magma. Exposure of the underlying strata on the eastern flank of the volcano reveals very irregular, circular fault patterns that place Cretaceous and Paleocene strata outside of the circular pattern against upper Eocene through lower Miocene strata inside the circular pattern. The Cretaceous and Paleocene strata contain no intrusive rocks; the Eocene through Miocene strata are up to 50% replaced by Miocene intrusive diabase. The irregular, circular faults are interpreted as the bounding walls of small diabasic magma chambers. The magma in these chambers intruded and partially melted the rocks underlying the volcano allowing unmelted portions of Eocene through Miocene rocks to settle into the chamber to the level of the Cretaceous and Paleocene strata outside of the magma chamber. Because the youngest rocks melted are only a few million years older than the volcanic rocks, the magma must have risen to a level very near to the prevolcanic surface. By all definitions this is an example of hypabyssal magmatic stoping.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California