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- Massironi, Matteo, et al.
(författare)
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Geological map and stratigraphy of asteroid 21 Lutetia
- 2012
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Ingår i: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 66:1, s. 125-136
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Tidskriftsartikel (refereegranskat)abstract
- The OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) images acquired during the recent Rosetta fly-by of Lutetia (10th of July 2010), enabled us to unravel the long geological history of the asteroid. This is recorded on its highly varied surface which displays geological units of disparate ages. In particular, using images of the closest approach, five main regions (in turn subdivided into minor units) have been discriminated on the basis of crater density, overlapping and cross-cutting relationships, and presence of linear features (i.e., fractures, faults, grooves, troughs). Other regions, with still unclear stratigraphic position, were also recognized on images of lower resolution on the bases of geomorphological properties such as crater density, relationship with scarp and ridges, and sharp morphological boundaries. In this work the geological evolution of Lutetia surface is reconstructed through the description of its main units and related contacts. The oldest regions imaged during the closest approach (Achaia and Noricum) are pervasively affected by fractures and grooves and display surfaces so heavily cratered to be dated back to a period not far from the Late Heavy Bombardment (yielding Achaia a crater retention age of 3.6-3.7 Ga). A crater of 55 km diameter, named Massilia and corresponding to the Narbonensis region, cuts both Achaia and Noricum regions and probably represents the most prominent event of the Lutetia history. The considerable crater density on its floor and walls, the absence of discernable deposits related to the impact event, and the intense deformation of it floor - all attest to its relatively great age. The North Polar Cluster (Baetica region) is associated with smooth ejecta broadly mantling the surrounding units and displays few craters and no linear features, demonstrating its relatively young age (estimated at less than 300 Ma). The North Polar Crater Cluster is the product of superimposed impacts: the last one of 24 km of diameter excavated the pre-existing ejecta up to the bedrock which locally outcrops at the crater rim. The ejecta of this last impact were involved in several gravitational phenomena testified by the great variety of deposits made up of mega-boulders diamictons, fine materials, gravitational taluses and debris, and landslide accumulations. A part from the big cratering events generating Massilia and the North Polar Crater Cluster, the Lutetia geological history is also punctuated by minor events still recorded by its stratigraphic record well imaged by the closest approach data.
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| 2. |
- Rosenbaum, Gideon, et al.
(författare)
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Dating deformation in the Gran Paradiso Massif (NW Italian Alps) : Implications for the exhumation of high-pressure rocks in a collisional belt
- 2012
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 144, s. 130-144
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Tidskriftsartikel (refereegranskat)abstract
- The Gran Paradiso massif, situated in the internal part of the Western Italian Alps, records a complex tectono-metamorphic history involving high-pressure metamorphism and subsequent exhumation during retrograde metamorphism. The exact timing of deformation and, consequently, the geodynamic evolution of this part of the Western Alps is still debated and is addressed here by the application of Rb/Sr geochronology, Ar-40/Ar-39 step heating and Ar-40/Ar-39 total fusion dating techniques. Geochronological results are presented from shear zone samples in the core of the Gran Paradiso massif (Piantonetto Valley). and in the area closer to the contact with the overlying Piedmont ophiolitic domain (south and southwest of Pont Valsavarenche). The shear zones operated during crustal thinning and exhumation of the Gran Paradiso massif. Ar-40/Ar-39 step heating results from shear zones in the Piantonetto Valley show acceptable plateau ages that are interpreted to represent two events of mica growth. Similar ages, and an additional younger age cluster, are recognised in the Ar-40/Ar-39 total fusion analyses, indicating that specific cleavage domains operated at 39.2 +/- 0.2, 36.5 +/- 0.6 and 33.3 +/- 0.4 Ma. P-T pseudosections show a progressive decrease in metamorphic conditions during deformation, suggesting that the age of incipient exhumation and the related deformation in the Piantonetto Valley is equal to or older than 39.2 +/- 02 Ma. In the Pont area, the last increments of deformation in a top-to-W shear zone postdate 36.6 +/- 0.6 Ma (Rb/Sr mineral data), whereas the present-day top-to-W contact of the Gran Paradiso massif with the overlying Piedmont domain is dated at 41.2 +/- 1.1 Ma (Rb/Sr multi-mineral isochron age). We propose a model that considers exhumation of the Gran Paradiso nappe at 41-34 Ma. During this period, the nappe was coupled with the Zermatt-Saas zone, forming an extruding wedge. The kinematics associated with this wedge involved top-to-W shearing within the Gran Paradiso nappe (e.g. Pont area shear zones) and top-to-E shearing at the top of the extruding wedge (e.g. Orco shear zone). Subsequent deformation (after similar to 34 Ma) was characterised by coaxial strain involving orogenic-scale backfolding and backthrusting.
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