Variations in deformation fields during development of a large-volume magmatic arc, central Sierra Nevada, California

Abstract

Mid- to Late Cretaceous plutons in the central Sierra Nevada magmatic arc show widely preserved magmatic foliation, whereas regionally developed solid-state foliation is absent. Relatively slow cooling of these plutons and expected strain rates (10^(−14)) suggest that the plutons were emplaced in a neutral or weakly extensional deformation regime. Domains of solid-state ductile shear of only slightly younger age than the plutons, on the other hand, indicate a contractional regime. Timing of pluton emplacement and movement on the shear zones have been constrained using Pb-U (zircon) and ^(40)Ar/^(39)Ar (hornblende and biotite) geochronology. Both plutons and ductile shear zones become younger toward the east. The four more westerly shear zones, which were active between ca. 100 and 90 Ma, show steeply plunging stretching lineations, whereas the most easterly and/or youngest zones, active between ca. 88 and 78 Ma, show mostly oblique and/or subhorizontal stretching lineations, indicating a change in kinematics at ca. 90 Ma. The above events define a complex deformation pattern in which strain regimes fluctuated in time and space between neutral or weak extensional and contractional. We propose a tectonic model in which thenospheric mantle corner flow produced eddy pairs in the mantle corner that transmitted a neutral or weak extensional regime to the overlying crust and facilitated the movement of granitic magma to mid- and upper levels, probably as dikes via fractures. Slab flattening caused the neutral or weak extensional regime to move eastward away from the trench. Increased coupling between upper and lower plates induced by the slab flattening promoted contractional strain in the cooling plutons, and domains of ductile shear formed in progressively younger plutons to the east. The above events were accompanied by an oblique convergence vector between North America and Farallon plates (Engebretson et al., 1985), which imposed a relatively small component of right-lateral shear onto the arc that increased with time. We estimate that at ca. 100 Ma the convergence vector made an angle (Φ_(obl)) ≈ 20° to the arc normal, and we suggest that around ca. 90 Ma Φ_(obl) passed through a critical value, conceivably (20° < Φ_(oblcrit) < 30°). At this juncture, the component of right-lateral shear became sufficiently large to induce significant arc-parallel strike-slip movement on the most easterly shear zones; these kinematics continued as the dominant scheme, possibly as late as ca. 78 Ma.

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