A network of fossil subduction plate interfaces preserved in the Central Alps (Val Malenco, N Italy) is herein used as a proxy to study deformation processes related to subduction and subsequent underplating of continental slices (in particular the Margna and Sella nappes) at depths reported to in the former brittle-ductile transition. Field observations, microfabrics, and mapping revealed a network of shear zones comprising mostly mylonites and schists but also rare foliated cataclasites. These shear zones are either located at the contacts of the two nappes or within the boundaries of the Sella unit. Microprobe results point to two different white mica generations, with higher-pressure (Si-rich) phengites rimming lower-pressure (Si-poor) phengites. Garnet is locally observed overgrowing resorbed pre-Alpine cores. Pressure-temperature estimates based on pseudosection modeling point to peak burial deformation conditions of ∼0.9 GPa and 350–400 °C, at ∼30 km depth. Rb/Sr geochronology on marbles deformed during the Alpine event yields an age of 48.9 ± 0.9 Ma, whereas due to incomplete recrystallization, a wide range of both Rb/Sr and 40Ar/39Ar apparent ages is obtained from deformed orthogneisses and micaschists embracing 87–44 Ma.
Based on our pressure-temperature, structural and geochronological observations, the studied shear zones last equilibrated at depths downdip of the seismogenic zone in an active subduction zone setting. We integrate these new results in the frame of previous studies on other segments of the same Alpine paleosubduction interface, and we propose that this system of shear zones represents deformation conditions along the subduction interface(s) in the transition zone below the seismogenic zone during active subduction.
<p>Exhumed subduction shear zones often exhibit block-in-matrix structures comprising strong clasts within a weak matrix (m&#233;langes). Inspired by such observations, we create synthetic models with different proportions of strong clasts and compare them to natural m&#233;lange outcrops. We use 2D Finite Element visco-plastic numerical simulations in simple shear kinematic conditions and we determine the effective rheology of a m&#233;lange with basaltic blocks embedded within a wet quartzitic matrix. Our models and their structures are scale-independent; this allows for upscaling published field geometries to km-scale models, compatible with large-scale far-field observations. By varying confining pressure, temperature and strain rate we evaluate effective rheological estimates for a natural subduction interface. Deformation and strain localization are affected by the block-in-matrix ratio. In models where both materials deform viscously, the effective dislocation creep parameters (A, n, and Q) vary between the values of the strong and the weak phase. Approaching the frictional-viscous transition, the m&#233;lange bulk rheology is effectively viscous creep but in the small scale parts of the blocks are frictional, leading to higher stresses. This results in an effective value of the stress exponent, n, greater than that of both pure phases, as well as an effective viscosity lower than the weak phase. Our effective rheology parameters may be used in large scale geodynamic models, as a proxy for a heterogeneous subduction interface, if an appropriate evolution law for the block concentration of a m&#233;lange is given.</p>
Frictional failure is the dominant deformation mechanism for rocks in the upper crust while in the middle crust rocks begin to deform viscously. Within this transition, brittle and viscous phases coexist, forming semi‐frictional materials. While semi‐frictional deformation on large scales might play an important role in understanding the transition between earthquakes and slow slip/creep, it can also be observed at smaller scales. Here, we use field observations of the Papoose Flat pluton in eastern California to study deformation of heterogeneous materials during shearing. Clast concentration varies between 2% and 12% by area. Field and microscopic observations show that the matrix deforms viscously, while the clasts fail in a brittle manner. We systematically document clast concentration and spacing with respect to clast fracturing and observe increasing frictional failure of clasts with increasing clast concentration. To test which matrix viscosities impose enough stresses on the clasts to lead to frictional deformation, we complement field observations with 2D numerical models. Maps with 7% by area randomly placed circular clasts are created and deformed under simple shear kinematic conditions. We test different matrix viscosities, from constant low and high viscosity (1017 and 1019 Pa.s, respectively), to dislocation creep for granite. Clasts in the vicinity of other clasts are affected by stresses around their neighbors. This effect decreases with increasing clast distance. Our field observations and numerical results suggest that the viscous phase can impose significant stresses onto the brittle phase, causing failure even at very low clast concentrations and in the absence of clast‐clast interactions.
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