DEFORMATION IN THE CORE OF A FOLD: IMPLICATIONS FOR FOLD-TIGHTENING MECHANISMS

Cores of folds often hold critical clues to an entire fold’s evolutionary history. Fold shape adjustments often begin in the fold core and continue to take place there because this is the region where space problems predominantly arise. Retrodeforming a fold to unravel its kinematic history can often be problematic because many stages of the deformation may not be adequately recorded in the rocks. For example, in the plastic regime, evidence for early stages of deformation may be masked as a result of recrystallization. Folds that form in the elastico-frictional (EF) regime, by fracturing and cataclastic flow, may preserve a fold’s history more completely; younger fractures simply cross-cut older fractures rather than erase the previous stages of deformation.

The Canyon Range (CR) syncline, part of an internal thrust sheet in the central Utah segment of the Sevier fold-thrust belt (FTB), fold tightened under shallow crustal conditions (at depths <4 km), so the fold evolved within the EF regime, predominantly by cataclastic flow. As the syncline tightened, synorogenic conglomerates eroding from an adjacent anticline to its west were deposited in its core. Together with the host beds, these synorogenic conglomerates were folded by cataclastic flow. The cataclasized rocks in the conglomerate and adjacent older beds interact minimally, even though both are intensely fractured. The rheology and type of cataclasis that took place in the two types of rocks are quite different; the conglomerates have a significant amount of matrix whereas the older beds essentially have none and the boundary between them is not significantly disturbed. In addition to the complex cataclasis that took place in the core, several types of macroscale structures, such as out of the core thrusts and parasitic folds were formed. Some of these structures cross cut lithologic boundaries while others are confined to individual rock types.

The fractures (micro- and meso-scales), conglomerates, small scale folds and out-of-the-core thrusts (all within the core of the fold) were carefully analyzed in order to incrementally retrodeform the CR syncline. Our analysis shows that the hinge of the CR syncline migrated and that there was a significant amount of deformation parallel to the fold hinge (i.e. non plane strain).