Results from Buffered Crater Counting
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EPSC Abstracts Vol. 9, EPSC2014-444, 2014 European Planetary Science Congress 2014 EEuropeaPn PlanetarSy Science CCongress c Author(s) 2014 Linking the timing of volcanic and tectonic features on Mercury: results from buffered crater counting E. R. Fegan (1), D. A. Rothery (1), S. J. Conway (1), M. Anand (1,2) and M. Massironi (3) (1) Department of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK ([email protected]). (2) Department of Earth Science, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK. (3) Dipartimento di Geoscienze, Università di Padova, Via Giotto 1, 35137 Padova, Italy. 1. Introduction un-named crater designated ‘c1’. Lobate scarps on Mercury are curvilinear tectonic 2. Method features that are interpreted as thrust faults intersecting the surface, formed as the planet Here we use the buffered crater count method on the contracted. In some places these scarps are found at lobate scarps [1] together with classical crater counts the edges of flooded impact structures, where of surface units (e.g. [3]), on the volcanic plains of compression has resulted in tectonic activation of the b37 and Dario, and also Beethoven, Shakespeare and boundary between fill and original basin surface as c1. Count areas include only surfaces we judge not to indicated in Fig.6 of [2]. The timing of volcanic and have been obscured by subsequent impact ejecta or tectonic activity is poorly constrained on Mercury; dominated by secondary craters. we aim to use stratigraphic relationships and crater counting to address this uncertainty. One example of a volcanically filled and tectonised impact structure is b37 (see Fig.1). There are clear superposition relationships of three craters and the reactivated edge of the basin fill - the lobate scarp - demonstrating the sequence of formation events [2]. Additional examples of tectonised basin-fills identified by [2] are Shakespeare (see Fig. 2, north- western edge of fill is tectonised), Beethoven and an Figure 2: Shakespeare basin, sinusoidal projection of MDIS mosaic centred 150° W. The north-western edge of the volcanic fill has been tectonised (red line). Volcanic plains crater count area is delineated in blue. 3. Results Cumulative crater counts for the structures in basin and structures in Fig. 1 are plotted in Fig. 3, using the Neukum Production Function (NPF). The plot Figure 1: b37 and Dario basins, sinusoidal projection of indicates that b37 volcanic fill is older than that of MDIS mosaic centred 3° W. The west edge of the volcanic Dario, as expected from their superposition (Fig.1). fill has been elevated above the original rim, forming a Since the lobate scarp deforms Dario’s volcanic lobate scarp (red line). Volcanic plains crater count areas are delineated in green (b37) and blue (Dario). plains unit, it must be younger and would be expected to plot to the left of Dario’s plains in Fig. 3. However, data from the lobate scarp define a trend rim of that basin. For c1, the separation is 0.2 indistinguishable (within error) from that of the Dario Ga. data, showing that the scarp was activated within less - The volcanic fills of the 4 tectonised basins than 0.1 Ga of the Dario filling event. are dated at approximately 3.8-3.9 Ga. Dario basin, which overprints b37 but does not itself have a tectonised rim, was in-filled more recently at about 3.6 Ga. - These findings indicate that this type of basin-filling volcanism and compression occurred around the end of the Late Heavy Bombardment. Figure 3: Cumulative crater count plots and model ages for b37 (green), Dario (blue) and the lobate scarp on the western edge of b37 (red). Chronology and Production Functions as [3]. We found that the model-ages for the volcanic plains and lobate scarps of Beethoven- and Shakespeare are also indistinguishable, within error (3.8-3.9 Ga). The Figure 4: Cumulative crater count plots and model ages for model-ages for c1 are more clearly separated: 3.9 Ga Shakespeare volcanic fill (blue) and the associated lobate for its volcanic plains and 3.7 Ga for the lobate scarp. scarp (red). Chronology and Production Functions as [3]. The errors quoted in Figs. 3 and 4 have been rounded References to 2 significant figures and relate to the model-age calculations. [1] Fassett C.I. and Head (III) J.W.: The timing of Martian valley network activity: Constraints from buffered crater 4. Conclusions counting, Icarus, 195, 61-89, (2008). - In the cases of Beethoven and Shakespeare, [2] Rothery, D.A. and Massironi, M.: A spectrum of tectonised basin edges on Mercury, LPSC, Houston, Texas, emplacement of the volcanic units and lobate USA, (2013). scarp formation occurred over a relatively short timescale. [3] Neukum G., Oberst, J., Hoffmann, H., Wagner, R. and - For b37, 0.3 Ga separated the emplacement Ivanov, B.A.: Geologic evolution and cratering history of of volcanic fill and reactivation of part of the Mercury, PSS, 49, 1507-1521, (2001). .