Depth of faulting and ancient heat flows in the Kuiper region of Mercury from lobate scarp topography a d e Isabel Egea-Gonzalez .•, Javier Ruiz b, (arlos Fernandez c, Jean-Pierre Williams , Alvaro Marquez , a Luisa M. Lara a Instituto de Astror1Sica de Andaluda, CSIC, Glorieta de la Astronomia sin, 18008 Granada, Spain b Departamento de Geodinamica, Facultad de Ciencias Geo16gicas, Universidad Complutense de Madrid, 28040 Madrid, Spain C Departamento de Geodinamica y Paleontologla, Universidad de Hue/vG, Campus de El Cannen, 21071 Hue/vG, Spain d Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095, USA e Area de Geolog{a, Universidad Rey Juan Car/os, Mostoles, Madrid, Spain ABSTRACT Mercurian lobate scarps are interpreted to be the surface expressions of thrust faults formed by planetary cooling and contraction, which deformed the crust down to the brittle-ductile transition (BOT) depth at the time of faulting. In this work we have used a forward modeling procedure in order to analyze the relation between scarp topography and fault geometries and depths associated with a group of prominent lobate scarps (Santa Maria Rupes and two unnamed scarps) located in the Kuiper region of Mercury for which Earth-based radar altimetry is available. Also a backthrust associated with Keywords: one of the lobate scarps has been included in this study. We have obtained best fits for depths of Mercury faulting between 30 and 39 km; the results are consistent with the previous results for other lobate Lobate scarps scarps on Mercury. Depth of faulting The so-derived fault depths have been used to calculate surface heat flows for the time of faulting, Brittle-ductile transition taking into account crustal heat sources and a heterogeneous surface temperature due to the variable Heat flow insolation pattern. Deduced surface heat flows are between 19 and 39 mW m-2 for the Kuiper region, and between 22 and 43 mW m-2 for Discovery Rupes. Both BOT depths and heat flows are consistent with the predictions of thermal history models for the range of time relevant for scarp formation. 1. Introduction 10, using a forward modeling procedure for fitting the topography above a thrust fault to stereographically deduced topography. The most representative tectonic features of Mercury are lobate Recently, Ritzer et al. (2010) obtained a depth of faulting of35 km scarps, which are characterized by a steeply rising scarp face, a for two lobate scarps located near the equator using a similar gently declining back scarp and a trailing syncline (Strom et al., procedure and a topographic profile deduced from the Mercury 1975; Cordell and Strom, 1977; Melosh and McKinnon, 1988; Laser Altimeter (MLA) onboard the MESSENGER spacecraft. Watters et al., 2001, Watters and Nimmo, 2010), and were mostly The so-obtained depths of faulting provide constraints on the formed in the Tolstojan and Calorian periods (Watters and Nimmo, mechanical and thermal properties of the lithos ph ere at the time 2010), corresponding to an age between 3.2 and 4 Gyr (e.g., Tanaka when the lobate scarps were formed (e.g., Schultz and Watters, and Hartmann, 2008). Lobate scarps are interpreted to be the sUlface 2001; Grott et aI., 2007; Ruiz et aI., 2008). Watters et al. (2002) expressions of thrust faults formed by planetary cooling and con­ deduced surface heat flows of 10-43 mW m-2 from the depth of traction (e.g., Strom et al., 1975 ) and estimates of their depth of faulting beneath Discovery Rupes using a linear thermal gradient faulting suggest that they defonned the crust down to the brittle­ and assuming a wide range of temperatures at the BDT. Nimmo ductile transition (BDT) depth at the time of fault formation, and Watters (2004) derived an upper limit of 50 mW m-2 for the providing important clues about the geological and thermal history mantle heat flow using the BDT depths calculated by Watters of Mercury (Watters et al., 2002; Nimmo and Watters, 2004). et al. (2002), a strength envelope procedure and considering heat Watters et al. (2002) obtained a depth of faulting of 35-40 km generation within the crust. The obtained BDT depths and heat for Discovery Rupes, a prominent lobate scarp imaged by Mariner flows can be compared with predictions from thermal history models (e.g., Hauck et al., 2004; Williams et al., 2011) in order to further constraint the thermal history of Mercury. .. Corresponding author. Tel.: +34 958121566; fax: +34958814530. In this work we use a forward modeling procedure in order to E-mail address: [email protected] (I. Egea-Gonzalez). analyze fault geometries and depths associated with a group of prominent lobate scarps located in the Kuiper region of Mercury for which Earth-base radar topographic profiles are available (Harmon et al., 1986): Santa Maria Rupes