Origin and Evolution of Long Lobate Lava Flows on Syria Planum, Mars

Introduction: On Mars, long lava flows have been described as the first volcanic stage at the formation of a plains-style volcanic region (e.g. [1]). They have been identified and described on the eastern flanks of the Tharsis Montes (e.g.[2 4]), as well as on Tempe Terra [5] and Syria Planum [6]. This type of volcanism have been described as a consequence of magmatism near larger magmatic provinces near-plume or postplume (e.g. [1], [7]). On the Earth, the plains-style volcanic provinces are characterized by three main phases (e.g. [7 8]); 1 st : vent-elongated fissures, where flows follow the topography; 2 nd : low shield volcanoes form on slightly higher slopes; 3 rd : final volcanic resurfacing forming steeper slopes with a lower extrusive rate. The fact that plains-style volcanism on Syria Planum is peripheral to the large Tharsis Montes (located at the topographic summit after an extended eruptive phase) supports the idea of small batch magmatism under lithospheric conditions [8]. Volcano A Description: The Syria Planum region reveals an original pattern of volcanic features distinct from the other areas over the Tharsis bulge; a large volcano (Figure 1) (described as volcano A in [6]) was identified west of a swarm of small shield volcanoes [6]. An extensive field of lava flows, identified between approximately 15o-19oS and 105o102oW, covers an area of approximately 100,000 km 2 . Through the study of HRSC and THEMIS IR-day and IR-night images, we can identify elongated and lobate shapes for these flows with preferential NW to SE orientation, following the regional slope. Using THEMIS and MOLA data, we observe that these lava flows erupted from an isolated volcano – Volcano A on the northeast of Syria Planum [6]. The summit cone of volcano A is about 40 km in diameter, and it lies at an elevation of approximately 6.7 km (Figure 1). This volcano is located outside of a HRSC mosaic, although THEMIS and a HIRISE image enables its study. On Figure 1 is shown a MOLA profile from “a” to “b”. From the center of volcano A to the side “a”, the slope is about 1.4o while from the center to the side “b” the slope decreases to 0.7o, to the side where the lava flows extend downslope. Post-flow tectonic deformation is detected on the volcano’s western flank. In Figure 1 it is of note that the flanks facing east are considerably smoother than the western flanks. Regional Geology: Syria Planum formation was described by [6] as the result of successive magmatic and tectonic events. From the Early to the Late Hesperian period we have: I) extensional field stress that produced grabens; II) eruption of volcano A resulting in lavas that spread all over Syria Planum. Volcano A has similar characteristics to the Tharsis Montes, such as the same effusion rate (see [6]), except that it did not grow as high as the Tharsis Montes. Previous work [6] concluded that these lava flows are likely to be basaltic to andesitic in composition, similar to Martian Montes lava flows; III) tectonic deformation of the emplaced lava flows by the formation of several fractured patterns such as NE-SW en-echelon faults, troughs and adjacent grabens; IV) new episodes of volcanic activity, forming a coalesced swarm of small shields that bury preexisting faults.