Lava filling of Gale crater from Tyrrhenus Mons on Mars

Journal Pre-proof Lava filling of Gale crater from Tyrrhenus Mons on Mars

Daniele Gasparri, Giovanni Leone, Vincenzo Cataldo, Venkat Punjabi, Sangeetha Nandakumar PII:

S0377-0273(19)30187-8

DOI:

https://doi.org/10.1016/j.jvolgeores.2019.106743

Reference:

VOLGEO 106743

To appear in:

Journal of Volcanology and Geothermal Research

Received date:

27 March 2019

Revised date:

10 September 2019

Accepted date:

19 November 2019

Please cite this article as: D. Gasparri, G. Leone, V. Cataldo, et al., Lava filling of Gale crater from Tyrrhenus Mons on Mars, Journal of Volcanology and Geothermal Research(2018), https://doi.org/10.1016/j.jvolgeores.2019.106743

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© 2018 Published by Elsevier.

Journal Pre-proof Lava filling of Gale crater from Tyrrhenus Mons on Mars Daniele Gasparri1, Giovanni Leone1*, Vincenzo Cataldo2, Venkat Punjabi1, and Sangeetha Nandakumar1 1

Instituto de Investigación en Astronomia y Ciencias Planetarias, Universidad de Atacama 2

School of Earth and Space Exploration, Arizona State University

* Corresponding author: [email protected]. Copayapu 485, Copiapó, Atacama, Chile

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Keywords: Gale crater, Tyrrhenus Mons, Elysium Mons, volcanic channels, lava flows, Martian volcanism

Declaration of interest: We wish to confirm that there are no known conflicts of interest associated with this publication

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and there has been no significant financial support for this work that could have influenced its outcome. We confirm that the

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manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by

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all of us. We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual

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property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property.

Abstract

Gale crater shows infilling of lava of basaltic origin mainly coming from the south via Farah Vallis. Using available Thermal Emission Imaging System (THEMIS) images, Mars Orbiter Laser Altimeter (MOLA) topographic data, Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) mineralogical data, and geochemical analyses taken in situ by the Mars Science Laboratory (MSL) in different locations of the crater, we focused on the possible origin and the main path of the lava that filled Gale crater. We found that: 1) the K/Ar age of the basaltic rocks on Gale’s floor is consistent with the age of formation of Tyrrhenus Mons derived from the southern polar giant impact (SPGI) model; 2) the Aeolis Mensae region does not show evidence for interaction between lava coming from the north

Journal Pre-proof (Elysium Mons) and lava coming from the south (Tyrrhenus Mons); 3) the geomorphological analysis shows that Farah Vallis is the convergence of a complex network of volcanic channels that can be tracked back to the lava fields of Tyrrhenus Mons; 4) a one-dimensional model of lava along the observed path, using an Adirondrack basalt composition for the substrate, shows that lava from Tyrrhenus Mons is thermally capable of flowing the entire distance to Gale before cooling down. This

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evidence is consistent with the lava fill observed at Gusev crater.

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1. Introduction

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The discovery of the lava filling through the Spirit rover at Gusev crater (Greeley et al., 2005;

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McSween et al., 2003, 2006; Ming et al., 2008) supported the idea that a similar lava filling may have occurred within Gale crater. Lava filling of pre-existing impact craters is a common process observed

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on Mars. Palos crater (Leverington, 2006) and Gusev crater (Gellert et al., 2004; McSween et al., 2004;

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Greeley et al., 2005) are well-known examples but there are many more unnamed craters that experienced different amounts of lava filling in other regions (Leverington and Maxwell, 2004; Leone,

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2016), particularly in the Amenthes and Tyrrhena Regions (Craddock and Maxwell, 1990). The

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infilling of Gale crater was geologically mapped by Le Deit et al. (2013), although the interpretation was that of sediments deposited by fluvial transport (Anderson and Bell III, 2010; Le Deit et al., 2013). Fluid lava flows are rheologically similar to flows of water producing channels and fluvial networks (Dietterich and Cashman, 2014). We have observed volcanic channels and dendritic networks coming from both the lava fields of Tyrrhenus Mons and Hesperia Planum heading towards Gale crater. Le Deit et al (2013) have mapped the infill of Gale crater distinguishing several geologic units. The most relevant to our work are the Crater Floor Units 1-4, henceforth Cf1-4, defined as “sediment transported in fluvial valleys and deposited downstream” (Le Deit et al., 2013). We have known for a long time how problematic it is for water to stay liquid on the surface of Mars (Conrath et al., 1973; Hess et al.,

Journal Pre-proof 1976; Carr, 1987; Richardson and Mischna, 2005). Thus, we propose lava flows as an alternative explanation to fluvial transport for the crater floor (Cf1-Cf3) units shown in the map of Le Deit et al (2013). Gale crater is a ~150 km wide impact basin located along the Martian dichotomy boundary between two large volcanic provinces: Elysium to the north and Tyrrhena to the south. Crater counts suggested a period of formation from 3.8 to 3.6 Ga for Gale crater (Thomson et al., 2011; Le Deit et al., 2013). Radiometric in situ K-Ar measurements dated its lava filling at Cumberland, corresponding to

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the Cf1 unit in the map of Le Deit et al (2013), to 4.21 +/- 0.35 Ga (Farley et al., 2014) thus suggesting

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the hypothesis that the crater formation might be older than previously estimated with crater counts.

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The hypothesis that older igneous material could have been transported in a younger crater from

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elsewhere is excluded by the geomorphological observation of the infilling connected to the lava fields of Tyrrhenus Mons and Hesperia Planum. It is very likely that the crater already existed at the time of

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the volcanic eruption that filled it. In situ geochemical analyses made by the Curiosity rover showed a

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predominant basaltic composition within the Gale crater floor with detected presence of pyroxene and unaltered olivine (Grotzinger et al., 2014; McLennan et al., 2014; Ollila et al., 2014; Sautter et al.,

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2014; Vaniman et al., 2014; Rampe et al., 2017; Morrison et al., 2018; Udry et al., 2018). Since the age

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of the basaltic floor of Gale crater appear consistent with the age of formation and early activity (4.254.20 Ga) of Tyrrhenus Mons (Leone, 2016), we examined the hypothesis that the filling of the crater floor may be connected to the lava fields of Tyrrhenus Mons and Hesperia Planum. According to the model of the southern polar giant impact (SPGI), later verified with the discovery of twelve volcanic alignments, a migrating plume formed the main volcanic centres of Elysium Mons and Tyrrhenus Mons along the Alignment 2 of Mars (Leone, 2016). The volcanic alignments follow loxodromic trajectories, that is, volcanoes aligned along trajectories forming the same angle with all the crossed meridians. Some alignments may cross each other, due to the migration of different mantle plumes in different times, Tyrrhenus Mons for example is located at the crossing point between the Alignments 2

Journal Pre-proof and 5 of Mars (Leone, 2016). The high temperature resulting from the SPGI favoured the production of fluid lava that crossed the highlands as far as the lowlands and then built the volcanic centres (Leone et al., 2014). There was an initial phase during the Pre-Noachian in which fluid lava reached long distances then followed by an edifice building phase during the Noachian through superposition of lava flows. The SPGI age of 4.25-4.20 Ga for the first formation of Tyrrhenus Mons and the lava field of Hesperia Planum (Leone, 2016) is consistent with the age of 4.21 Ga obtained by Farley et al (2014)

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from the K/Ar measurements of the basaltic infill of the Gale crater floor. The age of formation of Gale

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obtained from crater counts, 3.8-3.6 Ga (Thomson et al., 2011; Le Deit et al., 2013) is consistent with

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the age of formation of Tyrrhenus Mons obtained from crater counts, 4.0-3.6 Ga (Greeley and Guest,

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1987; Greeley and Crown, 1990; Williams et al., 2008; Werner, 2009; Robbins et al., 2011; Milbury et al., 2012). However, the ages obtained from crater counts are not consistent with the age obtained from

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K/Ar measurements. The K/Ar age of the crater infill is consistent with the initial age of formation of

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Tyrrhenus Mons and the Hesperia Planum lava field obtained with the SPGI model. Despite the difference in absolute age obtained through different chronological methods, there is a correlation in

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time between the formation of Tyrrhenus Mons-Hesperia Planum and the infill of Gale crater.