and two unnamed lobate scarps referred to as S_K3 and S_K4 scarps. Calculations of surface heat flow have been performed from the BOT depth beneath these lobate 6°0'O"N scarps (and beneath Discovery Rupes for comparison) by assuming heat sources homogeneously distributed in the crust. Crustal heat sources abundances are based on preliminary surface measure­ ments of Th and K performed with the MESSENGER Ganuna-Ray Spectrometer (GRS) (Peplowsky et al., 2011). Indeed, previous works have pointed out the importance of taking into account crustal heat sources in this kind of calculations, since the obtained surface heat flow increases with the proportion of heat sources within the crust 4°0'O"N (Ruiz et al., 2006, 2009). Finally, we discuss the implications of our results for the history of Mercury. 2. Topographic profiles Santa Maria Rupes (3.5"N, 19"W), and the scarps S_K3 (10.3"N, Fig. 2. Detailed vision of the backthrust fault associated to the S K4 scarp. The backthrust crosscuts slightly shortens two impact craters. B"W) and S_K4 (4"N, 15"W) are three lobate scarps situated in the same region of the Kuiper's quadrangle. These three lobate scarps have similar features: all of them are over 200 km long, have a relief of � 700 m and the associated thrust faults dip to the west (Fig. 1 a and b). To the west of the S_K4 scarp we identify another structure, which runs parallel. Offset of the walls and floors of transected impact craters suggest that this structure is also a thrust fault (Fig. 2), which dips to the east and is �200 km long, and could be a backthrust associated with the S_K4 scarp. a Topographic profiles across these three scarps have been derived by applying the delay-Doppler method to the data obtained from the Arecibo antenna in the period 1978 -1984 (Harmon et al., 1986). These authors collated overlapping profiles and averaged them over 0.15" longitude bins to produce a single profile with surface resolutions of 0.15" and 2.5" in longitude and latitude, respectively (i.e., 6.4 x 106 km). The altitude resolution of the topographic profiles is variable, usually lower than 50 m for the S_K3 scarp, �100 m for the S_K4 scarps, but very variable for Santa Maria Rupes, with resolutions ranging from � 50 to �400 m. Fig. la and b shows our four faults and the locations of topographic profiles used in the model. The topographic profile across the S_K3 scarp, A-N, is situated between 12.0"W, 10.4"N and 17.1 "W, 10.0"N; Santa Maria Rupes and the S_K4 scarp are crossed by a profile, B-B/, located between 14.4"W, 3.9"N and 21.6"W, 3.9"N. Topographic profiles across Santa Maria Rupes, 2S·0'O"W 20·0'O''W 1S·0'O"W and the S_K4 and S_K3 scarps exhibit a regional slope towards the b west. The origin of this slope is beyond the scope of this work, so 20'O'O''W 1S'O'O''W 10'O'O"W we filter the regional slope to obtain detrended topography, which is used for forward modeling. Inspection of the topography A A' suggests that a simple linear detrending is sufficiently accurate in this case. The topographic profile across Santa Maria Rupes shows a small impact crater in the back of the lobate scarp. This crater produces a low area, postdating fault, in the back of the scarp. To obtain the fault geometry we have restored crater effects and the Santa Maria '\ Backthrust low area has not been considered. " S"O'O"N B __!,- __'-:- ---"�B.' 3. Depth of faulting In this section we use the topographic profiles described in the 100 '. previous section and a forward modeling procedure to analyze fault _Kilometers displacements, dip angles and depths of faulting of the analyzed lobate scarps. We use the mechanical dislocation program Coulomb (Lin and Stein, 2004; Toda et al., 2005; available online at http:// Fig. 1. (a) Mariner 10 mosaic showing the location of the Arecibo radar altimetry earthquake. usgs.gov /research/modeling/coulomb/download.php) to profiles (Harmon et aI., 1986) used in this study. (b) This map shows the location of the Santa Maria Rupes, S K4 and S K3 scarps (dashed lines). Solid lines indicate predict the surface displacement associated with faulting. This the trace of the topographic profiles. program has been previously used to study thrust faults beneath lobate scarps on Mars (Schultz and Watters, 2001; Ruiz et al., 2008) Table 1 and Mercury (Watters et al., 2002). Results of mechanical modeling. Coulomb models the lithos ph ere assuming an elastic homo­ Feature Displace- Dip Depth of Depth of geneous and isotropic half-space. A range of significant para­ ment (km) burial (Ion) faulting(km) meters such as dip angle, vertical depth of faulting, magnitude and sense of offset along the fault and elastic constants are Santa Maria Rupes 1.0-1.1 28"_32" 0.0 36-39 S K4 scarp 1.0-1.1 40"_44" 0.0 30-36 specified in the model.