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FP7

RHEOMANTLE Résumé de rapport

Project ID: 329183
Financé au titre de: FP7-PEOPLE
Pays: Netherlands

Periodic Report Summary 1 - RHEOMANTLE (Evaluation of mantle rheology in exhumed strike-slip faults)

Description of the project objectives

The strength of the lithosphere and, in particular, how strength varies with depth has been a topic of considerable interest, and controversy, over several decades of studies. In lithospheric-scale, strike-slip fault zones, upper crustal strength is well constrained but lower crustal and mantle strength is less well known. Through characterization of the relative strength of the different lithospheric layers, a series of conceptual rheological models have been developed: 1) The upper crust and uppermost mantle are both strong, but separated by a weak lower crust; 2) The lower crust is strong and uppermost mantle is relatively weak; and 3) A strong undeformed lithosphere is “split” by a weak (low stress) fault zone. Most conceptualizations of lithospheric strength require that the different lithospheric layers are independent of each other and that the rheology of each layer is constant through time. The former assumption overlooks the observation that crustal deformation in major strike-slip fault zones (e.g., San Andreas Fault and North Anatolian Fault) continues into the lithospheric mantle. The latter assumption overlooks that deformation in viscous shear zones below major strike- slip faults is unlikely to occur at constant strain rates, particularly given the evidence for rapid-slip events and low-frequency earthquakes in the lower crust beneath the faults.
In the Marie Curie IOF-project RHEOMANTLE, I study exhumed, naturally-deformed mantle rocks to understand the rheological behavior of lithospheric-scale, strike-slip faults zones, and particularly how mantle deformation can affect seismicity, and vice versa. I use three major strike-slip fault zones as “natural laboratories”: 1) the San Andreas Fault (SAF) system (California, USA); 2) the Bogota Peninsula shear zone (New Caledonia); and 3) the Mavrovouni shear zone (Greece). The study of exhumed mantle xenoliths from beneath the SAF system provides a unique opportunity to compare the strength of different lithospheric layers within the same continental transform fault zone. The Bogota Peninsula shear zone allows studying lateral variations in mantle rheology across an oceanic transform fault. The study of the Mavrovouni shear zone will inform on the rheological transitions across a lateral ramp, which formed at high-temperature conditions. The results from the three field areas, when combined, will advance our understanding of the rheological structure of lithospheric-scale, strike-slip fault zones.

Description of the work performed since the beginning of the project
In 2014–2015, I conducted research in all the three study areas, determining mantle deformation pattern in the field and mantle rheology in the laboratory through microanalytical work. In particular, during the first two years of the project RHEOMANTLE, the work performed involved (not necessarily in chronological order):
San Andreas Fault (SAF) system
1) Estimation of mantle strength (using olivine recrystallized grain size paleopiezometry) and water content (using FTIR) from the upper mantle xenoliths exhumed beneath the SAF system.
2) Estimation of strain rate (and viscosity) using experimentally derived flow laws for olivine.
3) Synthesizing the new results for the SAF system and proposing a conceptual rheological model, which explains results of both, field-based and geophysical studies.
Bogota Peninsula shear zone
4) Structural mapping and characterization of mantle fabric across the shear zone.
5) Fabric analysis in 32 samples across the shear zone using X-ray computed tomography (XRCT); determination of lateral variations in the degree of fabric anisotropy and fabric shape geometry.
6) Analysis of crystallographic preferred orientations (CPOs) in the harzburgites of the shear zone by means of Electron Backscatter Diffraction (EBSD).
7) Estimation of mantle strength, strain rate, and viscosity across the shear zone.
8) Estimation of the equilibration temperature using Electron Probe Micro-Analysis (EPMA) of the mantle rocks from different fabric domains across the shear zone.
9) Synthesizing the new results for the Bogota Peninsula shear zone with implications on strain localization processes in lithospheric-scale, strike-slip fault zones and stress transfer between different lithospheric layers.
Mavrovouni shear zone
10) Mapping the shear zone in the field, and particularly mapping structural domains characterized by variations in mantle fabric and degree of deformation.
11) Carrying-out detailed sampling (more than 80 samples were collected) across, along, and at different depths in the shear zone.
Statistical analysis of 3D geological data
12) Testing the homogeneity of spinel fabric data in each analyzed sample.
13) Testing the existence of statistically meaningful domains with specific fabric characteristics.

Description of the main results achieved so far
In the San Andreas Fault (SAF) system, mantle strength (~20 MPa) estimated from the upper mantle xenoliths is similar to upper crustal strength, constrained using paleopiezometers. These data support an interpretation for a uniform stress distribution throughout the lithosphere in major strike-slip fault zones, which does not fit with any existing lithospheric strength profile that supports a mechanically stratified lithosphere. I have proposed a new rheological model - the Lithopsheric Feedback model – in order to explain the observed lithospheric strength distribution in the SAF system and its bulk behavior, as deduced from geophysical, geodetic, and geological methods. The essence of the lithospheric feedback model is that in major strike-slip faults the upper crust and lithospheric mantle act together as an integrated system (Chatzaras et al., 2015a). Mantle flow loads the upper crust, where failure limits stress to <20 MPa. Stress redistribution during seismic events imposes a similar stress level throughout the lithospheric system. These mantle-crust feedbacks control the overall rheology of the integrated lithospheric-scale fault system.
In the Bogota Peninsula shear zone, differential stress increases (doubles) and viscosity decreases toward the high strain part of the shear zone (Chatzaras et al., 2015b). Importantly, the differential stress in the shear zone mylonites is similar to the stress determined for the upper mantle beneath the SAF system. Higher (but still low) stress and reduced viscosity during high-strain-rate deformation in the shear zone does not support a weakening-induced strain localization mechanism, which would require high strain rate but decrease of stress. Alternatively, strain localization could be the result of displacement-rate loading induced by earthquake rupturing in the upper crust.

Description of the expected final results and their potential impact and use
The most important contribution of the project RHEOMANTLE will be the development of a new rheological model for strike-slip fault systems, which will be based on observations from natural fault/shear zones. The proposed rheological model will take into account the time-dependent spatial (vertical and lateral) variation of lithospheric strength, as well as the mechanical interaction between the different lithospheric layers. This integrated approach will advance our understanding of seismic cycle in lithospheric-scale, strike-slip fault zones.
References
Chatzaras, V., Tikoff, B., Newman, J., Withers, A.C., Drury, M.R., 2015a. Mantle strength of the San Andreas fault system and the role of mantle-crust feedbacks. Geology, 43, 891–894, doi:10.1130/G36752.1.
Chatzaras, V., Tikoff, B., Newman, J., Titus, S.J., Withers, A.C., Kruckenberg, S.C., Drury, M.R., 2015b. Seismic cycle is controlled by stress and displacement loading between crust and mantle: The Lithospheric Feedback model. Geotectonic Research, 97, 7, doi:10.1127/1864-5658/2015-03.

Contact

Martijn Deenen, (Research support officer)
Tél.: +31302535169
E-mail

Thèmes

Life Sciences
Numéro d'enregistrement: 182261 / Dernière mise à jour le: 2016-05-24
Source d'information: SESAM