Fracturing in Entrada Sandstone at Arches National Park, Utah, and simulation of high-amplitude folding in viscous multilayers

Kenneth Menzies Cruikshank, Purdue University

Abstract

Fracture mechanics is used to interpret conditions responsible for minor cracks that adorn joints and faulted joints in the Entrada Sandstone. Some of the minor cracks are due to faulting, and others are due solely to interaction of joint segments at the time they formed. One type of minor crack associated with faulting is the kink, a short crack at the termination of a straight joint whose trend is abruptly different from that of the joint. The kink angle reflects the sense and magnitude of the shear stress responsible for kinking. The simplest minor fracture where there has been insignificant shearing is a veer, where the end of one joint segment turns gradually toward a nearby joint segment. Along band faults in Entrada Sandstone all stages in the development of a duplex structure are preserved--from isolated, stepped segments, to segments joined by tens of ramps. Mechanical and kinematic models used to simulate duplex structures along thrust faults reproduce the essential features of the band-fault duplexes, even though bedding had no significant role in constraining their geometry. A theory of folding involving layers of viscous fluids has proved useful as a rational basis for analyzing folding of multilayers. The solutions are approximated so that the analyses are limited to folding to low amplitudes, with maximum limb slopes of less than 30$\sp\circ$. I develop a numerical method, which is motivated by the basic theoretical development, for simulating high-amplitude folds that develop in a viscous multilayer during shortening. The method can better match boundary conditions along irregular interfaces, so it extends the range of slopes over which viscous-folding theory can be applied.

Degree

Ph.D.

Advisors

Johnson, Purdue University.

Subject Area

Geology|Geophysics

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