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The Oligocene orogenic volcanism in the Western Alps: a paleosurface and tectonic implications

English title The Oligocene orogenic volcanism in the Western Alps: a paleosurface and tectonic implications
Applicant Mercolli Ivan
Number 124331
Funding scheme Project funding (Div. I-III)
Research institution Institut für Geologie Universität Bern
Institution of higher education University of Berne - BE
Main discipline Geology
Start/End 01.05.2009 - 30.04.2010
Approved amount 59'033.00
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Keywords (9)

Oligocene magmatism; Sesia Zone; Exhumation; Fission track dating; Ivrea Zone; Canavese Line; Southern Alps; Oligocene paleosurface; exhumation of high-pressure terrains

Lay Summary (English)

Lead
Lay summary
We intend to approach the reconstruction of the spatial and temporal succession of exhumation, volcanism, plutonism and tectonics during the Early Oligocene in the Western Alps. This interplay between plutonism, volcanism, exhumation of high-pressure terrains and fluid circulation is crucial to understanding the dynamics of collisional orogeny and surface processes. The region of Biella (Piemonte, NW Italy) offers a unique geological setting to perform a systematic, multidisciplinary study of the emplacement, cooling, hydrothermal overprint and deformation of magmatic rocks intruding and extruding in and on exhumed high-pressure gneisses of the Sesia Zone. The uplift tectonics and magmatic activity are closely related to the dynamics along the Canavese Line, the western branch of the Periadriatic Lineament. The products of all these orogenic processes and their mutual geometrical relationships are well exposed in this region, yet the rates and mechanisms of orogenic evolution in the Western Alps remain controversial. To resolve the problem we intend to use the following methods: (1) petrological analysis of the magmatic rocks; (2) relative timing of the hydrothermal overprint of the volcanic rocks; (3) structural analysis of the mutual contact between magmatic, metamorphic and hydrothermal rocks; (4) fission track analysis; (5) isotope dating of volcanic rocks.The preservation of a 30 Ma old paleosurface offer the unique chance to study surface processes in the Oligocene. Moreover, the paleosurface is the doubtless marker for subsequent tectonic processes affecting this surface. The clarification of the relative and absolute timing of magmatic and tectonic activity of this portion of the Western Alps will have consequences for the whole Alps and their geodynamic evolution. As the Alps are frequently used as a model case to interpret other orogenic belts the results of our study are likely to have widespread resonance.
Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants

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Associated projects

Number Title Start Funding scheme
111644 The Lower Oligocene, orogenic volcanism in the Western Alps: a paleosurface and tectonic implications 01.05.2006 Project funding (Div. I-III)

Abstract

The present research proposal is intended as continuation of the project “The Late Oligocene orogenic volcanism in the Western Alps: a paleosuface and tectonic implications” (NF 200021-111644) started in April 2006 and that will terminate in March 2009. In the light of the result of the first two years we apply for another year for completion of the analytical and interpretative work indispensable to approach the reconstruction of the spatial and temporal succession of exhumation, volcanism, plutonism and tectonics during the Early Oligocene in the Western Alps. The interplay between plutonism, volcanism, exhumation of high-pressure terrains and fluid circulation is crucial to understanding the dynamics of collisional orogeny and surface processes. The products of all these orogenic processes and their mutual geometrical relationships are well exposed in the region of Biella (Piemonte, NW Italy), yet the rates and mechanisms of orogenic evolution in the Western Alps remain controversial. This project intends to draw evidence from the Oligocene magmatic activity and its related hydrothermal fluids to elucidate the exhumation of the Sesia high-pressure terrain in the Western Alps. Despite the favorable field relationships, our evaluation of the literature and of field observations has revealed a major temporal conflict. Convincing field evidence shows that subaerial lavas of reputedly Oligocene age extruded onto the erosional surface of exhumed Alpine metamorphic rocks of the Sesia Zone, which contain eclogitic relics. Equally, strong field evidence shows that the subsequent emplacement of two plutonic complexes into the gneisses of the Sesia Zone led to hydrothermal fluid circulation, which altered the gneisses and the volcanic rocks and is therefore younger than the volcanism. Published radiometric data on the plutons and zircon- and apatite-fission track data on the metamorphic rocks indicate that the plutonism and the hydrothermal alteration occurred when the Sesia Zone was buried to 6-8 km depth. The conflict is that all these processes were apparently simultaneous during the Middle Oligocene: the lavas flowed over an erosional surface which, paradoxically, was still at >6 km depth and at a temperature of approximately 200 °C. The results of the first two years of the project have evidenced the extremely closeness of the events. The new radiometric age (high precision U/Pb on single grain zircon) of the volcanic rocks is comprised between 32.44 und 32.89 Ma, very close to the intrusion age of the Valle del Cervo Pluton (31 ± 0.2 Ma, Romer et al. 1996). Our fieldwork has demonstrated the undisturbed stratigraphic nature of the contact surface between Sesia gneiss and volcanic rocks. Furthermore, hydrothermal veins, probably related to the intrusion of the Valle del Cervo Pluton (Rossetti et al. 2007), crosscut the already steeply tilted (up to 60o) volcanic suite. A confident reconstruction of the sequence of geological processes active within a very limited lapse of time (2-3 Ma) at the beginning of the Oligocene along the southern margin of the Western Alps will provide new insight into the timing and rates of exhumation and processes at or near the surface, because the preserved paleosurface.To resolve the problem we intend to use the following methods: (1) petrological analysis of the magmatic rocks; (2) relative timing of the hydrothermal overprint of the volcanic rocks; (3) structural analysis of the mutual contact between magmatic, metamorphic and hydrothermal rocks; (4) fission track analysis; (5) isotope dating of volcanic rocks. The combined results will allow us to reconstruct coherently the exhumation path of the Sesia Zone. The subaerial volcanic cover of the Sesia Zone clearly indicates that this unit was already exposed at the surface in Oligocene times. As these rocks are presently at the surface, the question arise if the Sesia Zone remained near the surface since that time or if it was buried again following the volcanism and re-exhumed later, once the plutons had been emplaced? The results of the apatite and zircon fission track dating will represent the backbone of the reconstruction. The detailed petrographic investigation of the volcanic rocks and their alteration should help to clarify their post-emplacement evolution from a petrological point of view. New insight into the influence of hydrothermal alteration on fission track data would also have methodological consequences. The alkaline volcanic and plutonic rocks in the region of Biella show a clear geochemical mantle signature. We aim to use the results on the near-surface magmatic activity and their link to the exhumation tectonics to constrain the possible migration paths of the melts from the mantle to the surface. Understanding the geometry and distance of melt migration in orogenic belts is of fundamental importance for the reconstruction of their dynamics. In the Alps, in particular, where the orogenic magmatism is concentrated along the Periadriatic Lineament, a clear representation of the near-surface relationships in the studied area would help to reconstruct a profile through the southern Western Alps in Oligocene times. As the Alps are frequently used as a model case to interpret other orogenic belts the results of our study are likely to have widespread resonance.
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