Interactions between continent-like ‘drift’, rifting and mantle flow on Venus: gravity interpretations and Earth analogues

Abstract

Regional shear zones are interpreted from Bouguer gravity data over northern polar to low southern latitudes of Venus. Offset and deflection of horizontal gravity gradient edges (‘worms’) and lineaments interpreted from displacement of Bouguer anomalies portray crustal structures, the geometry of which resembles both regional transcurrent shear zones bounding or cross-cutting cratons and fracture zones in oceanic crust on Earth. High Bouguer anomalies and thinned crust comparable to the Mid-Continent Rift in North America suggest underplating of denser, mantle-derived mafic material beneath extended crust in Sedna and Guinevere planitia on Venus. These rifts are partitioned by transfer faults and flank a zone of mantle upwelling (Eistla Regio) between colinear hot, upwelling mantle plumes. Data support the northward drift and indentation of Lakshmi Planum in western Ishtar Terra and >1000 km of transcurrent displacement between Ovda and Thetis regiones. Large displacements of areas of continent-like crust on Venus are interpreted to result from mantle tractions and pressure acting against their deep lithospheric mantle ‘keels’ commensurate with extension in adjacent rifts. Displacements of Lakshmi Planum and Ovda and Thetis regiones on Venus, a planet without plate tectonics, cannot be attributed to plate boundary forces (i.e. ridge push and slab pull). Results therefore suggest that a similar, subduction-free geodynamic model may explain deformation features in Archaean greenstone terrains on Earth. Continent-like ‘drift’ on Venus also resembles models for the late Cenozoic–Recent Earth, where westward translation of the Americas and northward displacement of India are interpreted as being driven by mantle flow tractions on the keels of their Precambrian cratons.Supplementary material:Bouguer gravity and topographic images over a segment of the Mid-Atlantic ridge and Ross Island and surrounds in Antarctica, principal horizontal stress trajectories about mantle plumes on Earth, map and interactive 3D representations of cratonic keels beneath North America from seismic tomography, and a centrifuge simulation for comparison with Venus in support of our tectonic model are available at http://www.geolsoc.org.uk/SUP18736.

The Physics of Changing Tectonic Regimes: Implications for the Temporal Evolution of Mantle Convection and the Thermal History of Venus

“…Producing the crustal thickening required to support Lakshmi implies a minimum of 2000‐3000 km of crustal convergence (Kiefer , ), with an even larger amount of convergence required to explain the high mountain belt topography. This interpretation is also supported by recent geologic mapping (Harris & Bédard, , ). Structural relationships and stratigraphy in the complexly deformed tessera terrain of Tellus Regio, 2000 by 1600 km across and ~1 km high, indicate that Tellus formed by lateral transport and assembly of three distinct tessera blocks (Gilmore & Head, ).…”

Volcanism and tectonism across the inner solar system: an overview

“…Schaber 1982;Head & Crumpler 1987;Hansen & Phillips 1993;Baer et al 1994;Hamilton & Stofan 1996;Aittola & Kostama 2000;Magee & Head 2001;Stofan et al 2001;Krassilnikov & Head 2003;Harris & Bédard 2014a). On the basis of structural observations and gravitational data, further major extensional rifts have been proposed to follow plains' depressions that were later covered by volcanic materials (Sullivan & Head 1984;Harris & Bédard 2014b).…”

“…Finally, Romeo et al (2005) and Chetty et al (2010) identified conjugate shear zones in Ovda Regio associated with tear-folds and imbricate duplexes. Continental drift may account for the substantial horizontal tectonism required by prominent strikeslip belts (Harris & Bédard 2014b). …”

“…Interactions between continent-like 'drift', rifting and mantle flow on Venus: gravity interpretations and Earth analogues Harris & Bédard (2014b) identify major strikeslip shear zones at Ishtar and Afrodite Terrae and at Sedna Planitia, using offsets of Bouger gravity anomalies and gravity gradient edges. Their observations call for a new conceptual model capable of satisfying Venusian subduction-free geodynamics, dominant convective upwellings and the substantial horizontal tectonism required by the observed strike-slip belts.…”

“…This process accounts for the fold-andthrust belt that bounds Lakshumi planum to the north, as well as the transpressive regimes recognized at its eastern and western margins. Harris & Bédard (2014b) …”