P43C-1408
Differential Rotation of Lithosphere for One-plate Planets and its Implications for the Tharsis Rise on Mars
Geological and geophysical observations suggest that Tharsis formation is initiated in the highlands at ~40S in the early Noachian and the volcanic centers subsequently migrate to the dichotomy boundary during the Noachian. This pattern of volcanic activities is similar to what is observed along the Hawaiian volcanic chain on Earth and suggests a relative motion between the lithosphere and the plume center. However, a basic tenet in planetary sciences is that the tectonics and dynamics of one-plate planetary bodies such as Mars are controlled by stagnant-lid convection in which there cannot be large-scale horizontal motion of lithosphere relative to the underlying mantle. Here I demonstrate that a unique mode of horizontal motion of lithosphere, differential rotation of lithosphere, is permissible, and is readily excited in a planetary mantle that has a very long-wavelength convective planform and lithospheric thickness variations. This mechanism provides a dynamically self-consistent explanation for large horizontal migration of Tharsis volcanic centers during the Noachian on Mars and for its final stable location near the boundary of the crustal dichotomy. It also suggests that the formations of Tharsis Rise and the crustal dichotomy, two most important features on Mars, are closely related and that the formation of the crustal dichotomy has had a direct effect on the formation and evolution of Tharsis volcanism. The proposed large-scale horizontal plate motion for one-plate planetary bodies has broad implications for volcanisms, tectonics and thermal evolution for these planetary bodies.
P43C-1409
Investigation of Pickering Crater (Mars) by Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS)
The Mars Express (MEX) Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) is a low- frequency (1.8, 3, 4, 5 MHz) radar capable of ground-penetration. The instrument records echoes returning to MEX from Martian interior depths as big as 5 km at nadir, well within the crust. It can thus provide fundamental information for the search and identification of geological structures and rock layering inside the Martian crust, being particularly useful on wide flat expanses such as the vast volcanic fields surrounding the Tharsis Montes. Pickering is an approximately 150 km diameter crater located about 1500 km SW of Arsia Mons, the oldest of the Tharsis volcanoes. The crater has been modified by tectonic activity and it has been infilled, with the most surficial strata consisting of volcanic rocks. To investigate structures deeper in the material filling Pickering Crater, we used data from MARSIS orbits 4192 and 4932. Their parallel tracks run along the eastern side and the central portion of the crater, respectively. After accounting for the effects of high intensity nadir surface reflections and off-nadir clutter, an apparent low angle N-dipping reflector of approximate horizontal length of 10 km was identified at depths 0.5-1 km below the surface in both orbits. Because the orbit tracks do not intersect, no unique structural reconstruction is possible. However, the presence of similar reflections in both orbits, and the geometric constraints provided by the morphology of the crater, are useful in narrowing the field of possible interpretations. Combining the MARSIS evidence with other datasets furthermore constrains plausible tectonic scenarios.
P43C-1410
Is recent volcanism at Central Elysium Planitia similar to large terrestrial igneous provinces?
