Superimposed extension and shortening in the southern Salinas Basin and La Panza Range, California: A guide to Neogene deformation in the Salinian block of the central California Coast Ranges

Fault geometry and cumulative offsets in the central Coast Ranges, California: Evidence for northward increasing slip along the San Gregorio-San Simeon-Hosgri fault

“…These may be the rocks that were displaced to the southeast by the San Juan-Chimineas-Russell fault. The nearly identical ages of the alaskite that intrudes the gneiss at Barrett Ridge (U/Pb age of 80 Ma; Mattinson and James, 1985) and the La Panza granodiorite (79 Ma; Colgan et al, 2012) indicate that these rocks were of the same intrusive event, possibly now displaced by post-80 Ma displacement. Given that the offset estimate from the magnetic anomalies is similar to the estimate of 26-29 km between 23 and 4 Ma on the Russell fault to the south (Yeats et al, 1989), we suggest that the Chimineas-San Juan fault offset occurred during the same time frame.…”

The Point Sal–Point Piedras Blancas correlation and the problem of slip on the San Gregorio–Hosgri fault, central California Coast Ranges

“…(4) The northern part of the Salinian block (east of the San Gregorio fault) has not undergone a major change in length parallel to the San Andreas Fault since the middle Miocene. The Salinian block was mostly deformed by Miocene extension and latest Miocene to Pliocene and younger shortening perpendicular to the San Andreas Fault (e.g., Compton, 1966;Graham, 1978;Page et al, 1998;Colgan et al, 2012). It is cut by some strike-slip faults subparallel to the San Andreas Fault, but these account for minor Miocene offset (tens of kilome ters) at the scale of the ~500 km long block (e.g., Langenheim et al, 2013).…”

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

“…Low-temperature thermochronology clearly indicates that this event must have occurred prior to 6 Ma when Salinian basement adjacent to the Gamboa fault at Limekiln Creek was at ~110 °C (Ducea et al, 2003). However, because the northern Salinian block was characterized by a transtensional tectonic setting recorded by early Miocene basins that cover much of the northern Salinian block (Graham, 1978;Colgan et al, 2012), we suggest that these sub-greenschist facies contractional structures are pre-Miocene. As an alternative to achieving sub-greenschist facies temperatures through contraction and tectonic burial alone, it is possible that these elevated temperatures could be related to a local thermal perturbation associated with the Los Burros mining district.…”

“…These marble mylonites are thermally comparable to the disjunctive cleavage developed in Big Creek conglomerate mudstone, which lacks synkinematic mica growth, and are consistent with deformation at sub-greenschist facies temperatures of 200-300 °C. These temperatures of deformation are higher than elsewhere in the Santa Lucia Range where Salinian basement and overlying Late Cretaceous-Neogene strata have been at temperatures below which fission tracks Johnston et al | Sur-Nacimiento fault structural geology GEOSPHERE | Volume 15 | Number anneal in apatite (~100 °C) since the Late Cretaceous (Naeser and Ross, 1976;Colgan et al, 2012). Low-temperature thermochronology clearly indicates that this event must have occurred prior to 6 Ma when Salinian basement adjacent to the Gamboa fault at Limekiln Creek was at ~110 °C (Ducea et al, 2003).…”

“…In contrast, the NNE-SSW P-axis girdle associated with late brittle faults near Big Creek is suggestive of dextral strain with a component of normal slip, which is inconsistent with the regional nature of transpressional strain that is expected adjacent to the San Simeon segment of the San Gregorio-San Simeon-Hosgri fault (e.g., Johnson et al, 2018). However, minor faults with normal displacement have been noted before along the Big Sur coast (Trasko and Cloos, 2007), and this subtle component of normal strain may (1) indicate that dextral displacement along the Gamboa fault initiated as early as the Miocene when the predominant strain pattern throughout the central California coast was transtensional (e.g., Graham, 1978;Colgan et al, 2012), or (2) be related to localized transtensional strain associated with releasing bends developed along the Gamboa fault (e.g., at Dolan Creek: Plate S1 [footnote 1]; south of Partington Point: Johnson et al, 2018, their figure 11). Regardless, given the similar dominantly dextral pattern of strain associated with recent seismic events and with noncoaxial shear zones and brittle faults along the Gamboa fault, we suggest that the 8-11 km of dextral slip along the Gamboa fault is Miocene and younger, and that the fault may be currently active.…”