Development of Mylonites and Pseudotachylites in Granitic Bodies: Brittle Deformation by Reactivation of Pre-existing Ductile Fabrics Defined by Mica

Abstract:

Pseudotachylites are common in brittle deformation zones and represent rapidly crystallized melts produced by friction-induced heating, possibly associated with dynamic rupture and slip during earthquakes. Thus, pseudotachylites may provide insight into how such localized deformation occurs. Here, we investigated the micron to cm scale structure and geochemistry of granitic plutons in the Eastern Peninsular Ranges shear zone (California), which is a large scale thrust rooted in the lower crust. Our U-Pb chronology and Zr thermometry of sphene porphyroblasts in the mylonite indicate that the plutons were deformed at near solidus conditions (780-750 ^◦C) between 89 and 78 Ma, which coincides with the last pulse of magmatism. This ductile deformation resulted in a foliated fabric defined by biotite-rich layers. Subsequent brittle deformation is superimposed on this ductile fabric as exemplified by pseudotachylites and ultracataclasites cross-cutting or parallel to the foliation planes. The pseudotachylites are characterized by a glassy matrix with microlites, and the ultracataclasites are characterized by a fine-grained matrix of angular mineral and rock fragments. We measured major and trace compositions of these brittle deformation products using LA-ICPMS. The pseudotachylites are more mafic (lower Si, but higher Fe, Ti and K) than the host granitoid, while the ultracataclasites are intermediate between the pseudotachylites and the host. The mafic compositions of the pseudotachylites indicate that they form almost solely by melting of biotite. The compositions of the ultracastaclasites also suggest preferential involvement of biotite, but with a greater component of feldspar.

The compositional discrepancy between the brittle deformation products and the host granitoid begs the question of why brittle deformation appears to preferentially involve biotite. One possibility is that brittle deformation occurred on weak zones inherited from the biotite-rich mylonitic foliation planes. Our observations indicate that such ductile deformation occurred at near-solidus conditions and may have been associated with the assembly of the Cretaceous magmatic arc. We suggest that brittle faulting inherits pre-existing ductile fabrics that, once formed, define long-lived weak shear zones in the crust.