Evidence in favour of recent volcanism at Central Elysium Planitia (CEP), previously referred to as the Ceberus Plains, dates back to the era of the Viking orbiters [1]. At the present time controversy surrounds the origin of some of the CEP surfaces (lava flows, surface pack-ice?) [e.g, 2,3]. A summary of the principal arguments in favour of a volcanic origin will be presented, before consideration of our results concerning (1) the rheology of these lavas, (2) the volume of volcanic material, (3) the timing of these eruptions. Most of the studies of the rheologic properties of lava flows at CEP agree with the idea that these lavas are emanating from both fissure vents and shield volcanoes, and that they are among the most fluid lavas at the Surface of Mars. From topographic modelling of the basins and large craters infilled by the lavas, we estimate that the volume of volcanic material at CEP ranges from 1.3-1.7 105 km, indicating a total volume similar to that of Elysium or Syrtis Major. Ages from crater counts have been derived in many places selected in order to infer the volcanic history at CEP. Ages are distributed from 260 Ma to several Ma with some gaps, indicative of relatively long periods of inactivity. The distribution of ages does not suggest spatial migration of the volcanic activity with time. The onset for volcanic activity in the case of central volcanoes is generally difficult to establish as recent lavas cover older ones. However, in the case of Cerberus, our result suggests surface magmatism over a period of more than two hundred millions years. It is found that the style of volcanism alternates between large lava sheets and shield volcanoes, invalidating the idea that volcanism at CEP might have evolved from large sheets to central eruptions. Among the 22 shield volcanoes documented, 7 were dated, and it is significant that their ages cover the entire history of CEP (261 My, 111 My, 88 My, 52 My, 50 My, 7My, 6.5 My). The older volcanoes are generally buried under more recent lava sheets. This therefore implies that the style of volcanic activity at CEP is notably different than that of large terrestrial igneous provinces associated with mantle plumes. In CEP, approximately 1.5*105 km3 or lava were erupted during several large events over a period of a few hundreds of millions of years, separated by long periods of inactivity. In contrast, the compilation of data concerning the terrestrial large igneous provinces show that about 107 km3 of lava are produced, in different locations, each 30 millions of years, the duration of each event being only few millions years for the peak of activity [4]. The characteristic times and volumes of volcanic activity at CEP that we derived do not appear consistent with plume associated magmatism as described on Earth, arguing in favour of alternative mechanisms for recent volcanism, such as that proposed by Schumacher and Breuer [5]. It remains to be understood how fluid lavas can be produced in this context. [1] Plescia, J.B., Icarus, 88, 465-490, 1990. [2] Jaeger, W.L. et al., Science, 317 (1709), 2007. [3] Paige, D., Science, 320, 1588b, 2008. [4] White, R., and D. McKenzie, Journ. Geophys. Res., 94 (B6), 7685-7729, 1989. [5] Schumacher, S., and D. Breuer, Geophys. Res. Lett., 34, L12202, 2007.
P43C-1411
Volcanic Variety: The Tyrrhena Patera Region of Mars is the Candidate for Change
The volcano Tyrrhena Patera (22°S, 108°E), Mars, is a low-relief structure with a central caldera complex, and whose flanks are dissected by broad (< 5 km wide), flat-floored radiating channels. The morphology of the volcano flanks is consistent with eroded, differentially welded pyroclastic flows. Hesperian- aged plains materials embay the volcano flanks, and locally embay the channel floors. The origin of these plains remains unclear: they may be fluid lava flows or reworked volcaniclastics deposited in a lucustrine environment. Amazonian-aged lava flows, connected to the summit caldera complex, crosscut the volcano flanks and overlie the Hesperian-aged plains. Tyrrhena Patera's deposits and its environs may therefore represent the full range of volcanic eruption styles on Mars through most of the planet's history. To date, only mafic products have been identified in this region. Tyrrhena Patera's flanks have been interpreted to be layered pyroclastic deposits because they appear to be easily eroded and lack any primary lava flow morphologies. Recent mapping efforts reveal that the flank materials can be identified at least as far as 450 km from the Tyrrhena Patera summit, possibly as far as 750 km. These observations raise unanswered questions about the dynamics of pyroclastic flow emplacement on Mars, because the longest pyroclastic flow yet identified on Earth is <<200 km. Lava flows are clearly identified in a large (1000 km x 300 km) field that extends southwest from the Tyrrhena Patera summit. Lobate scarps, leveed channels, and a 600-km-long tube/channel system have been observed. Flow lobe and lava channel dimensions suggest that the individual eruptions were volume-limited, rather than cooling-limited. However, ambient conditions on Mars caution against the use of terms such as "a'a" and "pahoehoe" to describe these lavas. Both the ancient, dissected flanks and the comparatively young lava flow field indicate the presence of a long-lived, large-volume magma chamber beneath Tyrrhena Patera. To date, however, there is no evidence of evolved lavas erupting at any time, in contrast to similar long-lived, large-volume systems on Earth.
