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Active foundering of a continental arc root beneath southern Sierra Nevada Zandt et al, 2004

Active foundering of a continental arc root beneath southern Sierra Nevada Zandt et al, 2004. Removal of ultramafic root beneath southern Sierra Nevada Initiated between 10 and 3 Mya via Rayleigh-Taylor instability Process is consistent with observables at surface and depth

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Active foundering of a continental arc root beneath southern Sierra Nevada Zandt et al, 2004

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  1. Active foundering of a continental arc root beneath southern Sierra Nevada Zandt et al, 2004

  2. Removal of ultramafic root beneath southern Sierra Nevada Initiated between 10 and 3 Mya via Rayleigh-Taylor instability Process is consistent with observables at surface and depth Drip is asymmetric and migrating to the southwest Key points

  3. Garnet pyroxenite xenoliths of Miocene age found in local volcanic fields– density of 3.5-3.6 g/cm3 Later xenoliths from pliocene show no evidence of ultramafic root Subsiding sub-basin in Great Valley migrating southwest Alignment of features along direction of root migration Surface observables

  4. Disappearance of moho beneath western foothills (“hole” in moho) Increasing depth of moho (crustal thickening) in direction of hole Maximum crustal thickness beneath Kings Volcanic Field (42km crustal welt) Negative polarity arrivals move downward to the south (?) Anisotropic layer at base of crust (?) Seismic observables

  5. 2 possible explanations: -Localized zone of low S wave velocity in uppermost mantle -Moho topography scatters Ps conversion -Synthetic modelling supports 2nd explanation

  6. Created synthetic receiver functions for various structures Processed synthetics in the same way as true data Size of Moho cusp based on volume of entrained material seen in cross sections and previous models V-shaped bulge of crust 50km wide and 25km deep would be sufficient to cause disappearing Moho Verification

  7. Mesozoic: ultramafic phase of batholith formation produces dense root Batholith + root formed strong microplate that persisted into Miocene Miocene destabilization of root causes active foundering through present Sequence of events

  8. Movement of volcanic are to north may have cause flux in Sierran lithosphere Passing of Mendocino triple junction 20-10Mya exposed lithosphere to asthenosphere from slab window Extensional faulting beginning at 15Mya weakens top of root Possible causes of destabilization

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