Magmatic arc structure around Mount Rainier, WA, from the joint inversion of receiver functions and surface wave dispersion

Abstract

The deep magmatic processes in volcanic arcs are often poorly understood. We analyze the shear wave velocity (V S ) distribution in the crust and uppermost mantle below Mount Rainier, in the Cascades arc, resolving the main velocity contrasts based on converted phases within P coda via source normalization or receiver function (RF) analysis. To alleviate the trade-off between depth and velocity, we use long period phase velocities (25-100 s) obtained from earthquake surface waves, and at shorter period (7-21 s) we use seismic noise cross correlograms. We use a transdimensional Bayesian scheme to explore the model space (V S in each layer, number of interfaces and their respective depths, level of noise on data). We apply this tool to 15 broadband stations from permanent and Earthscope temporary stations. Most results fall into two groups with distinctive properties. Stations east of the arc (Group I) have comparatively slower middle-to-lower crust (V S 5 3.4-3.8 km/s at 25 km depth), a sharp Moho and faster uppermost mantle (V S 5 4.2-4.4 km/s). Stations in the arc (Group II) have a faster lower crust (V S 5 3.7-4 km/s) overlying a slower uppermost mantle (V S 5 4.0-4.3 km/s), yielding a weak Moho. Lower crustal velocities east of the arc (Group I) most likely represent ancient subduction melanges mapped nearby. The lower crust for Group II ranges from intermediate to felsic. We propose that intermediate-felsic to felsic rocks represent the prearc basement, while intermediate composition indicates the mushy andesitic crustal magmatic system plus solidified intrusion along the volcanic conduits. We interpret the slow upper mantle as partial melt.

Shear Velocity Structure From Ambient Noise and Teleseismic Surface Wave Tomography in the Cascades Around Mount St. Helens

“…Figures h and i, c and 11d, and Figure S7), although relative Vs patterns are stable and the forearc is consistently slower than farther east. Most upper mantle Vs values are 4.2–4.5 km/s, similar to observations beneath Mount Rainier (Obrebski et al, ). The higher values, observed east of MSH and near Mount Adams, resemble typical regional (Shen & Ritzwoller, ) and global (Christensen & Mooney, ) upper‐mantle Vs .…”

“…(b) The gradient model or Vs R for a smoothed Moho. The original starting model (dashed black) is a moving average of the Group 1 average Vs from Obrebski et al (). Solid black line is the resulting Vs R , referred to as the “gradient model.” Monte Carlo uncertainties shown in solid red, as 1‐σ (dark) and 2‐σ (light) uncertainties.…”

“…The reference dispersion curve c R ( ω ) is then inverted to produce a reference shear‐velocity ( Vs ) model with depth z , Vs R ( z ) (Figure ; Table S1) using an iterative nonlinear one‐dimensional (1‐D) inversion method (Herrmann, ). A smoothed Vs ( z ) model from a nearby study (Group 1 average of Obrebski et al, ) is used as the starting model (dashed black line in Figure b), with layer thicknesses increasing from 2.5 to 5 km with depth (Table S1). This inversion imposes a first‐derivative smoothing constraint on Vs R ( z ).…”

Seismic evidence for a cold serpentinized mantle wedge beneath Mount St Helens

“…Several previous active and passive-source seismic studies have observed weak or absent Moho arrivals at other locations within the Cascadia forearc region141618 (Fig. 1), including a reversal of the Moho velocity contrast in central Oregon13.…”

“…Locations of the major Quaternary volcanoes active are plotted as red triangles and the grey regions depict the distribution of volcanic vents7. Yellow boxes show where previous seismic experiments have observed a weak Moho arrival131418. The red shaded region denotes the location of the Southern Washington Cascades Conductor2829.…”