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Available Mars Orbiter Laser Altimeter (MOLA) data of Gale crater show how the northern side of the crater floor is deeper than the southern side with the main inlet coming from the south (Palucis et al., 2014; Grotzinger et al., 2015). A secondary channel merging the main channel, probably coming from a hidden secondary volcanic source, was associated to past fluvial activity originating from the Herschel crater area (Irwin et al., 2005). We do not exclude the possibility of secondary hidden volcanic sources formed by the same plume between Elysium Mons and Tyrrhenus Mons, as postulated by Leone (2016), but we focus on the more important and visible contribution coming from Tyrrhenus Mons via Hesperia Planum. High concentrations of X-ray amorphous material, compatible with a mix of allophane, ferrihydrite, rhyolitic, and basaltic glass have been found on the floor of Gale crater,

Journal Pre-proof revealing a composition similar to that of the lava fields of Mauna Kea on Earth (Bish et al., 2013). The Curiosity rover discovered and analysed many igneous rocks on the floor of Gale crater, including trachybasalts (Edwards et al., 2017). Edwards et al. (2017) concluded that the trachybasaltic composition, similar to the one found in Gusev crater, is the major constituent of the early Martian lavas in the highlands, excluding the Tharsis region. Williams et al. (2005) studied the origin of the basaltic rocks found within Gusev crater, concluding that they must have been originated by low

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viscosity (about 2 Pa·s) lava with a temperature of 1270°C. A comparison between Gusev crater and

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Gale’s basaltic composition is shown in Table 1. The aim of this paper is: 1) showing both possible

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origin and path of the lava infilling of Gale; 2) showing the main ways of entrance of lava to Gale

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crater; 3) showing how fluid lava can travel long distances on Mars. We analysed a wide area surrounding Gale crater included in the Mare Tyrrhenus quadrangle, in the

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Aeolis quadrangle, and in the Elysium quadrangle. More in detail, we focus on the lava filling coming

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from the two main visible sources relatively closer to Gale: Elysium Mons from the north and Tyrrhenus Mons from the south-west. We examined the labyrinth-like terrains of Aeolis Mensae,

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which could have provided a way in for lava to Gale via its northern rim, and the south entrance

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contribution via Farah Vallis, which channelized lava coming from the highlands. We have used the available geochemical analyses of the MSL “Curiosity” rover and the above-mentioned physical properties of typical highlands lavas in order to present the 1.5-dimensional model of Williams et al. (1998, 2005) that will reproduce the extension of lava from Tyrrhenus Mons to Gale. We show how low viscosity lava can travel as far as the distance of about 2000 km separating Gale from Tyrrhenus Mons along the main path that we found across the highlands.

Journal Pre-proof 2. Methods For the geomorphological analysis and the reconnaissance of the lava flows along the path we have used CTX imagery, and THEMIS imagery where CTX data coverage was not available, combined with MOLA data in order to track the gradient of topography (Figs. 1a-1b). We have divided the territory between Elysium and Tyrrhenus, where Gale crater is located, in two broader regions: a) The southern one, centred on the Tyrrhenus quadrangle, to study and track the lava path from

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Tyrrhenus to Gale. In order to reconstruct in detail the path of the lava through the highlands,

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we have subdivided this territory into four areas of investigation, along with their topographic

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gradient (Fig.1a, 1b);

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b) The northern one, centred on the Elysium Planitia lava fields and the Aeolis Mensae, in order to study the contribution from Elysium and to see if there is interaction with the lava flows coming

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from the highlands (Fig. 2a, 2b). We have analysed in detail the southern part of Aeolis Mensae

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and inside Gale with daytime IR THEMIS data. For the geochemical composition of the lava we have used in situ analysis made by the Curiosity rover

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on the various igneous rocks found on the crater’s floor and we have summarized them in Table 1. In

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more detail, Edwards et al. (2017) concluded that “… All of these igneous rocks resulted from lowpressure, olivine-dominated fractionation of Adirondack (MER) class-type basalt compositions.” This conclusion will justify the use of the Adirondack basaltic mean composition in the Williams et al. (1998, 2005) model. The geomorphological data have been used to reconstruct the slope of the path followed by lava flows from Tyrrhenus Mons to Gale.

2.1. MOLA data We have observed both regional and local topographic gradients as indicators of the main direction followed by lava flows from their original source. We have used the NASA’s Solar System Mars Trek

Journal Pre-proof portal, which also allows to produce topographic profiles along curvilinear paths typical of the sinuous channels carved by lava flows on the planetary surfaces (Day and Law, 2017), to track and visualise the topographic gradients. The access to gridded 0.2° × 0.2° MOLA data from the Mars Trek portal (https://trek.nasa.gov/mars/index.html) enables the drawing of profiles on a map maintaining the resolution of those data, the vertical precision is 1.5 m with an horizontal precision of 30 m (Zuber et al., 1992; Smith et al., 2001). The gridded (contoured) data used in our work is ~460 m/px (horizontal)

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and 10 m (vertical). We have traced the topographic profile to show the gradient of topography along

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the main path. The main path is indicated with a thick yellow line in each area of investigation (Fig. 1b,

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Fig. 4, Fig. 5, Fig. 6, and Fig. 7). The thinner yellow lines indicate breakouts or tributaries that do not

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have any profile tracked along their courses. For reasons of better data visualisation, due to the higher elevation of Elysium with respect to the surrounding transition topography (i.e., Leone 2015), the

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topographic profile C-C’ does not start directly from the top of Elysium Mons to avoid too much

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flattening of the topographic profile at Gale (Fig. 3a). This is particularly important to see better slope details in the distal parts of the Elysium lava fields near to Gale crater. The preliminary observation of

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MOLA colour maps has given a first visual indication of all the possible paths that lava may have

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followed through the highlands before reaching the lowlands from Tyrrhenus Mons. This first observation at regional scale was then followed by another accurate observation at local scale using CTX or THEMIS imagery as explained in the next section.

2.2. THEMIS imagery data In order to follow adequately the path of the lava flows from their sources, we have divided the broad region located between Tyrrhenus Mons and Gale crater in smaller areas of investigation (Fig. 2b, Figs. 4-7). Where CTX imagery was not available we have used THEMIS imagery data because they have nearly full coverage of the Martian surface at a resolution of 100 m/pix (Christensen et al., 2004),

Journal Pre-proof which is good to have a regional context. We have also indicated in the THEMIS images the ghost craters (dashed circles) resulting from the lava filling. The observation of the ghost craters and of the impact craters filled by lava facilitated the finding of the main path on the surface. Other craters that appeared deeper and with (more or less) intact rims were less affected by the main lava flows because they were away from the main path. The observation of channels, chaos terrains, and dissected terrains

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helped to individuate all the possible locations affected by the passage of the lava flows.

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2.3. CRISM mineralogic data

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We have also used CRISM mafic data, where available, to track the global mineralogical composition

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of the observed terrains along the main path. The spectral library of minerals ready for interpretation is made available through the CRISM website (http://crism.jhuapl.edu/) and archived in the Planetary

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Data System (PDS) node (Viviano-Beck et al., 2014). The mafic minerals are characterized by a

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different colour in order to facilitate the identification on the ground: red for olivine, green for low-Ca pyroxene, blue for high-Ca pyroxene. These CRISM mafic data supported the observation that the main

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fluid eroding the terrains and creating the channels was lava because the mafic minerals pyroxenes and

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olivine are typically found in lava. CRISM images showing mafic minerals like olivine and pyroxenes are available in Figs. 6d and 7d.

