SHORT-TERM AND LONG-TERM QUATERNARY UPLIFT RATES FROM URANIUM-SERIES, STRONTIUM ISOTOPE, AND AMINO ACID AGES OF MARINE TERRACES, WITH IMPLICATIONS FOR THE TIMES OF EMERGENCE OF THE CALIFORNIA CHANNEL ISLANDS (Invited Presentation)

Marine terraces on the California coast record interglacial high-sea stands superimposed on tectonic uplift. Some of the best-preserved terraces in California are on the Channel Islands (including the Palos Verdes Hills, once an island as well). Younger (late Quaternary) terraces can be dated by uranium-series methods on fossil corals and older (middle and early Quaternary) terraces can be dated by strontium isotopes in fossil mollusks. Amino acid geochronology of fossil mollusks provides highly complementary data for assessments of ages for both of the isotopic dating techniques. At relatively low elevations, all islands host terraces dating to the last major interglacial period, ~120 ka. At higher elevations, the oldest fossil-bearing terraces are as old as ~2.0 Ma (Santa Cruz Island and San Clemente Island) and as young as ~660 ka (Palos Verdes Hills). The highest fossiliferous terraces on the other islands range in age from ~1.5 Ma to ~1.0 Ma. These ages, along with elevation measurements, indicate that long-term (early Quaternary) and short-term (late Quaternary, since ~120 ka) uplift rates are similar on each island. Nevertheless, uplift rates are variable from island to island, with the highest rate (~0.55 m/kyr) on the Palos Verdes Hills, intermediate rates (0.24-0.12 m/kyr) on San Nicolas Island, San Clemente Island, San Miguel Island, and Santa Rosa Island, and low rates (0.05-0.04 m/kyr) on Santa Cruz Island, Santa Barbara Island, and Anacapa Island. These uplift rates, along with measurements of the highest island elevations, allow estimates of the times of island emergence. Results show that Santa Cruz Island likely emerged the earliest, possibly as long ago as ~13 Ma, whereas neighboring Anacapa Island probably did not emerge until ~7-5 Ma. San Clemente, Santa Rosa, and Santa Barbara emerged ~5-3 Ma, and San Miguel and San Nicolas likely became islands ~2-1 Ma. The Palos Verdes Hills emerged most recently, around 0.8 Ma, and was still an island as recently as ~120 ka. These estimates should provide useful information for understanding the chronology of biotic migrations (mainland-to-islands, and island-to-island) and the origin of insular endemics.