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Terrestrial Analogs and Thermal Models for Martian Flood Lavas Edinburgh Research Explorer Terrestrial analogs and thermal models for Martian flood lavas Citation for published version: Keszthelyi, L, McEwen, AS & Thordarson, T 2000, 'Terrestrial analogs and thermal models for Martian flood lavas', Journal of Geophysical Research, vol. 105, no. E6, pp. 15027-15049. https://doi.org/10.1029/1999JE001191 Digital Object Identifier (DOI): 10.1029/1999JE001191 Link: Link to publication record in Edinburgh Research Explorer Document Version: Publisher's PDF, also known as Version of record Published In: Journal of Geophysical Research Publisher Rights Statement: Published in Journal of Geophysical Research: Planets by the American Geophysical Union (2000) General rights Copyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. Take down policy The University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorer content complies with UK legislation. If you believe that the public display of this file breaches copyright please contact [email protected] providing details, and we will remove access to the work immediately and investigate your claim. Download date: 09. Oct. 2021 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. E6, PAGES 15,027-15,049, JUNE 25, 2000 Terrestrial analogs and thermal models for Martian flood lavas L. Keszthelyiand A. S. McEwen Lunar andPlanetary Laboratory, University of Arizona,Tucson T. Thordarson Divisionof Explorationand Mining, CSIRO MagmaticOre DepositsGroup, Floreat, Western Australia Abstract. The recentflood lavason Mars appearto havea characteristic"platy-ridged" surfacemorphology different from that inferredfor mostterrestrial continental flood basalt flows. The closestanalog we havefound is a portionof the 1783-1784Laki lava flow in Icelandthat hasa surfacethat was brokenup andtransported on top of movinglava during major surgesin the eruptionrate. We suggestthat a similarprocess formed the Martian flood lava surfacesand attemptto placeconstraints on the eruptionparameters using thermal modeling. Our conclusionsfrom this modelingare (1) in orderto produceflows > 1000 km longwith flow thicknessesof a few tensof meters,the thermophysicalproperties of the lava shouldbe similarto fluid basalt,and (2) the averageeruption rates were probably of the order of 104m3/s, with the flood-like surges having flow rates of the order of 10s - 106m3/s.We alsosuggest that thesehigh eruptionrates should have formed huge volumes of pyroclastic depositswhich may be preservedin the MedusaeFossae Formation, the radar"stealth" region,or eventhe polar layeredterrains. 1. Introduction Some of the most recent and best preservedflood lavas lie The objective of this paper is to provide initial qualitative in eastern Elysium Planitia (the Cerberus Formation of and quantitativeinterpretations of new data on Martian flood Plescia [1990]) and in a portion of Amazonis Planitia lavas. The Mars Orbital Camera (MOC) on board the Mars centeredat 30ø N, 160ø W [McEwen et al., 1999b]. These Global Surveyor (MGS) spacecrafthas returnedspectacular two regions are connectedby Marte Vailis, which is also new images of Mars [Malin et al., 1998]. We were partially filled by flood lavas. The surfaceof the Cerberus particularlyinterested in imagesof the Martian flood lavasin plainsrecords a complexhistory of volcanic,fluvial, impact, order to comparethe emplacementof Martian and terrestrial tectonic,aeolian, and possiblylacustrine and glacialprocesses flood volcanism. [e.g., Mouginis-Mark et al., 1984; Tanaka and Scott, 1986; Flood volcanism is one of the most significant crust- Plescia, 1990; Scott and Chapman, 1991; McEwen et al., forming processesidentified on Mars [e.g., Greeley et al., 1998, 1999c]. In this paper we focus solely on the best 2000; Greeleyand Spudis,1981 ], and the extremepaucity of preserved(and presumablyyoungest) volcanic features. cratersin someof the imagesof the flood lavassuggests that Well-preservedflood lavasof similarappearance (although this style of volcanismmight plausiblypersist to this day on probably older) are also presenthigh on the Tharsis Bulge, Mars [Hartmann, 1999; Hartmann et al., 1999; Hartmann near Tharsis Tholus. Layers exposedin the walls of Valles and Berman, this issue]. Understandingthe eruptionsthat Marineris are also likely to be yet older flood lavas [McEwen formedthese flood lavasis a critical stepin understandingthe et al., 1999a]. Figure 1 showsthe locationof theseflood lava evolutionof the surface,interior, and atmosphereof Mars. exposureswith respect to other major features on Mars. Before proceeding,we shouldnote that we use the term Crater countingand modelssuggest ages