TILTING, BURIAL, AND UPLIFT OF THE GUADALUPE IGNEOUS COMPLEX, SIERRA-NEVADA, CALIFORNIA

It is often incorrectly assumed that plutons have a relatively unevent ful structural history after emplacement. The 151 Ma Guadalupe Igneous Complex (GIC) in the Foothills Terrane, California, was involved in t hree post-emplacement events: (1) approximately 30-degrees of southwes t-side-up tilting during ductile regional faulting and contraction, (2 ) burial of the pluton from approximately 4 to 12 km during crustal th ickening of the wall rocks, and (3) uplift with only minor tilting in the late Cretaceous. Tilt of the pluton is indicated by (1) southwest to northeast gradational changes from layered gabbros and diorites to granules and granophyres; (2) northeastward dips of layering in gabbro , internal contacts, and bedding of overlying coeval(?) volcanic rocks ; (3) northeastward decrease in wall-rock metamorphic grade, and (4) p aleomagnetic data from 14 localities across the pluton. We argue that tilt occurred between 146-135 Ma during southwest-northeast-directed r egional contraction. This contraction is indicated by widespread folds and cleavages and by reverse motion on the Bear Mountains fault zone (BMFZ), a large northeast-dipping shear zone that bounds the GIC on it s southwest side. Burial of the GIC, which overlapped in time but outl asted tilting, is suggested by (1) post-emplacement contractional faul ting, folding, and cleavage development; (2) analyses of strains assoc iated with widespread cleavage that indicate shallow thickening of app roximately 100%; and (3) microstructural and mineral assemblage data t hat indicate shallow emplacement of the GIC, in contrast to mineral as semblage and limited geobarometric data from adjacent 120-110 Ma pluto ns that indicate moderate emplacement levels. Late Cretaceous uplift i s indicated by 95-75 Ma sedimentary rocks that unconformably overlie t he 120-110 Ma plutons. This geologic history is interesting for severa l reasons. First, although the GIC participated in extensive post-empl acement deformation, it lacks internal structural evidence of these ev ents, except locally along the Bear Mountains fault zone. Second, the agreement between paleomagnetic and structural evidence for tilting su ggests that no large latitudinal displacement of the GIC is required. Third, the paleomagnetic data also help to define the geometry of the magma chamber now represented by the GIC. Lack of streaking of paleoma gnetic site-mean directions demonstrates that the pluton acted as a si ngle unit after cooling through the blocking temperature (450-560-degr ees-C) of low-titanium titanomagnetite; however, variations in the dip of internal layering and contacts, from 70-degrees at the base to 30- degrees near the top of the pluton, indicate that not all of these fea tures were horizontal and planar when they formed. We propose that thi s variation in dip of layering is most consistent with sidewall crysta llization of magma resulting in drape of layering along the walls of t he intrusion. Therefore, internal layering within this pluton does not record paleohorizontal.