NOACHIAN TECTONICS OF SYRIA PLANUM AND THE THAUMASIA PLATEAU

Syria Planum has previously been identified as a volcano-tectonic center partially surrounded by a raised annulus. MOLA data show a cap of over 100 low volcanic shields over the eastern half of the summit, and a continuous slope of upper Hesperian lava plains from Syria Planum out to underlying ridged plains, which are as old as Noachian in age. These observations suggest that Syria Planum was an active volcanic center from the Noachian until Late Hesperian. The annulus could represent the surface of a Noachian Syria Planum volcanic edifice which became gravitationally unstable. Southeastward lateral movement of the Thaumasia Plateau starting in the Noachian and tapering off in the Early Hesperian likely caused extension in Syria Planum, creating the annulus, and compression in the south-Tharsis ridge belt. This collapse of the Syria Planum edifice is similar to gravitational collapse common in terrestrial volcanic complexes.

We propose that the strain from Syria Planum was transferred along proto-Valles Marineris forming a sinistral transtensional zone which provided tectonic control for later valles formation. At the east end of Valles Marineris, the Coprates Rise is a lithospheric buckle with a thrust fault along the eastern edge. We also interpret the southern edge of the Thaumasia Highlands as the surface exposure of a thrust fault. The compressional structures of the Coprates Rise appear to extend into the Thaumasia Highlands. The thrust faults likely cut deep into the crust and may represent the décollment for later wrinkle ridge faulting in Sinai and Solis Planum. In this hypothesis, Claritas Fossae represents a dextral transpressional zone and acts as a boundary between the eastern and western halves of the south-Tharsis ridge belt identified by Schultz and Tanaka (1994, JGR 99, p. 8371), which includes the Coprates Rise, Thaumasia Highlands, and ridges in Daedalia Planum. Compression in Daedalia Planum likely resulted from the same forces affecting the Thaumasia Plateau in the Noachian, but was arrested at an earlier stage of development, perhaps due to the buttressing effect of early Tharsis Montes construction.