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exhumed foSsiL shear zones: a key to Investigate Present middle-crustal seismicity

Periodic Reporting for period 1 - SLIP (exhumed foSsiL shear zones: a key to Investigate Present middle-crustal seismicity)

Reporting period: 2019-06-03 to 2021-06-02

With approximately 500,000 earthquakes happening each year and as many as 100,000 of those being felt, seismic events represent a major hazard for humanity. In the last twenty years, geoscientists have discovered and characterized by means of indirect geophysical and geodetic techniques peculiar seismic phenomena occurring at convergent margins, which are referred to in the scientific community as “episodic tremor and slow slip events” (ETS). Tremors are a persistent low-frequency seismic signal associated with slow slip, a geodetically detected slip larger than the average plate motion. Although ETS do not represent a direct threat to humans, the fact that they are located in proximity of megathrust earthquakes and are believed to mark stress transfer to the megathrust faults makes them of great interest to geoscientists. Therefore, their better characterization can help to shed light on the very hazardous subduction seismicity. Unfortunately, direct observation of the geological record of these phenomena is only possible by analysing rocks exhumed from fossil subduction zones, such that the rock record of deep seismic activity is scanty because subducted rocks commonly re-equilibrate during exhumation en-route to the surface.
The Italian Northern Apennines (Italy) expose deeply subducted rocks exhumed back to the surface that still preserve mineralogical assemblages and structures acquired at great depth in the subduction channel. The studied rocks display evidence of coeval discontinuous and continuous deformation in the form of veins and pervasive foliation, respectively. Veins are composed of quartz and carpholite fibres oriented parallel to the stretching lineation of the host rock and display crack-seal textures. Veins thus formed by incremental growth of the constituent fibres by repeated brittle failure (fracture opening) and sealing of the fracture. These veins display strong analogies with dilational hydroshear veins, a key indicator and well known geological record of ETS occurring at shallow depth within accretionary wedges in subduction channels. By using mineralogical assemblages, thermodynamic modeling and age dating, we constrained the formation of both veins and foliation to pressure and temperature conditions of ~1 GPa and 350 °C at c. 20 Ma. These results suggest depths of 30-40 km and cold geothermal gradients typical of subduction zones. We propose that episodic pulses of aqueous fluids released due to destabilization of hydrated minerals during subduction can be held responsible for the formation of the observed veins.
Concluding, we suggest that these diagnostic veins can be regarded as a powerful fingerprint of deep ETS occurring in subduction zones. Based on the widespread occurrence of these veins, we propose that deep ETS are indeed common at the scale of the entire Apennine orogen. Therefore, the results of this Action require some degree of reinterpretation of the seismotectonic role of several metamorphic units worldwide and, at the same time, assist in doing it.
Fieldwork aimed at collecting structurally controlled samples along a SW-NE transect through the Tuscan Metamorphic Units in the Northern Apennines. At Giglio Island, the section southwesternmost portion, field observations revealed the presence of a metamorphosed “broken formation” composed of boudinaged metaconglomerate and metarenite layers embedded in a metapelite matrix displaying a pervasive mylonitic foliation. In the Monticiano-Roccastrada Unit, the section northeasternmost termination, a mesoscopic compressional duplex deforming with a top-to-the-NE sense of shear metasandstone, metaconglomerate and minor metapelite was documented. In both study sites, dilational hydroshear veins were found to occur. Microstructural analyses of veins reveal that quartz is generally little deformed, except close to phyllosilicate layers or carpholite grains, where quartz grain size reduction occurs via subgrain rotation recrystallization together with phase mixing and pinning. Geometrical, cross-cutting and petrographic relations suggest that there occurred cyclic and repeated switches between brittle and viscous deformation conditions, with the veins forming broadly syn-mylonitic shearing. These cycles are related to the fluctuation of pore pressure, which repeatedly reached lithostatic values, triggered by metamorphic dehydration reactions. Dilational hydroshear veins developed in subducted metasediments are already known to be a key indicator of ETS at shallow depth along the subduction channel (a few km of depth, above the locked seismogenic zone). Remarkably, the presence of carpholite constrains the development of these newly documented structures to blueschist facies condition. Thermodynamic modeling constrains ~1 GPa and 350° C for the formation of both hydroshear veins and the mylonitic foliation. These structures can therefore be considered as a geological record of deep ETS occurring at >30 km of depth in the Apenninic subduction channel. Based on the widespread occurrence of dilational hydroshear veins diagnostic microstructure, we suggest that deep ETS are common at the scale of the entire Apennine orogen.
The results of the action were disseminated participating and presenting at six international conferences, three workshops and one seminar as invited speaker. The project aims and results were also disseminated and communicated by means of two websites:
Two scientific articles are currently at the second review stage, a third contribution is being currently finalized and two additional papers are being written. Additionally, I organised three scientific sessions and two short courses at EGU international conference and delivered a seminar for a master course at University of Bologna. I participated at two European Researchers' Night in Bologna, co-supervised a Bachelor student and collaborated with a PhD student of Prof Viola’s research group.
The results of the Action represent a major contribution to the state of the art, as we provide the first ever petrographic and microstructural characterization of dilational hydroshear veins formed in deeply subducted continental crust. Additionally, we could constrain for the first time the pressure and temperature conditions of dilational hydroshear vein formation by means of quantitative X-Ray mapping and local bulk composition determination linked to thermodynamic modelling. We also constrained the deformation mechanisms acting in these veins by electron backscattered diffraction analyses and the formation of those structure with Ar-Ar age dating. Therefore, we expanded the use of dilational hydroshear veins as a diagnostic microstructure for deep ETS occurring at depths of 30-40 km in the subduction channel.
Summarizing, the present Action achieved a better characterization of the metamorphic and microstructural geological record of deep ETS occurring in subduction zone below the locked seismogenic zone. This is needed in order to merge geological and geophysical information aiming at better characterising seismicity along the subduction channel. Additionally, this piece of information can be useful to better investigate the spatial role that is believed to exist between ETS and megathrust earthquakes.