Deep-Seated Thrust Ring Faults Bound Elevated Mantle Plug Beneath Several Lunar Basins

52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 2447.pdf

DEEP-SEATED THRUST RING FAULTS BOUND ELEVATED MANTLE PLUG BENEATH SEVERAL LUNAR BASINS. Matthew S. Collins1, Paul K. Byrne1, Christian Klimczak2, and Erwan Mazarico3, 1Planetary Research Group, Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC, 27695, USA; 2Department of Geology, University of Georgia, Athens, GA 30602, USA; 3NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.

Introduction: Wrinkle ridges are broad, low-relief single morphology consistently characterizes these topographic landforms thought to reflect crustal landforms. shortening by some combination of thrust faulting and folding [e.g., 1]. On the Moon, wrinkle ridges are abundant within the mare deposits and, at several mascon basins, demarcate the inner edge of an elevated topographic bench that parallels at least a portion of the perimeter of the basin [2–5]. At Mare Crisium, the bench-bounding ridges were found to be situated atop outward-dipping, large (~20 km-deep) thrust faults that bound the elevated mantle plug beneath the basin [2]. The methodology established for Mare Crisium [2] was applied to a number of other mare-filled mascon basins. Here, we present the results for Mare Serenitatis and Mare Nectaris.

Figure 2. Model solutions are shown for four landforms within Mare Serenitatis (a–d; Figure 1a), along with the corresponding crust–mantle model depicting the geometric relationship between the “best-fit” fault solution and the elevated mantle plug (e–h; Figure 1a). RMSE values were calculated for the domain represented bounded by the black (start) and white (finish) circles along each cross-section (a– d). Figure 1. The distribution of mapped wrinkle ridges (thin black lines) relative to topography (left) and GRAIL-derived Bouguer gravity anomaly (right) for Mare Serenitatis. Solid Fault Modeling: Coulomb 3.3, an elastic ® black lines (a–d) in Figure 1a are lines of section along dislocation modeling package for Matlab , was used to which elevation values were extracted for use in modeling model vertical surface displacement as the function of landforms (Figure 2a–d). Dashed black lines (e–h) in Figure slip along an underlying thrust fault [2,6]. Fault 1a are lines of section along which elevation data and crustal parameters, including dip angle, penetration depth, thickness values (Wieczorek et al., 2013) were extracted to burial depth, and total displacement, were varied for visualize the relationship between our modeled fault each model iteration. solutions and the crust–mantle interface (Figure 2e–h). Topographic profiles from the SLDEM2015 digital elevation model [7] were extracted perpendicular to the Mapping: Shortening structures were identified and strike of several shortening structures within each of the mapped on the basis of a morphological and study basins (Fig. 1). The mismatch between observed topographic signature generally characterized by a topography and modeled results was quantified by steeply dipping forelimb and tapered backlimb [1] not calculating the root-mean-squared error (RMSE) for obviously related to cratering, ejecta blanketing, or each fault configuration. The fault model with the normal faulting. However, the asymmetry of these minimum RMSE value was taken to be representative structures is variable and likely related to along-strike of the real-world fault geometry (Fig. 2). changes in the geometry of underlying faults; no one 52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 2447.pdf

Modeling Results: Ridges were selected based on that, as for Mare Crisium, an outward-dipping reverse their relatively preserved morphology and approximate ring-fault system localized by the isostatic adjustment collocation with the maximum gravity gradient. Our of a superisostatic mantle plug and subisostatic crustal model solutions indicate that typical faults responsible collar is characteristic of several large mascon basins on for these landforms have dip angles of 12–28° and the Moon. penetrate 14–28 km into the lunar lithosphere (Figs. 2, 4).

Figure 3. The distribution of mapped wrinkle ridges (thin black lines) relative to topography (left) and GRAIL-derived Bouguer gravity anomaly (right) for Mare Nectaris. Solid Figure 4. Model solutions are shown for three landforms black lines (a–d) in Figure 3a are lines of section along within Mare Nectaris (a–d; Figure 3a), along with the which elevation values were extracted for use in modeling corresponding crust–mantle model depicting the geometric landforms (Figure 4a–d). Dashed black lines (e–h) in Figure relationship between the “best-fit” fault solution and the 3a are lines of section along which elevation data and crustal elevated mantle plug (e–h; Figure 3a). RMSE values were thickness values (Wieczorek et al., 2013) were extracted to calculated for the domain represented bounded by the black visualize the relationship between our modeled fault (start) and white (finish) circles along each cross-section (a– solutions and the crust–mantle interface (Figure 4e–h). d). Subsurface Geometry: We also compared the Implications: The results of this and previous work subsurface geometry of our modeled fault solutions with [2] shows that circumferential, deep-seated thrust faults the geometry of the elevated crust–mantle interface coincide with the boundary of an elevated mantle plug beneath each basin. Elevation values (from the beneath maria Crisium, Serenitatis, and Nectaris. This SLDEM2015 topographic model [7]), as well as crustal work demonstrates that such a fault geometry persists thickness values from ref. [8], were extracted along for a number of lunar mascon-bearing impact basins; radial lines of section that parallels the cross-sections indeed, such an arrangement may also be present within used for fault modeling and extends outward from the mascon basins on other terrestrial bodies, including center of the basin (Fig. 1) [2]. Crustal thickness values Mercury and Mars. were then subtracted from the elevation values to References: [1] Schultz R.A. (2000) JGR, 105, generate a model of the crust–mantle interface. If the 12035–12052. [2] Byrne P.K. et al. (2015) EPSL, 427, model developed for Mare Crisium [2] holds true for the 183–190. [3] Solomon S.C. and Head J.W. (1980) Rev. Geophys. Space Sci., 18, 107–141. [4] Bryan W.B. basins analyzed in this study, differential stress was th concentrated along the crust–mantle boundary to form (1973) Proc. Lunar Sci. Conf., 4 , 93–106. [5] Maxwell faults that parallel this boundary [2]. In the case of Mare T.A. et al. (1975) Geol. Soc. Am. Bull., 86, 1273–1278. Serenitatis and Mare Nectaris, our modeled faults do [6] Lin and Stein (2004) JGR: Solid Earth, 109, indeed parallel the crust–mantle boundary beneath the B02303. [7] Barker M.K. et al. (2016) Icarus, 273, 346– basin (Figs. 2, 4). This geometric arrangement suggests 355. [8] Wieczorek et al. (2013) Science, 339, 671–675.