A geological history connecting: Caldera formation, with an ignimbrite or pluton base. Post-caldera dacite flows, resurgent dome, and mafic ring fault volcanism

Balme, M. R.

Fawdon, P.

Jordan, C. J.

Skok, J. R.

Vye-Brown, C.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThe Nili Patea caldera; evolving magma, explosiveeruptions and hydrothermal deposits on Mars.Conference or Workshop ItemHow to cite:Fawdon, P.; Skok, J. R.; Balme, M. R.; Vye-Brown, C. and Jordan, C. J. (2014). The Nili Patea caldera;evolving magma, explosive eruptions and hydrothermal deposits on Mars. In: 45th Lunar and Planetary ScienceConference, 17-21 Mar 2014, The Woodlands, Texas.For guidance on citations see FAQs.c© 2014 The AuthorsVersion: Version of RecordLink(s) to article on publisher’s website:http://www.hou.usra.edu/meetings/lpsc2014/pdf/1967.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 NILI PATEA CALDERA; EVOLVING MAGMAS, EXPLOSIVE ERUPTIONS AND HYDROTHERMAL DEPOSITS ON MARS.  P. Fawdon*1, J. R. Skok2,  M.R. Balme1, C. Vye-Brown3, D.A. Rothery1,C. J. Jordan4 1Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, UK. MK7 6AA; peter.fawdon@open.ac.uk, 2Department of Geology & Geophysics, Louisiana State University, Baton Rouge, LA 70803., 3British Geological Survey, Murchison House, West Mains Road, Edinburgh, UK. EH9 3LA, 4British Geological Survey, Nicker Hill Keyworth, Nottingham UK. NG12 5GG  Introduction: Nili Patera is a 45 km diameter cal-dera at the centre of the Syrtis Major Planum volcanic province [1]. Nili Patera is located at the northern end of a central caldera depression at Syrtis Major and is unique amongst martian volcanic terrains because it exhibits:  Evidence of both effusive and explosive vol-canism.  Compositional diversity ranging from olivine rich basalts to dacite [2] and felsic units [3] at the lowest levels of the stratigraphy.  Outcrops with spectral features indicative of hydrothermal silica [4].  Asymmetric collapse with a maximum subsid-ence of 1800 m; dropping the caldera floor below the surrounding volcanic shield.  A 300 m high resurgent dome in the western caldera floor.  An active barcan dune system [5]. Our work addresses the stratigraphic and structural context of these discoveries and investigates Nili Patera’s formation, magmatic evolution, caldera floor volcanism and the role of volatiles in the caldera. We present a geological map (figure 1) and history of Nili Patera to put these geological findings into context.  Data and methords:  We used six 6 m/pixel Con-text Camera (CTX) images from Mars Reconnaissance Orbiter (MRO) to generate three 18 m/pixel Digital Elevation Models (DEMs) using SocetSet software. These DEMs were then used to orthorectify three CTX images as a base layer to produce the geologic map. Detail is provided, where available, from HiRISE (0.25 – 0.5 m/pixel) images, and mineralogical information is derived from all Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) visible-infrared spec-trometer data available for the study area.   Observations and discussion: Figure 1 shows a simplified map summarising the geology and geomor-phology of Nili Patera. Units are characterised at the highest resolution possible on a mineralogical and ge-omorphological basis, depending on the availability of derived mineralogy. The extents of units are mapped on to the CTX basemap. Units are then organised into formations based on their spatial, temporal or genetic associations. General physiography. The caldera is subdivided into two parts. The western section (figure 1b; W), which formed first, is defined by normal and reverse faults systems (figure 1a; black lines), and has experi-enced the largest subsidence. The floor has a 300 m  Figure 1: a. Simplified geological map of Nili Patera. Units and normal faults described in text, b. Topogra-phy from merged CTX DTMs over the hillshade.  high resurgent dome mirroring the tectonic boundaries of the western caldera floor. The eastern caldera sec-tion (figure 1b; E) is defined by subsidence associated with trapdoor collapse. Faulting at the margins cross-cuts faulting associated with the western caldera sec-tion. The eastern caldera floor is deformed by late-stage thrust-bounded wrinkle ridges and some minor normal faulting.  