2.4.1 1.5-dimensional model of lava flow For the purpose of determining whether or not a turbulently flowing lava can travel out to the final distance separating Tyrrhenus from Gale, identified in Fig.1a-1b, we have used the model of thermal erosion by channelized lava of Williams et al. (1998, 2005). The model can calculate erosion rates at the channel bed, but here it is only used to provide an estimate of the maximum distance traveled by the flowing lava. In the model, lava is erupted as a very low viscosity and turbulent flow with a thermally

Journal Pre-proof mixed interior, and convective heat transfer occurs to the top and the base of the flow. Although the model was designed to simulate the behavior of turbulently flowing lava, the Reynolds number of the flow decreases with progressively increasing downstream distances from the lava source. Moreover, the Reynolds number can decrease until it reaches a value of 2000 at a distance that marks the transition to a laminar flow regime. Thermal erosion occurs at the base of the flow, providing that the lava temperature is greater than the melting temperature of the substrate, and latent heat is released as

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the flow crystallizes. The model includes: (1) the effects of lava rheology changes due to assimilation

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of eroded substrate and crystallization of mafic minerals in the flowing lava, (2) the lava temperature

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decrease as the flow moves downstream, and (3) the flow thickness increase as velocity decreases

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(thickness is used as proxy for flux, which is conserved as the flow moves downstream). Several algorithms are used to calculate initial values of important temperature- and composition-dependent

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thermal-physical lava properties, such as solidus and liquidus temperatures, liquid density and

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viscosity, lava specific heat and thermal conductivity, etc. (see Table 2). However, these algorithms require an initial lava major oxide composition (see Table 1). Additionally, topographical parameters

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associated with the flow are required to run the model, which are described in sections 2.4.2 and 3.1.

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To follow, we illustrate the Williams et al. (1998) model equations that were used to determine the maximum distance traveled by the lava flow. Lava crystallinity (X) is given by the ratio of the degree of undercooling (difference between liquidus and lava temperature) to the range of crystallization (difference between liquidus and solidus temperature, Williams et al., 1998), as follows:

X=

(Tliq − T) (Tliq − Tsol )

(1)

Journal Pre-proof in which Tliq is liquidus temperature, Tsol is solidus temperature, and T is lava temperature at the vent and then at progressively increasing downstream distances. Crystallinity is calculated by assuming constant liquidus and solidus temperatures rather than a range of values, and a linear growth (in the crystal fraction) with cooling, the latter assumption giving an adequate approximation for lavas crystallizing a single silicate phase (olivine) (Williams et al., 2000). The bulk viscosity of the flow, µb,

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is expressed as a function of the initial liquid viscosity, µl, by the Roscoe-Einstein equation (2a):

0.48 X X µb = µl exp [2.5 + ( ) ] 0.6 − X 0.6

(2a, b)

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X −2.5 µb = µl (1 − ) , 0.6

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at crystal fractions X < 0.3 and by the Pinkerton and Stevenson (1992) relation (2b), at crystal fractions

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X > 0.3. Equations (2a) and (2b) assume that the crystals remain in suspension during flow emplacement, which is strongly indicated during turbulent flow (Huppert and Sparks, 1985). Following

4 g h sin(α) , λ

λ = [0.79 ln(Re) − 1.64]−2 , Re =

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𝑢=√

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this, flow velocity u, friction coefficient λ, and Reynolds number Re (3a,b,c) are calculated iteratively:

ρb u h cl µb , Pr = µb k eff

(3a, b, c, 4)

in which g is the acceleration due to gravity, h is flow thickness, α is ground slope, and ρb is bulk density of the flow (liquid + crystals). The value of Re = 2000 is chosen as the theoretical limit of turbulent flow in conduits. Another calculated composition-dependent thermal-physical property of the lava is the molecular Prandtl number Pr, the ratio of momentum diffusivity to thermal diffusivity (4), in which cl is lava specific heat and keff is the effective lava thermal conductivity in the thermal boundary layers at the base and top of the flow, respectively (5a, b) (Williams et al., 1998). The effective thermal conductivity is expressed as:

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k eff =

0.0013 (T − Tmg ) , ln (2.16 − 0.0013 Tmg ) (2.16 − 0.0013 T)

k eff =

0.0013 (T − Tsol ) ln (2.16 − 0.0013 Tsol ) (2.16 − 0.0013 T)

(5a, b)

In (5a), Tmg is the effective melting temperature of the substrate (which for a given value of µg maximizes thermal erosion). Unlike Hulme (1973) and Huppert and Sparks (1985), Williams et al.

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here one that includes the effects for turbulent sheet flows:

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(1998) adopt more than one expression for the convective heat transfer coefficient (h T), and we use

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(6)

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0.0296 k eff Re4/5 Pr1/3 µb hT = ( ) h µg

in which µg, the viscosity of the melted substrate, is calculated as a function of Tmg and is equal to 35.6

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Pa s for a substrate that is basaltic in composition (Williams et al., 1998). The ratio of the lava bulk

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viscosity to the viscosity of the melted substrate in (6) has the effect of reducing heat transfer compared

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to that found in fluids with constant physical properties. Lava thermal erosion rate, um, as modified from Huppert and Sparks (1985), is given by:

um =

hT (T − Tmg ) , Emg

Emg = ρg [cg (Tmg − Ta ) + Lg ]

(7a, b)

in which Emg is the energy required to melt the substrate, ρg is substrate density, cg is substrate specific heat, Ta is ambient temperature of the surface, and Lg is heat of fusion required to melt the substrate. The erosion rate is used to calculate the degree of contamination of the lava by assimilated substrate, given by:

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S(x) = 1 −

Q0 , Q(x)

𝑥

Q(x) = Q0 + ∫ um dx

(8a, b)

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in which Q0 is the initial flow rate and Q(x) is the flow rate at any given distance from the source.

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Finally, the following mass expressions:

Mnew = Mold (1-ΔX) + Molv (ΔX)

(9a,b)

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Mnew = Mold (1-ΔS) + Masm (ΔS),

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are used to calculate the compositional change in the liquid lava due to the assimilation of thermally

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eroded substrate S and the crystallization of minerals (olivine) X at each model distance increment.

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Here, Mnew is the major oxide composition of the lava at the current distance from the source, Mold is the major oxide composition of the lava at the previous distance increment, M asm is the major oxide

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composition of the substrate, and Molv is the olivine major oxide composition. Equation (9b) is used in

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conjunction with partition coefficient and stoichiometric algorithms to calculate Molv at each model

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increment of distance. The newly calculated lava composition from (9b) is then used to recalculate the temperature- and composition-dependent thermal, rheological, and fluid dynamic properties of the lava at each distance increment downstream. Lava temperature is the key parameter that advances the model, which decreases as the flow moves downstream. This model of lava cooling with distance is given by the following 1st order ordinary differential equation (modified from Huppert and Sparks, 1985):

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ρb cl hT (T − Tmg ) dT dT Ll X ′ (T) ρb cl hu = −hT (T − Tmg ) − hT (T − Tsol ) − + ρb cl hu dx Emg dx cl

(10)

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in which Ll is the lava heat of fusion and X’(T) represents the increasing volume fraction of olivine crystals in the lava with decreasing temperature, equal to -1/625°C-1 (derived from the slope of the liquidus, Figure 2, Usselman et al. 1979). Because the physical properties of the lava are changing with distance, (10) must be solved at each increment of distance from the eruption source using a fourthorder Runge-Kutta numerical method. Once a new temperature (from equation (10)) and a new lava

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composition (from equations (9)) are obtained, the new thermal, rheological, and fluid dynamic

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parameters are calculated at that distance. In doing so, the physical and geochemical evolution of the

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lava flow at progressively increasing downstream distances from the source is simulated.

