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Could Pantheon Fossae Be the Result of the Apollodorus Crater-Forming Impact Within the Caloris Basin, Mercury?

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Could Pantheon Fossae Be the Result of the Apollodorus Crater-Forming Impact Within the Caloris Basin, Mercury? Earth and Planetary Science Letters 285 (2009) 320–327 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl Could Pantheon Fossae be the result of the Apollodorus crater-forming impact within the Caloris basin, Mercury? Andrew M. Freed a,⁎, Sean C. Solomon b, Thomas R. Watters c, Roger J. Phillips d, Maria T. Zuber e a Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907, USA b Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA c Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, DC 20560, USA d Planetary Science Directorate, Southwest Research Institute, Boulder, CO 80302, USA e Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA article info abstract Article history: The ~40-km-diameter Apollodorus impact crater lies near the center of Pantheon Fossae, a complex of Accepted 20 February 2009 radiating linear troughs itself at the approximate center of the 1500-km-diameter Caloris basin on Mercury. Available online 9 April 2009 Here we use a series of finite element models to explore the idea that the Apollodorus crater-forming impact Editor: T. Spohn induced the formation of radially oriented graben by altering a pre-existing extensional stress state. Graben in the outer portions of the Caloris basin, which display predominantly circumferential orientations, have Keywords: been taken as evidence that the basin interior was in a state of horizontal extensional stress as a result of Mercury uplift. If the Apollodorus crater formed at the time of such a stress state, impact-induced damage to basin fill Caloris basin material would have caused basin material to move radially outward, leading to a decrease in the radial Pantheon Fossae extensional stress and an increase in the circumferential stress. If this change in differential stress was Apollodorus crater sufficient to induce failure, the predicted style of faulting would be radial graben extending outward from the exterior crater rim. The ~230-km radial extent of Pantheon Fossae implies, by this scenario, that the Apollodorus impact generated a large damage zone, extending to perhaps three crater radii (~60 km) or more. The calculations also suggest, under this scenario, that the Caloris basin fill had greater strength than the surrounding crust and that the basin uplift and extensional stress field prior to the Apollodorus impact were close to azimuthally symmetric. The location of Pantheon Fossae very near the center of the Caloris basin appears to be coincidental; any crater similar in size to Apollodorus and located within ~300 km of the basin center could have produced a radiating set of graben by the mechanism explored here. © 2009 Elsevier B.V. All rights reserved. 1. Introduction is the ~40-km-diameter Apollodorus crater. The near-coincidence of the center of symmetry for Pantheon Fossae and Apollodorus crater One of the most remarkable surface features imaged by the raises the question as to whether the two features are causally related. MErcury Surface, Space ENvironment, GEochemistry, and Ranging Although explanations for Pantheon Fossae unrelated to Apollodorus (MESSENGER) spacecraft during its first Mercury flyby (Solomon crater have been suggested (Head et al., 2008; Murchie et al., 2008; et al., 2008) was Pantheon Fossae (Fig. 1), a complex of troughs Watters et al., 2009-this issue), in this paper we examine the radiating from a location near the center of the Caloris impact basin hypothesis that the two features are physically linked. Specifically, (Murchie et al., 2008; Watters et al., 2009-this issue). The troughs are we explore whether the Apollodorus crater-forming impact modified interpreted as simple graben formed in response to near-surface the state of stress within the central Caloris basin floor in a manner extensional stresses in the basin interior. The radially oriented graben that led to the formation of many, if not all, of the graben of Pantheon appear to be part of a larger pattern of extensional features within the Fossae. basin floor; beyond about 300 km radial distance from the basin We begin with a brief overview of the imaging observations of center the graben are more typically circumferential to the basin Pantheon Fossae, Apollodorus crater, and their geological relations to rather than radial, although radial trends are also seen and the distal other features within the floor of the Caloris basin. We then explore graben in places form a polygonal pattern (Watters et al., 2005). constraints on and models for the state of near-surface stress within Pantheon Fossae is noteworthy as well because within a few the Caloris basin at or near the time of the Apollodorus crater-forming kilometers of the projected center of the radiating pattern of troughs impact. We apply the concept of impact-induced damage to