MIOCENE TO QUATERNARY VERTICAL MOTION OF THE CHANNEL ISLANDS: SIGNATURES OF TRANSFORM TECTONICS

Sediments on and surrounding the Channel Islands of southern California record differential vertical motion within the Southern California Continental Borderland. These motions provide valuable insights to the transition from Miocene transrotation and extension to late Miocene-present translation between the Pacific and North American plates. The Quaternary uplift rates demonstrated by fossil material recovered from marine terraces disagree with decadal GPS vertical motions on Santa Rosa, Santa Cruz, San Nicolas, and San Clemente Islands. Although it has been suggested that this disagreement is due to glacio-isostatic adjustment, we propose that the differential vertical motions of the Channel Islands are primarily modulated and by each island’s location with respect to the modern anastomosing system of strike-slip faults that dissect the Southern California Continental Borderland and by Miocene tectonothermal events.

We present comparative geomorphology of all the Channel Islands and compare GPS data to uplift/subsidence rates implied by marine terraces. We have correlated Santa Catalina Island's marine terraces with an ice-volume eustatic sea level curve and derive a subsidence rate of ~0.25 mm/yr for the last 1.15Ma. The paleoshoreline from the last glacial maximum has been identified offshore of Catalina at a depth of ~125 m. The presence of terraces at depths of >350 m overlain by retrogradational parasequence sets suggests that Catalina Island has subsided 220 m or more. In contrast San Clemente Island has a series of emergent terraces up to 550 m above mean sea level and one progradational submarine shelf of sediment at ~-130m.