Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..463h&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. The geological features, structures, thermal conditions, interpreted processes, and outstanding questions related to both the Earth's Archean and Venus share many similarities [1-3] and we are using a problem-oriented approach to Venus mapping, guided by insight from the Archean record of the Earth, to gain new perspectives on the evolution of Venus and Earth's Archean. The Earth's preserved and well-documented Archean record [4] provides important insight into high heat-flux tectonic and magmatic environments and structures [5] and the surface of Venus reveals the current configuration and recent geological record of analogous high-temperature environments unmodified by subsequent several billion years of segmentation and overprinting, as on Earth. Here we address the nature of the Earth's Archean, the similarities to and differences from Venus, and the specific Venus and Earth-Archean problems on which progress might be made through comparison. The Earth's Archean and its Relation to Venus. The Archean period of Earth's history extends from accretion/initial crust formation (sometimes called the Hadean) to 2.5 Ga and is thought of by most workers as being a transitional period between the earliest Earth and later periods largely dominated by plate tectonics (Proterozoic and Phanerozoic) [2, 4]. Thus the Archean is viewed as recording a critical period in Earth's history in which a transition took place from the types of primary and early secondary crusts seen on the Moon, Mars and Mercury [6] (and largely missing in the record of the Earth), to the style of crustal accretion and plate tectonics characterizing later Earth history. The Archean is also characterized by enhanced crustal and mantle temperatures leading to differences in deformation style and volcanism (e.g., komatiites) [2]. The preserved Archean crust is exposed in ~36 different cratons [4], forming the cores of most continental regions, and is composed of gneisses, plutons and greenstones. The geological record of the Archean Earth is considerably different than the Phanerozoic record and ongoing processes [1, 7]. The Archean record is characterized by evidence for enhanced mantle temperatures, different styles of crustal deformation (localized belts of high intensity deformation, tight high and low angle folds, diapiric-related deformation, significant lateral differences in lithospheric thickness (implied by 'cold' keels), significant evidence for crustal thickening processes and the burial and exhumation of thickened crust, abundant hightemperature komatiites, greenstone belts, "mafic plains"-type greenstones, positive gneissic and felsic diapirs, abundance of a distinctive TTG (tonalitetrondhjemite- granodiorite) assemblage, layered gabbro- anorthosite igneous intrusions, very abundant plume-derived basalts, unusual events interpreted to represent mantle instability and overturn, late stage granodiorites and granites derived from intracrustal melting, epicratonic basins, and production of large volumes of continental crust [1,4,5]. A major question in the study of the Archean is the nature of the geodynamic processes operating during this time. Do the geodynamic processes represent a steady-state accommodation to the Archean thermal environment, or do they represent a transitional or evolutionary phase? Does the Archean represent a particular unique style of vertical tectonics, as on oneplate planets, lateral tectonics (perhaps early plate tectonics) as on later Earth, or is it transitional in time (and perhaps in space), changing from one style to another during the Archean? What role do the enhanced mantle and crustal temperatures play in volcanism and tectonism during this period? Do global crustal and lithospheric density instabilities play a major role in the transition [8], perhaps causing catastrophic foundering and crustal overturn [9], as thought to have occurred on the Moon and Mars? Does vertical crustal accretion dominate over lateral crustal accretion, leading to density instabilities and planet-wide diapiric upwelling and downwelling, as has been postulated for Venus [10]? Many of these critical questions related to the Archean also face investigators studying Venus: 1) What is the nature of the tectonic and geodynamic evolution of Venus? (e.g., episodic plate tectonics [11]; vertical crustal accretion and depleted mantle layer overturn [10]; transition from mobile lid to stagnant lid convection [12]). 2) What is the nature of the geological evolution of Venus? (e.g., "nondirectional/ patchwork" [13], or displaying broad evolutionary, or "directional" trends [14]). 3) What is the nature of the volcanic record of Venus? (what is the significance in time and space of the wide range of different volcanic feature and deposit morphologies, ranging from extremely fluid (komatiite-like?) to much more viscous (rhyolite or dacite-like?). 4) What is the nature of the tectonic record of Venus? (e.g., what is the role and geodynamic linkage of the major deformation belts [15]; how do the protocontinent-like deformed crustal plateaus form (e.g., upwelling [16] or downwelling [17] ?). 5) What is the role of hot spots/rises in heat loss, volcanism, and mantle dynamics on Venus? [16]. 6) What is the role of crustal/lithospheric instabilities and diapirism? (e.g., are coronae mantle derived hot spots, or could they represent crustal/lithospheric instabilities and their evolution [18]. The geological and geophysical characteristics and global record of Venus provide a critical resource in which to explore these common questions between Venus and the Earth's Archean. Data from the Venera and Magellan missions have shown that Venus is characterized by atmosphere-induced high upper crustal temperatures and abundant and varied global ductile and brittle deformation [15, 19]. A wide range of tectonic features include tessera, intensely deformed high-standing terrains of thicker crust, linear fracture belts of extensional origin arrayed in narrow interconnected zones thousands of km long, narrow ridge belts of contractional origin arrayed in zones adjacent to and cross-cutting the fracture belts, folded mountain belts arrayed around the margins of tessera plateaus, young rift zones often associated with recent mantle upwelling, and zones of radial fractures and graben interpreted to be due to dike emplacement around a central intrusion. These data have shown that volcanism played a major role in the resurfacing of Venus and the building of its upper crust [20]. Extensive tracts of regional volcanic plains comprise the vast majority of the current surface. A wide range of basaltic volcanic landforms are observed, including small shields apparently representing small-scale broadly distributed melting, vast featureless flood-basalt-like plains filling lows with evidence for very high effusion rates in sinuous rilles hundreds to thousands of kilometers long, and huge lobate flow complexes emerging from rift zones and coronae (diapiric-like upwellings). Despite the current lack of a hydrosphere on Venus and evidence for dry crustal rocks, massive steep-sided domes and huge festoons are observed, suggesting evolved, more felsic compositions, often in areas of protocontinentlike crustal thickening. Broad topographic rises characterized by associated rifting and volcanism are interpreted to represent hot spots and mantle upwelling [21]. Hundreds of coronae dot the surface of Venus [18], and are interpreted to represent mantle upwellings or diapiric structures representing crustal and/or lithospheric instabilities and whose depth of origin is uncertain. Abundant shield volcanoes may be transitional to coronae. In summary, Venus and the Earth's Archean have both similarities and differences. Among the similarities are upper crustal temperatures, the importance of ductile deformation styles, evidence for both vertical and lateral tectonic structures and styles, abundant radial dike swarms, diapirism as an important candidate process, an important role for hot spots, and shared uncertainties about the geological and geodynami
Basilevsky Alexander T.
Head James W.
Hurwitz Debra M.
Ivanov Michael A.
Kumar Senthil P.
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