Lunar and Planetary Science XLVIII (2017) 2798.pdf Spatial and Temporal relationships among low shield volcanoes in the Ceraunius Fossae region of Tharsis: the last gasp of Martian volcanism. J. M. Christoph1 and W. B. Garry2, 1School of Earth and Space Exploration, Arizona State University ([email protected]), 2NASA Goddard Space Flight Center ([email protected]). Introduction: We have mapped the extent of lava flows from low shield volcanoes near Ceraunius Fossae in Tharsis, statistically analyzed the spatial distribution of their vents, and obtained age crater age estimates for magma production events. We find that these low shields are some of the youngest yet studied on Mars, indicating that Tharsis has been host to volcanic activi- ty in recent geologic time. Background: Tharsis is a massive volcanic plat- eau on Mars that extends for thousands of kilometers Fig. 1: MOLA false-color elevation map of Mars; study across Mars’ western hemisphere and rises up to seven area in black rectangle. kilometers (not counting major volcanic peaks) above the surrounding terrain [1]. Contained within Tharsis 31°N, where they meet the fracture systems surround- are hundreds of volcanoes, ranging in size from 25 ing Alba Mons. Immediately west of Ceraunius, origi- kilometers tall and 600 kilometers wide (Olympus nating at approximately 26°N, 111°W, is Olympica Mons) down to smaller, but more widespread monoge- Fossae, a fluvial-volcanic channel system running netic volcanoes known as low shields. Low shield vol- WSW for approximately 300 km [8]. To the southwest canoes are individually very small in scale, only a few of Ceraunius are other volcanic plains surrounding kilometers in diameter and a few hundred meters tall Jovis Tholus, a 2km tall, 58 km wide volcanic peak [9] [2]; however they are frequently found together in clus- interpreted to be Hesperian in age [10]. ters of tens or hundreds. The structural geology of Ceraunius is associated To date, four major sites of low shield volcanism in with an extension episode in the Noachian period [11]. Tharsis have been catalogued and studied using the The underlying basement rocks are known to be Early most recently available imagery: Syria Planum, age Noachian [12], while crater age estimates suggest the 3500-2600 Ma [3]; Tempe Terra, age 2000-100 Ma fractured terrain of the graben emerged later during the [4]; the slopes of Pavonis Mons, age >1000 Ma [5]; Late Noachian/Early Hesperian [13]. In some areas the and the caldera of Arsia Mons, age 200-100 Ma [6]. graben have been partially filled with younger Hesperi- The age disparity between the low shields at Arsia, an/Amazonian material [13]. Analysis of topographic Pavonis, Syria, and Tempe is significant. The present profiles across the graben system taken from MOLA understanding of Martian geologic history holds that data indicates significant crustal extension of 22 km to active volcanism in Tharsis peaked at approximately 44 km [14]. It is possible that volcanic activity accom- 3700 Ma [1], whereas the range of low shield eruptions panied Noachian extension in Ceraunius, although span a somewhat later period from 3500 Ma [3] to 100 global maps interpret low shields present at Ceraunius Ma [6]. Although the Tharsis Montes are somewhat now as much younger than the Noachian [10]. younger than the bulk of Tharsis, with peak eruptive The low shields in the Ceraunius Fossae region activity at approximately 3400 Ma [7], recent low were not identified until laser altimetry data from the shield eruptions may not be associated with the erup- MOLA instrument became available [15]; thus, they tions that formed their larger neighbors. Further study have seen comparatively little direct study relative to of low shields in Tharsis is necessary to determine the low shields elsewhere in Tharsis. The most recent degree to which Tharsis has hosted active volcanism study of low shield volcanism at Ceraunius Fossae only subsequent to the emplacement of its bulk. examined twelve vents and utilized HRSC imagery Study Area: A major region of low shields is centered [16]. Small-scale volcanism in the vicinity of Jovis in the Ceraunius Fossae area in northern Tharsis (25°N, Tholus has been studied by way of fissure eruptions 110°W) (Fig 1). This region of low shields is extensive [17], and volcanic flows have recently been identified and contains numerous other geologic features. The as a major component of Olympica Fossae [8]. Ceraunius Fossae themselves are a graben system run- Study Methods: We mapped vent positions and ning N-S from approximately 19°N latitude through flows for 67 low shields in Ceraunius Fossae. We start- ed