Formation and evolution of the midlands of Venus: Geological features and structures, stratigraphic relationships and geologic history of the Fredegonde area (V-57)

The topographic midlands on Venus comprise about 80% of the surface and an understanding of their mode of formation is essential to unraveling the geologic and geodynamic history of the planet. We explore this question by undertaking a comprehensive geological mapping of the Fredegonde Quadrangle (V-57, 50–751S, 60–1201E, 1:5M scale) that represents the transition zone from the midlands to the lowlands at the edge of Lada Terra. We report on the geologic units and structures and the sequence of events and, thus, the major stages in the evolution of this region of the midlands. At earlier stages of evolution of the long-wavelength topography, broad (hundreds of kilometers wide) and relatively low (1–1.5 km high) topographic ridges formed due to sequential development of deformation zones, first of contractional ridge belts (NW orientation) and then crosscut by extensional groove belts (NE orientation). Arcuate swarms of graben within groove belts often form the rims of coronae and represent their tectonic component. This suggests that groove belts and coronae within the quadrangle formed simultaneously. Intersections of these deformation zones caused separation of the topography of the region into a series of broad, shallow equidimensional basins many hundreds of kilometers across and currently hundreds of meters up to a kilometer deep. Thus, the principal topographic features within the quadrangle were established near the beginning of its observable geological record. The basins then remained sites of accumulation of successive volcanic plains units such as shield plains (psh) and the lower unit of regional plains (rp1). The flows of the younger plains, such as upper unit of regional plains (rp2) and lobate plains (pl), are less voluminous, and flow down the current topographic gradients. This implies that the major topographic pattern of the Fredegonde quadrangle has been stable since its establishment. Further evidence for this is that the vast volcanic plains units (psh and rp1) that postdate the heavily tectonized units of the deformation zones are only mildly deformed. This suggests that since the emplacement of shield plains, volcanism has been the primary geologic process and that the time of formation of unit psh corresponds to a major change from the earlier regime dominated by tectonics to the later volcanically dominated regime. Consistent age relationships among the main volcanic units within the quadrangle from older shield plains, through regional plains, to lobate plains, documents an evolution in volcanic style. Shield plains were formed from small eruptions from ubiquitous small shield volcanoes and are interpreted to be derived from broadly distributed and shallow magmatic sources. The lower unit of regional plains is widely distributed but vents and flow fronts are rare; this unit is interpreted to represent massive and probably short-lived flood basalts-like eruptions that filled in the lowlands basins. The upper unit of regional plains (rp2) and lobate plains (pl) are associated with localized and distinctive sources, such as late-stage volcanic activity at coronae. Thus, the tectonic stage of evolution of coronae (formation of the rims) and the volcanic stage when coronae served as magmatic centers and sourced lava flows, were separated in time by the emplacement of the shield and lower regional plains. How and when did the major components of Venus midland topography form? Clearly, in the Fredegonde quadrangle, regional deformation produced the deformation belts and groove belts/coronae in the earliest phases, and this topography formed the basis for the next, volcanic stage of emplacement (filling of the basins), with coronae-associated volcanism following this phase. The broad topography resulting from this early phase has persisted until the present. We compare this tectonic-volcanic sequence and history of ll rights reserved. ssian Academy of Sciences, 119991 Kosygin St., 19, Moscow, Russia. Tel.: þ7 499 1374995.