Macpherson, C.G.

Malliband, C. C.

Martin, P.

McCaffrey, K. J. W.

Stofan, E.R.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThe Geological History of Aleksota Mons, VenusConference or Workshop ItemHow to cite:Malliband, C. C.; Martin, P.; McCaffrey, K. J. W.; Macpherson, C.G. and Stofan, E.R. (2017). The Geological Historyof Aleksota Mons, Venus. In: 48th Lunar and Planetary Science Conference, 20-24 Mar 2017, The Woodlands,Houston, Texas.For guidance on citations see FAQs.c© [not recorded]Version: Version of RecordLink(s) to article on publisher’s website:https://www.hou.usra.edu/meetings/lpsc2017/pdf/1395.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukTHE GEOLOGICAL HISTORY OF ALEKSOTA MONS, VENUS.  C. C. Malliband1*, P. Martin1, K. J. W. McCaffrey1, C. G. Macpherson1 and E. R. Stofan2. 1 Department of Earth Sciences, Durham University, South Road, Durham, DH1 UK. 2 Department of Earth and Planetary Sciences, UCL, WC1E 6BT. *Now at: School of Physical Sciences, Open University, Walton Hall, Milton Keynes, MK6 1AA, UK. (chris.malliband@open.ac.uk)  Overview:  Aleksota Mons is an area of extensive small scale volcanic activity centred at 308.5oE, 9oS, and of radius 250km in the Navka Planitia region of Venus. It is located approximately 520 km ESE of the centre of the Venera 13 landing ellipse. While the area may be characterised as a shield field, with many small (<5 km), broadly conical shields, it has also been iden-tified by [1] as the location of a type 2 corona.  We propose that the majority of volcanic vents in the Aleksota Mons shield field are aligned with local faulting in Perunitsa Fossae. Therefore, it is likely that both formed within the same stress field. Regional Geology: The area around Aleksota Mons (Fig 1) shows complex tectonism, with extensive rifting in both the adjacent Perunitsa Fossae and the more distant Khosedem Fossae. The Perunitsa and Khosedem Fossae systems appear to meet and termi-nate at Vostrukha Mons in the northwest. They also appear to join south of Panina Patera and then continue south as a single system until terminating to the south at Iweridd Corona. Between Vostruka Mons and Ale-ksota Mons, Perunitsa Fossae runs approximately NW-SE. South of Aleksota Mons, Perunitsa Fossae runs approximately N-S.  There is evidence for volcanism throughout the ar-ea studied. In Perunitsa Fossae this mainly takes the form of isolated small scale shield vents that do not appear to show any clear preferential alignment on local scales. In contrast, in Khosedem Fossae there are large flows in the style of ridge volcanism identified by [2] and numerous pit chains.  Methods:  A history of the area was determined using geological mapping of tectonic and volcanic fea-tures, and examining geological relationships. We used stratigraphic relationships and a novel statistical tech-nique to determine age relationships. 2-point Azimuth Method: Two-point azimuth meth-ods are used to determine and quantify preferred orien-tations in clusters of point features. The initial method was developed by [3], and a modified method was pre-sented by [4]. Work by [5] created a MATLAB script compiled with a GUI (Graphical User Interface) to facilitate use of both methods on point data from Ve-nus, Earth, Mars, and a unit sphere. The software cor-rects for box shape by normalising the data using a specified number of Monte Carlo models. It also incor-porates a Student’s t distribution to find the 95th per-centile critical threshold, to determine a significance. In this work, we have used the Lutz method [3] which calculates azimuths between each point and eve-ry other point identified. This means for N points, there are N(N-1)/2 orientations. We have normalised for box shape using 200 Monte Carlo models. Styles of volcanism: The mapping of vents in the study area used 3 classes, which could then be further examined. These were: a) central pit with flows or ejecta, b) pit only and c) flow/ejecta centre. These clas-ses were chosen to provide the best representation of the styles of volcanism, following a preliminary study of the area. Results: The orientations for the amalgamated da-taset determined from the Lutz 2-point azimuth analy-sis are shown in Fig 2A. This