Abstract

Nappe piles of lower Mesozoic pelite in the Carson region of Nevada contain sheet-like carbonate complexes (breccia, unbrecciated masses, minor gypsum), referred to as carbonate breccia nappes and the rocks within them collectively as rauhwacke, after similar rocks in the Alpine chains. In the Carson region, rauhwacke occurs only in nappe piles of pelitic rocks, suggesting that nappe tectonics and rauhwacke origin were related.

The largest and most thoroughly studied carbonate complex, the Muttlebury nappe, consists of masses of deformed carbonate rocks, 2 m to 10 km in outcrop length, that are irregularly distributed through dominant carbonate breccia. The breccia, largely polymict and massive but locally stratified, is probably of intraformational origin. It is argued that rauhwacke of the Muttlebury nappe and elsewhere in the Carson region is derivative from detached masses of the Jurassic Lovelock Formation. Gypsum in the Lovelock Formation is believed to have been partially calcitized, yielding marble, the most abundant lithology of the Muttlebury nappe. Breccia is proposed to have originated by dissolution of gypsum that escaped calcitization and by subsidence and collapse of the intercalated carbonate rocks.

The Muttlebury nappe was emplaced as the sole of a thick pelite nappe, perhaps contemporaneously with solution brecciation. After emplacement, both nappes were folded with their substrate. It is hypothesized that translation of the nappe pair depended on the low strength of the gypsiferous proto-Muttlebury nappe. As gypsum dissolved, the rheologic contrast of the Muttlebury nappe and its surroundings diminished, and translation of the nappe pair was progressively hindered. Once the proportion of solution breccia was sufficient, further displacement was taken up by folding.

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