with a database of small Tharsis volcanic vents [18], Lunar and Planetary Science XLVIII (2017) 2798.pdf and used CTX and THEMIS image mosaics to verify vent positions and map the extent of flows originating from each vent. Due to the spatial extent of these vents, we hypoth- esized multiple magma production events formed the low shield fields. In the absence of spectroscopic data capable of distinguishing volcanic units compositional- ly, we determined the relationships between the low shield vents through nearest neighbor analysis. Given the assumption that vents in a field of volcanoes from the same magma production event will be randomly distributed [19], volcanoes which originate from dif- ferent magma production events can thus be distin- guished from each other by grouping vents which con- form to a random distribution [3] as defined by two Fig. 1: NE (3) and SW (2) vent groups on THEMIS statistics [20]. First is the ratio between the observed basemap; nearest neighbor vents connected by lines. and expected average nearest neighbor distances, where the expected average nearest neighbor distance some time after 1000 Ma [5]; at Tempe Terra over the is determined by 1/(2√ρ), where ρ is the vent distribu- period from 2000 Ma to 100 Ma [4]; and at Syria tion density, i.e. the number of vents per unit area. Planum over the period from 3600 to 2500 Ma [3]. Second is the test statistic, defined by the difference Prior to the Syria Planum eruptions would have been between observed and expected average nearest neigh- the active periods of the major volcanic edifices bor distances divided by the standard error for a popu- (Olympus, Tharsis Montes) as well as the emplacement lation of the observed density ρ. Both the test statistic of the bulk of the eruptive volume of Tharsis [1]. and the ratio of observed and expected average nearest Thus, the eruptive record of Tharsis low shields, in- neighbor distances should approach a value of 1 if the cluding those at Ceraunius Fossae, indicates a history vents conform to a random distribution [20]. of continuous volcanic activity in Tharsis, continuing Once we had obtained vent groups corresponding well after the major edifices went dormant and into to magma production events, we performed crater age recent geologic time. dating on each group. We selected eight vents with References: [1] Carr M. H. and Head J. W. (2010) flows which were entirely visible in single CTX frames EPS 294 185-203. [2] Greeley R. (1982) JGR 87 2705- (so as to avoid counting the same crater more than 2712. [3] Richardson J. A. et al. (2013) JVGR 252 1- once), counted all impact craters on their respective 13. [4] Manfredi, L. (2013) ASU M.S. Thesis. [5] flows larger than 100 m in diameter, and obtained ages Bleacher J. E. et al. (2009) JVGR 185 96-102. [6] using the CraterStats2[21] software and the crater age Richardson J. A. et al. (2015) LPS XLVI, Abstract # model from [22]. For comparison, we also obtained 2801. [7] Scott D. H. et al. (1998) USGS I-2561. [8] combined ages for all vents belonging to each magma Plescia J. B. (2013) LPS XLIV, Abstract # 2478. [9] production event. Wilson L. et al. (2009) JVGR 185 28-46. [10] Tanaka Discussion: Nearest neighbor analysis indicates K. L. (2014) USGS I-3292. [11] Anderson R. C. et al. that Ceraunius Fossae low shields originate from at (2001) JGR 106(E9) 20,563-20,585. [12] Scott D. H. least two separate magma production events: a North- and Tanaka K. L. (1986) USGS Misc. Inv. Ser. Map 1- east Group of 38 vents and a Southwest group of 29 1802-A. [13] Tanaka, K. L. (1990) LPS XX p.515-523. vents (Fig. 1). Our crater counts dated the Northeast [14] Borraccini F. et al. (2005) JGR-Planets 110. [15] Group at 200 + 30 Ma and the Southwest Group at 170 Head, J. W. (2001) GSA Abstracts with Programs + 20 Ma. #178-0. [16] Wong M. P. et al. (2001) LPS XXXII, When examined in the context of eruption ages Abstract #1563. [17] Wilson L. et al. (2009) JVGR 185 elsewhere in Tharsis, the age of Ceraunius Fossae low 28-46. [18] Bleacher J. E. (2014) USGS Catalog of shields provides further evidence that volcanic activity small volcanic vents in the Tharsis Province. [19] Lutz in Tharsis has occurred throughout Martian geologic T. M and Gutmann J. T. (1995) JGR 100 17,659- history. Low shields in the Arsia Mons caldera [6] 17,670. [20] Clark P. J. and Evans F. C. (1954) Ecolo- were active at approximately the same time as those in gy 35 445-453. [21] Michael, G. G. and Neukum G. Ceraunius Fossae, and these would have been preceded (2010) EPSL 294 223-229. [22] Hartmann W. K. by low shield activity on the flanks of Pavonis Mons (2005) Icarus 174 294-320. .