Detrital zircon geochronology and evolution of the Nacimiento block late Mesozoic forearc basin, central California coast

Abstract

Forearc basins are first-order products of convergent-margin tectonics, and their sedimentary deposits offer unique perspectives on coeval evolution of adjacent arcs and subduction complexes. New detrital zircon U-Pb geochronologic data from 23 sandstones and 11 individual conglomerate clasts sampled from forearc basin strata of the Nacimiento block, an enigmatic stretch of the Cordilleran forearc exposed along the central California coast, place constraints on models for forearc deformation during evolution of the archetypical Cordilleran Mesozoic margin. Deposition and provenance of the Nacimiento forearc developed in three stages: (1) Late Jurassic-Valanginian deposition of lower Nacimiento forearc strata with zircon derived from the Jurassic-Early Cretaceous arc mixed with zircon recycled from Neoproterozoic-Paleozoic and Mesozoic sedimentary sources typical of the continental interior; (2) erosion or depositional hiatus from ca. 135 to 110 Ma; and (3) Albian-Santonian deposition of upper Nacimiento forearc strata with zircon derived primarily from the Late Cretaceous arc, accompanied by Middle Jurassic zircon during the late Albian-Cenomanian. These data are most consistent with sedimentary source terranes and a paleogeographic origin for the Nacimiento block south of the southern San Joaquin Basin in southern California or northernmost Mexico. This interpreted paleogeographic and depositional history of the Nacimiento block has several implications for the tectonic evolution of the southern California Mesozoic margin. First, the Nacimiento forearc depositional history places new timing constraints on the Early Cretaceous unconformity found in forearc basin strata from the San Joaquin Valley to Baja California. This timing constraint suggests a model in which forearc basin accommodation space was controlled by accretionary growth of the adjacent subduction complex, and where tectonic events in the forearc and the arc were linked through sediment supply rather than through orogenic-scale wedge dynamics. Second, a paleogeographic origin for the Nacimiento forearc south of the southern San Joaquin Valley places new constraints on end-member models for the kinematic evolution of the Sur-Nacimiento fault. Although this new paleogeographic reconstruction cannot distinguish between sinistral strike-slip and thrust models, it requires revision of existing sinistral-slip models for the Sur-Nacimiento fault, and it highlights unresolved problems with the thrust model.

Forearc strike-slip displacement as an alternative to subduction erosion, with examples from Mexico and California (sinistral Nacimiento fault)

“…In addition, having typical Franciscan Complex juxtaposed against Salinia directly west of a fenster exposing the meta-arkosic schist of Sierra de Salinas (e.g., Kidder et al 2003;Kidder and Ducea 2006;Ducea et al 2009) presents insurmountable geometric obstacles for the reversefault model. The only architecture consistent with these observations involves thrusting the schist over the "missing" western arc and forearc (e.g., Johnston et al 2018), in which case the protolith for the POR schist would consist of forearc sand, rather than trench sand. In this scenario, there would be no mechanism to incorporate oceanic components such as metabasite, metachert, marble, serpentinite, and talc-actinolite rock into the POR schist, together with the dominant arkosic sand that was eroded, transported, deposited, and rapidly accreted and metamorphosed during subduction (Jacobson et al 2011).…”

“…5 and in fig. 10 in Johnston et al (2018). Increasing the angle between the continental margin and the Nacimiento fault decreases the slip needed to superpose the Nacimiento Franciscan block with Salinia, thus allowing the Nacimiento Franciscan block to be restored farther south prior to Nacimiento fault slip (more consistent with detrital zircon provenance signatures) (e.g., Chapman et al 2016;Johnston et al 2018).…”

Geologic map and structural development of the northernmost Sur-Nacimiento fault zone, central California coast

“…Despite this significant offset along the fault, the slip history of the Sur-Nacimiento fault is poorly understood, and a variety of models have been proposed to explain its kinematic evolution. Dip-slip models suggest that the Nacimiento block originated outboard of the Salinian arc, and that the forearc was thrust beneath the arc along the Sur-Nacimiento fault (Hall, 1991;Johnston et al, 2018), which may have behaved as a normal fault during Late Cretaceous extensional collapse of the southern California arc (Chapman et al, 2016). Sinistral-slip models suggest that the Nacimiento block originated at the modern-day latitude of San Francisco (Seiders, 1983;Dickinson, 1983;Jacobson et al, 2011) or the southern San Joaquin Valley (Johnston et al, 2018), and slipped obliquely across the forearc basin along the Sur-Nacimiento fault and southeastward into contact with the Salinian arc.…”

“…Dip-slip models suggest that the Nacimiento block originated outboard of the Salinian arc, and that the forearc was thrust beneath the arc along the Sur-Nacimiento fault (Hall, 1991;Johnston et al, 2018), which may have behaved as a normal fault during Late Cretaceous extensional collapse of the southern California arc (Chapman et al, 2016). Sinistral-slip models suggest that the Nacimiento block originated at the modern-day latitude of San Francisco (Seiders, 1983;Dickinson, 1983;Jacobson et al, 2011) or the southern San Joaquin Valley (Johnston et al, 2018), and slipped obliquely across the forearc basin along the Sur-Nacimiento fault and southeastward into contact with the Salinian arc. Alternatively, models that originally place the Nacimiento block at distant equatorial latitudes suggest removal of the forearc basin during allochthonous arrival of the Salinian and Nacimiento blocks in southern California via margin-parallel dextral slip along the Sur-Nacimiento and related faults (McWilliams and Howell, 1982;Page, 1982).…”

“…Given these Maastrichtian-Oligocene age constraints for sub-greenschist facies contraction within the Salinian block, it is possible that this event was associated with dextral transpression during the original juxtaposition of the Salinian and Nacimiento blocks along the Sur-Nacimiento fault. However, detrital zircon provenance studies of Nacimiento block trench and forearc basin sediments suggest an origin in southern California, and argue against allochthonous dextral transport of the Nacimiento block accretionary complex and forearc basin within the southern California late Mesozoic margin (Chapman et al, 2016;Johnston et al, 2018). As such, we suggest that this contractional event may have been associated with an enigmatic phase of Paleogene dextral displacement along the central California margin (Vedder et al, 1991;Sharman et al, 2013) possibly related to an increased component of dextral transport during the final stages of Farallon subduction (Doubrovine and Tarduno, 2008).…”