P43C-1412
Composition of recent lava flows in the Tharsis region of Mars and comparison to terrestrial flows
We present the first detailed analysis of the composition of some of the Tharsis lava flows. This vast volcanic region displays locally young lava flows which demonstrate the presence of a recent internal activity. These lava flows are among those involved for SNC meteorites such as the shergottites. However, the broad dust cover distributed over the Tharsis region limits any spectral study to the aeolian mantling. We have found within the Tharsis region several areas of interest using OMEGA spectrometer as pyroxene rich. These areas are small outcrops of few km large. They display a high thermal inertia and a low albedo as expected for lava flows devoid of dust. The first region is located on the Echus Chasma floor. This location presents platy textures as seen in several locations on Mars. This texture which is composed by pieces of km sized shallow plateaus separated by smooth interplates. This texture resembles that of rafts of solid material moving over fluids. For this reason, the platy texture is currently under debate between two hypotheses: (1) lava flows with solidified lava rafts over fluid lavas (2) ice raft with icy blocks over liquid water. Here, the spectral data display a strong enrichment in pyroxenes, especially in areas of high thermal inertia that can not correspond to eolian material. Close-ups on MOC show no sandy material that could hide the surface. Therefore, this region is likely a solidified lava flow with a recent age (Late Amazonian). Pyroxenes were also identified in four canyon floors of Noctis Labyrinthus in locations out of apparent sand mantling. These outcrops are rocky with a high thermal inertia, they display flow patterns, and a relative flat topography that embay residual hills. These outcrops therefore likely correspond to lava flows. These textures is unique in Valles Marineris canyons. Crater counts give ages in the Late Amazonian as in Echus Chasma floor. Both regions were studied using models developed to calculate the modal composition of the rock (Poulet et al., LPSC, 2008). These models show a composition dominated by pyroxenes and plagioclases, with a lack of olivine in spectra. Compositions of both areas mainly fit that of classical tholeiitic basalts. Platy textures are very flat and suggest a low viscosity of the lava flows. These lavas with platy textures on Mars can be compared to those observed locally on Earth, as in Iceland. Nevertheless, from the composition found, they do not seem to require a specific olivine-rich composition to explain the low viscosity. Strong lava flux, high temperatures, or other chemical factors might be involved to explain the low viscosity. Of interest is also the HCP/LCP ratio which is of about 4:1 for both regions which is part of the highest on Mars. This ratio is much higher than those of old regions, or that of SNC meteorites. These meteorites are therefore unlikely to come from such magmatic areas. Details on the composition and their analogy to terrestrial lava flows will be given at the conference.
P43C-1413
Discrete Element Simulations of Density-Driven Volcanic Deformation: Applications to Martian and Terrestrial Volcanoes
We have carried out 2-D numerical simulations using the discrete element method (DEM) to investigate density-driven deformation in volcanic edifices on Earth (e.g., Hawaii) and Mars (e.g., Olympus Mons and Arsia Mons). Located within volcanoes are series of magma chambers, reservoirs, and conduits where magma travels and collects. As magma differentiates, dense minerals settle out, building thick accumulations referred to as cumulates that can flow ductilely due to stresses imparted by gravity. To simulate this process, we construct granular piles subject to Coulomb frictional rheology, incrementally capture internal rectangular regions to which higher densities and lower interparticle friction values are assigned (analogs for denser, weaker cumulates), and then bond the granular edifice. Thus, following each growth increment, the edifice is allowed to relax gravitationally with a reconfigured weak cumulate core. The presence and outward spreading of the cumulate causes the development of distinctive structural and stratigraphic patterns. We obtained a range of volcanic shapes that vary from broad, shallowly dipping flanks reminiscent of those of Olympus Mons, to short, steep surface slopes more similar to Arsia Mons. Edifices lacking internal cumulate exhibit relatively horizontal strata compared to the high-angle, inward dipping strata that develops within the cumulate-bearing edifices. Our simulated volcanoes also illustrate a variety of gravity driven deformation features, including regions of thrust faulting within the flanks and large-scale flank collapses, as observed in Hawaii and inferred on Olympus Mons. We also see significant summit subsidence, and of particular interest, distinct summit calderas. The broad, flat caldera and convex upward profile of Arsia Mons appears to be well-simulated by cumulate-driven volcanic spreading. In contrast, the concave upward slopes of Olympus Mons are more challenging to reproduce, and instead are attributed to volcanic spreading along a pore-fluid- pressurized decollement with low basal friction.