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TitleA large mantle water source for the northern San Andreas fault system: A ghost of subduction past
AuthorKirby, S H; Wang, KORCID logo; Brocher, T M
SourceEarth, Planets and Space vol. 66, 2014 p. 1-18, Open Access logo Open Access
Alt SeriesEarth Sciences Sector, Contribution Series 20140092
PublisherSpringer Nature
Mediapaper; on-line; digital
File formatpdf
AreaSan Andreas fault; United States of America
Lat/Long WENS-125.0000 -120.0000 42.0000 35.0000
Subjectstectonics; igneous and metamorphic petrology; geophysics; subduction; subduction zones; mantle; serpentinites; lizardite; antigorite; chrysotile; ophiolites; seismicity; seismic interpretations; San Andreas Fault System
Illustrationslocation maps; profiles; plots; photographs; schematic diagrams
ProgramPublic Safety Geoscience Western Canada Geohazards Project
Released2014 07 07
AbstractRecent research indicates that the shallow mantle of the Cascadia subduction margin under near-coastal Pacific Northwest, USA is cold and partially serpentinized, storing large quantities of water in this wedge-shaped region. Such a wedge probably formed to the south in California during an earlier period of subduction. We show by numerical modeling that after subduction ceased with the creation of the San Andreas Fault System (SAFS), the mantle wedge warmed, slowly releasing its water over a period of more than 25 Ma by serpentine dehydration into the crust above. This deep, long-term water source could facilitate fault slip in San Andreas System at low shear stresses by raising pore pressures in a broad region above the wedge. Moreover, the location and breadth of the water release from this model gives insights into the position and breadth of the SAFS. Such a mantle source of water also likely plays a role in the occurrence of non-volcanic tremor (NVT) that has been reported along the SAFS in central California. This process of water release from mantle depths could also mobilize mantle serpentinite from the wedge above the dehydration front, permitting upward emplacement of serpentinite bodies by faulting or by diapiric ascent. Specimens of serpentinite collected from tectonically emplaced serpentinite blocks along the SAFS show mineralogical and structural evidence of high fluid pressures during ascent from depth. Serpentinite dehydration may also lead to tectonic mobility along other plate boundaries that succeed subduction, such as other continental transforms, collision zones, or along present-day subduction zones where spreading centers are subducting.
Summary(Plain Language Summary, not published)
The San Andreas Fault (SAF) in California is a well-known example of plate boundary faults that are very weak and generate large earthquakes. The weakness is widely thought to be due to the presence of fluids that "lubricate" the fault, but the source of the fluids has been an elusive subject, because meteroic water does not penetrate to seismogenic depths. In this paper, we use a numerical model to demonstrate the presence of a deep-seated long-lasting fluid source. The strike-slip SAF system evolved from a subduction zone over the past few tens of millions of years. The mantle wedge in that ancient subduction zone was hydrated and held a large amount of water in hydrous minerals (serpentine), just like our modern Cascadia subduction zone today. Since the cessation of subduction, the system has been warning up causing the hydrous minerals to break down to release fluids. Today, fluids are still being released from this ancient "reservoir". The role of fluids in weakening faults and facilitating earthquakes is an important issue of earthquake science.

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