STATISTICS AND SEGMENTATION: A ROBUST AND UNBIASED ANALYSIS OF CASCADE ARC VARIABILITY

Previous work suggests that the Cascade Arc can be partitioned into geochemically distinct segments which may reflect along-arc differences in subduction geometry, mantle flow, lithospheric composition and/or regional tectonics (Schmidt et al. 2008). Since the original study used a relatively small dataset (n=466), we have re-examined this issue using a considerably larger dataset of 7,689 Quaternary Cascades volcanic (WR, glass) samples, available through the GEOROC and EarthChem databases. In order to minimize sampling bias – the effect where well-studied locations heavily weight conclusions, we used a weighted bootstrap Monte Carlo approach in which the probability of a sample being selected to the posterior distribution was inversely proportional to the number of samples within its 0.25° latitude bin. Thus, a more uniform and unbiased distribution was produced, from which we could asses Cascade Arc variation.

Our results for mafic compositions generally support the initial work of Schmidt et al. (2008), however, our study also investigates back arc trends, includes analysis of felsic compositions, and presents unbiased characterizations of segments. For example, the geochemistry of Columbia segment basalts (Mt. Rainier to Mt. Jefferson), is skewed heavily by the primitive Sr and Nd isotopic compositions of Mt. Adams, which comprises 74% of all 957 basalt analyses for the segment. The bootstrapped median reduces the apparent isotopic differences between the Columbia and Central segments, revealing a more continuous North to South trend towards more evolved isotopic values. Trace element analysis suggests that the Columbia segment is dominated by low degree partial melts of an enriched mantle source (possibly Siletz LIP Terrain) with little subduction influence (OIB-like HFSE, higher LREE and Nb/Yb, lower Ba/Nb). Our study also shows that hot and dry rhyolites (High Sr/Zr, Y, and FeOT/MgO) dominate in the extensional Central segment, whereas more arc-typical wet rhyolites dominate in the South. Our approach demonstrates statistically significant trace element and isotopic differences between the arc segments proposed by Schmidt et al. (2008). However, future work will include the use of spatial hierarchical cluster analysis to assess if there is a more statistically robust clustering scheme.