THE EARLY PROTEROZOIC PAYSON OPHIOLITE AND THE MAZATZAL CRUSTAL BLOCK, CENTRAL ARIZONA: COINCIDENTAL OR CAUSAL RELATIONSHIP?

Along the east flank of the Mazatzal Mountains, siliciclastic sediments of the Mazatzal Group unconformably overlie the 1.73 Ga Payson Ophiolite (PO) and basin-filling turbidites and volcaniclastic sediments. This unconformity extends across the Mazatzal crustal block (MCB) that lies east of the Moore Gulch fault and is characterized by an extensive granite-rhyolite suite and very low metamorphic grades. In contrast to the MCB, much greater uplift and erosion exhumed quarrtzite-rhyolite sequences with similar ages but higher grades in other parts of the Early Proterozoic orogenic belt. Also unique in the belt, the PO has the sheeted dike complex characteristics of Phanerozoic ophiolites and modern submarine crust formed by seafloor spreading. After ca. 1.70 Ga Yavapai convergent deformation, the seafloor crust of the PO was exposed at sea level and buried by siliciclastic sediments. Preservation of Late Proterozoic and Cambrian unconformities at similar elevations attests to the lack of significant uplift, until formation of the Colorado Plateau. In contrast, many Phanerozoic ophiolites are thrust over older crust, and at high structural levels, erosion may erase them from the geologic record, quickly relative to the age of the PO. Likewise, the Early Proterozoic orogen may have lost many ophiolites, now represented by rare fragments preserved along structures in the more deeply exhumed crustal blocks. As a result, the unique crustal profile and uplift history of the MCB may be responsible for preservation of the PO. On the other hand, the tectonostratigraphic history recorded by the PO and MCB, leading up to convergent deformation, may have predisposed the crustal section to its subsequent history of minimal uplift and erosion. The presence of older crust within the PO and arc-parallel extension support a model of formation and emplacement of the ophiolite within the arc (an intra-arc basin) along arc-parallel strike-slip faults, prior to convergent deformation. According to this model, the intra-arc basin crust, modified by magmatic events, responded to convergent deformation to create a crustal profile with minimal over-thickening.