2.4.2 The input parameters and assumptions of the model

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The key input parameters of the model are (Williams et al., 1998, 2005): a) The major oxide composition of the erupted lava and underlying substrate; b) Liquidus and solidus temperatures for the

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lava and substrate; c) An initial lava pressure (at time of eruption) of 6.4x10-6 kbar, assumed to be

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equal to the environmental pressure on the Martian surface; d) The thickness of the lava; e) The slope of the substrate and; f) Thermal and rheological parameters of both flowing lava and substrate (see Table 2). The composition of both lava and substrate is assumed to be that of the Adirondack basalt, with an estimated liquid viscosity of 2.3 Pa s and a liquidus temperature of 1270˚C (1543 K) (McSween et al., 2004; Gellert et al., 2004), which well suit a turbulent emplacement regime.. The composition of the igneous rocks found at Gale is slightly different from that of the Adirondack basalt (Table 1), which is normal and might likely result from the evolution of magmas with time. The liquidus temperature (Tliq) was calculated using MELTS (Ghiorso and Sack, 1995); the solidus temperature (Tsol) is from Arndt (1976); liquid density (ρl) was calculated using the method of Bottinga

Journal Pre-proof and Weill (1970), and density changes as a function of evolving pressure, temperature, and composition were obtained by adopting the partial molar volume coefficients of Mo et al. (1982); liquid viscosity (µl) was calculated using the method of Shaw (1972); specific heat (c) was calculated from the heat capacity data of Lange and Navrotsky (1992; and the temperature-dependent heat of fusion (Ll) is approximated using expressions for the mineral representing the highest volume percentage of the rock (here forsteritic olivine, from Navrotsky, 1995). The lava substrate is also assumed to be dry and

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consolidated. No real constraint on the eruption temperature exists, which has led us to choose four

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values of eruption temperature (1270˚C, 1260˚C, 1250˚C, 1240˚C) to investigate the extent to which a

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similar temperature variation may affect the key physical parameters of the flow along with the total

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distance traveled by the lava. The lava flow is assumed to bear no gas bubbles within it. The likely impact of gas bubbles on a flow traveling the distance separating Tyrrhenus Mons from Gale Crater is

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discussed in section 4.3.1.

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For the path identified in Fig. 1a, we calculated a weighted average for the slope of the substrate, over the entire flow length, and obtained a value of 0.0578°. The maximum thickness of the lava (~150 m,

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see section 3.1) likely results from the superposition of more than an individual flow unit and, because

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of this, the assumed maximum lava thickness is always lower than 150 m. The minimum lava thickness has been chosen after measuring the thickness of the lava at Gale crater (~20 m, see section 3.1) as in this study we are focusing on a lava flow that might have traveled the distance separating Tyrrhenus Mons from Gale Crater.

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Fig.1 [Colour] a) Context MOLA map of the first macro region of investigation, from Tyrrhenus Mons (left) to Gale (upper right corner). b) Context MOLA map as above, the white rectangles indicates the location of the regions showed in figures 4-7, the yellow line indicates the main path of lava from Tyrrhenus (A) to Gale (A’). c) The topographic profile A-A’ shows a general average trend favourable for lava to flow from Tyrrhenus to Gale. In figures 4-7 we will show in more detail the flow of lava through this main path as far as Gale.

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Fig. 2 [Color] a) Context MOLA map of another broad region of investigation, from Elysium (top) to Gale (bottom). The white rectangle indicates the location of the region north of Gale, which includes Elysium Planitia and Aeolis Mensae. The yellow lines are three significant topographic profiles, coming from the highlands (BB’, DD’) and from Elysium (CC’), which are showed in fig. 3. b) THEMIS day IR image of the region north of Gale which shows the fronts of the northern lava flows and the southern lava flows; there is no interaction between the two main fronts except for an additional front of

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lava coming from the east which interacts with the main northern front. The dashed white lines indicate the largest ghost craters identified in the region. The thick white lines indicate the main fronts of the lava flows, the white arrows indicate the direction of movement of the lava flows inferred from the various fronts composing them. The lava flows coming from the north (Elysium) stopped at about 300 km north of Gale crater due to the unfavourable topographic gradient along the profile C-C’.

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Fig. 3 [B/W] MOLA topographic profiles of the region showed in Fig. 2. a) Profile from the Elysium lava fields (C) to Peace Vallis (C’), the topographic gradient is unfavourable for a lava filling of Gale via Elysium; b) topographic profile from Licus Vallis (B) to the lowlands (B’); c) topographic profile east of Gale, coming from the south (D) as far as to Aeolis Mensae (D’) from lava contribution coming from Tyrrhenus Mons. Both profiles shown in panels 3b) and 3c) show steep topographic gradients favouring the movement of lava from the highlands to the lowlands.

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Fig. 4 [Color] a) THEMIS day IR image of the transition region between the younger lava fields of Tyrrhenus Mons and the

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highlands as shown in the rectangle 4 of Fig. 1. The yellow line shows the topographic profile through the main path. The points 1-4 indicate regions where the local topographic gradient seems unfavourable to the main direction of lava from Tyrrhenus to Gale only because it was subsequently modified by impact craters. The labelled white rectangles refer to the panels c and d in this figure. b) MOLA altimetric profile of the path followed by lava from Tyrrhenus to the highlands. It is possible to see how the general trend is flat with local modifications of the topography labelled with the numbers 1-4. These critical points are discussed in the text and are due to subsequent modification of the original topography by impact cratering. c) CTX image P11_005405_1585_XN_21S232W showing part of a filled crater just north of the main path, a smaller ghost crater is visible in the image, the texture of the lava filling is very similar to the texture observed in other filled craters of the region, i.e. Palos crater (Leverington, 2006). d) CTX image G18_025210_1608_XN_19S227W showing the texture of the lava filling within an almost completely filled crater (upper part of the panel d) along the main path, the yellow line indicates the main path and its direction, the black arrow indicates a flow front.

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Fig. 5 [Color] a) THEMIS day IR image of the main path of lava obliterated by an unnamed crater (point 5), the area is shown in the rectangle 5 of Fig. 1. This crater shows a poorly eroded rim without lava infilling on the floor. The impact that generated this crater was then subsequent to the lava flooding from Tyrrhenus that formed the main path. The yellow, partially dashed, line indicates the main path of the lava flows from south toward north and filling another unnamed crater at the top of the image. The thinner solid yellow line represents another possible path for lava into the unnamed filled crater. Inside this crater there are flow fronts interacting (black arrows) and a ghost crater (dashed white line). The white arrows indicate filled craters along the main path of the lava. b) The topographic profile of the main path through the younger unnamed crater. The original topography was nearly flat, allowing lava from Tyrrhenus to flow towards the north and the north-east. c) CTX image G22_026845_1646_XN_15S224W showing a lava flow front along the main path, the morphology of the flow front is similar to the wrinkle ridges identified as lava flows in Leverington (2006). d) HIRISE image PSP_006631_1625 showing a blow up of the floor of a filled crater located south of the main path, once more the texture of the lava filling is very similar to that observed in Leverington (2006).

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Fig. 6 [Color] a) THEMIS day IR image of the lava channels around the Herschel crater region (lower left corner), the

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location of this area is shown in the rectangle 6 of Fig. 1. The main path coming from Tyrrhenus Mons is shown as a thick

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yellow line. The yellow thinner lines indicate secondary channels and/or breakouts, the blue line indicates the channel discovered by (Irwin et al., 2005) and associated to fluvial activity. All these channels converge to the main path in an unnamed valley visible on the top right corner of the image. The arrows indicates the filled craters along the main path. Lava flows are also marked. White rectangles indicate an area of interest presented panel c; b) Elevation profile of the main path from the Herschel crater (G) to the unnamed valley (G’). From this point onwards there is no significant topographic modification of the main path by subsequent impact cratering; c) THEMIS day IR blow-up of the lava outflow from the filled unnamed crater shown in Fig. 5, a network of smaller (outlet) channels finds a way out from a breach in the northern rim. On the right side of the image it is possible to note a partially filled, younger, crater that interrupted the lava flux towards the east; d) CRISM mafic composition of a region inside the valleys that collected all the lava flux from the channels, red refers to olivine, green to low-Ca pyroxene, blue to high-Ca pyroxene (Viviano-Beck et al., 2014). The presence of olivine suggests an igneous origin of the Farah Vallis channel’s bedfloor. CRISM image ID: 0000A4F2.

