14 Lost Oceans, Shifting Continents: Interdisciplinary Records of Paleo-Pacific Plate Histories in the American Cordilleras
This session integrates tectonics, petrology, geochronology, stratigraphy, and paleontology to illuminate how Paleo-Pacific plates drove subduction dynamics, crustal evolution, and paleogeographic reconstructions across the complex geological architecture of the American Cordilleras.
Wednesday, 22 April 2026
1:30 PM - 4:00 PM
LMH, Meeting Room
Initiation of Subduction Recorded in the Franciscan Complex: Plate Tectonic Models and Context in the Cordillera of Alta California
Paper number: 14-1
Program: Technical Sessions
Time: 1:50 PM – 2:10 PM
Authors
Abstract
The Franciscan subduction complex (FC) of the California Coast Ranges records >150 My of continuous subduction. Tectonics connected with initiation of FC subduction remain debated. FC subduction/accretion has been proposed as the last stage of continuous east-dipping subduction (with or without ridge subduction events) that began in late Permian time, with a subduction initiation horizon far east of the Coast Ranges. Evidence for early-mid Jurassic subduction initiation from the petrologic-geochemical characteristics of the ca. 161-168 Ma Coast Range ophiolite (CRO), that structurally overlies the FC refutes this model. Subduction initiation in the FC is recorded by high-temperature (HT)/high-pressure (HP) metamorphic (metamorphic sole) conditions and (≤176 Ma) ages in the highest-grade FC rocks. The remaining models propose E-dipping versus W-dipping pre-FC subduction. An E-dipping subduction zone E of the Franciscan cannot explain the island arc protoliths of FC high-grade rocks. A pre-FC, E-dipping subduction zone W of FC subduction initiation horizon may explain the arc-protoliths of the high-grade rocks, but the lack of pre-176 Ma accretion age rocks structurally below the subduction initiation (structurally highest) horizon makes this alternative unlikely. In contrast, a ca. 215-218 Ma metamorphic sole (including eclogite) and ≤207 Ma ophiolite and associated island arc rocks structurally above the sole, in the western Sierra Nevada, E of the Coast Ranges, suggests initiation of W-dipping subduction at ca. 218 Ma. The protoliths of Franciscan high-grade rocks may have formed over such a W-dipping subduction zone. West-dipping subduction may have terminated by arc-arc collision with the intervening ocean basin consumed by subduction zones dipping away from each other. This collision was largely complete by 180 Ma. East-dipping Franciscan subduction initiated by ca. 176 Ma, followed by generation of the Coast Range ophiolite above it. HT metamorphism of Franciscan high-grade rocks persisted to ca. 155 Ma. The long duration of HT metamorphism may have resulted from ridge subduction after initiation of subduction, consistent with the petrologic evolution of the CRO, and formational age versus subduction age of accreted FC volcanic units.
Geological Society of America Abstracts with Programs. Vol. 58, No. 3
doi: 10.1130/abs/2026CD-410797
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