Fracture character and distribution in detachment folds

Dr. Catherine Hanks, Research Associate Professor

Graduate Students: Joe Brinton (M.S.); J. Ryan Shackleton (M.S.)

Abstract from December 2001 DOE report:

Timing and character of mesoscopic structures in detachment folds and implications for fold development--an example from the northeastern Brooks Range, Alaska.

C.L. Hanks, W.K. Wallace, J. Lorenz, P.K. Atkinson, J. Brinton and J.R. Shackleton

In detachment-folded Lisburne Group carbonates of the northeastern Brooks Range, different mesoscopic structures formed at different times in the evolution of individual detachment folds, providing clues to the mechanism of folding and the conditions under which the fold formed. Rocks in advance of the visible thrust front experience similarly oriented differential stresses as in the fold-and-thrust belt, but of lower magnitude, resulting in orogen-perpendicular extension fractures. These rocks are later incorporated into the thrust belt, where they are thrust-faulted and folded. The distribution of fractures and other mesoscopic structures in individual folds suggest that folding occurs by both flexural slip and homogeneous flattening. Flexural slip and associated fracturing occurs early in the development of the fold and/or in the outer arc of the fold. These early fractures may be overprinted and/or destroyed by ductile strain as later homogeneous flattening accommodated additional shortening. The penetrative strain is in turn overprinted by late extension fractures, which probably formed during the waning phases of folding and/or unroofing of the orogenic wedge.

Abstract submitted to 2002 Pacific Section, AAPG

Limitations on the thermal conditions during formation of fractures and detachment folds in the northeastern Brooks Range

C. L. Hanks, University of Alaska, Fairbanks, AK, T.M. Parris, U.S. Geological Survey, Denver, CO, and W. K. Wallace, University of Alaska, Fairbanks, AK

Fractured and detachment-folded Mississippian-Pennsylvanian Lisburne Group carbonates and overlying Permo-Triassic clastic rocks are potential exploration targets in the foothills of the Brooks Range of northern Alaska. Along-strike examples of these folds and associated fractures are well exposed in the nearby northeastern Brooks Range. While paleo-thermal indices in the host rock limit the conditions of folding to temperatures equal to or less than 250 deg. Celsius, field and petrographic relationships suggest that different fracture sets formed under different temperature-pressure conditions. Many early fractures that formed on fold limbs due to flexural slip were subsequently overprinted by penetrative strain. While these fractures may have developed at lower temperatures, subsequent penetrative strain probably occurred at the upper end of the temperature range, suggesting that folding culminated at relatively high temperatures. The apparent genetic relationship of early fractures to structures that formed at high temperatures, and the absence of oil-bearing fluid inclusions in cemented early fractures suggest that early fractures postdated any oil migration that might have occurred in the area.

Later pervasive extension fractures show little sign of subsequent deformation and probably formed at lower temperatures and pressures during the waning phases of folding and/or unroofing. These late extension fractures also likely postdated local oil migration.

Abstract from January 2001 DoE report:

The relationship between fracturing, asymmetric folding, and normal faulting in Lisburne Group carbonates:
West Porcupine Lake Valley, northeastern Brooks Range, Alaska

J.R. Shackleton, C.L. Hanks, and W.K. Wallace

Abstract

The area of Porcupine Lake Valley in the northeastern Brooks Range (NEBR) is at a major structural transition between symmetric detachment folds that are characteristic of the NEBR proper, and asymmetric thrust truncated folds resembling those along the main axis of the Brooks Range. Lisburne Group carbonates in the western end of Porcupine Lake Valley are locally folded into strongly asymmetric NE striking and plunging (detachment?) folds characterized by short, steep to overturned forelimbs, and long (up to1 km) gently dipping backlimbs. Only one thrust fault was documented in the NW end of the field area that places a long, relatively flat panel of Lisburne Group carbonates above the Sadlerochit Group. NE and NW striking normal faults with relatively small displacements cut folds in West Porcupine Lake Valley.

Four major sets of extension fractures were documented in West Porcupine Lake valley, the majority of which dip steeply between 60º-90º in both directions: 1) a N-S striking set; 2) an E-W striking set; 3) a N-S to NW striking set; and 4) a NE striking set. While the relative timing of each of these fracture sets is unclear, some generalities can be made. The NW set appears to be younger than the N-S and E-W sets. E-W fractures terminate against N-S fractures at most sample locations. However, the opposite relationship was documented elsewhere in the field area, possibly suggesting multiple generations of N-S and E-W fracturing. All three fracture sets were found in en echelon sets of extension fractures, which indicate a component of shear during formation. Shear sense on these sets was commonly normal or strike slip, suggesting that many fractures are related to normal faulting in the area. The N-S and NW striking fractures were often found in 3-5 meter wide swarms of en echelon fractures, each swarm spaced approximately 10-20 meters apart. NE striking fractures were well developed in the lower portions of one of the major synclines in the area, although the timing of these
fractures is unclear. Other major mesoscopic-scale structures indicate some period of penetrative semi-ductile deformation, including dissolution cleavage, deformed crinoid stems, sheared stylolites, and elongated and transposed chert nodules.

Normal faulting in West Porcupine Lake Valley is atypical for the NEBR, and may have influenced fracture character and distribution. Cross cutting relationships suggest that NE striking faults occurred after thrusting, whereas folds truncated by hinge sub-parallel normal faults suggest that normal faulting may have occurred during folding, or may have significantly modified fold geometries after a previous phase of compressional deformation. Changes in fold geometry were observed across NW striking normal faults, suggesting that either the normal faulting modified fold geometries, or that these faults