Figure 2: a. The bright fractured unit Nili Patera, b. Exposed ignimbrite in the Atacama, Lasca, Chili.  Caldera-external lavas. The oldest deposits are the pre-caldera lavas (figure 1; purple). Within this unit are large lava flows emplaced prior to caldera formation. This unit is crosscut by the normal fault forming the north-western caldera rim. To the south east are lavas emplaced after caldera formation in the central caldera complex (figure 1a; pink).  Caldera-forming deposits. Two units are associat-ed with caldera formation. The lowest of these, ex-posed in the south east by 10-40 m of denudation, is a bright, fractured and layered unit with a characteristi-cally felsic composition [3] (figures; 1a; white, 2a). Outcrops of this unit can be seen in ejecta from impacts within the caldera, in fault scarps of the resurgent dome and (possibly) as thin layers on parts of the interior caldera rim faults. This unit has been considered an anorthosite plu-ton, supported by the compositional similarity to lunar anorthosites [6] and textural similatries to terrestrial granite [3]. However the unit could instead be a welded felsic ignimbrite, associated with initial caldera for-mation in the west, and trapped in the topographic low in the east. This is consistent with the observed compo-sitions, facilitates formation hypotheses for the western caldera, and is more consistent with detailed textural characterises of the unit, especially considering terres-trial analogues (figure 2b). Overlying these units is a rubbly basaltic pitted cal-dera floor unit which crops out across the south of the caldera floor and underlies later volcanism.  Caldera floor volcanism. Since caldera formation, there have been three phases of volcanism. In the north is an early basaltic flow field with associated effusive vents that has subsequently been dissected and uplifted by the resurgent dome in the western caldera floor (fig-ure 1a; red). Following the formation of this unit, magma chemistry evolved, and the dacitic flow in the centre of the caldera was emplaced as was the 500 m high volcanic cone (figure 1a; yellow). Hydrothermal deposits [4] are found at the foot of the cone and on the dacite flow. The dacite flow now dips at 2°, tilted when the western caldera floor was uplifted to form the 300 m high resurgent dome. Next, the northeast quadrant was resurfaced (figure 1a; blues) with rough and smooth textured volcanic material of uncertain origin. This unit onlaps the resur-gent dome and is studded with ~ 15 low Cones, each 60 – 150 m wide. A unit with a very strong olivine signature was also emplaced around this time (figure 1a; greens). The location on the western rim, the strength of the spectral response of the unit and late stage of emplacement suggest entrained crystal cumulates as opposed to na-tively high olivine content. Finally, further subsidence resulted in a conjugate pair of wrinkle ridges bounded by thrust faults, cross-cutting and deforming units in the eastern caldera floor. This was probably a local response to shortening caused by regional subsidence of the caldera complex. Surficial units and erosion. Since ~1.9 Ga [7] an aeolian processes have stripped back material in the south-eastern quadrant and formed the dune field, and which is active at the present day [5] (figure 1a; dots).  References: [1] Hiesinger, H. and J.W. Head, III, J. (2004) JGR 109(E1) E01004. [2] Christensen, P.R., et al., (2005) Nature, 436(7050) 504-509. [3] Wray, J.J., et al., (2013) Nature Geosci, 6(12): p. 1013-1017. [4] Skok, J.R., et al., (2010) Nature Geosci, 2010. 3(12): p. 838-841. [5] Silvestro, S., et al., (2010) GRL 37(20): p. L20203. [6] Cheek, L. C., et. al., (2013) JGR 118, 1805-1820. [7] Robbins, S.J., et. al,. (2011) Icarus,. 211(2): p. 1179-1203. 

The Nili Patea caldera; evolving magma, explosive eruptions and hydrothermal deposits on Mars.

http://oro.open.ac.uk/40625/1/Fawdon_LPSC2014.pdf

The Nili Patea caldera; evolving magma, explosive eruptions and hydrothermal deposits on Mars.

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