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Fig. 7 [Color] a) THEMIS day IR image from the unnamed valley to Farah Vallis, the location of this area is shown in the

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rectangle 7 of Fig. 1. The yellow thick line again indicates the main path from the unnamed channel located at the bottom left of the panel a (H) to Gale crater (H’). The blue line represents the channel investigated by (Irwin et al., 2005), which coincides with the main path in this sector. Smaller tributaries (yellow thin lines) come from a filled unnamed crater located to the west from the main path. The white rectangle located on top of panel a indicates the area inside Gale shown in panels c and d. b) Topographic profile from the unnamed valley (H) to the floor of Gale crater (H’) showing how the topographic gradient allows lava to run down and fill the crater via Farah Vallis. c) CRISM IR surface brightness data; d) CRISM mafic mineralogy data data, red is olivine, green on the left bottom side of the image is low-Ca pyroxene. CRISM image ID: 00004371.

Journal Pre-proof 3. The sources of lava flows In this section we discuss the main path of the lava flows to Gale crater from their main volcanic visible sources, Tyrrhenus Mons-Hesperia Planum and Elysium Mons. There would also be a third source (Fig. 8a), minor if compared to its giant “neighbours” of alignment, an unnamed caldera located between Elysium Mons and Tyrrhenus Mons along the Alignment 2 (Leone, 2016). There could be other hidden sources, as postulated by Leone (2016), but we prefer to focus only on the main visible

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sources that provide the main significant contributions to the infilling of Gale crater.

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3.1. Tyrrhenus Mons

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The first set of smaller areas of investigation is located along the ≈ 1900 km of distance connecting Tyrrhenus Mons to Gale crater. The main path starts directly from the most distal lava fields of

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Tyrrhenus Mons located in Hesperia Planum (Fig. 4a). Lava is channelized for a significant portion

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(~1,100 km in length) of the path connecting Tyrrhenus Mons to Gale crater, though channelized segments are frequently interrupted/obliterated by impact craters. The total thickness of the lava is

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measured to vary between ~150 m at Hesperia Dorsa (initial part of the path) and ~21 m at Gale, and

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the width of the channelized segments is equal to ~2.68 km (average value). The main path of the lava flows, indicated by a thick yellow line, was subsequently obliterated by several impact craters that modified the topography as indicated by the points 1-4 in both Figures 4a and 4b. Lava filled other preexisting impact craters encountered along the main path. In fact, filled craters are clearly visible in Fig. 4a, 4c, and 4d. The lava flow fronts were indicated where visible (Fig. 4d), although some of them were in the past interpreted as wrinkle ridges. The wrinkle ridges were originally interpreted as formed by feeder dikes or lava flows (Peterson, 1978; Scott and Tanaka, 1986; Hartmann and Berman, 2000; Hartmann et al., 2001; Williams et al., 2009), although a tectonic origin was also suggested (Watters, 1991; Mège and Masson, 1996; Head et al., 2002; Hiesinger, 2004; Basilevsky et al., 2006), but other

Journal Pre-proof studies have definitely recognized them as flow fronts or formed by volcanic origin (Leverington and Maxwell, 2004; Leverington, 2006; Korteniemi et al., 2010; Kostama et al., 2010; Williams et al., 2010; Leone, 2014, 2016, 2017). The volcanic origin is also supported by the absence of plate tectonics on Mars (O’Rourke and Korenaga, 2012) that cannot thus generate compressive stresses at global scale in the crust. Moreover, magma rise in the crust generates stress fields forming radial patterns either on Earth (McGuire and Pullen, 1989) or Mars (Ernst et al., 2001; Cailleau et al., 2003) whilst the wrinkle

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ridges have generally concentric or complex or random patterns (Chicarro et al., 1985; Watters and

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Robinson, 1997) more consistent with lava spreading away from volcanic centres (Hiller, 1979;

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Leverington, 2006; Leone, 2016). Thus, we favour the interpretation of the wrinkle ridges as lava flow

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fronts.

The main path has been obliterated along its way to Gale by another impact crater (the inferred path is

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indicated by the thicker yellow dashed line, point 5, in Fig. 5a), which modified the topographic

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gradient (Fig. 5b), and which may have created a breakout towards the east or simply concealed the western main branch (Fig. 5a). The eastern breakout filled another pre-existing crater and formed a

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ghost crater inside it; also, the main path filled the same crater from the west through small inlets (see

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top of Fig. 5a). Figs. 5c and 5d show the lava flows along the main path and the texture of the lava within a filled crater, respectively. From this point onwards the main path was not obliterated anymore by subsequent impact craters (Fig. 6a), so that a more straightforward observation was possible in the topographic profile F-F’ (Fig. 6b). In the lower right corner of Fig. 6a there is a large filled crater, whose overflows along its northern rim have formed smaller tributaries of the main path. The western rim of this crater was obliterated by another smaller crater (Figs. 6a and 6c). This smaller crater is also filled, sign that the eruption was still ongoing during its formation or maybe that another eruption followed the previous one. Furthermore, such a smaller crater obliterates and thus hides the entrance of the lava that previously filled the larger crater. The crater filled from the eastern breakout that we

Journal Pre-proof observed in Fig. 5a formed a couple of outlets out of its northern rim (Fig. 6c). These outlets reached the main path further to the north. The blue line in Fig. 6a indicates the channel interpreted by Irwin et al. (2005) as of fluvial origin. Other channels depart from the same source of the “Irwin’s” channel. These channels originated from a large lava filled crater located to the west of their heads and joined the main path just before reaching a filled crater located to the south of Farah Vallis. In the upper right corner of Fig. 6a we can see a valley formed by the main path and the convergence of another western

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tributary both leading towards Farah Vallis. CRISM data indicate the presence of olivine on the floor of

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Farah Vallis (Fig. 6d). The whole Farah Vallis is then shown in Fig. 7a. The topographic gradient

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shows again a steep slope in proximity of Gale crater (Fig. 7b). CRISM data show the presence of both

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pyroxenes and olivine at the entrance of Farah Vallis into Gale crater (Figs. 7c and 7d). The mafic composition observed in the CRISM images of these flows is very similar to the mafic composition

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observed in the CRISM image 0000B6F1 of the floor filling sampled by the Curiosity rover, which is

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3.2. Elysium Mons

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shown in Fig. 9b ahead, suggesting that they might be made by the same lava composition.

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The second set of areas of investigation starts from the middle of the southern lava fields of Elysium Mons as far as Gale crater (Fig. 2a). The southern Elysium Planitia is particularly important to investigate this area in order to understand whether any interaction occurs between the lavas coming from Elysium Mons with those coming from the Tyrrhenus Mons. We have traced three topographic profiles (Fig. 2a) and showed them in Fig. 3a, 3b, and 3c to show how the topographic gradient may have affected the direction and the maximum distance reached by the lava flooding coming from Elysium Mons as well as from Tyrrhenus Mons. The profile in Fig. 3a shows how the topographic gradient is gradually increasing in its distal part, thus making difficult the advancement of lava coming from Elysium Mons to Gale. Profiles in Fig. 3b and 3c show how the topographic gradients along Licus

Journal Pre-proof Vallis and from a series of unnamed valleys to the east of Gale crater favour the flow of lava coming from the highlands directed to the lowlands. In the blow up of Fig. 2b there is a THEMIS daylight IR showing the region between the lava fields of Elysium Mons all over the Aeolis Mensae. Lava fields formed by the floodings coming from the unnamed valleys are labelled as “Tyrrhenus” because indeed came from there (Fig. 2a, profile D-D’), whilst the lava flows coming from Licus Vallis and Elysium Mons are labelled with their own names “Licus Vallis” and “Elysium” respectively. The area shows

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two main lava flow fronts that do not interact among them. Only the lava flows coming from the east

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effectively interacted and somehow shielded Gale crater from the lava flows coming from the north

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(Fig. 2a). The decreasing topographic gradient from the south to the north, as it would be expected

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along the transition topography of the Martian dichotomy (Leone, 2015), favours the flows coming

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from the highlands to the lowlands and stops the flows coming from the opposite direction.