shows a significant pref-erential orientation of vent alignments sub-parallel with the direction of NW-SE faulting of the northern Peruni-tsa Fossae (Fig 2B). The trend is stronger when the ‘central pit with flows or ejecta’ class is analysed sepa-rately (Fig 2C). These orientation relationships suggest that the majority of vents at Aleksota Mons and the faulting in northern Perunitsa Faulting developed under the same NW-SE trending stress field. Therefore, we infer that most of this volcanism at Aleksota Mons was broadly contemporaneous with the northern Perunitsa Fig 1: Map of area with named volcanic and tectonic features labelled. North directly up. Image: NASA/JPL.  1395.pdfLunar and Planetary Science XLVIII (2017)Fossae faulting. This is supported by our observations of superposition relationships.  In contrast to the overall trend, vents mapped as ‘pit only’ show a preferential N-S alignment (Fig 2D). Thus, pits without visible flows formed under the influ-ence of a N-S stress field. Many of these ‘pit only’ vents appear to overlie traceable lava flows associated with Aleksota Mons, and are therefore interpreted to be younger than those flows.  Some pits outcrop as N-S trending pit chains. Cur-rent work [6] considers pit chains are most likely sur-face expressions of lateral dyke propagation. If this interpretation is correct, the pits and pit chains ob-served in this study may show dyke propagation in a N-S stress field. Given stratigraphy, this would be during the waning stages of magmatism in Aleksota Mons. Similarly to these pit chains, faulting in northern Khos-edem Fossae (near Vostrukha Mons) trends broadly N-S. Our preliminary analysis of stratigraphic relation-ships near Vostrukha Mons indicate that faulting asso-ciated with Khosedem Fossae formed later than fault-ing associated with Perunitsa Fossae in this area. We therefore infer that both the pit chains and the faulting in northern Khosedem Fossae formed under the same N-S trending stress field. Discussion: Aleksota Mons is isolated from the abundant N-S pit chains in Khosedem Fossae. It is pos-sible that N-S pit forming processes, potentially dyke emplacement, were occurring simultaneously in both Khosedem Fossae and Aleksota Mons. This could mean that volcanic processes were long lived and were active through changing stress fields, and/or that there were two distinct stages of magmatism at Aleksota Mons: small shield emplacement within a NW-SE trending stress field, followed by later pit formation in a N-S trending stress field.  Conclusion: Stratigraphic and orientation relation-ships indicate that volcanic vents at Aleksota Mons were principally emplaced concurrently with faulting in northern Perunitsa Fossae, under a NW-SE trending stress field. This demonstrates volcanism and tectonics occurred concurrently. There is evidence for a later stage of pit formation, perhaps from lateral propagation of dykes that occurred concurrently with the formation of Khosedem Fossae faulting (trending N-S). References: [1] Stofan, E. R. et al (2001) GRL, 28, 4267-4270. [2] Grindrod P. M. et al (2010) JGR, 37, L15201. [3] Lutz, T. M. (1986) JGR, 91, 421-434. [4] Cebriá, J. M. et al. (2011) J. Volcanol. Geotherm. Res., 201, 73-82. [5] Thomson, B. J. and Lang, N. P. (2016) Comput. Geosci., 93, 1-11. [6] Patterson C.W. et al (2016) LPS XLVII #2097. Fig 2: (A), Histograms of preferential orientation of vents in the overall dataset. (B), is a histogram of orienta-tions of fault segments in the study area.  Histograms of preferential orientation for: (C), Pits with clearly associ-ated flows or ejecta; (D), pits without any attributable flow or ejecta. This suggests magmatic processes occurred under two different stress regimes. Orientations and stratigraphic relationships indicate that the formation of the majority of vents associated with Aleksota Mons is contemporaneous with the formation of NW-SE trending Perunitsa Fossae faulting, followed by later-stage pit forming process occurring under a N-S trending stress field. 1395.pdfLunar and Planetary Science XLVIII (2017)

The Geological History of Aleksota Mons, Venus

Abstract

Similar works

Full text

Available Versions

Open Research Online (The Open University)