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Description
The Whipple Mountain Thrust Fault separates two lithologically distinct metamorphic rock associations. Although partly covered by Tertiary volcanic rocks, the severely mylonitized zone which defines the trace of the fault is sufficiently exposed to permit the reconstruction of the fault plane. The rocks above the mylonitic zone are non-cataclastic quartzofeldspathic gneisses; those below are compositionally banded quartzofeldspathic and amphibolitic gneisses and schists. The mineralogy of both rock associations is consistent with amphibolite facies metamorphism. The mylonitic zone varies in thickness from 80 to 400 feet and consists of a mineral assemblage typical of greenschist facies metamorphism. The trend of large and small scale fold axes and elongated quartz grains in the rocks of the lower plate are consistently N 40° E although the strike of the foliation is N 40° E on the northeast side of the range and N 50° W on the south side. The strike of the foliation in the upper plate rocks is generally northwest with a southwestern dip; there is no folding or mineral lineation. The strike of the foliation in the mylonitic zone is N 50° W, the trend of the lineation of the elongated quartz grains is N 40° E, both the plunge and dip are to the southwest. The above information suggests that there have been at least two periods of pre-Tertiary deformation in the Whipple Mountains. The structure and mineralogy of the upper and lower plate rocks are indicative of a period of regional metamorphism in the amphibolite facies. The attitude of the fold axes in the lower plate rocks indicates that the stress came from the northwest or southeast. Mineral lineations in the mylonitic zone, combined with the greenschist facies mineralogy, are suggestive of a subsequent episode of thrusting directed in a northeast direction. Tertiary volcanic flows and sedimentary rocks commonly rest directly on the thrust zone mylonite and upper plate rocks, but not on those of the lower plate. This is probably explained by the development of a pre-volcanic Tertiary erosion surface controlled by the relative erosional resistance of mylonitic rocks. Post-Miocene northwest-trending normal faults in the eastern sector of the range have tilted both the volcanic and underlying basement rocks. The present configuration of the Whipple Mountains is that of an eroded elongate dome exposing a window of lower plate rocks. This doming occurred after the deposition of the Tertiary rocks and was probably produced at the same time as the normal faults. The Whipple Mountains and several lithologically and geologically similar northeast trending ranges in adjacent Arizona are bounded by two northwest trending lineaments. These ranges constitute a distinct tectonic province. K-Ar dates obtained on the rocks of the Whipple Mountains are consistent with the 70-140 m.y. dates obtained on rocks involved in thrusting in the Sevier orogenic belt. This combined with similarity in tectonic style and direction of movement suggests that the Whipple Mountain Thrust Fault is related to the Sevier orogenic belt.
