Interactions Between Regional Transcurrent Shearing, Rifting, and Mantle Flow on Venus ‒ Radar and Gravity Interpretations and Earth Analogues
Lyal Harris, INRS-ETE, Québec, Canada
Venus is dominated by upwelling and downwelling mantle plumes within a stagnant lid or transitional convection regime. NASA's Magellan radar imagery does not show any evidence for single-sided subduction zones, spreading ridge-transforms, nor arcuate volcanic chains that typify present-day plate tectonics on Earth and there is consensus that plate tectonics does not occur on Venus. Radar data over part of W Ishtar Terra portrays brittle-ductile shear zones within orogenic belts on the margins of the ʻcraton-likeʼ Lakshmi Planum. A fold-thrust-transpressional shear belt and high plateau, with structural and Bouguer gravity similarities to the Himalayan orogen and Tibetan Plateau on Earth, developed N of Lakshmi Planum. On the western N- to NE-trending planum margin, the offset of lithological layering and early structures along shear zones and fold-shear relationships indicate sinistral transpression. Dextral shearing along the NNW trending E planum margin and sinistral shearing on sub-parallel structures 500 km E of Lakshmi Planum are interpreted. Folds, shear zones, and rifts are similar in geometry to those associated with regional indentation and lateral escape during formation of the Himalayas and the Eastern Alps on Earth. Northward displacement of Lakshmi Planum refutes the conjecture that there have been no large horizontal displacements on Venus.
Bouguer gravity images and horizontal gravity gradient edges ('worms') portray hitherto unrecognized crustal structures on Venus. Belts of high Bouguer gravity and thinned crust (comparable to the Mid-Continent Rift in N America) suggest underplating of denser, mantle-derived material beneath extended crust. Rifts are partitioned by transfer faults and flank a zone of mantle upwelling between colinear upwelling mantle plumes. Gravity data support the model for northward drift and indentation of Lakshmi Planum. Reversal of transcurrent displacement senses and fold and thrust overprinting imply an orthogonal change in principal strains. The large displacements of areas of continent-like crust are interpreted to result from mantle tractions / pressure acting against their deep lithospheric mantle 'keels' commensurate with extension in adjacent rifts. Displacements of the continent-like Lakshmi Planum and Ovda and Thetis regiones on Venus cannot be attributed to plate boundary forces (ridge push, slab pull). Continent-like 'drift' on Venus resembles the westward translation of the Americas and continued northward displacement of India which are driven by mantle flow tractions on Precambrian cratonic keels. This new perspective of Venus provides an analogue for an Archean Earth without plate tectonics.