查看更多>>摘要：This study integrates field, geochronological and geochemical data to constrain fluid circulation in the damage and core zone of the seismogenic Monte Morrone Fault System (MMFS), central Apennines (Italy). Faulting along the MMFS evolved from a diffuse deformation at the damage zone towards progressive localisation of a narrower fault core and, finally, to (re)activation of discrete slip surfaces at shallower crustal conditions. Multiple generations of carbonate mineralisations, including veins and slickfibers, occur along the main fault surfaces. Carbonate mineralisations are locally fractured and incorporated in the surrounding cataclasites, documenting repetitive structurally-controlled fluid infiltration during transient episodes of permeability creation and destruction. Stable carbon and oxygen isotopes of the carbonate mineralisations document a dominant meteoric water source probably mixed with deeper circulating waters having longer residence time. Clumped-isotope yield formation temperatures of vein and slickenfibers in the range between 23 and 40 degrees C. U-Th dating of carbonate mineralisations yield Middle Pleistocene ages (from 268 to 189 ka BP), with a 10-15-ka cyclicity that we link to the coseismic rejuvenation of the structural permeability in the fault zone. We propose that fault-related mineralisations recorded the interactions among tectonic deformation and climate during the Quaternary. Our study is the first documentation of fault-controlled recurrence intervals in fluid infiltration in a seismically active fault of central Apennines.
查看更多>>摘要：As for most upper mantle shear zones, mylonitic and ultramylonitic peridotites exposed in the northern Lanzo shear zone display thoroughly mixed, fine-grained (< 25 mu m) bands. The weak crystallographic preferred orientations (CPO) of all their phases are indicative for deformation by diffusion creep. Although interpreted as significantly decreasing the strength of the upper mantle if interconnected, the formation process of fine-grained phase mixtures is still debated. Microstructural analysis of gradually higher strained peridotites of the northern Lanzo shear zone revealed the formation of fine-grained (< 25 mu m) polymineralic tails adjacent to clino- and orthopyroxene porphyroclasts in mylonitic samples. Phase mixtures (av. 65% phase boundaries) are dominated by neoblasts derived from the porphyroclast (Opx/Cpx) and olivine, with minor abundances (< 10 area-%) of plagioclase, spinel +/- amphibole. Neoblast microstructures and mixing of phases directly at the border of the pyroxenes, indicate a reaction of pymxene porphyroclasts to neoblast phase mixtures. Neoblast temperature estimations (similar to 860 degrees C), their systematic change in composition, and plagioclase/spinel abundances suggest that continuous net-transfer reactions, enhanced by the spinel lherzolite to plagioclase lherzolite transition are likely the main driving forces for the formation of ultramylonitic phase mixtures in the Lanzo shear zone. These reactions were potentially enhanced by the presence of fluid as evidenced by pargasitic amphibole crystallization. Comparing these results to other upper mantle shear zones and deformed xenoliths, the importance of reactions (melt-rock, fluid-rock and/or metamorphic) for the formation of polymineralic, ultramylonitic bands is confirmed.
查看更多>>摘要：The depth extent of low velocity fault zones is still a matter of debate due to the lack of vertical resolution for most seismic methods. Autocorrelations and cross-correlations of seismic ambient noise provide us new constraints on this issue by obtaining the shallow subsurface reflections without the need of active sources or earthquakes. The horizontal-to-vertical spectral ratio (HVSR) method can also give constraints on this interface and does not need any extra cost of data collection. In this study, we used one-month continuous seismic data from a dense linear array deployed crossing the Chenghai Fault (CHF) in Yunnan, southwest China. Single station autocorrelation and adjacent station pair cross-correlation methods were used to retrieve body wave reflections from the interface of the shallow crustal low-velocity zone (LVZ). The HVSR method was also performed to delineate the shape of the LVZ. Results show an inverted trapezoidal LVZ which extends to similar to 1.0 km depth across the fault zone with a lateral extent of similar to 3 km. Previous studies based on ambient noise surface-wave tomography and teleseismic travel time analysis in the same area show similar characteristics, consistent with the interpretation of our results.
NSTL
Elsevier