At Rancho San Marcos, halfway between Tecate and Ensenada in northwestern Baja California, a 1 km by 5 km group of giant olistoliths of Early Ordovician age occurs within phyllite and metasandstone of Mesozoic(?) age. This group of giant olistoliths is underlain by a mélange of olistolith-derived granule to boulder-size fragments in a foliated, phyllitic matrix. Granitic rocks and andesite/dacite dikes of the Cretaceous Peninsular Ranges arc and batholith intrude both autochthonous and allochthonous rocks. The olistoliths of Ordovician rock are resistant, moderately to well-sorted, blue-gray quartzite; brown, gray, and black, commonly argillaceous, bedded chert; medium to dark gray, finely to coarsely recrystallized, carbonate rock; minor amounts of brown to gray-green metaargillite; and clast-supported cobble conglomerate. North Atlantic and Midcontinent province conodonts from the carbonate rock indicate a medial Arenigian (Early Ordovician) age. Both the Ordovician (allochthonous) and Mesozoic (autochthonous) rocks have undergone low greenschist grade regional metamorphism of Cretaceous age. The quartzite has been openly folded, but argillaceous units are pervasively foliated and isoclinally folded with strikes N30 to N70°W, and dips to the northeast. Field relationships suggest that the mélange is sedimentary, not tectonic, in origin. Emplacement occurred at a time of tectonic unrest during which debris was shed westward off an unstable continental margin into flysch basins. The allochthonous rocks of Rancho San Marcos appear similar in age and lithology to portions of the eugeoclinal Valmy Formation of north-central Nevada. If these strata are correlative, palinspastic reconstruction appears to require large-scale left-lateral displacement. Proposed sinistral displacement on the medial Jurassic Mojave-Sonora megashear, plus northward translation on the San Andreas fault system in the Neogene, would place Valmy-equivalent rocks at roughly the same latitude as San Marcos.

Previous structural interpretations of the Sonoma Range in north-central Nevada have concluded that the Tobin thrust fault—regarded as the equivalent of the Golconda thrust fault—is younger than other thrust faults of post-Triassic age in the range. However, thrust emplacement of the distinctive oceanic upper Paleozoic rocks of the Golconda allochthon over a large region in western and north-central Nevada, and perhaps even beyond, seems to have taken place prior to deposition of Triassic strata in the region. Hence, the structural relationships in the Sonoma Range that bear on the age of the Golconda thrust fault have been questioned. Restudy of the critical part of the Sonoma Range in the vicinity of Clear Creek shows that the oldest faults in the area that bound rocks of the Golconda allochthon and therefore may represent the Golconda thrust fault are, in fact, segments of a single fault that has been displaced by several successive slices of the Clear Creek thrust fault, the north end of which cuts Triassic rocks exposed in the northwestern Sonoma Range. Furthermore, the geometry of rocks displaced since Triassic time on the Clear Creek system of thrust faults suggests that the faults regarded as parts of the Golconda thrust fault in the Sonoma Range are offset segments of the type Golconda thrust fault as exposed about 15 km to the northeast. Consequently, the Golconda thrust fault in its type locality, as well as in the Sonoma Range, is evidently older than faults that cut Triassic rocks, and its age relationships do not conflict with the generally accepted Late Permian or Early Triassic time of emplacement of the Golconda allochthon. Integrated into this structural reinterpretation of the Sonoma Range are several other conclusions and findings of more than local significance, including the following: (1) Prior to emplacement of the Golconda allochthon, lower Paleozoic rocks in the Sonoma Range area, such as the Harmony and Valmy Formations and perhaps the Preble Formation, were intricately deformed and faulted together, presumably during the middle Paleozoic Antler orogeny. (2) Coarse clastic detritus derived from the Harmony and Valmy Formations occurs in the Golconda allochthon of the Sonoma Range, which suggests that it was originally deposited along the North American continental margin. (3) Radiometric ages of plutonic rocks in the Sonoma Range suggest that post-Triassic displacement, perhaps as gravity slides, of parts of the Golconda allochthon on the Clear Creek system of thrust faults took place between about 170 and 100 m.y. ago.