A multicomponent Isabella anomaly: Resolving the physical state of the Sierra Nevada upper mantle from Vp/Vs anisotropy tomography
File(s)
Published version
Author(s)
Type
Journal Article
Abstract
The Isabella anomaly, a prominent upper-mantle high-speed P-wave anomaly located within the southern Great Valley and southwestern foothills of the Sierra Nevada, has been interpreted either as foundering sub-Sierran
lithosphere or as remnant oceanic lithosphere. We used Vp/Vs anisotropy
tomography to distinguish among the probable origins of the Isabella anomaly.
S waveforms were rotated into the Sierran SKSFast and SKSSlow directions
determined from SKS-splitting studies. Teleseismic P-, SFast-, SSlow-, SKSFast-, and
SKSSlow-wave arrival times were then inverted to obtain three-dimensional
(3-D) perturbations in Vp, Vp/VsMean, and percent azimuthal anisotropy using
three surface wave 3-D starting models and one one-dimensional (1-D) model.
We observed the highest Vp/Vs anomalies associated with slower velocities
in regions marked by young volcanism, with the largest of these anomalies
being the Mono anomaly under the Long Valley region, which extends to
depths of at least 75 km. Peak Vp/Vs perturbations of +4% were found at 40
km depth. The low velocities and high Vp/Vs values of this anomaly could
be related to partial melt.
The high wave speeds of the Isabella anomaly coincide with low Vp/Vs
values with peak perturbations of −2%, yet they do not covary spatially. The
P-wave inversion imaged the Isabella anomaly as a unimodal eastward-plunging
body. However, the volume of that Isabella anomaly contains three separate
bodies as defined by varying Vp/Vs values. High speeds, regionally average
Vp/Vs values (higher than the other two anomalies), and lower anisotropy
characterize the core of the Isabella anomaly. The western and shallowest
part has high wave speeds and a lower Vp/Vs values than the surrounding
mantle. The eastern and deepest part of the anomaly also contains high
speeds and lower Vp/Vs values but exhibits higher anisotropy. We considered
combinations of varying temperature, Mg content (melt depletion), or modal
garnet to reproduce our observations. Our results suggest that the displaced
garnet-rich mafic root of the Mesozoic Sierra Nevada batholith is found in the
core of the Isabella anomaly. If remnant oceanic lithosphere exists within the
Isabella anomaly, it most likely resides in the shallow, westernmost feature. Within the Sierra Nevada, the highest upper-mantle anisotropy is largely
contained within the central portion of the range and the adjacent Great
Valley. Anisotropy along the Sierra crest is shallow and confined to the lithosphere
between 20 and 40 km depth. Directly below, there is a zone of low
anisotropy (from 170 to 220 km depth), low velocities, and high Vp/Vs values.
These features suggest the presence of vertically upwelling asthenosphere
and consequent horizontal flow at shallower depths. High anisotropy beneath
the adjacent western foothills and Great Valley is found at ~120 km depth
and could represent localized mantle deformation produced as asthenosphere
filled in a slab gap.
lithosphere or as remnant oceanic lithosphere. We used Vp/Vs anisotropy
tomography to distinguish among the probable origins of the Isabella anomaly.
S waveforms were rotated into the Sierran SKSFast and SKSSlow directions
determined from SKS-splitting studies. Teleseismic P-, SFast-, SSlow-, SKSFast-, and
SKSSlow-wave arrival times were then inverted to obtain three-dimensional
(3-D) perturbations in Vp, Vp/VsMean, and percent azimuthal anisotropy using
three surface wave 3-D starting models and one one-dimensional (1-D) model.
We observed the highest Vp/Vs anomalies associated with slower velocities
in regions marked by young volcanism, with the largest of these anomalies
being the Mono anomaly under the Long Valley region, which extends to
depths of at least 75 km. Peak Vp/Vs perturbations of +4% were found at 40
km depth. The low velocities and high Vp/Vs values of this anomaly could
be related to partial melt.
The high wave speeds of the Isabella anomaly coincide with low Vp/Vs
values with peak perturbations of −2%, yet they do not covary spatially. The
P-wave inversion imaged the Isabella anomaly as a unimodal eastward-plunging
body. However, the volume of that Isabella anomaly contains three separate
bodies as defined by varying Vp/Vs values. High speeds, regionally average
Vp/Vs values (higher than the other two anomalies), and lower anisotropy
characterize the core of the Isabella anomaly. The western and shallowest
part has high wave speeds and a lower Vp/Vs values than the surrounding
mantle. The eastern and deepest part of the anomaly also contains high
speeds and lower Vp/Vs values but exhibits higher anisotropy. We considered
combinations of varying temperature, Mg content (melt depletion), or modal
garnet to reproduce our observations. Our results suggest that the displaced
garnet-rich mafic root of the Mesozoic Sierra Nevada batholith is found in the
core of the Isabella anomaly. If remnant oceanic lithosphere exists within the
Isabella anomaly, it most likely resides in the shallow, westernmost feature. Within the Sierra Nevada, the highest upper-mantle anisotropy is largely
contained within the central portion of the range and the adjacent Great
Valley. Anisotropy along the Sierra crest is shallow and confined to the lithosphere
between 20 and 40 km depth. Directly below, there is a zone of low
anisotropy (from 170 to 220 km depth), low velocities, and high Vp/Vs values.
These features suggest the presence of vertically upwelling asthenosphere
and consequent horizontal flow at shallower depths. High anisotropy beneath
the adjacent western foothills and Great Valley is found at ~120 km depth
and could represent localized mantle deformation produced as asthenosphere
filled in a slab gap.
Date Issued
2019-11-08
Date Acceptance
2019-08-09
Citation
Geosphere, 2019, 15 (6), pp.2018-2042
ISSN
1553-040X
Publisher
Geological Society of America
Start Page
2018
End Page
2042
Journal / Book Title
Geosphere
Volume
15
Issue
6
Copyright Statement
© 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY-NC license.
Sponsor
The Leverhulme Trust
Grant Number
RPG-2013-332
Subjects
0402 Geochemistry
0403 Geology
0404 Geophysics
Geochemistry & Geophysics
Publication Status
Published
Date Publish Online
2019-11-09