Age estimates and uplift rates for Late Pleistocene marine terraces: Southern Oregon portion of the Cascadia Forearc

Abstract

Marine terraces are prominent landforms along the southern Oregon coast, which forms part of the forearc region of the Cascadia subduction zone. Interest in the Cascadia subduction zone has increased because recent investigations have suggested that slip along plates at certain types of convergent margins is characteristically accompanied by large earthquakes. In addition, other investigations have suggested that convergent margins can be broadly classified by the magnitude of their uplift rates. With these hypotheses in mind, we generated new uranium series, amino acid, and stable isotope data for southern Oregon marine terrace fossils. These data, along with terrace elevations and two alternative estimates of sea level at the time of terrace formation, allow us to determine terrace ages and uplift rates. Uranium series analysis of fossil coral yields an age of 83 _ 5 ka for the Whisky Run terrace at Coquille Point in Bandon, Oregon. A combination of amino acid and oxygen isotope data suggest ages of about 80 and 105 ka for the lowest two terraces at Cape Blanco. These ages indicate uplift rates of 0.45-1.05 and 0.81-1.49 m/kyr for Coquille Point and Cape Blanco, respectively. Late Quaternary uplift rates of marine terraces yield information about deformation in the overriding plate, but it is unclear if such data vary systematically with convergent margin type. In order to assess the utility of the southern Oregon uplift rates for predicting the behavior of the Cascadia subduction zone, we compared late Quaternary uplift rates derived from terrace data from subduction zones around the world. On the basis of this comparison the southern Oregon rates of vertical deformation are not unusually high or low. Furthermore, late Quaternary uplift rates show little relationship to the type of convergent margin. These observations suggest that local structures may play a large role in uplift rate variability. In addition, while the type of convergent margin may place an upper limit on possible uplift rate, greater upper limits serve to increase the range of possible uplift rates. In the case of the southern Oregon coast, variability in uplift rate probably reflects local structures in the overriding plate, and the rate of uplift cannot be used as a simple index of the potential for great earthquakes along the southern Cascadia subduction zone.

A cool eastern Pacific Ocean at the close of the Last Interglacial complex

“…Amino acid ratios in the bivalve Saxidomus reported by Kennedy et al (1982) suggested that the Cape Blanco terrace could be younger than the Whisky Run terrace at Coquille Point. Later amino acid and oxygen isotope studies, also on Saxidomus valves, suggested that the two terraces could be correlative (Muhs et al, 1990). Correlation of the Cape Blanco and Whisky Run terraces is also supported by soils data (Bockheim et al, 1992;Kelsey and Bockheim, 1994).…”

“…Addicott (1964Addicott ( , 1966, Zullo (1969), and Kennedy (1978) studied fossils from the Whisky Run and Cape Blanco terraces. In a previous study, a coral from the Whisky Run terrace at Coquille Point yielded an alpha-spectrometric U-series age of 80,000 yr BP (Muhs et al, 1990). Amino acid ratios in the bivalve Saxidomus reported by Kennedy et al (1982) suggested that the Cape Blanco terrace could be younger than the Whisky Run terrace at Coquille Point.…”

“…Kennedy (1978Kennedy ( , 1981 studied fossils from the lowest terrace deposits (our mapping unit Qt 1 ) at Point Arena. Kennedy et al (1982) and Muhs et al (1990Muhs et al ( , 1994 presented amino acid and U-series data correlating this terrace with that at Coquille Point, Oregon. Alphaspectrometric U-series analyses of marine terrace corals from Point Arena are 76,000 yr BP and 88,000 yr BP.…”

Coastal tectonics on the eastern margin of the Pacific Rim: late Quaternary sea-level history and uplift rates, Channel Islands National Park, California, USA

“…The Cascadia subduction zone also seems to fit the model reasonably well, as it has an intermediate type of subduction zone geometry (Jarrard, 1986) and the data presented here show variable, but generally intermediate uplift rates when compared to the two extremes of the Ryukyu Islands. Nevertheless, other subduction zones do not seem to fit the model very well (see review in Muhs et al (1990)). For example, the uplift rate in a coastal area of northcentral Chile that can be categorized (as its name implies) as a "Chilean" type of subduction zone has, in fact, a rather modest late Quaternary uplift rate of 0.16 m/ka (Leonard and Wehmiller, 1992).…”

“…Paleo-sea levels, relative to present, used for uplift rate calculations are þ6 m (~120 ka), À11 m (~80 ka), and À62 m (~49 ka), derived from data in Muhs et al (2012). Abbreviations and sources of data, south to north: CP, Cabo Pulmo (Muhs et al, 2002a); LP, La Paz (Sirkin et al, 1990); BH, Bahía Magdalena (Omura et al, 1979); IC, Isla Coronados and PC, Punta Chivato (Johnson et al, 2007; see also Table 2); MU, Mulege (Ashby et al, 1987); BT, Bahía de Tortugas (Emerson et al, 1981); PB, Punta Banda (Rockwell et al, 1989;Muhs et al, 2002a); PL, Point Loma (Kern, 1977;Muhs et al, 2002a); SCI, San Clemente Island (Muhs et al, 2002a; NB, Newport Bay (Grant et al, 1999); SNI, San Nicolas Island ; PV, Palos Verdes Hills (Muhs et al, 2006); NCI, Northern Channel Islands (this study); V, Ventura Kennedy et al, 1982;Wehmiller, 1982); IV, Isla Vista (Gurrola et al, 2014; see also Table 2); SB, Shell Beach (Stein et al, 1991;Hanson et al, 1994); PSL, Point San Luis (Hanson et al, 1994;Muhs et al, 1994); C, Cayucos (Stein et al, 1991;Muhs et al, 2002a); AN, Año Nuevo (Muhs et al, 2006); PA, Point Arena (Muhs et al, 2006); PD, Point Delgada (McLaughlin et al, 1983a(McLaughlin et al, , 1983bMerritts and Bull, 1989); CC, Crescent City (Kennedy et al, 1982;Polenz and Kelsey, 1999); CB, Cape Blanco (Kelsey, 1990;Muhs et al, 1990); B, Bandon (McInelly and Kelsey, 1990;…”