Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..186i&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Logic
Scientific paper
Introduction: Lakshmi Planum is a highstanding plateau (3.5-4.5 km above MPR) surrounded by the highest mountain ranges on Venus [1-6]. Lakshmi represents a unique type of elevated region different from dome-shaped and rifted rises and tessera-bearing plateaus. The unique characteristics of Lakshmi suggest that it formed by an unusual combination of processes. Lakshmi was studied with Venera-15/16 [7-10, 5,11] and Magellan data [12-14], resulting in two classes of models, divergent and convergent, to explain its unusual characteristics. Divergent models explain Lakshmi as a site of mantle upwelling [10,15-18] due to rising and subsequent collapse of a mantle diapir; such models explain emplacement of a lava plateau inside Lakshmi and, in some circumstances, formation of the mountain ranges. The convergent models consider Lakshmi as a locus of mantle downwelling, convergence, underthrusting, and possible subduction [19,11,20-29]. Key features in these models are the mountain ranges, high topography of Lakshmi interior, and the large volcanic centers in the plateau center. These divergent and convergent models entail principally different mechanisms of formation and suggest different geodynamic regimes on Venus. Almost all models make either explicit or implicit predictions about the type and sequence of major events during formation and evolution of Lakshmi and thus detailed geological mapping can be used to test them. Here we present the results of such geological mapping (the V-7 quadrangle, 50- 75N, 300-360E; scale 1:5M) that allows testing the proposed models for Lakshmi. Material units: Eleven material units make up the V-7 quadrangle. (1) Tessera (t), exposed inside and outside Lakshmi appears to be the oldest material. (2) Densely lineated plains (pdl) postdate tessera and form one of the oldest units; patches occur outside Lakshmi Planum. (3) Ridged plains (pr) postdate pdl and occur outside Lakshmi. (4) Shield plains (psh) display abundant small shields (small volcanoes), embay the previous units and occur outside Lakshmi. (5) Pitted and grooved material (pgm) displays small pits and is cut by broad and shallow groove; it occurs inside Lakshmi in association with mountain ranges. (6) Lower unit of regional plains (rp1) has a smooth surface, is cut by wrinkle ridges; this most widespread unit occurs inside and outside of Lakshmi Planum. (7) Upper unit of regional plains (rp2) is also deformed by wrinkle ridges but has lobate boundaries and higher radar albedo than rp1; occurs both inside and outside Lakshmi. (8) Lobate plains (pl) is characterized by lobate flows that embay the most tectonic structures including wrinkle ridges; form fluctuses outside Lakshmi and surround Colette and Sacajawea Paterae inside the plateau. (9) Smooth plains (ps) have uniform and low radar albedo, embay wrinkle ridges; largest occurrence in southern portion of Lakshmi. (10) Impact craters (c) and (11) crater outflow deposits (cf); peppered throughout without preferential concentrations. Structures: Extensional structures. In places, fractures and graben form belts (groove belts, gb) that extend for hundreds of kilometers mostly within the southern regional slope of Lakshmi where they cut pdl and pr and are embayed by psh and rp1. Contractional structures. Wrinkle ridges mildly deform psh and regional plains; broader and more linear ridges dominate ridged plains (pr). The most important occurrences of contractional structures are mountain belts (unit mt) that surround the interior of Lakshmi and consist of densely spaced ridges 5-15 km wide, tens of km long. Regional plains usually embay the ridges. Sequence of major events during evolution of Lakshmi Planum: Various plains units heavily embay fragments of tessera in all localities inside and outside Lakshmi. The consistent relationships of embayment and the complex and unique surface deformational pattern suggest that tessera represents the oldest material. Tessera distribution patterns suggest more extensive presence under younger plains units, forming basement. Densely fractured plains (pdl) appear younger than tessera; the largest massifs of pdl occur in Atropos and Itzpapalotl where plains are further deformed by broad ridges and to some degree resemble the tessera deformation patterns. The ridges are generally conformal to the strike of Akna/Freyja Montes, occur within large areas of pdl adjacent to the mountain ranges, and clearly relate to the orogenic phase of formation of mountain belts [10,16-19,11,20- 23,25,26,29]. Shield plains and regional plains embay mountainous ridges both outside and inside Lakshmi Planum, which implies that the orogenic phase was toward earlier stages of the observable geological history. Shield plains were emplaced after the main phase of mountain belt formation and before regional plains, but exclusively outside of the plateau. The lower unit of regional plains (rp1) postdates shield plains; occurrences are concentrated S of Lakshmi Planum and in the interior of Lakshmi. The thickness of unit rp1 is small because outliers of older units occur within the broad regional plains. Youngest units, smooth/lobate plains, are superposed on regional plains and undeformed by tectonic structures; they were emplaced after cessation of major tectonic activity. Smooth/lobate plains form extensive lava aprons around Colette and Sacajawea Paterae, representing the latest volcanic activity inside Lakshmi Planum. Testing models of Lakshmi Planum formation: Detailed geological analysis thus allows definition of map units, establishing the sequence of major events during formation of Lakshmi, and testing the suite of models proposed to explain the mechanisms of formation of this structure. The interpreted nature of units and the sequence of events strongly contradict the predictions of divergent models: 1) The very likely presence of an ancient (craton-like) tessera massif in the core of Lakshmi; such a core is inconsistent with the rise and collapse of a mantle diapir [10,15,16]. 2) The absence of a rift zone in the interior of Lakshmi; these zones appear to be a natural consequence of growth of surface topography due to diapiric rise [e.g. 30]. 3) The apparent migration of volcanic activity toward the center of Lakshmi; divergent models are consistent with the opposite trend. 4) The abrupt cessation of mountain range ridges at the edge and propagation over hundreds of kilometers outside Lakshmi in Atropos and Itzpapalotl Tesserae. Divergent models predict the opposite progression. Convergent models of formation and evolution of Lakshmi appear to be more consistent with the observations. The pure downwelling models [e.g. 23], however, faces three important difficulties. 1) The possibly unrealistically long time span that seems required to produce the major features of Lakshmi [31]. 2) The strongly asymmetrical N-S topographic profile of Lakshmi and striking difference in the height and thickness of the mountain belts to the NW and N (Akna and Freyja) and to the S of Lakshmi (Danu). The pure downwelling models would require formation of more symmetrical structure. 3) The absence of radial contractional structures (arches and ridges) in the interior of Lakshmi. These structures represent the predicted result of the downwelling models. Convergence models are most consistent with observations and explain the structure by collision and underthrusting/subduction of lower-lying plains with the elevated and rigid block of tessera [20-22]. These models are capable of explaining formation of the major features (e.g., mountain belts), the sequences of events, and principal volcanic and tectonic trends. To explain the N-S asymmetry of Lakshmi, however, these models have to consider major axes of collision to be N and NW of the plateau in Atropos and Itzpapalotl Tesserae. A plausible scenario for formation/evolution of Lakshmi Planum consists of the following stages (Fig. 1). (1) Pre-deformational configuration of western Ishtar; a layered suite of low-lying plains surrounded a tessera craton. (2) Compression from the N led to deformation of plain
Head James W.
Ivanov Michael A.
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