Estibalitz Ukar

Low-temperature blueschist-facies mafic blocks in the Franciscan mélange, San Simeon, California: Field relations, petrology, and counterclockwise P-T paths

Dozens of mafic blueschist blocks are found in the Franciscan melange, which is well exposed along 6+ km of nearly continuous sea cliffs and wave-cut benches near San Simeon, California. Thirty-four blocks were studied to discover all the varieties in this classic locality of melange. The Na-amphibole and lawsonite ± epidote–rich schists were dynamically deformed, folded, and veined (including lawsonite and aragonite) before becoming incorporated into the shale-matrix melange. Tectonically driven flowage of the matrix caused all lithologic components to pinch and swell, forming boudins that separated to become isolated ellipsoidal blocks. This megascopically ductile style of deformation was accommodated in mafic blocks by cataclasis concurrent with alteration to chlorite and pumpellyite. Shearing and alteration were most intense along block margins and within block faults. Petrographic analyses, aided by backscattered electron imaging, microprobe mineral analyses, bulk chemistry, and thermodynamic calculations, reveal these blueschists traveled along a counterclockwise pressure-temperature ( P - T ) path. Na-amphiboles have actinolitic cores that indicate an early greenschist-facies stage. Peak T and P conditions for the epidote-bearing blueschists were ∼350 °C at pressures of 5–9 kbar. Many blocks have textural evidence of lawsonite replacing epidote and Na-amphibole with Fe 3+ -rich rims. This indicates epidote replacement occurred as the rocks cooled below ∼250 °C at ∼5 kbar. It is evident that the phase of dynamic epidote-blueschist-facies recrystallization that nearly obliterated evidence of an earlier greenschist-facies stage was followed by retrograde recrystallization under much less dynamic to nearly static, but still high- P , conditions. Cooling and much of the observed veining must have happened after these rocks were deeply underplated along the bottom of the ophiolitic leading edge of the North American plate. Pieces of the underplated blueschist terrane were probably detached from the hanging wall as slabs that boudinaged and reboudinaged while entrained in shale-matrix melange upwelling from depths of at least 15 km.

First 40Ar-39Ar Ages from Low-T Mafic Blueschist Blocks in a Franciscan Mélange near San Simeon: Implications for Initiation of Subduction

The Franciscan Complex of California is a type example of an accretionary prism with widespread high-P/T subduction-zone metamorphism. Low-T, fine-grained, lawsonite-bearing mafic blueschists encased in a shale-matrix mélange near San Simeon have been dated for the first time. 40Ar-39Ar ages were obtained for phengite separated from four blocks and from an actinolitic rind on the margin of one of the blocks. The three blocks with lawsonite + epidote yield ages of between 154 and Ma, while the actinolitic rind yields an age of Ma. These ages are from the part of the Franciscan Complex west of the San Andreas Fault that moved northward at least 300 km with respect to the extensive Franciscan exposures east of the fault. The ages obtained in this study show that some fine-grained, low-T (lawsonite + epidote) Franciscan blueschist blocks formed at the same time as coarse-grained, high-T (garnet ± epidote) Franciscan blueschist blocks. These dated rocks indicate that both high- and low-T mafic blueschists are coeval and probably formed along 1000+ km of the North American Plate margin at ∼160–155 Ma. The episode of dynamic blueschist metamorphism was soon followed by high-Mg rind formation and a long period of nearly static high-P/T conditions.

Quartz c-axis orientation patterns in fracture cement as a measure of fracture opening rate and a validation tool for fracture pattern models

We evaluate a published model for crystal growth patterns in quartz cement in sandstone fractures by comparing crystal fracture-spanning predictions to quartz c-axis orientation distributions measured by electron backscatter diffraction (EBSD) of spanning quartz deposits. Samples from eight subvertical opening-mode fractures in four sandstone formations, the Jurassic–Cretaceous Nikanassin Formation, northwestern Alberta Foothills (Canada), Cretaceous Mesaverde Group (USA; Cozzette Sandstone Member of the Iles Formation), Piceance Basin, Colorado (USA), and upper Jurassic–lower Cretaceous Cotton Valley Group (Taylor sandstone) and overlying Travis Peak Formation, east Texas, have similar quartzose composition and grain size but contain fractures with different temperature histories and opening rates based on fluid inclusion assemblages and burial history. Spherical statistical analysis shows that, in agreement with model predictions, bridging crystals have a preferred orientation with c-axis orientations at a high angle to fracture walls. The second form of validation is for spanning potential that depends on the size of cut substrate grains. Using measured cut substrate grain sizes and c-axis orientations of spanning bridges, we calculated the required orientation for the smallest cut grain to span the maximum gap size and the required orientation of the crystal with the least spanning potential to form overgrowths that span across maximum measured gap sizes. We find that within a 10° error all spanning crystals conform to model predictions. Using crystals with the lowest spanning potential based on crystallographic orientation (c-axis parallel to fracture wall) and a temperature range for fracture opening measured from fluid inclusion assemblages, we calculate maximum fracture opening rates that allow crystals to span. These rates are comparable to those derived independently from fracture temperature histories based on burial history and multiple sequential fluid inclusion assemblages. Results support the R. Lander and S. Laubach model, which predicts that for quartz deposited synchronously with fracture opening, spanning potential, or likelihood of quartz deposits that are thick enough to span between fracture walls, depends on temperature history, fracture opening rate, size of opening increments, and size, mineralogy, and crystallographic orientation of substrates in the fracture wall (transected grains). Results suggest that EBSD maps, which can be more rapidly acquired than measurement of tens to hundreds of fluid inclusion assemblages, can provide a useful measure of relative opening rates within populations of quartz-filled fractures formed under sedimentary basin conditions. Such data are useful for evaluating fracture pattern development models.