18 Crossing Borders: Large-scale tectonic models of the North American Cordillera

This session focusses on the Phanerozoic evolution of the North American Cordillera, soliciting studies that address large-scale tectonic models associated with obliquely convergent and divergent movement along the margin.

Thursday, 23 April 2026
8:00 AM - 12:00 PM
LMH, Isla Carmen

Zircon Investigation of the Cretaceous Evolution of the Southern Sierra Nevada Arc, CA: Evidence for Local Tectonic Reorganization at ca. 100 Ma

Paper number: 18-6
Program: Technical Sessions
Time: 9:45 AM – 10:05 AM

Authors

Abstract

Isotopic and trace element data from detrital and plutonic zircon separated from Cretaceous granitoids and forearc deposits preserve the fingerprint of a mid-Cretaceous tectonic event. Secular variation of zircon initial hafnium isotopic values (eHfi) delineates a hafnium isotopic draw-down spanning the Early to Late Cretaceous, indicating an increasing component of partial melts of continental crust as the Cretaceous arc matured. At ca. 100 Ma this isotopic trend is interrupted by zircons with atypically positive and negative eHfi values, some of which also have elevated middle and heavy rare earth element (MHREE) contents (Clemens-Knott, Surpless, Barth and Wooden, 2021). These data signal the introduction of zircons crystallized from lower crustal and mantle-derived partial melts into upper crustal magmatic systems. Mid-Cretaceous extension likely facilitated upward transport of zircons crystallized at depth, some of which were erupted and/or freed by weathering and subsequently preserved in Late Cretaceous forearc deposits. Significantly, many of these geochemically unusual zircons also contain elevated Nb contents, a signature inconsistent with crystallization from arc magmas that are globally recognized as characterized by a negative Nb anomaly. Together these data suggest upward mobilization of zircon derived from partial melting of the lower North American crust, the underlying mantle wedge, and from relatively Nb-rich sub-slab asthenospheric mantle. These data can be explained by a slab detachment event at ca. 100 Ma, a result consistent with models for mid-Cretaceous collision of the Baja B.C. terrane against southern California. Timing of this event is constrained by the Nb-rich (10-65 ppm) zircon autocrysts that dominate two plutons (100.3 ± 0.4 Ma; 99.7 ± 0.5 Ma) collected ~10 km east of the proto-Kern Canyon fault, a ~130-km-long ductile shear zone along which  ~10 ± 5 km east-side-up offset and ~27 km dextral translation is documented between 95 and 80 Ma, with additional deformation possibly beginning at ca. 105 Ma (Nadin and Saleeby, 2008). Slightly younger granitoids do not contain geochemically atypical zircon, indicating prompt reestablishment of the Sierran arc magmatic system and suggesting that Late Cretaceous convergence was locally accommodated along either an existing or a rapidly established trench system located farther offshore.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3

doi: 10.1130/abs/2026CD-411057

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