scholarly journals 168 : Magnetic and Gravity Anomalies in the Great Valley and Western Sierra Nevada Metamorphic Belt, California

1975 ◽  
Keyword(s):  
Sierra Nevada ◽  
Gravity Anomalies ◽  
Metamorphic Belt ◽  
Magnetic And Gravity Anomalies ◽  
Great Valley

scholarly journals A multicomponent Isabella anomaly: Resolving the physical state of the Sierra Nevada upper mantle from Vp/Vs anisotropy tomography

Geosphere ◽  
2019 ◽  
Vol 15 (6) ◽  
pp. 2018-2042 ◽  
Author(s):  
Melissa V. Bernardino ◽  
Craig H. Jones ◽  
William Levandowski ◽  
Ian Bastow ◽  
Thomas J. Owens ◽  
...  
Keyword(s):  
Sierra Nevada ◽  
Upper Mantle ◽  
Oceanic Lithosphere ◽  
P Wave ◽  
Wave Speeds ◽  
High Wave ◽  
The Core ◽  
High Speeds ◽  
Great Valley ◽  
Isabella Anomaly

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 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.

Improved inversion of geopotential field anomalies for lithospheric investigations

Geophysics ◽  
1988 ◽  
Vol 53 (3) ◽  
pp. 375-385 ◽  
Author(s):  
R. R. B. von Frese ◽  
D. N. Ravat ◽  
W. J. Hinze ◽  
C. A. McGue
Keyword(s):  
Large Scale ◽  
Superposition Principle ◽  
Gravity Anomalies ◽  
Damping Factor ◽  
Stable Model ◽  
Memory Integration ◽  
Unstable Solutions ◽  
Ill Posed ◽  
Anomaly Analysis ◽  
Magnetic And Gravity Anomalies

Instabilities and the large matrices which are common to inversions of regional magnetic and gravity anomalies often complicate the use of efficient least‐squares matrix procedures. Inversion stability profoundly affects anomaly analysis, and hence it must be considered in any application. Wildly varying or unstable solutions are the products of errors in the anomaly observations and the integrated effects of observation spacing, source spacing, elevation differences between sources and observations, geographic coordinate attributes, geomagnetic field attitudes, and other factors which influence the conditioning of inversion. Solution instabilities caused by ill‐posed parameters can be efficiently minimized by ridge regression with a damping factor large enough to stabilize the inversion, but small enough to produce an analytically useful solution. An effective choice for the damping factor is facilitated by plotting damping factors against residuals between observed and modeled anomalies and by then comparing this curve to curves of damping factors plotted against solution variance or the residuals between predicted anomaly maps representing the processing objective (e.g., downward continuation, differential reduction to the radial pole, etc.). To obtain accurate and efficient large‐scale inversions of anomaly data, a procedure based on the superposition principle of potential fields may be used. This method involves successive inversions of residuals between the observations and various stable model fields which can be readily accommodated by available computer memory. Integration of the model fields yields a well‐resolved representation of the observed anomalies corresponding to an integrated model which normally could not be obtained by direct inversion because the memory requirements would be excessive. MAGSAT magnetic anomaly inversions over India demonstrate the utility of these procedures for improving the geologic analysis of potential field anomalies.

A GRAVITY MAXIMUM IN THE GREAT VALLEY OF CALIFORNIA DUE TO THE ISOSTATIC EFFECT OF THE SIERRA NEVADA

Geophysics ◽  
1957 ◽  
Vol 22 (1) ◽  
pp. 62-66 ◽  
Author(s):  
L. F. Ivanhoe
Keyword(s):  
Sierra Nevada ◽  
Sedimentary Basin ◽  
Gravity Data ◽  
Full Length ◽  
Gravity Effect ◽  
Great Valley

A gravity maximum extends along the full length of the Great Valley of California. This feature is believed to represent the western limit of the gravity effect due to the adjustment of the Sierra Nevada isostatic block. Recognition that such a maximum with a sedimentary basin may be due to isostasy rather than to shallow geologic features assists in the interpretation of gravity data.

Regional, structural and strain analyses of terranes in the Western Metamorphic Belt, Central Sierra Nevada, California

1989 ◽  
Vol 11 (3) ◽  
pp. 255-273 ◽  
Author(s):  
Scott R Paterson ◽  
Othmar T Tobisch ◽  
Tapas Bhattacharyya
Keyword(s):  
Sierra Nevada ◽  
Metamorphic Belt
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