Journal of Volcanology and Geothermal Research 337 (2017) 62–80 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores Mangala Valles, Mars: A reassessment of formation processes based on a new geomorphological and stratigraphic analysis of the geological units Giovanni Leone ETH Zurich, Institute of Geophysics, Geophysical Fluid Dynamics, Room H28, Sonneggstrasse 5, CH-8092 Zurich, Switzerland article info abstract Article history: Mangala Valles has always been viewed as the typical outflow channel formed by catastrophic floods of water. A Received 9 October 2016 new analysis has shown that the geomorphological traces of fluvial or lacustrine processes within Mangala Valles Received in revised form 1 March 2017 can be better explained by fluid lava flooding the channels and filling pre-existing impact craters. As for the Accepted 10 March 2017 circum-Chryse outflow channels, where no clear source of water or mechanism able to replenish water at its hy- Available online 11 March 2017 draulic head is observed, there is no geologic trace of a sudden removal of a volume of water (ice) necessary to carve Mangala Valles. Neither maars nor rootless cones, typical volcanic features indicative of interaction be- tween lava and ground ice, were found. Past works suggested that the formation of Mangala Valles occurred in late Amazonian age when the climate of Mars was similar to that seen today, that is absolutely not liquid water friendly. The present work shows how the origin of Mangala Valles may go back to Noachian or even Pre-Noachian when other studies have concluded that the climate was not liquid water friendly. Even assuming limited periods of obliquity favourable to liquid water in the history of Mars, which is at odds with the wide- spread presence of unaltered olivine and jarosite, it is very difficult to find plausible mechanisms of aquifer re- charge or signs of catastrophic water release at the Notch of Mangala Valles that could feed the multiple episodes, or even a single episode, of fluvial flooding suggested in the literature. This evidence and other analysis will show that the presence of water and, eventually, ground ice is not incontrovertible in the equatorial regions and should not be given for granted as commonly done so far in the literature. The geomorphological analysis of the Mars Reconnaissance Orbiter (MRO) images provided in this paper, combined with THEMIS and MOLA data, show how Mangala Fossa, from which Mangala Valles originated as a breakout, is an erosional channel formed by the flow of lava in a original tube coming from Daedalia Planum rather than a tectonic graben or the sign of a dike rupturing to the surface. © 2017 Elsevier B.V. All rights reserved. 1. Introduction and eolian erosion (Cutts and Blasius, 1981). Volcanic (Leverington, 2004)andmixedvolcanic-fluvial processes (Keske et al., 2015) Mangala Valles is a ~900–1000 km-long, 165 km-wide, outflow regained consideration. A description of all these mechanisms of forma- channel always thought as formed by catastrophic flooding of water tion will be given in the discussion section. Several other papers have (Baker and Milton, 1974; Ghatan et al., 2005) initiated through mobili- suggested the volcanic origin of the outflow channels (Jaeger et al., zation of shallow aquifers by volcanic intrusions (Tanaka and Chapman, 2007; Leverington, 2007; Leverington, 2011; Leone, 2014) and one of 1990; Dohm et al., 2001; Hanna and Phillips, 2006; Leask et al., 2007a, b; them mentions the problems related to the poor stability of liquid Basilevsky et al., 2009; Burr et al., 2009; Bargery and Wilson, 2011). The water in the low-pressure atmosphere of Mars (Leverington, 2011). head of Mangala Valles is located along the ~220 km-long, ~6 km-wide, New evidence supports a volcanic-only erosional origin for Mangala and ~1 km deep, Mangala Fossa on a volcanic flood plain sloping to- Valles without any water and tectonism involved. The most important wards Amazonis Planitia (Fig. 1). A volcanic-only origin was initially evidence is observed along Mangala Fossa and along the course of proposed for several other outflow channels located in the largest volca- Mangala Valles. Mangala Fossa was interpreted in the past as a graben nic provinces of Mars, but not for Mangala Valles (Carr, 1974). Other opened by the intrusion of a dike from which water trapped in the works suggested alternative hypotheses of formation, including glacial cryosphere spilled out to carve Mangala Valles (Tanaka and Chapman, erosion (Lucchitta, 1982), debris flow (Nummedal and Prior, 1981), 1990; Ghatan et al., 2005; Leask et al., 2007a). However a dike or a sill rupturing to the surface should leave trace of lateral flows (e.g. perpen- dicular to the fissure) all or partially along its length. Magma coming out E-mail