An Exhumation Pulse From the Nascent Franciscan Subduction Zone (California, USA)

Abstract

We investigated a suite of metabasite blocks from serpentinite matrix and shale matrix mélanges of the California Coast Ranges. Our new data set consists of 40 Ar/ 39 Ar dates of amphibole and phengite and U-Pb dates of metamorphic zircon. Combined with published geochronology, including prograde Lu-Hf garnet ages from the same blocks, we can reconstruct the timing and time scales of prograde and retrograde metamorphism of individual blocks. In particular we find that exhumation from amphibole-eclogite facies conditions occurred as a single episode at 165-157 Ma, with an apparent southward younging trend. The rate and timing of exhumation were initially uniform (when comparing individual blocks) and fast (with cooling rates up to~140°C/Ma). In the cooler and shallower blueschist facies, exhumation slowed and became less uniform among blocks. Considering the subduction zone system, the high-grade exhumation temporally correlates with a magmatic arc pulse (Sierra Nevada) and the termination of forearc spreading (Coast Range Ophiolite). Our findings suggest that a geodynamic one-time event led to exhumation of amphibole-eclogite facies rocks. We propose that interaction of the Franciscan subduction zone with a spreading ridge led to extraction of the forearc mantle wedge from its position between forearc crust and subducting crust. The extraction led to fast and uniform exhumation of subducted rocks into the blueschist facies. We also show that the Franciscan subduction zone did not undergo significant cooling over time and that its initiation was not coeval with blueschist-facies metamorphism of the Red Ant schist of the Sierra Nevada foothills. Plain Language Summary A subduction zone is where oceanic crust of one tectonic plate dives under the crust of another plate. These zones are important drivers of the movement of the continental plates. Special rocks-blueschists and eclogites-form in subduction zones at high pressure and relatively low temperature. The Franciscan Complex forms most of the geology of western California. It consists of rocks that dived into a subduction zone and returned back to the surface. We investigated the history of some of these rocks by determining the age and conditions of their movement. We find that the rocks that subducted to depths of up to 80 km all came up at the same time around 160 million years before present. This suggests that a specific tectonic event occurred at that time; other regional data show that the effects of this event were felt elsewhere along the paleo-Pacific margin. We also investigate rocks in the Sierra Nevada foothills that other researchers suggested to have formed at about the same time and found that they are not correlated.

Quartz-in-garnet barometry constraints on formation pressures of eclogites from the Franciscan Complex, California

“…At later times, sediments are underplated and exhumed, complicating this comparison due to the greater buoyancy of sediments. Rutte et al (2020) propose a single exhumation event for high-grade blocks of the Franciscan Complex based on amphibolite and phengite Ar-Ar dating, perhaps consistent with a coherent underplating and exhumation model.…”

“…Protolith age constraints suggest the CRO formed between ∼172-162 Ma (e.g., Shervais et al 2005;Hopson et al 2008), and the oldest reported ages of metamorphism from Franciscan high-grade blocks, i.e., blocks with significantly higher metamorphic grade (blueschist, eclogite, and amphibolites facies) relative to the surrounding matrix, are ∼176 Ma (mean U-Pb age of metamorphic zircon inclusions in garnet, Mulcahy et al 2018). However, metamorphic zircon U-Pb ages from high-grade blocks are primarily younger ( ∼166-157 Ma), and ∼180 Ma ages (U-Pb zircon core) have been related to protolith basalt crystallization (Rutte et al 2020). The similar age of metamorphism of Franciscan high-grade blocks and formation of CRO magmatic and sedimentary rocks, suggests that subduction of the Franciscan complex occurred simultaneously with CRO formation (Anczkiewicz et al 2004), or that initiation of subduction of the Franciscan occurred slightly before CRO formation (Mulcahy et al 2018).…”

“…The timing relationship between eclogite and garnet hornblendite metamorphism is debated: Wakabayashi (1992) inferred that the eclogite facies metamorphism occurred later, whereas Tsujimori et al (2006) considered the two assemblages to have formed coevally. Samples from the TIBB block were dated by Anczkiewicz et al (2004) and Rutte et al (2020), and thermobarometric estimates were obtained by Tsujimori et al (2006) and Cooper et al (2011). Garnet from the garnet hornblendite gave a Lu-Hf age of 153.4 ± 0.8 Ma (Anczkiewicz et al 2004), hornblende gave an Ar-Ar age of 159.19 ± 0.81 Ma (Rutte et al 2020), and U-Pb ages from zircon cores and rims suggest crystallization over the period of 166-157 Ma (Rutte et al 2020).…”

“…Samples from the TIBB block were dated by Anczkiewicz et al (2004) and Rutte et al (2020), and thermobarometric estimates were obtained by Tsujimori et al (2006) and Cooper et al (2011). Garnet from the garnet hornblendite gave a Lu-Hf age of 153.4 ± 0.8 Ma (Anczkiewicz et al 2004), hornblende gave an Ar-Ar age of 159.19 ± 0.81 Ma (Rutte et al 2020), and U-Pb ages from zircon cores and rims suggest crystallization over the period of 166-157 Ma (Rutte et al 2020). Sample PG22A is primarily composed of clinopyroxene + garnet + rutile, but exhibits variable degrees of retrogression as indicated by glaucophane replacement of clinopyroxene, garnet chloritization, and late-stage white mica and titanite growth.…”

“…Outer Rim: P trap (GPa) A Lu-Hf garnet age of 157.9 ± 0.7 Ma was obtained from sample PG31B by Anczkiewicz et al (2004), and a garnethornblendite from Jenner gave an inverse Ar isochron on hornblende of 158.1 ± 1.6 Ma (Rutte et al 2020). Eclogite on Santa Catalina Island is very rare-the main occurrences are layers and patches of garnet-clinopyroxenite within garnet hornblendite blocks (cm to ∼100 m) situated in zones of ultramafic melange.…”