Abstract

The Amazon fault passes eastward from south of Guayaquil on the Pacific to the Amazon mouth on the Atlantic coast. Along its eastern segment, a rift with pyroxenite intrusions opened up at the start of Paleozoic time; its Early Jurassic reactivation produced more than 100,000 km3 of diabase sills, intruded into the evaporitic upper Paleozoic simultaneously with North Atlantic rifting. Pliocene reactivation faulted the coastal Tertiary and sheared the subducted slab beneath the Pacific end of the fault. The northeast-trending Pisco-Juruá fault, first described here, cuts across the continent from Pisco on the Pacific to the Guyana-Surinam border on the Atlantic coast. In early Mesozoic time the fault formed the southwestern continuation of the North Atlantic Rift; its Guyanan end opened up as the North Atlantic Rift opened. The separation of North and South America caused northwestern South America to move southwest along the Pisco-Juruá fault, creating the Tacutu graben in the Guyanan Shield, the gently folded Juruá zone across the Amazon Basin, and the Pisco-Abancay deflection in the Andes. Pliocene reactivation at the Pacific end of the fault sheared the subducted slab of the Nazca plate and may have contributed to the formation of the Nazca Ridge.

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