Tellus Regio, Venus: Evidence of Tectonic Assembly of Tessera Terrain and Implications for Exploration

Introduction: Tellus Regio is a plateau-shaped highland in the northern hemisphere of Venus largely composed of tessera terrain characterized by complex deformation comprising at least two sets of intersecting ridges and grooves which contribute to high radar backscatter [1,2; Fig. 1]. Stratigraphic studies of tessera terrain occurrences establish that they are generally embayed by plains materials [1, 3-7], however, the density of craters on both terrains is similar where the tesserae yield a surface crater retention age of 1X [8] to 1.4X [9] the average Venus surface age of ~300 [8] to ~800 Ma [10]. Tessera terrain thus uniquely records the geologic history of an ancient (and tectonically extinct or dormant) Venus prior to and during plains emplacement, and constrains the character, magnitude and direction of surface strain and lithospheric structure at this time. Tellus can be subdivided into a number of distinct units based on unique structural fabrics [11,12; Fig. 1]. Several of these tessera units are identified at the 1:5M scale in the SW quadrant of Tellus Regio, where a fold belt comprised of layered volcanics lies between the tessera units [11-13]. We have undertaken a preliminary detailed analysis of this region [12] (~24°-36° N, 73°-83° E) to better understand the sequence of events recorded there. Methods: Mapping. Mapping of the region was completed in ArcGIS on a base of 100 m resolution FMAP SAR imagery (~1:100,000 scale). Map units are defined as geomorphologic units according to the interpreted textures, dominant structural patterns, (including recurrent features’ size and density), regional relief, and radar backscatter primarily and emissivity, slope, topography and reflectivity data secondarily [e.g., 14]. Width and Spacing Analysis. Graben widths were measured from the top of the radar bright wall to the top of the radar dark wall and used to calculate depth to mechanical discontinuity as described in [15]. The wavelength of features interpreted to be of compressional origin was calculated from the center of ridge to center of ridge or center of trough to center of trough. The thickness of the competent layer was then calculated after [16] and [17]. Results and Interpretation: Results of preliminary mapping [12] can be summarized thusly: 1) the fold belt (zones NEa and SEa, Fig. 1) lies between 3 regions of tessera terrain (region 1 = Indenter, Fig. 1; region 2 = NEb, Fig. 1; region 3= SEb, Fig. 1), each with a distinctive structural fabric which we interpret to represent distinct strain histories.