Abstract

The ca. 90 Ma Mount Givens pluton is one of the largest granodioritic to granitic intrusions in the Sierra Nevada batholith of California. Emplacement of the pluton occurred during a critical time in the tectonic evolution of the central Sierra Nevada magmatic arc, marked by a transition from regional contraction to dextral transcurrent shear. A model for the emplacement of the intrusion is developed based on detailed mapping of the pluton and its wall rocks and characterization of its internal structure by measurements of the anisotropy of magnetic susceptibility (AMS) at 351 stations. One of the key results of the study is documentation of a strong correlation between petrologic and structural fabrics in the pluton, and determination that these fabrics reflect internal magma chamber dynamics more than regional tectonic strain.

The ∼80-km-long, 15–30-km-wide pluton crystallized from a multiphase, three-segment magma chamber marked by a bulbous northern lobe and linear central and southern segments. The pluton is interpreted to be tabular in shape with a thickness of ∼5 km. Most of the space for the pluton was created by piecemeal block downdrop of the magma chamber floor along three principal fracture sets, the most important of which were steeply dipping, northwest-trending fractures formed parallel to the structural grain of the arc, and vertical, north-trending extension fractures formed in response to a component of arc-parallel dextral shear. Some of these fractures acted as magma conduits, episodically filling the pluton as source rocks became depleted in melt. An initial, voluminous intrusive event (stage 1) quickly filled the southern chamber with granodiorite magma, but only partially filled the northern and central chambers. Stage 2 magmatism involved underplating of megacrystic granite in the northern chamber and lateral flow of a large batch of this magma from the northern to the central chamber, the latter delineated by a 20-km-long belt of megacrystic granite containing subhorizontal magnetic lineations that connects the pluton segments. Floor downdrop eventually ceased to be an effective space-making process in the northern lobe, and renewed magmatism (stage 3) led to expansion and doming of the chamber. As the northern lobe cooled, a ring fault ruptured within the viscoelastic stage 1–2 carapace, allowing ring dike intrusion (stage 4) and sinking of a central flap of consolidated material. The temporal and spatial variations in emplacement mechanisms demonstrated for the Mount Givens pluton (i.e., fracture generation, floor downdrop, underplating, inflation, ring diking) suggest that end-member models (e.g., fracture vs. diapir) are oversimplifications of the pluton assembly process.

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