Provenance of and Age of Granitoid and Sandstone Clasts in Conglomerates of the Paleocene to Upper Cretaceous Yakutat Group, Russell Fjord, Alaska

The Paleocene to Upper Cretaceous Yakutat Group consists of a flysch facies. A conglomerate occurs in two places in Russell Fjord, and the composition and age of clasts bears on tectonic reconstructions. One site (#23) occurs in what is mapped as flysch and one in the mélange (#25), but the conglomerates are essentially identical. They contain clasts of sandstone, greenstone, limestone, marble, chert, and plutonic rocks that are clast supported, and interbedded with sandstones that contain coalified plant fragments. The maximum depositional age (MDA) of the U/Pb-dated zircons from the sandstone is 65.9 ± 1.8 Ma and 65.6 ± 2.2 Ma for the two samples, indicating deposition was Maastrichtian or younger. Grain-age distributions for these two samples yield populations at 69-74 Ma, 92-94 Ma,157-183 Ma, 1365 Ma, and 1710 Ma. The Jurassic population, which is subordinate, may be resolved into component populations at 154 Ma and 182 Ma. Clasts of sandstones and plutonic rocks were dated and analyzed. A well-rounded sandstone clast from the conglomerate at site #25 was dated and has an MDA of 71.7 ± 2.4 Ma, and the overall grain-age distribution is identical to sandstone elsewhere in the mélange. The similarity in MDAs and lithology of the sandstone clast and host mélange sandstone suggests that parts of the mélange may have been reworked. Three plutonic clasts from site #23 were analyzed for geochemistry in addition to two plutonic samples from tectonic slices (knockers) in the mélange. Based upon the geochemistry, the clasts are granite, trondhjemite, and tonalite, and both knockers are tonalite, and all plot as volcanic arc granites on discrimination diagrams. Two clasts and both knockers were U/Pb zircon dated. One clast has a date of 167.2 ± 2.3 Ma and the other has two zircon populations with modes at 156.2 ± 2.7 Ma and 179.5 ± 2.6 Ma. The bimodal age distribution is unexplained, but might be due to lead loss, mixing magma, or contamination. The granitic knockers are tectonic slices that have U/Pb dates of 174.9 ± 2.0 Ma and 173.8 ± 2.1 Ma. ?Hf(t) values on the 167 Ma clast and 175 Ma knocker range from +10.0 to +14.3 and +9.8 to +12.0, respectively. Thus, these data suggest that the source region for the granitoid clast and knocker are isotopically homogenous and juvenile. Potential provenances include the Jurassic Bonanza and Talkeetna arcs on Vancouver Island and Wrangellia, respectively. Geochemistry analysis on the plutonic rocks also show a correlation to fragments of similar lithology and age in the Western Mélange Belt (WA)

Defining historical earthquake rupture parameters and proposed slip distributions through tsunami modeling in south-central Chile

Reliable tsunami early warning forecasts rely on accurate initial modeling conditions and interpretations of subduction zone behavior in a multi-century perspective. GPS and seismologic data were introduced this past century to study rupture dynamics in detail, however limited information is known about ruptures that pre-date the 20th century. I propose a methodology that uses statistics to better understand these pre-20th century ruptures. This methodology applies the historical and geologic tsunami record as a means to select a suite of tsunami simulations from earthquake source solutions. I chose south-central Chile (46°S to 30°S) to test this new methodology; it has an extensive earthquake historical record at 47 different coastal sites, some of which date to the 16th century. Between 1570 and 1960, this region experienced at least 17 tsunamigenic earthquakes. In addition to evaluating possible source solutions for these earthquakes, my methodology also allows the test of whether subducted fracture zones, like the Mocha fracture zone (MFZ) in south-central Chile, controls rupture propagation (as previously hypothesized). For this research, I used GeoClaw, a numerical tsunami modeling code, to simulate 423 forward-modeled Mw 8.7 - 9.5 earthquake scenarios with stochastic, variable slip distributions. I used Akaike’s Information Criterion (AIC) to identify significant earthquake parameters (Mw and slip location) of 17 events by statistically selecting source models that had similar simulated wave heights to known observations in the historic and geologic record. For example, I concluded from AIC that the 1960 event was a Mw 9.3 rupture with high slip concentration (~ 30 m) at ~ 39-40ºS, and the 1730 event was a Mw 9.3 rupture with shallow maximum slip at ~ 36ºS; both solutions support the MFZ hypothesis. The AIC results generally agree with previously estimated magnitudes within the literature and were validated by using root mean square error RMSE values. I produced high resolution maps at three coastal sites with well-known tsunami observations for further refinement of potential rupture scenarios. Defining historical rupture characteristics gives insight regarding temporal and spatial variabilities of locking zones. This information may be useful for predicting future near-field tsunami wave heights for particularly vulnerable coastal regions

The giant 1960 tsunami in the context of a 6000-year record of paleotsunamis and coastal evolution in south-central Chile

The tsunami associated with the giant 9.5 Mw 1960 Chile earthquake deposited an extensive sand layer above organic-rich soils near Queule (39.3°S, 73.2°W), south-central Chile. Using the 1960 tsunami deposits, together with eye-witness observations and numerical simulations of tsunami inundation, we tested the tsunami inundation sensitivity of the site to different earthquake slip distributions. Stratigraphically below the 1960 deposit are two additional widespread sand layers interpreted as tsunami deposits with maximum ages of 4960–4520 and 5930–5740 cal BP. This \u3e4500-year gap of tsunami deposits preserved in the stratigraphic record is inconsistent with written and geological records of large tsunamis in south-central Chile in 1575, 1837, and possibly 1737. We explain this discrepancy by: (1) poor preservation of tsunami deposits due to reduced accommodation space from relative sea-level fall during the late Holocene; (2) recently evolved coastal geomorphology that increased sediment availability for tsunami deposit formation in 1960; and/or (3) the possibility that the 1960 tsunami was significantly larger at this particular location than other tsunamis in the past \u3e4500 years. Our research illustrates the complexities of reconstructing a complete stratigraphic record of past tsunamis from a single site for tsunami hazard assessment