Richard D. Law

Internal structure and emplacement of the Papoose Flat pluton: An integrated structural, petrographic, and magnetic susceptibility study

The Papoose Flat pluton of eastern California is commonly cited in the geologic literature as a classic example of a “forcibly” emplaced pluton, although the relative importance attributed to magmatic versus tectonic processes in controlling the structural evolution of the pluton has been controversial. Reexamination of this Late Cretaceous (83 Ma) pluton, using a combination of new structural, petrographic, microstructural, and anisotropy of magnetic susceptibility (AMS) data on the pluton–wall-rock system, has shown that the pluton is an inclined and internally zoned tabular structure, assembled by forcible intrusion of successive pulses of magma at a crustal depth of 12–15 km. Initial pluton formation involved magma ascent in a vertical west- northwest–striking feeder dike, which was arrested at a stratigraphically controlled mechanical discontinuity in the overlying Cambrian metasedimentary rocks, leading to formation of a southwest-dipping sill. Subsequent sill inflation, accompanied by horizontal infilling from the feeder dike at the base of the sill, resulted in deformation and vertical translation of previously emplaced magma pulses and local raising of the sill roof, facilitated by thermal weakening as the wall-rock temperatures progressively rose during emplacement of successive magma pulses. Cooling from the roof of the pluton downward resulted in cessation of vertical inflation on the west side of the pluton and promoted lateral expansion toward the northeast and floor depression below the eastern part of the pluton. We have been unable to document any regional-scale structures (e.g., of equivalence to similar-age synplutonic strike-slip shear zones in the Sierra Nevada batholith to the west) that may have controlled emplacement of the Papoose Flat pluton. However, this fact does not preclude the likelihood that the country rocks were subjected to a regional deviatoric stress field at this time; indeed, tectonic overpressuring was probably essential for producing sufficiently high magma pressures for laccolith formation at this midcrustal level. Simple thermal modeling, using microstructural and thermobarometric data, indicates that the total duration of emplacement of the pluton did not exceed 30u2009000 yr. This rapid emplacement rate may explain why the pluton appears to be anorogenic even though it was emplaced during a period of regional- scale deformation.

Laccolith-like emplacement model for the Papoose Flat pluton based on porphyroblast-matrix analysis

Detailed porphyroblast-matrix analysis within the concordant metasedimentary aureole rocks surrounding the Papoose Flat pluton of eastern California indicates that inclusion trails within porphyroblasts can be used as strain markers to restore the aureole rocks to their prepluton emplacement position. Using porphyroblast-matrix relationships in combination with measurement of stratigraphic sections and whole-rock geochemical analyses, we have determined the kinematics of rotation, the change in thickness and volume, and the amount of translation of the metasedimentary formations within the aureole. These data are consistent with initial emplacement of the magma as an inclined sill and subsequent inflation into a pluton or laccolith. The combination of structural and porphyroblast-matrix analysis leads to a three-dimensional kinematic history of the wall rocks wherein vertical upward translation represents a significant part of the pluton emplacement-related strain history.

Incremental assembly and emplacement of Mesozoic plutons in the Sierra Nevada and White and Inyo ranges, California

This field guide was created in coordination with the Geological Society of America Field Forum “Rethinking the Assembly and Evolution of Plutons: Field Tests and Perspectives,” held 7-14 October 2005 in the Sierra Nevada and White and Inyo ranges, California. The goal of this five-day field trip was to examine field relations and characteristics of plutons in the central Sierra Nevada and in the White and Inyo ranges as they relate to processes of pluton growth and emplacement and, more particularly, as they relate to the hypothesis that plutons are assembled slowly and incrementally.

Emplacement of the Santa Rita Flat pluton as a pluton-scale saddle reef

Regional mapping indicates that the Middle Jurassic Santa Rita Flat pluton, exposed in the Inyo Range of eastern California, is situated within the core of a south-plunging synform defined by bedding in the surrounding metasedimentary wall rocks, which dip beneath the pluton. However, bedding in rocks preserved above the pluton defines a south-plunging antiform. Anisotropy of magnetic susceptibility analysis revealed that magnetic foliation within the pluton also defines a south-plunging antiform, and the accompanying magnetic lineation plunges subparallel to the axis of this antiform. Our data indicate that the pluton was initially intruded as a sill in the hinge zone of the enveloping synform. Subsequent vertical inflation of the sill resulted in upward doming of the overlying roof and formation of the antiform now observed at the current erosion level in the pluton and preserved roof rocks. Emplacement of the pluton at 164 ± 1.5 Ma (U-Pb zircon age) overlaps in time with regional thrust faulting and folding ca. 185–148 Ma recognized in the southern Inyo Range. We speculate that space for initial emplacement of the pluton was produced during folding by layer-parallel slip and hinge-zone dilation, producing a saddle reef-like structure.