The Papoose Flat Pluton of eastern California: a reassessment of its emplacement history in the light of new microstructural and crystallographic fabric observations

Abstract

One of the most outstanding apparent examples in N America of a forcibly emplaced pluton is the Papoose Flat Pluton of eastern California. Sideways expansion of this granitic pluton, during emplacement into a series of Cambrian shelf strata, has been regarded by early workers as resulting in the observed intense crystal plastic deformation of the pluton's mylonitic border facies and surrounding country rocks. This deformation is evidenced by up to 90% thinning of individual stratigraphic layers within the pluton's metamorphic aureole, although such intense penetrative deformation of the country rocks is not observed outside the aureole.Previously published quartz c-axis fabrics associated with this deformation (and presented on projection planes oriented perpendicular to lineation) were interpreted as being symmetrical with respect to foliation and lineation, implying almost coaxial deformation histories. Such fabrics could be interpreted as indicating that the pluton evolved by "ballooning" as a result of new magma being intruded into its core during emplacement. However, a major problem with applying the strict ballooning model to the Papoose Flat Pluton is that while oblate strains would be expected to develop in association with a ballooning mechanism, the mylonitic rocks of this elongate WNW-ESE-trending pluton and its aureole are characterised by both a strongly developed foliation, which is concordant with the pluton's margin, and an intense, NW-SE trending, shallow plunging stretching lineation.Previously published fabrics from the Papoose Flat Pluton and its metamorphic aureole have been rotated on to a projection plane oriented parallel to lineation and perpendicular to foliation. Examination of the fabrics in this projection plane has revealed that they are in fact dominantly asymmetric, and that a constant sense of asymmetry is detected across the pluton, suggesting a consistent (top-to-the-SE) shear-sense. This new interpretation is strongly supported by microstructural and petrofabric analysis of additional L-S tectonites collected, during recent fieldwork, from both the aureole and quartz veins within the pluton's gneissic border facies. Thus mylonite formation around the Papoose Flat Pluton could have involved large-scale consistently oriented translation and associated shearing, rather than passive "blister-like" coaxial deformation associated with pluton ballooning. It should be noted that mylonitic deformation is restricted to the western half of the pluton, features indicative of a more "permitted" emplacement mechanism being found in the eastern portion of the pluton.The detected top-to-the-SE shear-sense could be interpreted as indicating that the granitic material forming the western part of the pluton was forcibly intruded in a northwestward direction from the pluton source as a nearly solidified wedge beneath a static cover of sedimentary rocks. Alternatively, the detected shear sense could also be interpreted as indicating SE-directed thrusting of the cover rocks over the unde...

Microstructures and quartz lattice‐preferred orientations in the eclogite‐bearing migmatitic gneisses of the D'Entrecasteaux Islands, Papua New Guinea

“…Considered in isolation, this predicts the wrong shear sense. We note that our observations on the relationship between on oblique quartz GSFs and deformation bands are remarkably similar to those made by Law et al [1992] in quartz veins in high-temperature gneissic rocks near the Papoose Flat pluton in eastern California (see especially their Figures 5b and 7). [38] Our EBSD data indicate that the oblique GSFs in recrystallized quartz aggregates are parallel to, and apparently controlled by, the orientation of deformation bands in that mineral, which, in turn are controlled by the LPO, which includes a unimodal concentration of [c] axes near X.…”

“…We note that our observations on the relationship between on oblique quartz GSFs and deformation bands are remarkably similar to those made by Law et al . [] in quartz veins in high‐temperature gneissic rocks near the Papoose Flat pluton in eastern California (see especially their Figures b and ).…”

Out-of-sequence deformation and expansion of the Himalayan orogenic wedge: insight from the Changgo culmination, south central Tibet

“…The opening angle of cross‐girdled quartz c axis fabrics can be related to the temperature at which the fabrics were developed [ Tullis et al , 1973; Lister and Hobbs , 1979; Lister and Dornsiepen , 1982; Kruhl , 1998; Law et al , 1992, 2004]. In the presence of a stable critical resolved shear stress during plastic deformation and dynamic crystallization and limited hydrolytic weakening, increased temperature results in an increased opening angle of quartz c axis fabrics.…”

Fabric transitions in quartz via viscoplastic self-consistent modeling part I: Axial compression and simple shear under constant strain

“…Bunge and Wenk, 1977;Law et al, 1992;Lister and Dornsiepen, 1982;Pennacchioni et al, 2010;Schmid and Casey, 1986;Schmid et al, 1981;Toy et al, 2008). In most cases, the Y-maxima fabric is attributed to a predominant activation of prismatic baN slip (e.g.…”