EVALUATING COUPLED INTERNAL GROWTH AND HOST ROCK DISPLACEMENT MODELS FOR THE TUOLUMNE INTRUSIVE COMPLEX, SIERRA NEVADA, CA

Growth models proposed for the ~1100 km², 12-40 km wide, 6-10 km deep, 95-85 Ma, Tuolumne Intrusive Complex (TIC) include: (a) vertical diking requiring W-E horizontal displacement of host; (b) horizontal sill emplacement requiring vertical host rock displacement and folding of sills; (c) fault space-making, and (d) early vertical sheeting evolving to active chambers and/or nesting of diapirs during downward movement of host rock. Both internal and host rock features must be evaluated to test these models. In host rocks, basement terrane boundaries, lithotectonic packages, unconformities, faults, contacts, bedding, and solid-state foliations define pre-emplacement markers in Mesozoic strata that are undeflected as they approach to <1 km of the TIC. A narrow (0.1 to 0.03 TIC radii) discontinuous structural aureole is present around the TIC in which common discordant, stepped margins bound regions of rare melting, magma wedging, stoping, local rotation of pendant structures and minor emplacement related ductile deformation. Solid-state deformation is largely absent at northern and southern margins, is present along the western Kuna Crest margin and is common in a shear zone along the eastern TIC margin.

Internally, the TIC’s “hour-glass” shape with protruding lobes or bulges, common sheeting in peripheral units, complex inward compositional zoning, inward and NE younging ages, sharp to gradational internal contacts, 1000’s of clustered, compositionally defined magmatic structures with local outward younging or migration directions, a widespread steeply plunging magmatic lineation, patterns of 2 localized and 3 regional magmatic foliations and a complex mineral geochemical record (Memeti et al., this session) provide an exceptional opportunity to test growth models for this batholith.

These observations rule out significant: (1) “space-making” by regional faults, (2) W-E displacements (no regional deflections or shear zones along S and N margins), (3) ductile expansion and (4) significant vertical uplift (no ductile or brittle shear zones along most margins). Internal patterns do not support formation nor folding of sills, provide numerous challenges for vertical dike models and require a dynamic, multi-stage evolution of TIC magmas linked to lower and higher crustal level magma systems.