1,721,245 research outputs found
Fault plane processes and mesoscopic structure of a strong-type seismogenic fault in tonalites (Adamello batholith, Southern Alps)
No full textThe Gole Larghe Fault is an exhumed paleoseismic fault crosscutting the Adamello tonalites (Italian Southern Alps). Ambient conditions of faulting were 9-11 km in depth and 250-300 degrees C. In the study area the fault accommodates similar to 1100 in of dextral strike-slip over a fault thickness of 550 m. Displacement is partitioned into three hierarchically different sets of discrete subparallel cataclastic horizons (faults (1-2-3)). Fault displacement is in the range of few centimeters (faults(3)) to a maximum of a few tens of meters in major faults (1). Faults(1-2) nucleated on pre-existing joints, whereas faults(3) are newly generated fractures produced during slip along faults(1-2). Each fault within the Gole Larghe Fault records the same evolution with development of indurated cataclasites precursory to pseudotachylyte production. Pseudotachylytes are usually generated at the host rock/ cataclasite boundary and within cataclasites the mean clast size decreases getting closer to pseudotachylyte fault veins. Pseudotachylytes and cataclasites have a similar chemical composition which is enriched in Loss On Ignition, K, Rb, Ba, U and Fe(3+) compared to host rock.
We envision two models for the evolution of the Gole Larghe Fault. In both models synkinematic fluid-rock interaction along a fault causes fault hardening by precipitation of abundant K-feldspar+epidote (and minor chlorite) in the cataclasite matrix conducive to final production of pseudotachylyte. In the first model, induration occurs progressively by differential precipitation related to fabric evolution in cataclasites. In the second model, induration occurs abruptly dependent on the development of full connectivity within the fault network and to fluid reservoir. Whatever the model, the Gole Larghe Fault represents a strong fault, where hardening processes resulted in a low displacement/fault thickness ratio and contrast with many mature weak faults where localized repeated seismic slip along the same weak horizons yields high displacement/fault thickness ratios. (c) 2005 Elsevier B.V All rights reserved
Superheated friction-induced melts in zoned pseudotachylytes within the Adamello tonalites (Italian Southern Alps)
No full textPseudotachylytes and cataclasites are present along a strike-slip fault zone in the tonalites of the Adamello intrusion (Italian Southern Alps). Ambient conditions during faulting were 0.25-0.3 GPa and 250-300degreesC. Pseudotachylyte veins thicker than 6 mm are zoned and consist of two symmetric microlitic domains towards the vein walls and a central spherulitic domain. The thickness of the microlitic domains (x) increases linearly with the half total thickness (a) of the pseudotachylyte vein (commonly < 2 cm) according to the relation: x = (0.29 +/- 0.12)a. The spherulitic and microlitic domains have similar chemical composition but the microlitic domain has a lower amount of plagioclase clasts. A numerical model explains zoning as the result of the different cooling rates and clast/melt interactions at the center and periphery of thick veins. Zoning is compatible with the injection of a single pulse of superheated friction-induced melt (T-melt congruent to 1450degreesC). Melt temperatures estimated by the clast/matrix ratio (O'Hara, 2001) are considerably lower (316-577degreesC). It is suggested that the difference in the temperature estimates reflects a more complex slip history than a single seismic slip event along the fault during production of cataclasites/pseudotachylytes. (C) 2004 Elsevier Ltd. All rights reserved
Amphibolite-facies pseudotachylytes in Premosello metagabbro and felsic mylonites (Ivrea Zone, Italy)
No full textPseudotachylytes (solidified friction-induced melts produced during seismic slip) that formed under high
temperature ambient conditions might retain information on earthquake mechanics beneath the brittle–
ductile (elasto-frictional/viscous-plastic) transition in the crust. Thin, sharply bounded pseudotachylytes
and ultramylonites occur together inside the granulite-facies Premosello metagabbro and inside country
felsic mylonites of the Ivrea Zone (Italian Southern Alps). Petrographic and microstructural data indicate
that the studied pseudotachylytes and ultramylonites are coeval and developed under amphibolite facies
(500–650 °C) metamorphic conditions. The occurrence of foliated pseudotachylytes and of local remnants
of cataclastic microstructures within ultramylonites suggests that pseudotachylytes might have represented
planar heterogeneities for nucleation of ultramylonites. The association of coeval pseudotachylyte and
mylonites is interpreted as caused by cyclic stages of coseismic high strain rate transients, related to downward
propagation of seismic ruptures from the upper brittle crust, followed by post-seismic stress relaxation
by ductile flow
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