Initiation and growth of strike-slip faults within intact metagranitoid (Neves area, eastern Alps, Italy)

Abstract

Exhumed strike-slip faults exposed in the Neves area of the Tauern Window (eastern Alps, Italy) formed in the lower brittle crust under hydrous conditions within intact metagranitoids, where any precursor fractures were already healed due to earlier amphibolite-facies metamorphism. Faults initiated as newly formed en-echelon fractures delineating shear bands, with segmentation occurring over scales of 0.001 to 100 m. Due to the initial en-echelon pattern, stepovers between fault segments were almost invariably contractional during subsequent slip accumulation. In initial low-slip (centimeter to decimeter) stages, synthetic slip on the segments was associated with the development of a set of antithetic faults in the contractional stepovers, oriented at an angle of 30 –45 to the bounding faults. Slip on this antithetic set did not fully compensate for the decrease in slip on the main faults, implying an additional deformation mechanism within the stepovers (e.g., block rotation and/or out-of-plane movement). In larger faults with slip on the order of a few meters, the antithetic faults within the stepover were crosscut by synthetic sigmoidal faults that connected the overstepped fault segments and accommodated most of the subsequent displacement transfer. This second stage of evolution involving fault linkage is well documented in the Mesule fault, which has a current maximum offset of ~10 m. In the Mesule fault, the total horizontal slip summed along all secondary faults within the stepover accounts almost entirely for the net slip decrease toward the tips of the overstepping faults. However, the boundary faults remain nearly straight in spite of the sigmoidal ramp-like geometry of the connecting faults. Since the adjacent blocks are little deformed and there is no evidence for appreciable volume loss or block rotation in the stepover, signifi cant out-of- plane movement is implied, although it is difficult to quantify. The Neves area provides unusually detailed field constraints on fault initiation, linkage, and displacement accumulation within nonbedded and relatively isotropic granitoid rocks at the base of the brittle crust, where neither a free upper surface nor substantial volume change (e.g., by veining and pressure solution) was a controlling factor in accommodating fault linkage and displacement transfer

Weak and Slow, Strong and Fast: How Shear Zones Evolve in a Dry Continental Crust (Musgrave Ranges, Central Australia)

“…Outside the high strain zones, two parallel fractures are observed, of which only one shows ductile reactivation (Figure c). The unsheared fracture is interpreted as an incipient shear fracture (Lockwood & Moore, ; Pennacchioni & Mancktelow, ; Pennacchioni & Zucchi, ), and because the resolved shear stress on these parallel features is presumed to be the same, the favored interpretation is that fracturing was progressive in time and that the unsheared fractures developed after ductile shearing.…”

Comparison of thermal modeling, microstructural analysis, and Ti‐in‐quartz thermobarometry to constrain the thermal history of a cooling pluton during deformation in the Mount Abbot Quadrangle, CA

“…First analyzed in the Mount Abbot Quadrangle (central Sierra Nevada, CA) [ Lockwood and Lydon , ; Segall and Pollard , ], this interplay of brittle and ductile deformation has been recognized in a number of exhumed granitoid bodies, including the Adamello Batholith [southern Alps; e.g., Pennacchioni , ], the Neves region of the Tauern Window [eastern Alps; Mancktelow and Pennacchioni , ; Pennacchioni and Mancktelow , ], the Mont Blanc Massif [western Alps; e.g., Guermani and Pennacchioni , ], the Gran Paradiso nappe [northwestern Alps; Menegon and Pennacchioni , ], the Roses granodiorite [northeast Spain; Segall and Simpson , ], and the Mooshla pluton [Abitibi Belt, Canada; e.g., Tourigny and Tremblay , ]. The presence of pseudotachylyte associated with faults in these settings [e.g., Di Toro and Pennacchioni , ; Griffith et al ., ; Pennacchioni and Mancktelow , ] suggests seismic slip occurred, possibly producing earthquakes of magnitude 5.7 or greater [ Kirkpatrick et al ., ]. Thus, investigating structural features in these settings benefits not only basic knowledge of how plutons accommodate deformation during cooling, but also assessment of seismic hazard associated with faults at evolving plate boundaries (e.g., magmatic arcs, suture zones).…”

Testing constitutive equations for brittle‐ductile deformation associated with faulting in granitic rock

“…Conversely, contractional right steps (Figure ) often contain a locally strong ductile fabric strictly confined to the area between the step‐bounding faults. Such localized mylonitization within contractional fault steps also is observed in amphibolite grade metagranitoids of the Neves region in the Tauern Window [ Pennacchioni and Mancktelow , ; Pennacchioni and Mancktelow , ] and the Lobbia region in the southern Italian alps [ Pennacchioni , ], suggesting that this may be a common structural feature for faults in the mid‐ to lower‐lithosphere.…”