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3.3. Unnamed volcanic centre

At last, we have observed the lava flows coming from an additional volcanic source, an unnamed crater

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located between Elysium Mons and Tyrrhenus Mons along the Alignment 2 (Leone, 2016). This is the

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volcanic source that fed Licus Vallis and another unnamed channel heading towards Aeolis Mensae via the Robert Sharp depression, which likely fed Peace Vallis (Fig. 8a, profile I-I’). We have traced a topographic profile I-I’ along this channel (thicker yellow line in Fig. 8a) and we showed it in Fig. 8b. The topographic gradient is favourable to the flow of lava for most of the path towards Aeolis Mensae and aided the volcanic channel to make its way through the rims of two craters, the second of which shows chaotic terrain and landslides that have subsequently modified the debouching zone. Lava coming from this channel, indicated as K-K’ in Fig. 9a, has partially turned back towards Peace Vallis but probably did not enter into it according to the profile shown in Fig. 10c; the distal part of this profile shows a slight increase in the topographic gradient. Another profile J-J’ of the same channel

Journal Pre-proof was traced to see if the local topography allowed lava flows moving to the north as well. Indeed, the flat topography that we can see along the J-J’ profile would allow some advancement to the north (Fig. 10a). In fact, lava flow fronts coming from this channel heading to the north were already observed in Fig. 2b. A different situation can be seen for the flows coming from Tyrrhenus to the east of Gale crater. We have explored two other possible entrances for the lava flows coming from Tyrrhenus Mons heading to

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Peace Vallis among the chaotic terrain of Aeolis Mensae, these are the paths L-L’ and M-M’ shown in

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Fig. 9a. According to their corresponding topographic profiles shown in Fig. 10b and 10d, respectively,

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the topographic gradient is favourable as far as Peace Vallis, disturbed only by an impact crater located

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just north of Gale which modified the local topography.

Fig. 8 [Color] a) MOLA map of the main contribution to Licus Vallis and the network of channels heading to Robert Sharp depression. The unnamed crater, according to (Giovanni Leone, 2016a) is a spreading point, consistent with the alignment of volcanic features subsequent the SPGI (Giovanni Leone, 2016b; Leone et al., 2014). The lava flux coming from this spreading point reached the lowlands through Licus Vallis and Robert Sharp depression, interacting with the flux coming

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from the unnamed spreading point (I) to the north-east exit of Robert Sharp depression (I’).

Fig. 9 [Color] THEMIS day IR image with MOLA colours of the area north of Gale. Yellow lines shows the path of lava from the highlands to the lowlands, toward north and inside Gale crater via Peace Vallis. The yellow thin line is the contribution of Farah Vallis. b) HIRISE ESP_018920_1755 image showing the lava filling of the crater floor coming from Farah Vallis, such a lava filling coincides with the Cf1 unit mapped by Le Deit et al (2013), the red line is the path of the Curiosity rover that took the geochemical analyses of basaltic composition all over the lava filling of the crater floor, the yellow line indicates the main path of the lava coming from Farah Vallis. c) Delta of lava formed by a branch of the main path (yellow line) which debouched into Gale crater from Farah Vallis.

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Fig. 10 [B/W] Altimetric profiles showed in fig. 9. a) The profile coming from the north-east boundary of Robert Sharp

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depression (J) to the north (J’). b) The profile from the same origin (K) but headed to Peace Vallis (K’), may have flooded

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Gale. c) The profile coming from a the northern side of labyrintus like formation of Aeolis Mensae (L) to Peace Vallis (L’). d) The profile from the southern portion of Aeolis Mensae (M) to Peace Vallis (M’). Any of these profiles show a global topographic gradient favourable to lava flows along the direction of the profiles. This implies that lava from the highlands, including the contribution from Tyrrhenus Mons, reached the lowlands and filled partially the northern side of Gale crater mainly from the south through Farah Vallis.

Journal Pre-proof 4. Discussion In order to better explain how the lava flows reached Gale crater from Tyrrhenus Mons, it might be useful to put together the tiles of new information coming from our observations with those of the literature produced with the results of the Curiosity mission. To do so it is necessary to establish some key points as a context framework:

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1) The presence of basaltic minerals on the floor of Gale crater (Payré et al., 2017), exactly on the

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Cf1-Cf3 units mapped by Le Deit et al (2013), in combination with the detected presence of

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tridymite formed at high temperatures (Morris et al., 2016), and a delta of lava at the east of the

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debouchment of Farah Vallis into Gale crater (Fig. 9d), shows a clear evidence of volcanic infilling by low viscosity lavas; CRISM mafic data taken at the debouching of Farah Vallis are

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consistent with CRISM data taken on the floor of Gale crater where the Cf1-Cf3 units are

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located; the delta of lava visible in Fig. 9d shows the thickness of the latest lava flooding reaching Gale crater in correspondence of the Cf3 unit of Le Deit et al (2013), which also

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helped to calculate the final thickness of the lava flooding that reached Gale as mentioned in

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section 3.1 before and in section 4.3 ahead. 2) Impact craters formed after the emplacement of the lava flooding that first reached Gale, as evidenced by the superposition of the various points labelled from 1 to 5 on the main path, modified the local topography creating subsequent barriers that might also have been circumvented by later lava flows. In the absence of these craters, there would likely have been a favourable gradient until younger ages; 3) The adopted 1.5-dimensional model with the chemical composition found on Gale’s floor showed how lava from Tyrrhenus can travel for two thousands of kilometres along the main path (Fig. 1a) and reach Gale through the network of channels that we observed on the ground.

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4.1. The role of Tyrrhenus Mons There is a striking consistency between the age of the initial emplacement of Tyrrhenus Mons (4.254.20 Ga), estimated through the SPGI model (Leone et al., 2014; Leone, 2016), and the age of the basaltic filling of Gale crater (4.21 ± 0.35 Ga) measured by Farley et al (2014). We exclude that this consistency may simply be due to a coincidence because there is a visual continuity between the lava

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fields of Tyrrhenus Mons and Hesperia Planum and the infilling of Gale crater where the K/Ar

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measurement was made. This continuity also suggests that Gale crater was already formed when the

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lava flows came down from Tyrrhenus Mons via Hesperia Planum and filled it. The evidence of huge

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lava fields, even on cold planetary surfaces, piercing the rims of and filling pre-existent impact craters (Leverington, 2006; Leone, 2016, 2017), and extending for thousands of kilometres around the main

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volcanic centres of Mars (Tharsis, Elysium, Tyrrhenus), shows how it is possible for fluid basaltic lava

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flows to travel even for longer distances than that separating Tyrrhenus Mons from Gale. We do not exclude the possibility of secondary vents along the main path that could contribute to the main flow

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towards Gale. However, their scarce visibility suggests that their contribution would have been minor if

Gale crater.