address: [email protected]. from tectonic fractures showing vertical and/or horizontal displacement http://dx.doi.org/10.1016/j.jvolgeores.2017.03.011 0377-0273/© 2017 Elsevier B.V. All rights reserved. G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80 63 Fig. 1. MOLA context map of the Sirenum Terra region, the black rectangles indicate the location of the panels shown in Fig. 3. The black arrows indicate the path of the lava flooding that formed the volcanic inter-crater plain indicated with the geologic unit Asp2 (Nsp2). was observed at Prometheus Patera (Leone et al., 2009) and Zal Patera explanation to water or to a dike rupturing onto the surface for the for- (Bunte et al., 2008) on Io, but does not seem to be the case at Mangala mation of both Mangala Fossa and Mangala Valles. The lack of any kind Fossa. Also, the hypothesis of a dike approaching but not reaching the of displacement shown by MOLA profiles along the sides of Mangala surface to potentially warm up the putative cryosphere to release Fossa (Fig. 2) raises doubts about a putative tectonic origin. This does groundwater (Wilson and Head, 2004) was analysed. These hypotheses not necessarily mean that other fossae on Mars might not be of tectonic raised several questions. How much water is required to carve Mangala origin, every situation must be evaluated case by case based on the Valles? Was this water potentially available underground? How can available evidence. water be replenished or a cryosphere be present in sufficient amounts The direct observation of lava flows through high resolution CTX and at the hydraulic head of Mangala Valles to feed multiple episodes of HiRISE imagery, combined with a global geomorphological and mineral- flooding suggested by Tanaka and Chapman (1990), Zimbelman et al. ogical analysis of the surface of Mars, was fundamental to understand (1992), and Basilevsky et al. (2009)? How can liquid water survive in the volcanic origin of Valles Marineris, of several other circum-Chryse the low atmospheric pressure of Mars after phreatomagmatic (explo- outflow channels, and of all the valley networks once thought to be flu- sive) activity? Why does magma coming from depth not raise at differ- vial networks (Leverington, 2004, 2007, 2009, 2011; Leone, 2014, ent heights the sides of Mangala Fossa? Why is no significant tectonic 2016). The same approach will be used here to better understand the displacement observed along Mangala Fossa (i.e. a large scale strike- origin of Mangala Valles. The available geological maps of Mangala slip fault)? Which type of tectonic stress might selectively produce frac- Valles made by Chapman and Tanaka (1993), and particularly by tures inside a pre-existing impact crater and none on its rim crossed by Keske et al. (2015), provided a basis for the discussion and the new in- Mangala Fossa? This paper will answer these questions and will show terpretation of the geological units. An alternative interpretation of sev- how the erosional power of lava flowing in a tube (Greeley et al., eral geologic units will be given in this paper and will be compared to 1998) or in channels (Hurwitz et al., 2010) can provide an alternative that made by Keske et al. (2015), every description and age of the 64 G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80 Fig. 2. A) MOLA profiles across the lower course of Mangala Fossa; the white arrows indicate Mangala Fossa in the corresponding profiles numbered from 1 to 5; all the profiles show no significant vertical displacement between the sides of the fossa. B) MOLA profiles across the median course of Mangala Fossa near the Notch of Mangala Valles and the ridge of separation with Daedalia Planum; the white arrows indicate Mangala Fossa in the corresponding profiles numbered from 6 to 10; the profiles show no significant displacement between the sides of the fossa, the difference in height is only related to the local topographic roughness of the ridge. C) MOLA profiles across the source region of Mangala Fossa; the white arrows indicate Mangala Fossa in the corresponding profiles numbered from 11 to 15; also here the profiles do not show any significant vertical displacement except for that related to the topographic roughness of the crater rim crossed by Mangala Fossa. units that will be discussed in this paper refers to their geologic map. section of Mangala Fossa just west of the Notch (Fig. 3a), a process pre- The description of the geomorphology will start from the source of viously seen along Valles Marineris (Leone, 2014). The geomorphology Mangala Fossa through the head of Mangala Valles and then will follow still shows a narrow section of Mangala Fossa immediately west of the its whole course, including all the distributaries, to the mouth at Notch and a wide section to the east (Fig.