Gail Morrissey Arizona State University Structural Attenuation in the Big Maria Mountains, Southeastern California: An Investigation with the Thermal Infrared Multispectral Scanner (TIMS) Gail Morrissey Arizona State University

Relationship between the syncline and the rock units in the range Introduction The Big Maria Mountains contain a spectacular example of large-scale folding; namely, a huge overturned syncline. The upper limb of the syncline has been severely attenuated, within which the Paleozoic units have been progressively thinned to less than 1% of their original thickness. The mechanism that caused this attenuation has never been studied. Relationship between the syncline and the rock units in the range

Introduction (Cont.) There are two possible mechanisms that could account for this thinning: ductile flow or brittle faulting. In the first, the rock unit would deform by stretching and flowing, with a taffy-like consistency. In the second, the unit would be broken into blocks by a series of parallel faults, thinning as the blocks slide past each other (similar to a row of books falling over.) Deformation occurred simultaneously with a metamorphic event. Geothermometry in the range shows that temperatures were between 450 to 600ºC during the time of deformation (Hoisch 1987, 1988 and Hamilton 1987). Given that quartz and calcite will behave ductilely at such high temperatures, I propose that attenuation was a ductile process.

Quartz and Calcite The Paleozoic section of the Big Maria Mountains is composed of Grand Canyon type rocks. Most of these units contains either quartz or calcite: there are almost pure sandstone units, such as the Coconino, and limestone units, such as the Redwall; there are mixed units, such as the Supai; and there are quartz rich pods and veins throughout the range. Quartz and calcite both become ductile at temperatures above 250°C. This is much lower than the calculated temperatures of metamorphism. In the Paleozoic section of the range, the other rock types are mainly clays and micas. These materials should also behave ductilely at lower temperatures. Elsewhere, there are granites and mafic dikes, but these have only a limited effect on the Paleozoic. Metamorphism will change the original composition of the units. However, the pure quartz and calcite units will remain essentially pure, preventing other minerals from changing the ductility.

Structural Attenuation Attenuation is in the upper, overturned limb of the syncline. Can be thinned to 1% of original thickness. All units are equally effected, yet remain coherent and distinguishable. Internal folding causes some units to appear thicker.

Previous Work - Syncline and Attenuation Theory 1: The syncline was caused by warping of the craton along zones weakened by fluids (Hoisch et al., 1988). Theory 2: The syncline was formed between two rising granitic Jurassic plutons. Attenuation initiated as subvertical stretching along the Paleozoic- Jurassic contact (A) (Ellis et al. 1981). Later underthrusting to the north contributed to the pre-existing attenuation. This later attenuation occurred subhorizontally (B).

Testing through TIMS Examine attenuated portions of the Thermal Infrared Multispectral Scanner (TIMS) lines for faults and folds. An exceptional example of the attenuation can be seen in the quartzite unit (red on the image) on the left side of the image. Any gaps in the unit may represent fault blocks. However, these can also be places where the unit is covered by float from another rock type. Continuity of the band, even where extremely thinned, might indicate the presence of ductile flow. Width = ~1000 m

Testing through Field and Lab Work Field mapping should show the presence of either faults or folds within the attenuated units. Analysis of additional structural features should support one of the two methods. For faulting, these include the presence of fault breccias and cataclasites. For folding, these include mylonites. Striations, lineations, and foliations will result in a sense of shear for the thinning. Thin sections taken from the thinned areas should show microstructures that support one of the methods. These would serve to reinforce the results of the macrostructure study or present new evidence.

Possible Results At finer resolutions, the attenuated units appear continuous. This would support a ductile method of attenuation. Small-scale folds are present in almost every unit in every area of the range. It would seem unlikely the attenuation would occur as a brittle process given the overall ductile nature of the rocks. The extreme temperatures would allow most, if not all, of the rock units to behave in a ductile manner.

Conclusions While much of the work remains to be done, preliminary mapping and analysis of the TIMS lines seems to indicate that attenuation was ductile in the range. This is also supported by the ductile nature of quartz and calcite at the temperatures present. These studies will serve to further constrain the structural history of the range, as well as the surrounding area. Additionally, the level of ductility of the varies minerals may serve to further constrain the temperatures of metamorphism in the range.