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compared to the main and well visible link of lava between Tyrrhenus Mons-Hesperia Planum and

As a consequence of the lava flowing from Tyrrhenus Mons we could observe: a) lava flows and dendritic networks of channels of every size as postulated and observed by other authors (Leverington, 2011; Dietterich and Cashman, 2014; Leone, 2016); b) that the filling of Gale would have been differentially distributed from south to north, being the northern side deeper and thus less filled by basaltic material than the southern side (Le Deit et al., 2013; Grotzinger et al., 2015). This is consistent with the major size of Farah Vallis, which would have favoured a larger influx of lava compared to Peace Vallis. The presence of small incisions inside and outside Gale crater, clearly small when

Journal Pre-proof compared to the larger outflow channels of volcanic origin present elsewhere on Mars (i.e. Valles Marineris, Mangala Valles), are directly proportional to the volumes of lava that reached Gale crater from Tyrrhenus Mons (Leone, 2014, 2017). Despite the northern rim of Gale shows heavier signs of fluid erosion than the southern one, the northern contribution of lava to the filling of the crater floor has been marginal. This is supported by: 1) the smaller size and depth of the main northern inflow channel, Peace Vallis; 2) the greater depth of the northern side of the crater respect to the southern one due to

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the minor infilling of lava. The main contribution to the infilling of the crater thus remains the southern

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one coming from Tyrrhenus Mons via Farah Vallis and its multiple tributaries.

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4.2. Mineralogical tracers for lava

Direct measurement made by the MSL Curiosity revealed the presence of minerals with basaltic

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composition with a clear dichotomy: a) the material sampled on Mount Sharp has an andesitic

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composition compatible with that of pyroclastic deposits (Ollila et al., 2014; Payré et al., 2017; Sautter et al., 2014, 2015, 2016); b) the samples on the floor at the Bradbury landing site as far as the foothills

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of Mount Sharp have a basaltic composition with the presence of jarosite, tridymite, and unaltered

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olivine (Blake et al., 2013; McLennan et al., 2014; Schmidt et al., 2014; Grotzinger et al., 2015; Morris et al., 2016). The ambiguity related to the possible co-existence of phyllosilicates and olivine, being the former previously ascribed to the presence of water (Bibring et al., 2006), is resolved by the argument that they can both have a common volcanic origin: a) the phyllosilicates as a result of thermal alteration by flowing lava (Che and Glotch, 2014); b) the olivine being a common component of mafic lava, of course.

4.3. Model of lava flow

Journal Pre-proof Table 2 shows that any lava that flowed from Tyrrhenus to Gale, following the path identified and shown in Fig. 1, might have reached the flow terminus for values of initial flow thickness in the range 21-65 m and initial lava temperatures from 1270°C to 1240°C. Lavas can be erupted at their liquidus temperature (here T=1270°C), though it is likely for lavas to contain a few crystals at time of eruption. In our simulations, the only crystal phase that occurs within the erupted lava is olivine (also according to the observations related to CRISM data), and - in the Martian environment - the next mineral phase

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expected to occur as the temperature of the lava decreases to a value of 1211°C is clinopyroxene. Here,

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the lowest temperature achieved by the flowing lava at the flow terminus is 1222°C. For eruption

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temperatures of 1270°C, 1260°C, 1250°C, and 1240°C, the proportion of crystals (vol.%) in the lava

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increases from 0% to 8%, 16.7%, and 25%, respectively. A higher proportion of crystals in the erupted lava is associated with a higher flow viscosity, a lower flow velocity and Reynolds number, and, hence,

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a less vigorous turbulence. In contrast, a higher flow thickness is conducive to a lower flow viscosity

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(keeping other factors equal), higher flow velocities and Reynolds number, and a more vigorous turbulence, all of which translate into a better ability of the lava to travel a longer distance from the

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source. For the same initial flow temperature, T, the higher flow thickness represents the minimum

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thickness for which the lava flows turbulently out to the final distance; the lower thickness is the minimum value for which the flow travels to the final distance – while the flow regime transitions from turbulent to laminar at some downstream distance from the lava source. Calculated flow rates and total lava volumes lie in the ranges 2.6 - 9.3 x 105 m3 s-1 and 1.1 – 3.3 x 102 km3, respectively. In this study, we are only concerned with lava that might have travelled the distance between Tyrrhenus and Gale during an individual eruptive event. As a result, in our calculations of total volumes, we have used a range of thickness values that refer to an individual flow unit. Remarkably, the measured thickness value of ~21 m (see section 3.1) at Gale suggests that total flow volumes might have been not significantly higher than the 110 km3 value referring to a 21-m-thick flow in Table 3. The whole range

Journal Pre-proof of calculated flow rates and volumes lie well within the upper limits obtained for channelized lava eruptions on the Moon (Wilson and Head, 2017). Also, maximum lava volumes fall within ~1.2 orders of magnitude of the upper values obtained for the 1,400-km-long Athabasca Valles lava on Mars (Jaeger et al., 2010). The Athabasca flood lava was likely emplaced as an individual eruptive event over a few weeks (Cataldo et al., 2015; Jaeger et al., 2010). By comparison, individual Columbia River LIP (Self et al., 1997) and Deccan Traps lava flows (Chenet et al., 2009) led to the rapid emplacement

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(at least a few years) of ~125 km3/year and 30-300 km3/year of lava, respectively. The turbulent-

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laminar transition is assumed to occur at a Reynolds number, Re, equal to 2,000.

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4.3.1 Uncertainties of the model

The difference in composition between the modelled Adirondack basalt and the basaltic lava that

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originally flowed from Tyrrhenus to Gale has a potential to introduce an error in the estimate of the

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total distance travelled by the lava. The Adirondack type of picritic basalts are a variety of highmagnesium, sub-alkaline olivine basalts (McSween et al., 2004). Because of their compositional make-

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up, they have a very-low viscosity at time of eruption and, hence, are more likely to flow turbulently

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and travel to greater distances than other basaltic lavas. Most of the igneous rocks found in the Gale crater catchment area are basaltic. However, felsic (silica-rich) and alkaline rocks, and trachy-basalts were also found recently (Cousin et al., 2017). The recently-found compositions imply a higher flow viscosity and a comparatively lower ability to flow turbulently. That being stated, using fractionation calculation, Edwards et al. (2017) suggested that fractionation of olivine from an Adirondack type basalt can yield the trachy-basalt composition found in Gale crater. Also, the observed felsic rocks can be obtained through different degrees of fractional crystallization of basaltic compositions (Udry et al., 2018). In other words, at least some of the earliest lavas that flowed from Tyrrhenus to Gale were likely similar in composition to that of the Adirondack basalt we used in our simulations.

Journal Pre-proof Perhaps the second most important source of errors is represented by the fact that the Williams et al. (1998, 2005) model is a one-dimensional model that cannot address the level of complexity that characterizes the flow of turbulent lava in a 3-D environment. From Tyrrhenus Mons to Gale, both the slope of the ground and the width of the channels change significantly. These variations likely lead to variations in the distribution of the thermal budget across the flow – something that only a 3-D model can attempt to reproduce. Besides – when it comes to channelized lava - any significant variation in

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flow width is likely accompanied by a variation in flow thickness. We saw how flow thickness affects

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flow velocity and, ultimately, the ability of the flow to travel greater distances (Table 2).

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Our simulations deal with a bubble-free flow of lava, which might also represent an additional source

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of error. The presence of bubbles in the lava is expected to decrease lava density and increase flow viscosity at low strain rates (Pinkerton and Stevenson, 1992), whereas at high strain rates bubbles will

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deform, reducing flow viscosity (Spera et al., 1988). In the case of turbulent flow, high strain rates

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rather than low ones should be expected, which will likely enhance turbulent flow conditions and cause the lava to travel even farther from the source (holding other factors the same). However, the behavior

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of bubble-bearing turbulent lava has not been reproduced in any experimental setting to date, and

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vesicularity likely decreases downstream of the source region and over time anyway. As a result, not assessing the impact of bubbles on the total distance traveled by the lava introduces an error that is likely smaller than the error introduced by assuming a 1.5-dimensional flow of lava over the distance of 1,963 km that separates Tyrrhenus Mons from Gale Crater. Another potential source of error is introduced by the assumption of effusion rates that – for an individual eruptive event characterized by a specific combination of eruption temperature and lava thickness - are held constant over time. However, effusion rates always vary with time. Although this is likely to introduce an error in our travel distance estimate, results are unlikely to be dramatically

Journal Pre-proof different, especially when considering that our flow travelled a distance of 1963-2210 km from the lava source. Over such a large distance, effusion rate variations likely cancel each other.

