Sample Characteristics
The La Calera pseudotachylyte sample (98109) was chosen to test whether existing 40Ar/39Ar ages of pseudotachylyte from this fault zone could be replicated using excimer laser-probe techniques. Two generations of pseudotachylyte are evident within the La Calera zone: a centimeters-thick, light brown pseudotachylyte with abundant quartz and feldspar-dominated lithic fragments (upper half of Figure 2b), and a dark brown, millimeters-thick pseudotachylyte that intrudes and crosscuts the thicker, light brown pseudotachylyte veins (center to lower right of (Figure 2b). The darker pseudotachylyte is visibly more homogenous with apparently fewer clasts and fragments and therefore was chosen for analysis. Lithic fragments within the darker pseudotachylyte include quartz±feldspar clasts and fragments of the light brown pseudotachylyte.
Pseudotachylyte samples from the Los Tuneles fault zone (9819, 9820) consist of dark, cryptocrystalline matrix with abundant cataclastic fragments of host rock. Clasts are predominantly quartz±feldspar, range from sub-microscopic to 500µm in diameter, and are uniformly distributed in center regions of pseudotachylyte veins (Figure 2c). Vein margins are composed of a much higher proportion of host-rock fragments (lower half of Figure 2c) and consequently were avoided in these analyses.
Ultramylonite samples (004, 006) contain a very fine-grained (~5-30µm) matrix of biotite±plagioclase±quartz, with 100-200µm, rounded porphyroblasts of plagioclase±garnet (Figure 2d). Close examination of the matrix fabric reveals mostly random, interlocking grains with little evidence of a well-defined foliation. 40Ar/39Ar analyses were performed on 10-20µm matrix biotite and avoided relict, coarser-grained biotite that occurs adjacent to, or as inclusions within, garnet porphyroblasts.