of about 10 Ma for "flood lava" to denotea packageof sheet-likelava flows that the youngestof the flows in the CerberusPlains [Hartmann inundatesa large region, producinga smoothplain without and Berman, this issue]. However, MOC imagesalso reveal •-100 m tall constructs. This use of the word is in line with that there are locationswhere the lava is eroding out from that presentedby Greeleyand King [1977] and its application beneath the Medusae Fossae Formation, which may have to terrestrial continental flood basalts [Bates and d•ackson, protectedthe lava from small impact craters. While many 1984] and is essentiallyidentical to the useof the term "mare" processescan complicate the age estimates from crater on the Moon (but withoutthe requiredlow albedo). However, counts, the crisp, undegradedmorphology of the flows someother workershave used "flood lava" to simply refer to supportsthe idea that theseflood lavas are geologicallyvery long lava flows with no clear channels or tubes [e.g., young. Together,the recent flood lavas appearto cover an Cartermole,1990]. areaof about107 km 2, roughly the size of Canada[McEwen et al., 1998]. The minimum thicknessof the Cerberusdeposits Copyright2000 by theAmerican Geophysical Union is estimated to be in the hundreds of meters, and the new Papernumber 1999JE001191. MOC images support the estimate by Plescia [1990] that 0148-0227/00/1999JE001191509.00 individualflow frontsare only of the orderof 10 m thick. 15,027 15,028 KESZTHELYI ET AL.' MARTIAN FLOOD LAVAS These flood lavas have a distinctive surfacemorphology dominated by plates and ridges [McEwen et al., 1999b] (Figures 2-9). The platy-ridgedmaterial is interpretedto be lava becauseit is boundedby lobatemargins characteristic of lava flows (Figure 4). At high spatial resolution(2 to 20 m/pixel), these flows exhibit a complexsurface morphology interpreted to include rare channels (Figures 3 and 4), common rafted crustal slabs (Figures 2, 3, 5, 6, 7, and 9), ubiquitous pressure ridges (Figures 2-9), some ponded surfaces(Figures 5-7), and possiblesqueeze-ups (Figures 2 .. .,.,,. and 5-7). Coherentslabs of crustappear to rangein size from a few hundred meters to a few kilometers.' Ridges are typically only tens of meters wide but up to kilometersin length. Most ridges are interpretedto have formed at the edges of plates as they are rammed into each other during :i:; 5• emplacement(i.e., pressureridges). Other ridges may have ., .,%• formedby liquid lava oozingup throughextensional cracks in ß.:.• ..,.•. the plates(i.e., squeeze-ups). ... These flows are remarkably flat, with about 400 m of vertical elevation over 1500 km of lateral distance(0.027% •:• ...... slope) [Smith et al., 1999]. However, the region is radar ß•.•(½•..:•:**..:.•,•;•. •:•.:..:s,:•:•:.•..:.•½::.•;•:•::s•....,-.• ...::......•:•::.:•,:,•a. ..•:•:•...e•:•½:.-.,. bright, implying a rough surface on the decimeter scale [Harmon et al., 1992, 1999]. .'• ....::-,•: •::• •;:•.................... Vents in the flood lava regions are difficult to identify, probably as a result of burial by the flows, their small size, and/or rapid removal of the relatively fragile vent structures. However, the CerberusRupes fracturesand nine low shields on the western side of the Cerberus Plains were identified as • •;•:.•:::•::::::::::::::::::::::::::::::::::::::: ..>:•:.:•:•;:•,•::•',-•:' .....-..'..,.: .• likely ventsfrom Viking data[Plescia, 1990; Edgettand Rice, 1995]. The most recentMOC imagesshow small lava flows • .:%;.•:•t•:;• ..... •:• ...... .' emanating from both sides of some of the CerberusRupes .:: ;"::•*•:.............. ,..?:.,•*'*$;:•;:• :• fissures,confirming that they indeedserved as volcanicvents [McEwen et al., 1999c]. However, there is still no conclusive :::. proof that most of the large flood lavas were fed from CerberusRupes. Also, the CerberusRupes fractures cut all the lava flows, indicatingthat it has been tectonicallyactive more recentlythan it hasbeen a volcanicvent. One of the ongoinglimitations in workingwith the Martian ,,• •,-,.•::e•.::.: [-/• ............ ........•, ................. flood lavas is our inability to trace individual flows. The Viking data do not have the resolutionto allow flow margins ..:......... •..-. -, ......... :...e?. :-:;,:.•;•;:•.:.:.,;•.• :;:.½,•.:,: .... to be confidently traced. The MOC data cover only a tiny fraction of the Cerberus Plains, allowing only extremely discontinuoussampling. Our bestestimate for the extentof a singleflow is shownin the lightestshading in Figure 1. This area correspondsto the brightestradar returns reportedby Harmon et al. [1999]. It also follows the local topographic gradient away from the presumed
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