sign in sign up
<<
Home , Moreux (crater)

Lunar and Planetary Science XXXVII (2006) 1425.pdf

MID-LATITUDE GLACIAL MODIFICATION OF MOREUX CRATER (44°E, 42°N; 135 KM): EVI- DENCE FOR POLYTHERMAL GLACIATION RELATED TO IMPACT-INDUCED ENHANCED THERMAL GRADIENTS. David R. Marchant1, James W. Head2 and Mikhail A. Kreslavsky2, 1Dept. Earth Sci., Boston Univ., Boston MA 02215 USA, 2Dept. Geol. Sci., Brown Univ., Providence RI 02912 USA (mar- [email protected]).

Introduction: The dichotomy boundary represents sources of the linear-ridged, lobate flow-like features. one of the most important geological and geophysical These features display parallel ridges oriented predomi- features of . We have been assessing the nature of nantly downslope; the features merge with one another, modification processes along the dichotomy boundary at and merging ridges deform into tight and often crenu- northern mid-latitudes in the Deuteronilus-Protonilus lated folds. Some lobes remain distinctive and have ar- region and have found evidence for extensive regional cuate concentric ridges at their termini (Fig. 4, lower glaciation in lobate debris aprons (LDA) middle), while others continue to converge (Fig. 4, left and (LVF) in the [e.g., side), ultimately merging into the large basal lobes (Fig. 1-6]. The major geological landforms found there sup- 1). These are interpreted to be debris-covered port an interpretation that involves cold-based glaciation derived from frost, snow, ice and debris accumulation in in valley and plateau icefield landsystems over much of alcoves, similar to LDA occurrences outside the crater this region earlier in the Amazonion [2-6]. Thus broad along the dichotomy boundary [e.g., 2-6]. environmental conditions (cold hyperarid polar desert) Crater rim crest and interior wall: Topography on at this time appear similar to those of the present, but the crater rim crest is characterized by rounded massifs changes in orbital parameters [7] led to sublimation and and intervening smooth plains (e.g., Fig. 3). Between mobilization of H2O and its redeposition as frost, snow the massifs the plains change morphology, transitioning and ice at low and mid-latitudes [e.g., 8] to produce the into lineated, flow-like lobes, extending 3-15 km down observed glacial landsystems. Analysis of these cold- the crater interior walls (Fig. 3, center). Often charac- based glacial deposits [2-6] raises the question of terizing the transition are scallop-like alcoves forming whether wet-based conditions ever prevailed in glacial convex-outward scarps. Where developed, the alcoves environments on Amazonian Mars. Two factors could mark the beginning of the lineated lobate deposits. In- lead to wet-based conditions under the cold polar desert ter-massif lineated lobate deposits are joined by similar conditions thought to have prevailed throughout much lobate deposits originating in alcoves farther down the of the Amazonian: 1) sufficient ice accumulation to crater wall, sometimes resulting in a broad digitate have caused basal melting [e.g., 9-10], or 2) localized apron deposit extending from the crater wall down to- heat sources, such as volcanism [e.g., 11-12] causing ward the crater floor (Fig. 3). Multiple lineated lobes melting. Here we investigate an additional example of originate on the crater rim in alcoves, converge and localized thermal anomalies, formation. show evidence for flow-like deformation of their linea- We show that the impact of the 135 km diameter crater tions, and then descend downslope through a major Moreux, superposed on the dichotomy boundary at break in the rim crest, with their sinuous pathways con- ~44°E, 42°N [13] (Fig. 1), provided a thermal anomaly trolled by wall topography. In some cases, the lobate [14-16] sufficient to cause elevated near-surface tem- deposits terminate in a distinctive moraine-like ridge peratures and wet-based glacial conditions in the crater (Fig. 3). These are interpreted to be late-stage debris interior for a period of time following its formation. We covered deposits. find that the Moreux impact crater environment pro- Evidence for wet-based conditions: Small sinuous duced polythermal glaciation in an otherwise cold-based channels are observed on the inner rim and down the glacial phase along the dichotomy boundary during the crater interior wall, generally occurring stratigraphically Amazonian. below the lobate deposits (lower and upper part of Fig. Moreux crater setting: Moreux is located at the 3), and suggesting the flow of liquid water. Also associ- edge of the plateau of the southern uplands and its ated with the crater wall deposits are 1) areas of pitted southern portion disrupts the regional scarp representing and collapsed terrain, 2) sinuous esker-like ridges that the dichotomy boundary (Fig. 1). To the north, ejecta lead into channels, 3) small delta-like features at esker can be seen on the summits of the plateaus and in the and channel termini, and 4) elongated, fractured lobes, intervening valleys in some places, but in others, lobate similar to gelifluciton lobes formed by water and ice debris aprons and lineated valley fill clearly postdate the laden sediment flow, arrayed along portions of the base ejecta, indicating post-Moreux regional glaciation. of the slope. Taken together, these features are consis- Thus, Moreux is interpreted to have formed at a time tent with wet-based glacial conditions localized in time subsequent to the formation of the current dichotomy and space, and caused by the enhanced geothermal gra- scarp and the abundant related mesas, but prior to the dients produced in the aftermath of impact cratering end of the modification of the scarp and mesas by LDAs events [14-16]. and LVF. The interior of Moreux is 2-3 km below the Summary and Conclusions: Features associated rim crest (Fig. 2). Although relatively sharp, the rim with the Moreux crater interior strongly suggest that crest has undergone significant local degradation where glaciation occurring in this regional environment [e.g., it has been eroded into a series of isolated massifs of 4] was locally interrupted by the creation of a major various sizes. thermal anomaly [16] in the Moreux crater. As surface Crater central peaks: At the broad scale, the flanks thermal conditions subsequently evolved toward ambi- and margins of the central peaks appear lobate (Fig. 1). ent values over millions of years [16], thermal gradients More detailed examination of the near summit areas at were sufficient to cause basal melting of subsequent ac- the top of the broad lobes (Fig. 4) reveals the presence cumulating snow and ice. Once regional gradients had of abundant arcuate alcove-like scarps that are the been re-established, cold-based glaciation returned. Lunar and Planetary Science XXXVII (2006) 1425.pdf

