On the origin of mixed-layered clay minerals from the San Andreas Fault at 2.5–3 km vertical depth (SAFOD drillhole at Parkfield,California) |
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| Authors: | A M Schleicher L N Warr B A van der Pluijm |
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| Institution: | 1.Geozentrum Nordbayern,Universit?t Erlangen-Nürnberg,Erlangen,Germany;2.Institut für Geographie und Geologie,Ernst-Moritz-Arndt-Universit?t Greifswald,Greifswald,Germany;3.Department of Geological Sciences,University of Michigan,Ann Arbor,USA |
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| Abstract: | A detailed mineralogical study is presented of the matrix of mudrocks sampled from spot coring at three key locations along
the San Andreas Fault Observatory at depth (SAFOD) drill hole. The characteristics of authigenic illite–smectite (I–S) and
chlorite–smectite (C–S) mixed-layer mineral clays indicate a deep diagenetic origin. A randomly ordered I–S mineral with ca.
20–25% smectite layers is one of the dominant authigenic clay species across the San Andreas Fault zone (sampled at 3,066
and 3,436 m measured depths/MD), whereas an authigenic illite with ca. 2–5% smectite layers is the dominant phase beneath
the fault (sampled at 3,992 m MD). The most smectite-rich mixed-layered assemblage with the highest water content occurs in
the actively deforming creep zone at ca. 3,300–3,353 m (true vertical depth of ca. 2.7 km), with I–S (70:30) and C–S (50:50).
The matrix of all mudrock samples show extensive quartz and feldspar (both plagioclase and K-feldspar) dissolution associated
with the crystallization of pore-filling clay minerals. However, the effect of rock deformation in the matrix appears only
minor, with weak flattening fabrics defined largely by kinked and fractured mica grains. Adopting available kinetic models
for the crystallization of I–S in burial sedimentary environments and the current borehole depths and thermal structure, the
conditions and timing of I–S growth can be evaluated. Assuming a typical K+ concentration of 100–200 ppm for sedimentary brines, a present-day geothermal gradient of 35°C/km and a borehole temperature
of ca. 112°C for the sampled depths, most of the I–S minerals can be predicted to have formed over the last 4–11 Ma and are
probably still in equilibrium with circulating fluids. The exception to this simple burial pattern is the occurrence of the
mixed layered phases with higher smectite content than predicted by the burial model. These minerals, which characterize the
actively creeping section of the fault and local thin film clay coating on polished brittle slip surfaces, can be explained
by the influence of either cooler fluids circulating along this segment of the fault or the flow of K+-depleted brines. |
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| Keywords: | SAFOD Illite– smectite Mixed-layered clays San Andreas Fault |
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