P52B-01 INVITED
Eruption Conditions During Fissure Eruptions in Tharsis, Mars.
We focus on six separate, but physically close, vent systems in eastern Tharsis, Mars, just to the east of the volcano Jovis Tholus. The vents are typically linear fissures a few to 20 km in length that have built small shields rising to 50-85 m above the level of the surrounding topography. Typical lengths of individual lava flows from these fissures are 15-20 km, with a maximum of 30 km. Mapping of the vent complexes reveals smooth material, interpreted to be spatter from fire fountaining, located on the rims of some of the fissures. In some places this spatter fed individual short (less than 5 km) lava flows; elsewhere spatter from segments of the fissure coalesced to form longer flows. Temporal evolution of the flow fields is indicated: the oldest parts of the basement at each center are built from a series of compound flows that cannot be subdivided into individual flows. In places activity was locally focussed and formed lava ponds and channelized flows from central vents. MOLA topographic data and photoclinometry topographic profiles made from THEMIS VIS images show that the flows are typically less than 5 m thick, 8-15 times thinner than flows elsewhere on Mars. We used standard models of magma rise through dikes, lava flow emplacement, and fire-fountain behavior to find lava viscosities and yield strengths (100 Pa s and 100 Pa, respectively), eruption rates (5,000 m3/s), flow speeds (1-2 m/s), durations of emplacement of individual flow units (5 hours), and an equivalent magma water content (0.1-0.2 mass percent). These eruption conditions are consistent with a wide range of possible depths of the magma reservoirs feeding the eruptions. Small vents like those studied here have been identified elsewhere in Tharsis (particularly just to the east of Olympus Mons), and the eruption conditions described above may characterize a previously poorly recognized style of volcanism on Mars only identifiable in images with spatial resolutions better than 10 m/pixel.
P52B-02
Was Apollinaris Patera the source for the Gusev crater basalts?
A possible source of the Gusev crater basalts analyzed by the Spirit rover could be Apollinaris Patera. We test this hypothesis by identifying and analyzing potential lava flow paths using THermal EMission Imaging System (THEMIS), Mars Orbital Camera (MOC), High Resolution Imaging Science Experiment (HiRISE), and Mars Orbiter Laser Altimeter (MOLA) datasets together with published geologic mapping. Image interpretation reveals three possible flow paths from Apollinaris onto Gusev crater floor via a breach in Gusev's northwest rim; an unnamed crater that must have post-dated any potential flow along the paths currently blocks the breach. After correcting for the unnamed crater, topographic profiles constructed along possible flow paths demonstrate that elevation along each path increases from ~80-300 m. Therefore, we conclude that the Apollinaris construct was not a direct source of the Gusev basalts. Instead, it seems more likely that the Gusev basalts were sourced from below the crater itself. The similar ages (as derived from crater counts) of Apollinaris' south flank and the Gusev plains lavas, however, suggest that the two volcanic regions may be part of a common magmatic feature.
P52B-03 INVITED
Gravity Evidence for Radiating Dike Swarms in the Thaumasia Region of Mars
Previous work has suggested that extension at many graben systems on Mars may be due to stresses
associated with the emplacement of volcanic dikes. If the magma filling a dike is denser than the surrounding
country rock, as is true in Hawaii, the dike will produce a positive gravity anomaly. For example, dike material
may be less degassed than its surroundings, and thus less vesicular, resulting in a greater density. Although
individual dikes can not be detected in orbital gravity data, the aggregate effect of dike swarms is detectable.
In this work, I consider gravity evidence for dike swarms in the Thaumasia region of Mars. Thaumasia is the
southeast portion of the Tharsis volcanic province and contains some of the earliest evidence for geologic
activity in Tharsis. The Claritas Fossae graben system in western Thaumasia has a residual gravity anomaly
(after removing the effects of both the surface topography and the compensating root) of up to 100 mGal,
implying the presence of dense subsurface material. Detailed modeling shows that plausible combinations of
dike height, density contrast, and filling factor (the fractional amount of dike material) can reproduce the
observed anomaly. Thus, subsurface dike swarms may be present along much of this 1500 km long graben
system. Maps of the residual gravity anomaly in the remainder of Thaumasia reveal the existence of a series
of elongated, positive anomalies that radiate from a common center point in southeastern Tharsis. The
common center point suggests a common origin. Their elongated planform, anomaly amplitudes, and origin
within the Tharsis volcanic province are all consistent with formation as dike swarms. In the Solis Planum
region of Thaumasia, there is no surface expression of these dikes, suggesting that the dikes pre-date the
youngest surface lavas. This implies that the dike swarms were emplaced in the late Noachian or earliest
Hesperian. On-going work is focused on assessing the physical volcanology conditions required to emplace
these dikes, which in some cases exceed 2000 km in length. Possible analog structures in the Tempe Terra
region are also being assessed.
http://www.lpi.usra.edu/science/kiefer/home.html
P52B-04 INVITED
Numerical Modeling of Dike-Induced Deformation and Graben Formation on Mars
The Tharsis region of Mars is characterized by large volcanic and tectonic centers with distinct sets of graben systems. These Tharsis-radial graben systems have a simple graben morphology with long narrow grabens bounded by normal faults and a down-dropped flat floor unbroken by antithetic faults. Many of the radially oriented grabens have been inferred to form in response to intrusion of magmatic dikes. This interpretation is based primarily upon early physical and numerical (boundary element) models that were originally developed to understand surface deformation associated with dike emplacement on Earth. In this study, we constructed and analyzed two-dimensional discrete element models to test the hypothesis of shallow dike emplacement and widening as a primary mechanism for the production of grabens on Mars. In particular, our models are designed to explore the extent to which a widening subsurface dike, in the absence of regional extension or pre-existing faults, will induce near-surface graben formation. The use of discrete element models allows for the permanent deformation and material heterogeneity to be captured as opposed to boundary element models that are limited by an assumption of homogeneous elastic behavior. Our analyses consider both homogeneous materials as well as mechanical stratification. The results indicate that forcible widening of a dike alone is unlikely to produce grabens at the surface. In our models, surface deformation took the form of a synclinal trough between two anticlines rather than a graben. Formation of the trough was accomplished primarily through compression adjacent to the dike, causing contractional fold development up to the surface. The model evolution indicates that the primary deformation style of dike widening is via trough margin uplift rather than trough center subsidence and that the most distinctive topographic signature of an underlying dike would be parallel ridges formed by contractional folding on either side of a trough. In addition, this shouldering aside of a large volume of subsurface material would require high internal dike pressures to attain significant surface deformation. The presence of a mechanically strong layer in the models produced an overall increase in trough width and a decrease in the amplitude of surface uplift. This effect of material heterogeneity would likely impact any estimates of dike dimensions based solely on measurements of surface graben widths.