Subduction zone and crustal dynamics of western Washington; a tectonic model for earthquake hazards evaluation

Stanley, Dal; Villaseñor, A.; Benz, H.

doi:10.3133/ofr99311

Cited by 17 publications

(20 citation statements)

References 88 publications

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“…Mount Rainier samples selected for their lower-than-typical K 2 O concentrations are also distinguished by low 206 Pb/ 204 Pb and approach Mount St. Helens Sr-Nd-Pb isotopic values. The buried eastern margin of Siletzia lies ~15 km west of Mount Rainier beneath the west Rainier seismic zone and the Carbon River anticlinorium (Stanley et al, 1995), whereas Mount St. Helens straddles the St. Helens seismic zone identifi ed as overlying Siletzia's buried eastern margin (Parsons et al, 1998), consistent with lower contributions from Siletzia basement to the Mount Rainier magmatic system. Mount Rainier is also farther from a possible offset in the subducted Juan de Fuca plate imaged at 90 km depth by S-wave tomography (Schmandt and Humphreys, 2010), so subordinate infl uence from slab-edge melts is also possible.…”

Section: Mount St Helens-type Magmas In the Mount Rainier Systemmentioning

confidence: 77%

Petrogenesis of Mount Rainier andesite: Magma flux and geologic controls on the contrasting differentiation styles at stratovolcanoes of the southern Washington Cascades

Sisson

Salters

Larson

2013

Geological Society of America Bulletin

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Quaternary Mount Rainier (Washington, USA) of the Cascades magmatic arc consists of porphyritic calc-alkaline andesites and subordinate dacites, with common evidence for mingling and mixing with less evolved magmas encompassing andesites, basaltic andesites, and rarely, basalts. Basaltic andesites and amphibole andesites (spessartites) that erupted from vents at the north foot of the volcano represent some of Mount Rainier's immediate parents and overlap in composition with regional basalts and basaltic andesites. Geochemical (major and trace elements) and isotopic (Sr, Nd, Pb, O) compositions of Mount Rainier andesites and dacites are consistent with modest assimilation (typically ≤20 wt%) of evolved sediment or sediment partial melt. Sandstones and shales of the Eocene Puget Group, derived from the continental interior, are exposed in regional anticlines fl anking the volcano, and probably underlie it in the middle to lower crust, accounting for their assimilation. Mesozoic and Cenozoic igneous basement rocks are unsuitable as assimilants due to their high 143 Nd/ 144 Nd, diverse 206 Pb/ 204 Pb, and generally high δ 18 O.The dominant cause of magmatic evolution at Mount Rainier, however, is inferred to be a version of in situ crystallization-differentiation and mixing (Langmuir, 1989) wherein small magma batches stall as crustal intrusions and solidify extensively, yielding silicic residual liquids with trace element concentrations infl uenced by accessory min-eral saturation. Subsequent magmas ascending through the intrusive plexus entrain and mix with the residual liquids and low-degree re-melts of those antecedent intrusions, producing hybrid andesites and dacites. Mount St. Helens volcanic rocks have geochemical similarities to those at Mount Rainier, and may also result from in situ differentiation and mixing due to low and intermittent longterm magma supply, accompanied by modest crustal assimilation. Andesites and dacites of Mount Adams isotopically overlap the least contaminated Mount Rainier magmas and derive from similar parental magma types, but have trace element variations more consistent with progressive crystallization-differentiation, probably due to higher magma fl uxes leading to slower crystallization of large magma batches, allowing time for progressive separation of minerals from melt. Mount Adams also sits atop the southern projection of a regional anticlinorium, so Eocene sediments are absent, or are at shallow crustal levels, and so are cold and diffi cult to assimilate. Differences between southwest Washington stratovolcanoes highlight some ways that crustal geology and magma fl ux are primary factors in andesite generation. Figure 1. Regional geologic setting of Mount Rainier volcano, simplifi ed from Schuster (2005) and Walker and MacLeod (1991). Geologic units are: chiefl y Mesozoic and Paleozoic igneous and metamorphic rocks (light blue; Tertiary plutons omitted for clarity); Paleocene-middle Eocene submarine basalts of the Siletzia terrain (purple); middle Eocene and younger sandst...

