Giuseppe
Emeritus
Low strength of deep San Andreas fault gouge from SAFOD core (Nature Publishing Group, abstract, edited)
[Source: Nature, full text: <cite cite="http://www.nature.com/nature/journal/v472/n7341/full/nature09927.html">Low strength of deep San Andreas fault gouge from SAFOD core : Nature : Nature Publishing Group</cite>. Abstract, edited.]
Low strength of deep San Andreas fault gouge from SAFOD core
* David A. Lockner,1 * Carolyn Morrow,1 * Diane Moore 1 * & Stephen Hickman 1 *
Journal name: Nature, Volume: 472, Pages: 82?85
Date published: (07 April 2011)
DOI: doi:10.1038/nature09927
Received 19 October 2010
Accepted 31 January 2011
Published online 23 March 2011
The San Andreas fault accommodates 28?34 mm yr−1 of right lateral motion of the Pacific crustal plate northwestward past the North American plate. In California, the fault is composed of two distinct locked segments that have produced great earthquakes in historical times, separated by a 150-km-long creeping zone. The San Andreas Fault Observatory at Depth (SAFOD) is a scientific borehole located northwest of Parkfield, California, near the southern end of the creeping zone. Core was recovered from across the actively deforming San Andreas fault at a vertical depth of 2.7 km (ref. 1). Here we report laboratory strength measurements of these fault core materials at in situ conditions, demonstrating that at this locality and this depth the San Andreas fault is profoundly weak (coefficient of friction, 0.15) owing to the presence of the smectite clay mineral saponite, which is one of the weakest phyllosilicates known. This Mg-rich clay is the low-temperature product of metasomatic reactions between the quartzofeldspathic wall rocks and serpentinite blocks in the fault2, 3. These findings provide strong evidence that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms1. The combination of these measurements of fault core strength with borehole observations1, 4, 5 yields a self-consistent picture of the stress state of the San Andreas fault at the SAFOD site, in which the fault is intrinsically weak in an otherwise strong crust.
Affiliations
1. US Geological Survey, Menlo Park, California 94025, USA * David A. Lockner, * Carolyn Morrow, * Diane Moore & * Stephen Hickman
Contributions
C.M. and D.A.L. conducted deformation experiments, D.M. provided petrographic analysis and XRD measurements, and S.H. provided borehole logging results and related stress analysis.
Competing financial interests
The authors declare no competing financial interests.
Corresponding author
Correspondence to: * David A. Lockner
-
------<cite cite="http://www.nature.com/nature/journal/v472/n7341/full/nature09927.html"></cite>
[Source: Nature, full text: <cite cite="http://www.nature.com/nature/journal/v472/n7341/full/nature09927.html">Low strength of deep San Andreas fault gouge from SAFOD core : Nature : Nature Publishing Group</cite>. Abstract, edited.]
Low strength of deep San Andreas fault gouge from SAFOD core
* David A. Lockner,1 * Carolyn Morrow,1 * Diane Moore 1 * & Stephen Hickman 1 *
Journal name: Nature, Volume: 472, Pages: 82?85
Date published: (07 April 2011)
DOI: doi:10.1038/nature09927
Received 19 October 2010
Accepted 31 January 2011
Published online 23 March 2011
The San Andreas fault accommodates 28?34 mm yr−1 of right lateral motion of the Pacific crustal plate northwestward past the North American plate. In California, the fault is composed of two distinct locked segments that have produced great earthquakes in historical times, separated by a 150-km-long creeping zone. The San Andreas Fault Observatory at Depth (SAFOD) is a scientific borehole located northwest of Parkfield, California, near the southern end of the creeping zone. Core was recovered from across the actively deforming San Andreas fault at a vertical depth of 2.7 km (ref. 1). Here we report laboratory strength measurements of these fault core materials at in situ conditions, demonstrating that at this locality and this depth the San Andreas fault is profoundly weak (coefficient of friction, 0.15) owing to the presence of the smectite clay mineral saponite, which is one of the weakest phyllosilicates known. This Mg-rich clay is the low-temperature product of metasomatic reactions between the quartzofeldspathic wall rocks and serpentinite blocks in the fault2, 3. These findings provide strong evidence that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms1. The combination of these measurements of fault core strength with borehole observations1, 4, 5 yields a self-consistent picture of the stress state of the San Andreas fault at the SAFOD site, in which the fault is intrinsically weak in an otherwise strong crust.
Affiliations
1. US Geological Survey, Menlo Park, California 94025, USA * David A. Lockner, * Carolyn Morrow, * Diane Moore & * Stephen Hickman
Contributions
C.M. and D.A.L. conducted deformation experiments, D.M. provided petrographic analysis and XRD measurements, and S.H. provided borehole logging results and related stress analysis.
Competing financial interests
The authors declare no competing financial interests.
Corresponding author
Correspondence to: * David A. Lockner
-
------<cite cite="http://www.nature.com/nature/journal/v472/n7341/full/nature09927.html"></cite>