VARIATIONS IN DEFORMATION FIELDS DURING DEVELOPMENT OF A LARGE-VOLUMEMAGMATIC ARC, CENTRAL SIERRA-NEVADA, CALIFORNIA

Mid- to Late Cretaceous plutons in the central Sierra Nevada magmatic are show widely preserved magmatic foliation, whereas regionally devel oped 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. D omains of solid-state ductile shear of only slightly younger age than the plutons, on the other hand, indicate a contractional regime. Timin g of pluton emplacement and movement on the shear zones have been cons trained using Pb-U (zircon) and Ar-40/Ar-39 (hornblende and biotite) g eochronology. Both plutons and ductile shear zones become younger towa rd the east. The four more westerly shear zones, which were active bet ween ca. 100 and 90 Ma, show steeply plunging stretching lineations, w hereas the most easterly and/or youngest zones, active between ca. 88 and 78 Ma, show mostly oblique and/or subhorizontal stretching lineati ons, indicating a change in kinematics at ca. 90 Ma. The above events define a complex deformation pattern in which strain regimes fluctuate d in time and space between neutral or weak extensional and contractio nal. We propose a tectonic model in which asthenospheric mantle corner flow produced eddy pairs in the mantle corner that transmitted a neut ral or weak extensional regime to the overlying crust and facilitated the movement of granitic magma to mid- and upper levels, probably as d ikes via fractures. Slab flattening caused the neutral or weak extensi onal 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 coolig plutons, and domains of ductile sh ear formed in progressively younger plutons to the east. The above eve nts were accompanied by an oblique convergence vector between North Am erica and Farallon plates (Engebretson et al., 1985), which imposed a relatively small component of right-lateral shear onto the arc that in creased with time. We estimate that at ca. 100 Ma the convergence vect or made an angle (phi(obi)) almost-equal-to 20-degrees to the arc norm al, and we suggest that around ca. 90 Ma phi(obi) passed through a cri tical value, conceivably (20-degrees < phi(oblcrit) < 30-degrees). At this juncture, the component of right-lateral shear became sufficientl y 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.