FRACTURE-CONTROLLED MAGMA CONDUITS IN AN OBLIQUELY CONVERGENT CONTINENTAL MAGMATIC ARC

Magnetic fabric patterns of two mid-Cretaceous nested plutons (102 +/- 1 and 96 +/- 3 Ma) in the central Sierra Nevada batholith provide evi dence that felsic magma emplacement (and ascent?) occurred via north-t rending, steeply dipping, planar fracture conduits oriented obliquely to the arc. U-Pb geochronology data indicate that emplacement of the p lutons was separated by 2 to 10 m.y. and that they were emplaced in pa rt via the same conduit. Magnetic fabrics in the younger pluton are re lated to its final emplacement, which was strongly influenced by a sys tem of host-rock joints. Formation of the north-trending conduits at s imilar to 100 Ma can be related to a small, arc-parallel, dextral-shea r component that produced tension fractures and that was associated wi th high-angle oblique convergence. By similar to 90 Ma, convergence ha d become sufficiently oblique for the development of nearly arc-parall el structures, which were then favored as magma conduits.