Jurassic rocks in Sonora, Mexico: Relations to the Mojave-Sonora megashear and its inferred northwestward extension

Abstract

The Mojave-Sonora megashear constitutes a regional boundary between lithologically distinct Jurassic assemblages of different ages. North of the Mojave-Sonora megashear, arc-related volcanic, volcaniclastic, and clastic rocks, intruded by plutons (175–160 Ma) compose part of the Middle Jurassic (commonly ca. 175 Ma) igneous province, previously recognized in Arizona and California. Distinct domains among Jurassic igneous rocks in northern Sonora are: (1) southern Papago, a region where pre-Jurassic rocks are unknown, (2) Nogales-Cananea-Nacozari, where Jurassic rocks are underlain by 1.7–1.4 Ga crystalline basement, and (3) Mojave-Sonora, where strata, including Oxfordian beds, along the north side of the Mojave-Sonora megashear are commonly strongly deformed, as recorded by thrust faults, mylonitic foliation, and recumbent folds. The Mojave-Sonora domain extends across the southwestern margins of the southern Papago and the Nogales-Cananea-Nacozari domains. Strong deformation that distinguishes the zone markedly declines within a few tens of kilometers northward. South of the Mojave-Sonora megashear, in central and southern Sonora, Lower Jurassic clastic and volcaniclastic rocks distinguish the Caborca domain. Upper Jurassic sedimentary rocks, commonly conglomeratic, are abundant north of Mojave-Sonora megashear; a single occurrence is known south of the Mojave-Sonora megashear. Waning of subduction-related Middle Jurassic magmatism was followed by the abrupt formation, ca. 165 Ma, of Coast Range, Josephine, Great Valley, and Devil's Elbow ophiolites and the Smartville Complex within oceanic pull-aparts west of the margin of the North America plate. The formation of ophiolitic rocks signaled the beginning of transtensional faulting. Almost contemporaneously (ca. 163 Ma) the lowest volcanic units and overlying coarse sedimentary beds began to accumulate in fault-bounded continental pull-apart basins such as the McCoy Mountains basin. Other transtensional basins, formed at releasing steps where pull-aparts formed, are well developed within the Papago domain and other parts of southwestern United States and northern Mexico. From Sonora northward into California the Mojave-Sonora megashear fault zone, developed generally within the Middle Jurassic arc-parallel to the former continental margin, is inferred to link with strands of the Melones and Bear Mountain faults of the Foothills fault system, the Wolf Creek fault, and the Big Bend fault. A protuberance of Proterozoic basement (the Caborca block) that was truncated from the continental margin records ∼800–1000 km of left-lateral offset. The displacement of the Caborca block took place south of a major releasing step along the Big Bend fault with the result that a regional pull-apart that coincides with the Great Valley of California developed. Inboard of the Mojave-Sonora megashear Late Jurassic magmatic rocks crop out near faults at some releasing steps and within floors of some pull-apart structures. The distribution suggests that magma rose along faults and into areas of thin crust. In southern Arizona these igneous rocks are included as part of the Artesa layered sequence and the Ko Vaya plutonic suite. Oxfordian and younger beds, which crop out north of the Mojave-Sonora megashear may contain exotic blocks and contractional structures that are contemporaneous with the Nevadan orogeny. The variation in the style and intensity of deformation of Middle and Upper Jurassic strata, and Upper Jurassic conglomerate rich in clasts derived from rocks of the Caborca domain, are postulated to record transpression near the Mojave-Sonora megashear that locally overlapped the more widespread transtensional structures in time and space. The cessation of strike-slip faulting locally began ca. 150 Ma, as shown by undeformed intrusive bodies that cut older deformed Middle Jurassic rocks. By the time that the Independence dikes and correlative rocks were emplaced at 148 Ma, scant evidence of lateral faulting is known. Intrusions, young volcanic cover, transecting strike-slip faults, and multiple generations of low-angle extensional and contractional faults obscure Jurassic structures in Sonora and southern California. Despite these complications, removal of the effects of superposed structures reveals a viable trace for an inferred Late Jurassic left-lateral fault linking the Mojave-Sonora megashear and more northerly fault segments. The position of this major inferred fault is constrained by distinctive tectonostratigraphic domains. The Middle and Late Jurassic and earliest Cretaceous plate tectonic history includes (1) subduction (175–165 Ma), (2) coupling (ca. 165 Ma), (3) rifting, transtension, lateral faulting, transpression, and contraction (165–145 Ma), and (4) renewed subduction (ca. 135 Ma) along the western margin of the North America plate and terranes (e.g., Wrangellia) to the west. The structures that record the diverse plate processes and that are preserved best in the overriding North America plate are compatible with a consistently maintained easterly directed maximum compressive stress.