Assembling the world’s type shallow subduction complex: Detrital zircon geochronologic constraints on the origin of the Nacimiento block, central California Coast Ranges

Abstract

Temporal and spatial patterns in the architecture of the Franciscan Complex provide valuable insights into the subduction processes through which such patterns arise. The Nacimiento Franciscan belt is an allochthonous sliver of subduction assemblages in the central California Coast Ranges displaced either: (1) from southern California by >300 km of Neogene dextral slip along the San Andreas fault system or (2) from central California to southern California and back again, by >500 km of Late Cretaceous-Paleocene sinistral slip along the Sur-Nacimiento fault followed by San Andreas-related motion. New U-Pb detrital zircon data from 20 (meta)clastic samples indicate that the Nacimiento Franciscan section was assembled between ca. 95 and 80 Ma. Abundant Cretaceous (particularly Late Cretaceous) and diminishing amounts of Jurassic and Proterozoic zircon grains point to a southern California origin for Nacimiento Franciscan protoliths, precluding significant sinistral strike-slip along the Sur-Nacimiento fault. Furthermore, the suite of detrital zircon ages reported here bears a strong resemblance to new and existing data from subduction complexes in southern California that were emplaced during Laramide shallow subduction (i.e.,

Transfer of Metasupracrustal Rocks to Midcrustal Depths in the North Cascades Continental Magmatic Arc, Skagit Gneiss Complex, Washington

“…In addition, recent studies link the presence of two distinct Proterozoic age peaks from forearc and accretionary wedge rocks from Washington to Alaska (i.e., the WMB, Yakutat Terrane, and Nanaimo Group) to sedimentary sources located in Southern California and southwestern Laurentia (Garver & Davidson, ; Matthews et al, ; Sauer et al, ) or southern Idaho and northwestern Laurentia (Dumitru et al, ). The same pair of Proterozoic age peaks and similar detrital zircon signatures are also observed in coeval sedimentary units in the accretionary wedge (Franciscan Complex), forearc (Great Valley), and underplated trench sediments (Pelona‐Orocopia‐Rand schist) near the southern end of the Sierra Nevada (Chapman et al, ; Dumitru et al, ; Jacobson et al, ; Sharman et al, ). Based on these observations, rocks now present in northwestern Washington and southern British Columbia potentially originated between the latitude of southern Oregon and southern Sierra Nevada and were translated northward.…”

Structural context and variation of ocean plate stratigraphy, Franciscan Complex, California: insight into mélange origins and subduction-accretion processes

“…Episodic accretion transferred material (OPS) from the downgoing plate to the subduction complex to form a structural stack of accretionary slices/units with different formational and accretion ages ( Fig. 3); Dumitru et al (2016); Chapman et al (2016). These sheetlike accretionary units, bounded above and below by narrow (<100 m thick) non-accretionary paleomegathrust horizons, have low-angle regional dips, but dip steeply in many areas as a result of later folding (Wakabayashi 2015;2016a) (Fig.…”

Detrital zircon record of mid-Paleozoic convergent margin activity in the northern U.S. Rocky Mountains: Implications for the Antler orogeny and early evolution of the North American Cordillera

“…The Northern Sierra and Eastern Klamath terranes may have composed part of the converging Antler arc; however, it is uncertain if the timing of Early to Middle Devonian deformation and metamorphism therein (e.g., Cashman, 1980;Saleeby et al, 1987;Wallin et al, 2000) is consistent with that required by models for Late Devonian-Mississippian arc-continent collision and foreland basin sedimentation. Because accretionary prism and subduction complex rocks typically show evidence of syndepositional magmatic activity (e.g., Amato et al, 2013;Chapman et al, 2016), Roberts Mountain allochthon strata in this model are expected to contain Paleozoic detrital zircons from the adjacent Sierra-Klamath arc. The Precambrian-dominated detrital zircon signatures of most Roberts Mountain allochthon units appear to be inconsistent with Paleozoic arc provenance; however, Upper Devonian Milligen Formation strata in the Pioneer Mountains yield Silurian detrital zircons that broadly support ties with known rock assemblages of the Northern Sierra, Eastern Klamath, and Quesnellia terranes (e.g., Saleeby et al, 1987;Roback et al, 1994;Wallin and Metcalf, 1998;Grove et al, 2008).…”