Earthquake nucleation in the lower crust by local stress amplification

Deep intracontinental earthquakes are poorly understood, despite their potential to cause significant destruction. Although lower crustal strength is currently a topic of debate, dry lower continental crust may be strong under high-grade conditions. Such strength could enable earthquake slip at high differential stress within a predominantly viscous regime, but requires further documentation in nature. Here, we analyse geological observations of seismic structures in exhumed lower crustal rocks. A granulite facies shear zone network dissects an anorthosite intrusion in Lofoten, northern Norway, and separates relatively undeformed, microcracked blocks of anorthosite. In these blocks, pristine pseudotachylytes decorate fault sets that link adjacent or intersecting shear zones. These fossil seismogenic faults are rarely >15 m in length, yet record single-event displacements of tens of centimetres, a slip/length ratio that implies >1 GPa stress drops. These pseudotachylytes represent direct identification of earthquake nucleation as a transient consequence of ongoing, localised aseismic creep

Spatial correlation bias in late-Cenozoic erosion histories derived from thermochronology

International audienceThe potential link between erosion rates at the Earth's surface and changes in global climate has intrigued geoscientists for decades1,2 because such a coupling has implications for the influence of silicate weathering3,4 and organic-carbon burial5 on climate and for the role of Quaternary glaciations in landscape evolution1,6. A global increase in late-Cenozoic erosion rates in response to a cooling, more variable climate has been proposed on the basis of worldwide sedimentation rates7. Other studies have indicated, however, that global erosion rates may have remained steady, suggesting that the reported increases in sediment-accumulation rates are due to preservation biases, depositional hiatuses and varying measurement intervals8-10. More recently, a global compilation of thermochronology data has been used to infer a nearly twofold increase in the erosion rate in mountainous landscapes over late-Cenozoic times6. It has been contended that this result is free of the biases that affect sedimentary records11, although others have argued that it contains biases related to how thermochronological data are averaged12 and to erosion hiatuses in glaciated landscapes13. Here we investigate the 30 locations with reported accelerated erosion during the late Cenozoic6. Our analysis shows that in 23 of these locations, the reported increases are a result of a spatial correlation bias—that is, combining data with disparate exhumation histories, thereby converting spatial erosion-rate variations into temporal increases. In four locations, the increases can be explained by changes in tectonic boundary conditions. In three cases, climatically induced accelerations are recorded, driven by localized glacial valley incision. Our findings suggest that thermochronology data currently have insufficient resolution to assess whether late-Cenozoic climate change affected erosion rates on a global scale. We suggest that a synthesis of local findings that include location-specific information may help to further investigate drivers of global erosion rates

The control of precursor brittle fracture and fluid-rock interaction on the development of single and paired ductile shear zones

Ductile shear zones can occur as relatively isolated single structures, as arrays, or as characteristic paired zones. In continuous glaciated exposures of metagranodiorites from the Tauern window (Eastern Alps), the control of initial dilatant brittle fracture and associated fluid\u2013rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established. Shearing occurred under amphibolite facies conditions. Fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres. Many are healed joints without shear offset. Others show minor (mm\u2013cm), discrete dextral offset. Such brittle faults commonly display a low-angle en-\ue9chelon arrangement, with displacement transferred between discrete fracture segments by ductile compressive bridges. The geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid\u2013rock interaction, which is related to fluid infiltration and veining along the primary fractures. With little fluid\u2013rock interaction, reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre-existing fracture. With increased fluid\u2013rock interaction, a bleached halo is developed symmetrically to either side of a central epidote\u2013quartz (\ub1garnet\ub1calcite) vein. Ductile shear zones commonly flank this bleached zone, to develop a characteristic paired pattern. Strain is partitioned, localizing in the central fracture/vein and the flanking shear zones. Paired zones may anastomose in accordance with changes in the width of the central bleached zone, but are always symmetrically spaced with regard to the central fracture/vein. With increasing deformation, the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone, which remains preserved as a low strain region. Paired shear zones can also develop to either side of aplite dykes. Examples of characteristic paired shear zones, usually with a clear central vein, are found in many areas ranging from greenschist to eclogite facies, suggesting that the mechanism of their formation is quite general

Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions

In the Neves area, the pre-Alpine intrusive protolith of the Zentralgneise unit (Tauern window, Eastern Alps) is well preserved in a kilometric-scale low-strain domain without pervasive Alpine deformation. It is compositionally heterogeneous, consisting predominantly of granodiorites, with lesser leucocratic granites, and different generations of lamprophyres and aplites. The intrusive rocks are crosscut by fractures that were locally infiltrated by fluids and surrounded by alteration haloes. Incipient Alpine amphibolite facies ductile deformation is strongly localized on these precursor fractures and on lithological planar heterogeneities, resulting in the development of several different types of shear zones. Fractures without alteration haloes initially accommodate slip entirely on the fracture itself. With increasing deformation, a foliation is progressively developed in the adjacent host rock, eventually producing a single heterogeneous \u201cductile\u201d shear zone with the typical sigmoidal foliation pattern. Strong layers (aplite dykes and bleached alteration haloes developed to either side of precursor fractures) localize shear on their boundaries to produce characteristic paired shear zones. Shearing is more evenly distributed within weak layers (lamprophyres and quartz veins), with a marked discontinuity in shear strain against the adjacent, little deformed granodiorite. Shear zone development was accompanied by the formation of new fractures and quartz-rich veins in the host rock, which in turn also localized shear. Magmatic contacts, fractures and quartz veins are mostly steeply dipping and effectively span the complete range of strike orientations. The kinematics of the overprinting (strike-slip) shear zones was determined by the orientation of the initial discontinuities relative to the local principal compressive stress axis \u3c31 (here oriented ca. 345\ub0). Discontinuities of almost all orientations show shear reactivation, even in the case of very low resolved shear stress, indicating an overall viscous response of the system without a specific yield stress. The geometry and kinematics of the shear zone network suggest that the overall deformation in low-strain domains was close to coaxial. During deformation along the shear zone network, compatibility was maintained by fracturing (with the development of new quartz veins) and by distributed ductile deformation of the host rock, especially within contractional domains of the shear network and at shear zone intersections. Deformation never propagates into the undeformed homogeneous granodiorite as discrete ductile shear zones but is limited by the original extent of the precursor discontinuities. Shear zone development in intact rock is always preceded by fracturing, which localizes subsequent shear reactivation

Why calcite can be stronger than quartz

In the Neves area (Eastern Alps), calcite forms asymmetric centimeter-scale single-crystal porphyroclasts in quartz mylonites developed during hydrous amphibolite facies metamorphism at ~550\ub0C. Under these conditions, coarse calcite was clearly stronger than the surrounding polycrystalline, dynamically recrystallized, quartz matrix. Experimental results indicate that coarse calcite is less strain rate sensitive than wet quartzite, consistent with an inversion in strength on extrapolation to natural strain rates. For this to occur, wet quartzite must be weak, flowing at differential stress of <10 MPa. The lack of high-temperature twins (showing bulging or recrystallization) in calcite clasts is consistent with such low stresses during shear zone development under near peak metamorphic conditions. The maximum effective viscosity ratio of coarse calcite to quartzite for these conditions is probably not large (<10). However, numerical modeling shows that ratios of around 2 are sufficient to maintain near rigid calcite clast behavior for power law rheology with stress exponents appropriate to quartz (n ~ 3\u20134) and coarse calcite (n 65 6). The inversion in relative strength reflects the difference in influence of water on the crystal plastic flow of calcite and quartz: water has a dramatic effect for quartz but little or no effect for calcite. Quartz-rich rocks under hydrous amphibolite facies conditions in the middle to lower crust are therefore relatively weak (in fact, weaker than coarse calcite) and flow at much lower stresses than dry quartz-rich rocks at similar crustal levels

Correction to "Why calcite can be stronger than quartz"

In the paper \u201cWhy calcite can be stronger than quartz\u201d by Neil S. Mancktelow and Giorgio Pennacchioni (Journal of Geophysical Research, 115, B01402, doi:10.1029/2009JB006526, 2010), Figure 10 requires clarification of the parameters used in its calculation to avoid misinterpretation. Figure 10 presents the deformed shapes at shear strain \u3b3 = 6 of cylindrical inclusions with an initially circular cross section. The results, calculated by the first author using a personally developed FEM code, were used to demonstrate that for power law materials in simple shear flow, nearly rigid behavior of isolated inclusions is possible even when the effective viscosity ratio is low (~2). In particular, the example with a power law stress exponent of n = 6 in the inclusion and n = 3 in the matrix was considered to be directly relevant to natural examples of coarse calcite clasts in quartz mylonites from the Neves area of the eastern Alps. This fundamental conclusion is correct, as are the calculated shapes for the parameters employed. However, with the aim of remaining concise, details of these parameters were not given: it was simply stated in the caption that \u201cthe effective viscosity ratio (\u3bci/\u3bcm),\u201d as listed on the left, was \u201cfor the case of equal strain rate in inclusion and matrix.\u201d This statement requires clarification because (1) for power law viscous material there is no specific material parameter \u201cviscosity\u201d (or \u201cviscosity ratio\u201d) independent of strain rate, as there is for linear viscous behavior, (2) the strain rate in the inclusion and matrix will not be the same, even at the very start of a numerical experiment, and (3) the strain rate in the inclusion will vary with its axial ratio and orientation

Strain-insensitive preferred orientation of porphyroclasts in Mont Mary mylonites

The shape preferred orientation (SPO) of porphyroclasts was determined in high temperature mylonites. The porphyroclasts approach rhomboidal (sillimanite) or elliptical (garnet, plagioclase, sillimanite) shapes, and exhibit aspect ratios (R) as high as 11. Particles with R>3 are dominantly rhomboidal. The long axis of the best-fit ellipse defines a very strong SPO inclined at 5\u201310\ub0 to the mylonitic foliation, in an antithetic sense with respect to the shear direction. This angle is independent of R. The inclination of the long sides of rhomboidal sillimanite increases from 10 to 20\ub0 with decreasing R. In contrast, the short sides have a constant orientation of 15 to 17\ub0 irrespective of R and are parallel to extensional crenulation cleavage. Low aspect ratio (mainly elliptical) objects show low intensity SPO close to the shear plane. The two SPOs appear strain-insensitive. In the case of R3, a stable position is acquired. This is not explained by any of the current theoretical and experimental models of SPO