The erosion factor in the emplacement of the Keystone thrust sheet (South East Nevada) across a land surface

Abstract

The Keystone/Muddy Mountain thrust of South East Nevada provides a spectacular example of a thrust sheet that has travelled across the earth's surface for a distance of at least 25 km, probably as much as 50 km. Along the thrust a Palaeozoic to early Mesozoic succession (> 4 km thick) has moved over a channelled erosion surface cut into Jurassic aeolian sandstone. Clasts in the channels are stratified, with those in upper layers being easily matched with the rocks in the thrust sheet – thus the clasts ‘herald’ the approach of the sheet. The clasts are only weakly deformed by the thrusting which belongs to the Sevier-Laramide orogeny.Erosion appears to be a significant if not crucial factor in the emplacement of the Keystone thrust sheet. Adopting an erosion rate of 1 mm/a (in accord with Alpine and Himalayan estimates) and a rate of thrusting of 5 mm/a (in accord with estimates from the Canadian Rockies) it follows that the thrust sheet was ‘half-eroded’ after travelling about 25 km.It is concluded that large displacement (i.e. > 25 km) may be impossible without erosion and one of two ‘erosion models’ would greatly facilitate transport up to 50 km, which is the likely maximum displacement.