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

Abstract

Slip on the Nacimiento fault of the central Coast Ranges of California has been variably interpreted as dextral, sinistral, or reverse. The currently prevailing interpretation is that the Nacimiento fault represents subduction erosion, by which the central to eastern part of the Cretaceous California batholith was thrust over the western part of the batholith and forearc basin, resulting in juxtaposition of the Salinian batholithic block against the Franciscan Complex, concurrently with Laramide flat-slab subduction (75–55 Ma) and underplating of the Pelona-Orocopia-Rand schist. No modern convergent plate margin includes such overthrusting. The closest modern analog to the likely configuration of the Salinian continental margin near the end of the Laramide deformation is southern Mexico, where arc plutons are exposed near the trench. Although commonly considered an example of subduction erosion, this margin is “missing” parts of the plutonic arc and forearc because they have been displaced to the southeast by sinistral slip. By analogy, the Nacimiento forearc was modified as a trench-trench-transform triple junction migrated southeastward along the continental margin during flat-slab subduction. This model makes testable predictions involving northwest-to-southeast younging of deep-marine deposits on batholithic crust underlain by contemporaneous schist. Correct restoration of later Cenozoic primarily dextral slip and Maastrichtian – Early Eocene primarily sinistral slip must result in realignment of north–south-trending belts of the Sierra Nevada – Salinia – Peninsular Ranges batholith, Great Valley forearc, and Franciscan Complex. These modern and ancient examples suggest that several “erosional” subduction zones are more plausibly explained by strike-slip truncation of forearcs.

Ali Mehmet Celâl Şengör: A geologist who unravels the histories of continents and oceans

“…The author suggests that the Nacimiento accretionary complex was juxtaposed against the Salinian continental block along the Nacimiento fault near the end of Laramide deformation. Ingersoll (2019) shows that the closest modern analog to the likely configuration of the Salinian continental margin near the end of Laramide deformation is in southern Mexico, where arc plutons are exposed near the trench. Ingersoll (2019) suggests that the Nacimiento forearc was modified as a trench-trench-transform triple junction migrated southeastward along the Salinian continental margin during the Laramide flat-slab subduction, resulting in truncation and displacement of the forearc by strike-slip faults.…”

“…Ingersoll (2019) shows that the closest modern analog to the likely configuration of the Salinian continental margin near the end of Laramide deformation is in southern Mexico, where arc plutons are exposed near the trench. Ingersoll (2019) suggests that the Nacimiento forearc was modified as a trench-trench-transform triple junction migrated southeastward along the Salinian continental margin during the Laramide flat-slab subduction, resulting in truncation and displacement of the forearc by strike-slip faults. The author concludes that several places in the geological record that are interpreted as "erosional" subduction zones are more plausibly explained by strike-slip truncation of forearcs.…”

The forearc ophiolites of California (USA) formed during trench-parallel spreading: kinematic reconstruction of the western USA Cordillera since the Jurassic

“…Between the San Gregio-Hosgri Fault and the San Andreas Fault, dextral deformation has been partitioned over the Rinconada, Eastern Huasna, and Nacimiento Faults (Figure 2, Table 3). The Nacimiento Fault forms the boundary between displaced continental margin arc plutons of the Salinian Block and the Nacimiento subduction-accretion assemblage (Chapman et al, 2016;Ingersoll, 2019;Johnston et al, 2018). Each of these faults has several tens of km of displacement (Table 3).…”

The birth of a forearc: The basal Great Valley Group, California, USA

“…In an arc-arc collisional model, the Smartville and Great Valley ophiolite segments of the CRO formed as backarc ophiolites atop a west-dipping subduction zone offshore western North America (Schweickert and Cowan, 1975;Ingersoll, 2000Ingersoll, , 2019Schweickert, 2015). These ophiolites and island arcs accreted onto the California margin during the Sierran phase of the Nevadan orogeny ca.…”