Variations in deformation fields during development of a large- volume magmatic arc, central Sierra Nevada, California Othmar T. Tobisch Earth Sciences Department, University of California, Santa Cruz, California 95064 Jason B. Saleeby Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 94115 Paul R. Renne* Institute of Human Origins, Geochronology Department, 2453 Ridge Road, Berkeley, California 94709 Brendan McNulty } Earth Science Department, University of California, Santa Cruz, California 95064 Weixing Tong 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 plu- tons and expected strain rates (10 14 ) sug- gest 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 plu- tons, on the other hand, indicate a contrac- tional regime. Timing of pluton emplace- ment and movement on the shear zones have been constrained using Pb-U (zircon) and 40 Ar/ 39 Ar (hornblende and biotite) geo- chronology. 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 defor- mation pattern in which strain regimes fluc- tuated in time and space between neutral or weak extensional and contractional. We propose a tectonic model in which astheno- spheric 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 move- ment of granitic magma to mid- and upper levels, probably as dikes via fractures. Slab flattening caused the neutral or weak exten- sional regime to move eastward away from the trench. Increased coupling between up- per and lower plates induced by the slab flattening promoted contractional strain in the cooling plutons, and domains of ductile shear formed in progressively younger plu- tons 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 esti- mate that at ca. 100 Ma the convergence vector made an angle ( obl )  20 to the arc normal, and we suggest that around ca. 90 Ma  obl passed through a critical value, conceivably (20 <  oblcrit < 30). At this juncture, the component of right-lateral shear became sufficiently large to induce significant arc-parallel strike-slip move- ment on the most easterly shear zones; these kinematics continued as the dominant scheme, possibly as late as ca. 78 Ma. INTRODUCTION The deformation environment of mag- matic arcs has been interpreted as varying from extensional to neutral to contractional (e.g., Dewey, 1980; Uyeda, 1982; Hamilton, 1988), mostly as a function of dip of the downgoing slab between 0 and 100 km depth, convergence rate, slab age and/or ab- solute motion, and other less influential fac- tors (Jarrard, 1986). It is clear that contrac- tional structures form in the accretionary wedge and are present in some forearc and backarc settings; however, whether the arc is continuously under extension or contraction during subduction over geologic time has re- mained uncertain. Further, conditions and/ or models applicable to the oceanic-arc environment may not hold for their conti- nental equivalents. Indeed, variable-rate or intermittent convergence, changing micro- plate geometry and kinematics, thickness variations of the crust, and the volume, lo- cation, and paths of magma being generated in the arc provide mechanisms by which the style and kinematics of deformation within a magmatic arc might vary in time and space. Reconstructing the dynamic environment in ancient arcs is difficult, therefore, because rapidly changing deformation regimes are likely to overprint each other as the arc develops. The Sierra Nevada represents an 600- km-long segment of a very long continental magmatic arc stretching discontinuously from southeastern Alaska to Baja California and characterized by substantial emplace- ment of granitic magma during middle to Late Cretaceous time. Over the past quarter century, extensive work by geologists from various sectors (but principally the U.S. Ge- ological Survey) have elucidated the gross nature and many detailed aspects of the Si- erra Nevada portion of this magmatic arc (Fig. 1; summarized by Bateman, 1992). Some of the major observations that came from this effort are relevant to the present paper: (1) the Cretaceous (ca. 120 – 85 Ma) batholith in the central Sierra Nevada (CSN) is a large-volume magmatic arc: in the CSN alone, an estimated minimum vol- ume of granitic magma is 4  10 5 km 3 ; (2) the thermal axis of the magmatic arc gener- ally migrated eastward at an average rate of 2.7 mm/yr (Chen and Moore, 1982; Stern et al., 1981); (3) there is widespread preser- vation of magmatic foliation in individual *Present address: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709. GSA Bulletin; February 1995; v. 107; no. 2; p. 148–166; 10 figures; 3 tables. 148