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Section: Mount St Helens-type Magmas In the Mount Rainier Systemmentioning

confidence: 77%

Petrogenesis of Mount Rainier andesite: Magma flux and geologic controls on the contrasting differentiation styles at stratovolcanoes of the southern Washington Cascades

Sisson

Salters

Larson

2013

Geological Society of America Bulletin

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“…It has previously been interpreted as only 6 km thick offshore Vancouver Island [ Hyndman et al , 1990]. Crescent‐Siletz terrane provides the backstop to a large accretionary sedimentary prism formed from the sediments scraped off the incoming oceanic plate [ Brandon and Calderwood , 1990; Hyndman , 1995b; Parsons et al , 1999; Stanley et al , 1999] (Figure 2). …”

Section: Geological and Tectonic Settingmentioning

confidence: 99%

Crustal structure beneath the Strait of Juan de Fuca and southern Vancouver Island from seismic and gravity analyses

et al. 2003

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[1] Wide-angle and vertical incidence seismic data from Seismic Hazards Investigations in Puget Sound (SHIPS), gravity modeling, and seismicity are used to derive twodimensional crustal models beneath the Strait of Juan de Fuca. The Eocene volcanic Crescent-Siletz terrane is significantly thicker than previously recognized and extends from near the surface to depths of 22 km or greater. For the northern strait, a weak midcrustal reflector, dipping east from 12-to 22-km depth, is inferred from wide-angle reflections. A stronger deeper reflector, dipping eastward from 23-to 36-km depth, is associated with the top of ''reflector band E,'' a zone of high reflectivity on coincident Multichannel Seismic (MCS) data, interpreted as a shear zone. A high-velocity zone (7.60 ± 0.2 km s À1 ) between these reflectors is interpreted as a localized slice of mantle accreted with the overlying Crescent-Siletz terrane. For the southern strait, no deep high-velocity layer is observed and the E-band reflectivity is weaker than to the north. A strong deep reflector, interpreted as the oceanic Moho dips eastward from 35 to 42 km. Seismicity within the subducting slab occurs mainly above the inferred oceanic Moho. Gravity modeling, constrained by the wide-angle seismic models and seismicity, is consistent with the inferred large thickness of Crescent-Siletz and high-density rocks (3030 kg m À3 ) in the lower crust.

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“…However, if mafic rocks occur in the upper and lower plates without intervening sediments, then the thrust region may not become ductile until temperatures exceed 600 ø C (Bergman and Soloman, 1988). Stanley et al (1999) have inferred from seismic velocity models that part of the subduction thrust beneath Puget Sound at depths of 30-50 km has mafic/ultramafic rocks in the upper plate. They reviewed petrological considerations for the thrust contact and concluded that this deep part of the thrust could be partially locked.…”

Section: New Models For Cascadia Thrust Couplingmentioning

confidence: 99%

Models of downdip frictional coupling for the Cascadia Megathrust

Stanley

Villaseñor

2000

Geophysical Research Letters

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Abstract. We have developed models of downdip frictional coupling along two transects across the Cascadia subduction zone in northern Washington and central Oregon. The models involve complicated downdip coupling profiles. Although not unique, our models closely predict available GPS shortening rates and vertical uplift data. We are able to explain relatively low horizontal shortening rates along the Washington coast and small vertical uplift rates in central Oregon. Our models depart from previous models by inclusion of a deeply coupled region assumed to be related to mafic rocks in both the upper and lower plates of the thrust.

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Subduction zone and crustal dynamics of western Washington; a tectonic model for earthquake hazards evaluation

Cited by 17 publications

References 88 publications

Petrogenesis of Mount Rainier andesite: Magma flux and geologic controls on the contrasting differentiation styles at stratovolcanoes of the southern Washington Cascades

Petrogenesis of Mount Rainier andesite: Magma flux and geologic controls on the contrasting differentiation styles at stratovolcanoes of the southern Washington Cascades

Crustal structure beneath the Strait of Juan de Fuca and southern Vancouver Island from seismic and gravity analyses

Models of downdip frictional coupling for the Cascadia Megathrust

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