47th Lunar and Planetary Science Conference (2016) 1022.pdf

THE EAST KAIBAB MONOCLINE: A TERRAN LOBATE SCARP? Paul K. Byrne1, Christian Klimczak2, and Julia K. LaFond1. 1Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, USA ([email protected]); 2Department of Geology, University of Georgia, Athens, GA 30602, USA.

Introduction: The surfaces of Mercury, Venus, the The East Kaibab monocline: Extending about 180 Moon, and Mars abound with examples of tectonic km from southern Utah to northern Arizona, the East shortening deformation [e.g., 1–8]. This style of Kaibab monocline is one of several large anticlines deformation is typically manifest as positive-relief, within the Colorado Plateau [13]. The monocline is arcuate-to-linear landforms (Figure 1), either in likely underlain by an originally normal, high-angle isolation [9], concentrated into narrow regions [2], or basement fault that was reactivated with reverse offset distributed broadly across entire regions [10]. during the Laramide orogeny [14]. The portion of the A range of nomenclature including “wrinkle ridge” landform in southern Utah is ~50 km long and, striking and “lobate scarp” has arisen in association with these north–northeast, possesses almost 1.5 km of reverse landforms, with the former describing arch-like displacement and as much as 8 km of right-lateral structures and the latter pertaining to cliff-like displacement [15]. escarpments. Despite the broad morphological variety Despite differences in tectonic setting and of such extraterrestrial landforms, there is a general lithology, there are substantial similarities between consensus that they collectively result from horizontal Terran monoclines like the East Kaibab structure and shortening involving some combination of reverse extraterrestrial lobate scarps. Both types of landform faulting and folding [e.g., 11]. are convex in cross section and arcuate to linear in plan view, represent considerable horizontal shortening strain, and likely result from thrust-fault-related folding. Observations of the regional- and outcrop- scale geometry of the East Kaibab monocline, together with inferences about its subsurface geometry from elastic half-space modeling, may provide insights into the kinematic and geometric properties of lobate scarps not currently available from remotely sensed data alone. Field survey: A preliminary field assessment of the monocline yielded 40 strike and dip measurements along a 25 km stretch of the structure (Figure 2). Measurements were taken to characterize both the cross- and along-strike pattern of deformation within the monocline. The general shape of the monocline is apparent in the distribution of dips across strike. The peripheral Lower Jurassic Navajo Sandstone and Upper Figure 1. Color-coded, shaded-relief map of an unnamed Cretaceous Kaiparowits Formations, which represent lobate scarp in Mars’ southern hemisphere. A topographic the strata above the hanging wall and footwall blocks, profile (red line) of the landform is shown below the image. respectively, show relatively shallow (i.e., ~10°) dips to the east. The units that compose the monocline’s Our own world has no lack of shortening structures, forelimb, including the Middle Jurassic Entrada although most are concentrated in orogenic settings Sandstone formation in the north of our field area, and reflecting convergent plate motion. Nonetheless, some the Upper Cretaceous Tropic Shale formation in the examples of landforms on Earth thought analogous to south, show the greatest dips, which typically range crustal shortening structures on other bodies have been ~60–80° to the east. These units outcrop in the center documented [e.g., 12]. Here, we explore a portion of a of the eroded portion of the monocline, along which shortening landform in southern Utah, the East Kaibab the Cottonwood Canyon road meanders (Figure 2). monocline, and appraise its morphology, internal At outcrop scale, the monocline is characterized by structure, and underlying fault system as a Terran substantial brittle deformation. Folding has been counterpart to the lobate scarps observed on Mercury, accommodated locally by both normal and reverse the Moon, and Mars. faulting, resulting in single deformation bands, 47th Lunar and Planetary Science Conference (2016) 1022.pdf

complex fracture sets through individual strata, meter- monocline. Notably, the use of a cumulative dip slip of thick shear zones, and even decameter-size blocks that 2 km (greater than the 1.6 km inferred previously [15]) rotated such that their constituent beds have near- allows us to closely match both the geometry and vertical dips. Previous field studies have also location of folded strata within the monocline, as documented sets of transpressive faults with both left- documented in geological maps of the area [e.g., 17] as and right-lateral displacements along the length of the well as geological cross sections [15]. These Utah portion of the monocline [15]. displacements together define the pre-erosion shape of the landform, which bears a strong morphological similarity to landforms identified as lobate scarps on other silicate bodies (compare Figures 1 and 3).

Figure 3. Simulated vertical displacements for the main stratigraphic units in the monocline; red: present-day topography; dashed lines: reconstructed monoclinal fold. High -angle, blind reverse fault not shown.

Outlook: Our field and model data suggest that there are strong geometric and kinematic similarities between large-scale shortening landforms on Earth and other worlds. The East Kaibab monocline has been interpreted as a fault-propagation fold [15]. Our understanding of crustal shortening processes on other planetary bodies will be enhanced by applying similar structural interpretations to extraterrestrial landforms historically labeled as lobate scarps. References: [1] Strom R. G. et al. (1975) JGR, 80, 2478–2507; [2] Byrne P. K. et al. (2014) Nature Geosci., 7, 301–307; [3] Solomon S. C. et al. (1991) Science, 252, 297– 312; [4] Hansen V. L. and Olive A. (2010) Geology, 38, 467–470; [5] Maxwell T. A. et al. (1975) Geol. Soc. Am.

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