PENNSYLVANIAN SYN-DEPOSITIONAL FOLDS IN FAR WEST TEXAS, ANCESTRAL ROCKIES DEFORMATION MAY EXPLAIN COMPLEX RELATIONSHIPS IN THE FRANKLIN MOUNTAINS

The Franklin Mountains are a west-tilted, normal-fault footwall in far west Texas. On the western side of the range, a complex fault system puts Permian and Cretaceous strata against strata ranging in age from Proterozoic through Permian. It has been difficult to restore this structure due to its complex history and due to the fact more than 2 km of relief is exposed along the western upthrown side. Much of this can be explained by deformation during Ancestral Rockies tectonism that has not been considered in the history of the range.

A northern extension of the Franklin Mountains, the Anthony Hills, exposes Ordovician through Permian strata in a relatively unfaulted block. The Ordovician through early Pennsylvanian strata are folded by several westward plunging anticlines and synclines. The folds are open (interlimb angles are 120º), forming a monocline stepping down from the North. Axial planes dip North (250 º; 15º). The axis plunges northwest (305º 23º) indicating southwest-northeast shortening. Small folds and faults in the core of the syncline similarly show compression to the Northeast.

The upper part of the Pennsylvanian strata are much less folded and apparently thin onto the flank of the monocline. Construction of a 3D model using the Leapfrog modelling package allowed measurement of thinning. The overall thinning was 150 m and one interval thinned from 210 m to 106 m along the flank of the monocline. This observation indicates syn-depositional formation of the folds and requires deformation during the Pennsylvanian Ancestral Rockies Orogeny. Neogene extension tilted the Franklins West 40º along a North-South axis. Restoration of the Permian strata to horizontal rotates the fold axis to a gentle southeast plunge (126º; 11º), with a near vertical axial plane.

The restored folds show compression toward the Ancestral Rockies Orogrande Basin centered 60 km northeast of the Franklin Mountains. This study suggests Ancestral Rockies deformation extended farther southwest than currently thought. If the Franklin Mountains formed part of the Orogrande basin, it would explain the younging of the west side of the range to the north. The Hueco Mountains, 45 km east show a similar relationship, with the Permian in angular unconformity cutting into older units to the south and then overlying a thickening Pennsylvanian section.