estimate, by means of finite element models, the change in lithospheric stress ⁎ Corresponding author. Tel.: +1 765 496 3738; fax: +1 765 496 1210. and strain that would accompany such an impact. We investigate the E-mail address: [email protected] (A.M. Freed). affect on those solutions of different assumptions regarding the 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.02.038 A.M. Freed et al. / Earth and Planetary Science Letters 285 (2009) 320–327 321 Fig. 1. (A) Tectonic map of the Caloris basin showing graben (black) and wrinkle ridges (red), overlaid on a high-resolution narrow-angle camera mosaic constructed from MESSENGER and Mariner 10 images. (B) MESSENGER image of Pantheon Fossae; the area of the image is indicated by the white box in (A). (C) Close-up of Apollodorus crater and its relation to the proximal graben of Pantheon Fossae. Adapted from Murchie et al. (2008). detailed state of stress at the time of impact, the radial extent of 3. Pre-Apollodorus stress state damage, the thickness of the mechanical lithosphere at the time of impact, and any contrast in strength between different rock types If the Apollodorus crater is related to the formation of Pantheon within the Caloris basin floor. We also explore the sensitivity of the Fossae, a likely explanation is that the Apollodorus impact modified a models for stress state change to the particular location of the pre-existing state of stress. We presume that the stress state Apollodorus impact relative to the center of the basin. We conclude immediately prior to the formation of the Apollodorus crater may with an assessment of the likelihood that the formation of the be specified by one of the recent models of Kennedy et al. (2008) for Apollodorus crater led to the formation of the graben of Pantheon stress and deformation in and around the Caloris basin. In that study a Fossae, as well as the future observations that would permit the idea series of axisymmetric finite element models were used to test a range to be tested further. of lithospheric loading scenarios to account for the observation from Mariner 10 images that predominantly basin-concentric contractional features predate dominantly basin-concentric extensional features on 2. Observations of faulting within Caloris basin the outer floor of the basin (neither the innermost portions of the floor nor the entire western half of the basin were imaged during the Pantheon Fossae consists of more than 200 linear troughs that Mariner 10 flybys). From these axisymmetric models, it was shown radiate out from a locus near the center of the Caloris basin (Fig. 1, that the earlier contractional features on the basin floor are consistent Murchie et al., 2008; Watters et al., 2009-this issue). The graben range with subsidence accompanying a flexural response to partial basin in length from about 5 to 110 km and in width from less than 1 km to infilling by smooth plains deposits, presumed to be volcanic in origin. as much as 8 km (Murchie et al., 2008). A large number of graben A variety of fill thicknesses, emplacement scenarios, and lithospheric extend outward from the center of the Caloris basin in virtually all thicknesses are consistent with the observed pattern of interior directions to radial distances as great at 230 km. Some graben contractional faulting associated with subsidence. continue outward to greater distances within limited azimuthal bands Models that accounted for the locations of contractional features to (Murchie et al., 2008; Watters et al., 2009-this issue), and the distal 500-km radial distance from the basin center required surface loading portions of some of the radiating graben were seen in Mariner 10 of nearly uniform thickness over most of the basin. This result images and mapped as part of the polygonal pattern of basin- suggested that the pre-fill Caloris basin floor was flatter than the concentric and basin-radial troughs in the eastern portion of the outer geometry inferred for similar-sized basins on the Moon (Watters et al., Caloris basin floor (Strom et al., 1975; Watters et al., 2005). Nowhere 2005; Fig. 2 of Kennedy et al., 2008). This inference of a flat basin floor are the graben embayed by interior smooth plains deposits, implying was in part based on an assumed basin diameter of 1300 km as that the faulting postdates the partial volcanic infilling of the basin indicated by Mariner 10 observations. MESSENGER observations, in (Murchie et al., 2008). contrast, suggest a basin diameter of about 1500 km (Murchie et al., The Apollodorus crater, 41 km in diameter, is located near the 2008), which enables basin fill to extend farther from the basin center. center of the Pantheon Fossae complex. Like several other large craters A larger basin allows for a partial relaxation of this implied flat-floor in the floor of Caloris, Apollodorus exposed low-reflectance material, geometry, as it becomes easier to explain contractional features to distinct in spectral properties from the Caloris interior plains 500 km if the fill extends to 750 km.
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