Latest Cretaceous and early Tertiary orogenesis in south-central Arizona: Thrust faulting, regional metamorphism, and granitic plutonism

Abstract

Rocks in a number of mountain ranges in southernmost central Arizona are juxtaposed by thrust faults, regionally metamorphosed, and intruded by garnet–two-mica granites. Field relations and K-Ar and U-Pb isotopic geochronology indicate that thrust faulting, metamorphism, and granitic plutonism were closely related aspects of a latest Cretaceous and early Tertiary orogenic episode. The regionally metamorphosed rocks, chiefly greenschist-facies quartzofeldspathic schists, were derived from Jurassic and Cretaceous sedimentary, volcanic, and plutonic rocks. In five ranges, these metamorphic rocks are overlain, along synmetamorphic thrust faults, by Precambrian gneiss or Late Jurassic or Late Cretaceous plutonic rocks. In three additional ranges, upward increase of textural grade within the metamorphic rocks and analogy with the areas of exposed thrust faults indicate the likelihood of a concealed thrust fault flanking the range. Seven of these eight thrust faults have single lower plates, but one thrust is underlain by a duplex consisting of several imbricate structural sheets separated by mylonitic tectonic slides. Areal structural relations and the regional arrangement of distinctive Jurassic igneous rock units strongly suggest that one large range, the Baboquivari Mountains, and several smaller ranges are fensters in a thrust-fault system of regional extent, and that several adjacent ranges are allochthonous. The regionally metamorphosed rocks are intruded by synmetamorphic to postmetamorphic early Tertiary leucocratic granites characterized by various combinations of accessory biotite, muscovite, and garnet. At least two of these granites contain inherited Precambrian zircon indicative of generation by crustal anatexis. K-Ar and(or) U-Pb isotopic ages of premetamorphic granites, metamorphic rocks, thrust-zone mylonites, synmetamorphic granites, and metamorphogenic fissure veins indicate that the orogenic episode commenced in Late Cretaceous time and culminated in early Tertiary time, 58 to 60 m.y. ago. The observed and inferred close spatial and temporal relations between thrust faulting, regional metamorphism, and granitic plutonism lead to the following hypothesis for orogenesis in south-central Arizona: crustal compression caused overthrusting of crystalline rocks, resulting in crustal thickening; and crustal thickening and conductive and magmatic heat flux from the mantle together set up a thermal regime within which regional metamorphism and the generation and emplacement of the anatectic granites took place.