Geomorphological criteria outlined in this analysis are ence, in press, 2006; [9] J. Fastook et al., LPSC 35, 1452, 2004; [10] currently being used to map similar crater deposits in D. Shean et al., LPSC 35, 1428, 2004; [11] D. Shean et al, JGR, other parts of the region [17]. doi:10.1029/2004JE002360, 2005; [12] L. Wilson et al., LPSC 36, 1189, 2005; [13] J. Head and D. Marchant, B/V Micro 42, 23, 2005; References: [1] B. Lucchitta, JGR, 89, B409, 1984; [2] J. Head et al., [14] B. Ivanov and A. Deutsch, GSA SP-339, 389, 1999; [15] H. EPSL, in press, 2005; [3] J. Head et al., GRL, in press, 2005; [4] J. Melosh and B. Ivanov, Ann. Rev. EPS, 27, 385, 1999; [16] J. Rathbun Head and D. Marchant, LPSC 37, 1127, 2006; [5] J. Head and D. and S. Squyres, Icarus, 157, 362, 2002; [17] G. Morgan, LPSC 37, Marchant, LPSC 37, 1126, 2006; [6] A. Kress et al., LPSC 37, 2006; 2006. [7] J. Laskar et al., Icarus, 170, 343, 2004; [8] F. Forget et al., Sci-

Fig. 2. Altimetric profile of Moreux interior. MOLA gridded data

Fig. 1. MOLA shaded topography and altimetry map.

Fig. 4. Central peaks; alcoves, and linear lobate flows converging toward base of peaks (THEMIS Fig. 3. Crater rim crest and wall with massifs V01122003). and lobate flows (THEMISV12805004).