GEOMORPHIC EXPRESSION OF A MIOCENE DIKE COMPLEX, SAN JOAQUIN HILLS, CALIFORNIA, USA

Miocene transtension during development of the North American-Pacific plate boundary in southern California coincided with extensive magmatism and emplacement of a 15-16 Ma basaltic to andesitic dike and sill complex in the San Joaquin Hills, Orange County. Intrusions cut through and altered a thick Mesozoic to Cenozoic marine and nonmarine siliciclastic sedimentary succession. Hydrothermally altered sandstone within 20 meters of the contact is cemented with secondary microcrystalline quartz and illite, and locally with calcite. Cementation plus removal of iron oxides from redbeds rendered the altered sandstones more resistant to erosion than the highly weathered dikes or unaltered sedimentary strata.

These Miocene dikes exert a profound influence on modern topography due to differential susceptibilities of the dikes and altered wall rock to chemical and physical weathering. At vegetated inland sites, where chemical weathering is important, plagioclase feldspar in dolerite intrusions alter to smectitic clays, and the dikes weather to recessive, brush-covered soils on valleys and slopes. In contrast, altered and hardened sedimentary wall rocks stand up in resistant relief. Many of the wall rocks form the high ridges of the uplifted and dissected San Joaquin Hills and control the geometry of drainages by forming resistant ledges that set local base level and by offsetting stream drainages. Differential erosion of the soft weathered mafic dikes and hard, resistant wall rocks produced a sharp contrast that formed most of the steepest slopes in the study area.

Coastal exposures of andesitic dikes, where physical weathering dominates, display a contrary behavior. Igneous dikes are more resistant to wave erosion and form prominent headlands jutting out into the ocean, whereas sedimentary wall rocks are eroded back to form flanking cliffs or sand-covered beaches.