5. Conclusions In this work we have found how: 1) the geochemical composition of Gale's floor is compatible with

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lava filling, as also shown by CRISM data (Figs. 6d and 7d); 2) the K-Ar age of such lava filling is

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consistent with the age of the volcanic activity of Tyrrhenus as predicted by Leone et al. (2014) and

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Leone (2016); 3) the mean topographic gradient is favourable to lava flowing from Tyrrhenus through

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the highlands down to the lowlands (Fig. 1 and Fig. 10); 4) the main path was identified by the numerous filled and ghost craters found along the way to Gale (Figs. 2b, 7a, and 9) and by

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reconstructing the complex network of channels heading to Farah Vallis; 5) the lava fields coming

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from the north did not interact with the lava flows coming from the south, the negative topographic gradient in the distal region of Elysium Planitia prevented thenorthern lava flows from reaching Gale

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(Fig. 2b); 6) the northern sector of Gale received a contribution from the highlands mainly via Peace

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Vallis, although lower than that received from the southern sector via Farah Vallis; 7) after other evidence in the literature in which lava travelled large distances on Mars (Griffiths, 2000; Leone, 2014), we have at last showed that lava from Tyrrhenus Mons has effectively reached Gale crater. Our model also considered a range of flow temperatures and initial lava viscosities that account for the errors introduced by our model assumptions. The lava flows departing from Tyrrhenus Mons covered almost two thousands of kilometres, as far as Aeolis Mensae, along a region where the massive volcanism that affected the planet was thought to be marginal. Such a finding makes us to question again the scale and the behaviour of the volcanic activity in the early history of the planet. Since there is no direct comparison in the Solar System that reveals

Journal Pre-proof such catastrophic events in a planet 10 times less massive than Earth, we should consider that something unique happened to Mars early in its history. The observation of the lava filling of Gale and its origin has confirmed once more that the SPGI theory, which caused a massive burst of volcanism that lasted several hundreds of millions of years (Leone et al., 2014), is a good explanation that accounts for the huge amount of lava erupted and for the alignment of the volcanic features of Mars (Leone, 2016).

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Acknowledgements. The authors are grateful to two anonymous reviewers for their useful contribution

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which greatly improved this paper.

Data set

Sio2

Mean MER APXSa

45.0 1.0

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Adirondack APXSc

9.6

3.0

0.4

1.4

6.1

0.9

0.3

17.8 7.4

5.9

2.9

1.1

1.0

5.8

1.1

0.4

15.2 12.8 7.49 2.79 0.06 0.52 N/A 0.09 0.41

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45.4 0.46 10.9

MnO

6.2

15.5 9.3

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Mean MSL igneous APXSb 45.5 0.9

10.5

Cl

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TiO2 Al2O3 FeO MgO CaO Na2O K2O P2O5 SO3

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Table 1. Comparison between the mean composition (APXS, Alpha particles X-ray spectrometer) of the basaltic rocks found in Gusev crater and igneous rocks identified in Gale’s floor by MSL. For reference, we also showed the composition of the Adirondack rock. Data from (Gellert et al., 2004; McSween et al., 2004; Edwards et al., 2017);

a

MER PSD,

b

MSL ChemCam LIBS Planetary Data

System, c Gellert et al., 2004, McSween et al., 2004.

Lava Properties Tliq

Tsol

T

ρl

ρb

μl

cl

Ll

Journal Pre-proof ºC

ºC

ºC

kg m-3

kg m-3

Pa s

J kg-1 ºC-1 J kg-1

1270

1150

1270

2820

2820

2.3

1560

5.3E+05

1260

2810

2814

3.7

1558

5.2E+05

1250

2800

2810

6.0

1557

5.2E+05

1240

2792

2806

8.2

1556

5.1E+05

Tmg

kg

ρg

μg

cg

Lg

ºC

J m-1s-1 ºC- kg m-3

Pa s

1.0

3180

35.6

1560

4.4E+05

-p

1080

J kg-1 ºC-1 J kg-1

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1

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Substrate Properties

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Table 2. Lava and substrate thermal and rheological parameters used to model flow from Tyrrhenus to

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Gale (from Williams et al., 1998, 2005). Liquidus temperature, Tliq, was calculated using MELTS (Ghiorso and Sack, 1995); solidus temperature, Tsol, is from (Arndt, 1976); T is lava eruption

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temperature; liquid density, ρl, was calculated using the method of Bottinga and Weill (1970) and varies

ur

with decreasing lava temperature; bulk density, ρb, is a function of pressure, temperature and

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composition; liquid viscosity, μl, was calculated using the method of Shaw (1972); specific heat, cl, was calculated using the method of Lange and Navrotsky (1992); heat of fusion, Ll, is approximated from data of Navrotsky (1995); Tmg is melting temperature of basalt substrate (Navrotsky, 1995); ρg is density of basalt substrate;. The value of μg is calculated as a function of Tmg and is equal to 35.6 Pa s for a basaltic substrate (Williams et al., 1998; cg is specific heat of basalt substrate, from Williams et al. (2005); Lg is heat of fusion of basalt substrate (Navrotsky, 1995); kg is thermal conductivity of basalt substrate.

Init. Flow Flow

Initial

Final

Initial

Initial

Lava Flow Total Flow Turb.-lam.

Journal Pre-proof Temperat. Thickness Viscosity Viscosity Velocity Reynolds N. Rates

Volumes

Transition

m

Pa·s

Pa·s

m s-1

#

m3 s-1

km3

km

1270

25.0

2.3

300.0

5.1

3.1E+05

3.4E+05

1.3E+02

> 1963

1270

21.0

2.3

502.1

4.6

2.3E+05

2.6E+05

1.1E+02

1492

1260

30.0

8.3

300.1

5.2

1.1E+05

4.2E+05

1.6E+02

> 1963

1260

25.0

8.3

481.4

4.6

8.2E+04

3.1E+05

1.3E+02

1646

1250

35.0

48.3

502.5

4.9

2.3E+04

4.6E+05

1.8E+02

> 1963

1250

30.0

48.8

680.8

4.4

1.7E+04

3.5E+05

1.6E+02

1450

1240

62.0

638.3

1,238

5.6

4.1E+03

9.3E+05

3.3E+02

> 1963

1240

47.0

640.0

1,558

4.6

2.7E+03

5.8E+05

2.5E+02

493

lP

re

-p

ro

of

ºC

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Table 3. Output parameters of the model (Williams et al., 2005, 1998) for lava that is erupted as a

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turbulent flow, and covers the total distance of 1,963 km that separates Tyrrhenus from Gale along the

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main path (Fig.1a). The slope of the ground (weighted average) is calculated to be ~0.06° over the entire flow length. For the same initial flow temperature, T, the higher flow thickness represents the minimum value for which the lava flows turbulently out to the final distance; the lower thickness is the minimum value for which the flow travels to the final distance, while the flow regime transitions from turbulent to laminar at some downstream distance from the lava source. Lava flow rates and total volumes are based on a measured value of 2.68 km for the width of the channelized segments that occur along the Tyrrhenus-Gale path. The turbulent-laminar transition is assumed to occur at a Reynolds number, Re, equal to 2000. “Initial” indicates a distance from the lava source of 0 km, “Final” refers to the distance of 1963 km (lava terminus).

Journal Pre-proof

Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The authors are grateful to two anonymous reviewers for their helpful comments which greatly improved the manuscript.

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Journal Pre-proof Highlights

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Gale crater is filled by low viscosity basaltic lava, similar to Gusev crater. Lava travelled from Tyrrhenus Mons directly to Gale crater Farah Vallis is the main entrance for lava coming from Tyrrhenus Mons. Computer simulations are consistent with the presented geological observations.

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