The Formation of Tharsis: What the Line-of-sight Data Is Telling

Introduction: Tharsis is a vast, complex topographic rise on Mars extending over 30 million square kilometers that dominates the western hemisphere of Mars. The region has been the locus of large-scale volcanism that has endured for the entirety of the planet’s history, resulting in pervasive fracturing of the crust from lithospheric loading by the voluminous intrusive and extrusive magmatic deposits [1,2,3,4]. In this study, we present results from analysis of lineof-sight (LOS) spacecraft acceleration profiles from the Radio Science Experiment of the Mars Global Surveyor (MGS) for the primary purpose of estimating the effective elastic thickness (Te) for various regions of Tharsis. This is done using two approaches: 1) best-fitting individual LOS profiles that cross the Thaumasia Highlands with a forward model, and 2) determining admittances from LOS profiles for various regions of Tharsis and fitting them with theoretical curves. The value of Te reflects the thermal state of the lithosphere at the time topographic loads are emplaced allowing temporal comparisons of features and crustal provinces to be made (e.g. [2]). The values derived in this study are used to infer the evolution of Tharsis over its ~4.5 billion year history. Thaumasia: The Thaumasia region, the southeastern portion of Tharsis, is a volcanic plateau bounded by an arcuate mountain belt, the Thaumasia Highlands, which extends southward from the region of the Tharsis Montes that then curves northeastward to form a quasicircular feature [5]. The Thaumasia Highlands, the oldest preserved portion of Tharsis [1], contains heavily cratered Noachian terrains that have survived resurfacing by younger volcanic flows presumably because of their high elevation. Immediately adjacent to the Thaumasia Highlands in the surrounding cratered plains, a negative free-air gravity anomaly flanks the high standing topography (Figure 1). This gravity anomaly reveals a possible flexural trench created by the load emplaced on the lithosphere by the formation of Thaumasia that has undergone subsequent burial [6]. We model the flexural response of the lithosphere to the emplacement of the volcanic plateau, represented by a disk load, to estimate Te at the time the Thaumasia Highlands formed. The model balances the initial applied load with buoyant, elastic, and membrane forces, and the LOS acceleration for a given MGS orbit is calculated from the resulting configuration of the topography and Moho. The disk thickness, disk density, and effective elastic thickness, are varied to achieve a best-fit with observed LOS accelerations profiles using ground tracks transecting the Thaumasia Highlands (Figure 2). We find the Thaumasia Highlands to be near-isostatic with Te < 20 km. Similar values are observed in Early Noachian terrains of the Southern Highlands [7], consistent with the Thaumasia Highlands having formed in the Early Noachian. The Admittance of Tharsis: Estimates of the admittance, the transfer function between topography and gravity, are used to constrain values of Te, the surface density ρs, and the ratio of bottom to top loading F with a best-fit theoretical admittance curve (see [8] and [9] for model description). Three study areas were analyzed: Thaumasia (240° to 320° E and -60° to 5° N), Olympus Mons (–5o to 45o N and 190o to 250o), and the western half of Tharsis (200° to 290° E and -20° to 45° N) selected to incorporate the largely Amazonian surface ages that are observed there [1]. The results are shown in Table 1.