Marine terraces and rates of vertical tectonic motion: The importance of glacio-isostatic adjustment along the Pacific coast of central North America

Abstract

Differences in marine terrace elevations across the Pacific coast of North America have long been assumed to be a result of differences in the rates of tectonic motion. However, other processes, particularly glacio-isostatic adjustment, lead to regional variations in sea levels. In this study, we compiled the ele va tions of marine isotope stage (MIS) 5e (ca. 119-129 ka), 5c (ca. 106 ka), and 5a (ca. 84 ka) terraces across the Pacific coast of central North America and compared these regional variations in elevation with model predictions of glacio-isostatic adjustment after correcting for tectonics. These predictions are generally consistent with the observed trends in the elevations of the terraces and show that this process created up to 20 m of coeval variation in local sea levels along the Pacific coast of central North America (between 20°N and 45°N) during MIS 5c and MIS 5a, but less, ~4 m, during MIS 5e. Accounting for glacio-isostatic adjustment reduces the variability in uplift rates calculated at individual locations using different-aged terraces as datums. Ignoring glacio-isostatic adjustment leads to overestimated uplift rates by an average of 40%, but up to 72%, across the Pacific coast of central North America. An understanding of regional variations in glacioisostatic adjustment-corrected sea levels also contributes to the correct identification of marine terraces with mistaken ages.

Little Islands Recording Global Events: Late Quaternary Sea Level History and Paleozoogeography of Santa Barbara and Anacapa Islands, Channel Islands National Park, California

“…In a recent study by Creveling et al (2015), sea level at ~127 ka to ~120 ka is estimated to have been as high as ~11 m above present (assuming a purely eustatic sea level of +6 m) to ~13 m above present (assuming a purely eustatic sea level of +8 m) on the southern California coast. These values are in good agreement with those of another modeling study where LIG (~120 ka) paleo-sea level estimates of +12 m to +13 m relative to present were reported for much of the Pacific Coast of North America, from northern California to northern Baja California (Simms et al 2016).…”

“…Lindberg et al (1980) report that emergent marine deposits on Isla Guadalupe have elevations ranging from 1 m to 8 m above sea level, with most localities described as 1-6 m above sea level. If sea level were truly +11 to +13 m above present during the LIG period (as the models of Creveling et al 2015 andSimms et al 2016 indicate for this island as well), Isla Guadalupe would have to have experienced significant subsidence in the past 120 ka. Given its geologic setting, there is simply no apparent mechanism that would have brought about late Quaternary subsidence.…”

“…The GIA estimates of paleo-sea level presented by Creveling et al (2015) and Simms et al (2016) can be assessed from the geologic records of the LIG period for SBI, ANA, and Santa Cruz Island presented here. Based on the amino acid data discussed previously, we assume that the initial formation of the lowelevation terraces on all three islands dates to the peak of the LIG period (MIS 5.5) at ~120 ka.…”

“…Glacial isostatic adjustment effects on apparent sea level will generate a geologic record that departs from a purely eustatic one most dramatically on coastlines near such ice sheets ("near-field" localities or "intermediate-field" localities, such as California), whereas coastlines distant from ice sheets ("far-field" localities, such as Barbados and New Guinea) will record a more purely eustatic sea level history. A combination of field studies and modeling shows that GIA effects have been important on the Pacific Coast of North America (Muhs et al 2012, Clark et al 2014, Creveling et al 2015, Reeder-Meyers et al 2015, Simms et al 2016.…”

Shelf evolution along a transpressive transform margin, Santa Barbara Channel, California

“…Clark et al (2014) and Reeder-Myers et al (2015) subsequently presented GIA models that proposed that LGM sea level was 100 m to 111 m lower than present for the Channel Islands. These GIA models do not incorporate information on LGM shorelines and instead rely at least in part on inferences derived from analysis of uplifted marine isotope stage (MIS) 5 terraces (e.g., Simms et al, 2016b). Only significant post-LGM subsidence (≥12-28 m, 0.6-1.4 mm/yr) could reconcile the observed depth of what we interpret as a shoreline angle on the outer shelf east of Hueneme Canyon (Fig.…”