Franciscan geologic history constrained by tectonic/olistostromal high-grade metamafic blocks in the iconic California Mesozoic-Cenozoic accretionary complex

Abstract

Subduction generated a NW-trending Andean arc along the Californian margin beginning at ~175 Ma. Coeval high-pressure (HP) transformation of oceanic crust in an east-dipping, subduction zone probably accompanied plate convergence, but recovered Franciscan eclogite and garnet blueschist blocks chiefly possess younger recrystallization ages of ~165-150 Ma. These Jurassic HP metamafic rocks were sequestered in a low-temperature environment well into Cretaceous time. Fragments of actinolitic rinds partially surround many such high-grade blocks. Only slightly younger than the HP metamorphism, these rinds reflect metasomatic exchange between metabasalt and serpentinized harzburgite along the dynamic oceanic crust-mantle hanging wall during storage of the mafic rocks at moderate depth. High-grade tectonic blocks later were brought toward the surface in circulating, low-density, sheared mud-matrix mélange and/or in buoyant serpentinite bodies. Most exotic HP metamafic blocks occur in mélanges of the Franciscan Central Belt, reflecting tectonic insertion within the subduction zone-not near-surface additions to the clastic section. However, rare, high-grade clasts in feebly recrystallized Franciscan conglomerates suggest erosion and sedimentary deposition for some HP blocks. The addition of dense metabasaltic olistoliths to the mid-and Upper Cretaceous section requires that these HP material were carried surfaceward first as tectonic fragments, perhaps immersed in low-density serpentinite or mud-matrix mélange, then eroded and transported into the trench. HP rocks are conspicuously lacking in coeval Great Valley strata. Whatever the origin of particular high-grade rocks, widespread post-depositional shearing has largely obliterated their original natures, but all dense metamafic blocks of Jurassic recrystallization age must have been supplied to the Cretaceous Franciscan accretionary complex by entrainment in a low-density, circulating muddy matrix or serpentinite body. The vast majority of exotic HP blocks resides in Central Belt mélanges, and appears to be tectonic rather than olistostromal in origin.

Early Onset of Franciscan Subduction

“…In abstracts, Ross and Sharp () reported a garnet Sm‐Nd age of 190 ± 10 Ma from garnet amphibolite, and Page et al () reported a zircon U‐Pb age of 178 ± 7 from eclogite. Indirect evidence for early onset of Franciscan subduction is supplied by convergence along the western North American inferred on the basis of 180–170‐Ma arc magmatism and Early Jurassic terrane accretion in the Sierra Nevada and Klamath Mountains (Day & Bickford, ; Ernst, ; Fagan et al, ; Hacker, ; Wright & Fahan, ) and by Shervais et al () based on Middle Jurassic ages derived from the Coast Range Ophiolite.…”

“…In contrast, Anczkiewicz et al () argued that garnet‐amphibolite formed during the earliest stages of subduction and interpreted a Lu‐Hf garnet age from a single amphibolite sample to date subduction initiation circa 169 Ma. More recently, Ernst () inferred that convergence began at 175 Ma based on the timing of early arc magmatism and terrane accretion in the Sierra Nevada and Klamath Mountains.…”

Sandstone-matrix mélanges, architectural subdivision, and geologic history of accretionary complexes: A sedimentological and structural perspective from the Franciscan Complex of Sonoma and Marin counties, California, USA

“…In contrast, thinner, especially mud-richer sections may convert to shale-matrix, fault-zone mélange, some of which may form and accrete incrementally (e.g., Rowe et al, 2013). The Subduction Channel model (Cloos, 1982(Cloos, , 1984Shreve, 1988a, 1988b) is used widely to explain formation and emplacement of laterally extensive and generally thick shale-matrix or serpentinite-matrix mélange units within accretionary complexes (e.g., Hansen, 1992;Vannucchi et al, 2008;Blanco-Quintero et al, 2011;Ukar, 2012;Ogawa et al, 2014;Ernst, 2015). In the Subduction Channel model, continuous ductile flow and pervasive deformation of the shale or serpentinite matrix are essential for offscraping, fragmentation, and mixing of high-and low-grade blocks and other rocks within mélange packages (Cloos, 1982;Cloos and Shreve, 1988a;Ukar, 2012).…”

“…The Franciscan Complex of western California was recognized during the development of plate tectonic theory as a subduction accretionary complex. For many it became the archetypical accretionary complex (e.g., Ernst, 1970Ernst, , 2015Wakabayashi, 2015). The Complex was initially divided into three (possibly fault-bounded) geographic belts-the Eastern, Central, and Western belts-with each belt characterized by particular rock types, rock ages, and a low-temperature metamorphic facies (blueschist, prehnite-pumpellyite, and zeolite facies-from east to west; Fig.…”

Geologic Setting of Eclogite-facies Assemblages in the St. Cyr Klippe, Yukon–Tanana Terrane, Yukon, Canada

“…La présence de phengite et de zircon permien cristallisé sous conditions métamorphiques du faciès à éclogite au sein de la roche hôte quartzofeldspathique indiquent que l'éclogite a été métamorphisée en place, avec le schiste comme unité cohérente du terrane de croûte d'arc continental de Yukon-Tanana, plutôt qu'un mélange associé à une subduction de croûte océanique du terrane de Slide Mountain. Des similarités pétrologiques, géochimiques, géochrono-logiques et structurales lient les éclogites de St-Cyr à d'autres lieux de hautes pressions au Yukon, ce qui indique que les INTRODUCTION High-pressure metamorphic assemblages formed in Pacifictype convergent margins, such as the iconic Franciscan complex (Ernst 2015), are found along the western North American margin from Alaska to Mexico . The Yukon-Tanana terrane in the Canadian Cordillera preserves both Mississippian and Permian high-pressure rocks.…”