Viscous Shear Zones Research Articles

How ductile are ductile shear zones?

Three premises of ductile deformation in the middle and lower crust are widely accepted: (1) rocks flow with power-law viscous rheology, (2) localization in ductile shear zones involves strain softening, and (3) fluid flows into and is channelized within ductile shear zones. Ductile (viscous) shear zones should therefore initially develop along planes of maximum shear stress, strain soften, and rotate as material planes into the field of progressive extension. However, the mean stress or pressure within a weak, elongate viscous band being stretched is higher than the surrounding matrix, and this is difficult to reconcile with premise (3). In contrast, Mohr-Coulomb brittle faults always have lower pressure within the fault zone. Flow of fluid and melt into high-temperature shear zones therefore implies that “ductile” shear zones are not perfectly viscous but have a pressure-dependent viscoplastic rheology. The continued pressure dependence may reflect significant microcracking on the grain scale even when localized deformation does not produce larger-scale discrete fractures.

Dynamics of Izmit Earthquake Postseismic Deformation and Loading of the Duzce Earthquake Hypocenter

We have developed dynamic finite-element models of Izmit earthquake postseismic deformation to evaluate whether this deformation is better explained by afterslip (via either velocity-strengthening frictional slip or linear viscous creep) or by distributed linear viscoelastic relaxation of the lower crust. We find that velocity-strengthening frictional afterslip driven by coseismic shear stress loading can reproduce time-dependent Global Positioning System data better than either linear viscous creep on a vertical shear zone below the rupture or lower crustal viscoelastic relaxation. Our best frictional afterslip model fits the main features of postseismic slip inversions, in particular, high slip patches at (and below) the hypocenter and on the western Karadere segment, and limited afterslip west of the Hersek Delta (Burgmann et al., 2002). The model requires a weakly velocity-strengthening fault, that is, either low effective normal stress in the slipping regions or a smaller value for the parameter describing rate-dependence of friction ( a - b ) than is indicated by laboratory experiments. Our best afterslip model suggests that the Coulomb stress at the Duzce hypocenter increased by 0.14 MPa (1.4 bars) during the Izmit earthquake (assuming right-lateral slip on a surface dipping 50° to the north), and by another 0.1 MPa during the 87 days between the Izmit and Duzce earthquakes. In the Marmara Sea region (within about 160 km of the Izmit earthquake rupture), this model indicates that the Coulomb stresses increased by 15%-25% of the coseismic amount during the first 300 days after the earthquake. Three hundred days after the earthquake, postseismic contributions to Coulomb stressing rate on the Maramara region faults had fallen to values equal to or less than the inferred secular stress accumulation rate. Our estimates of postseismic Coulomb stress are highly model dependent: in the Marmara region, the linear viscous shear zone and viscoelastic lower crust models predict greater postseismic Coulomb stresses than the frictional afterslip model. Near-field stress and fault-zone rheology estimates are sensitive to the Earth9s elastic structure. When a layered elastic structure is incorporated in our model, it yields a Coulomb stress of 0.24 MPa at the Duzce hypocenter, significantly more than the 0.14 MPa estimated from the uniform elastic model. Because of the higher near-field coseismic stresses, the layered elastic model requires a higher value of velocity-strengthening parameter ( A - B ) ([ a - b ] times effective normal stress r ′) to produce comparable postseismic slip. ( A - B ) is estimated at 0.4 and 0.2 MPa, respectively, for the layered and uniform elastic models. These results highlight the importance of understanding the Earth9s elastic structure and the mechanism for postseismic deformation if we wish to accurately model coseismic and postseismic crustal stresses.

Element mobility and volumetric strain in brittle and brittle–viscous shear zones of the superdeep well KTB (Germany)

Mass-balance studies of brittle and brittle–viscous shear zones of the superdeep well KTB (Germany) show that element mobility and associated volumetric strain is markedly different in metabasites and paragneisses. Shear zones in metabasites show thickening and volume increase due to mineralization of prehnite (±epidote±calcite) within open fractures and pore space. Gains in Al 2O 3, CaO and SiO 2 are compatible with these observations. Shear zones in paragneisses show either constant volume or volume loss. Volume loss of the paragneiss shear zones can be explained by pressure solution of quartz and by retrograde mica-forming reactions associated with significant gains in potassium-group elements (K, Rb, Ba) and losses in SiO 2. The differences in volumetric strain between paragneiss and metabasite shear zones can be explained by the different deformation mechanisms and rheology of paragneiss and metabasite at T=ca. 250–350°C. The rigid metabasites supported long-lived open fractures and pores where new minerals could precipitate from a fluid phase. The paragneisses, on the other hand, were weak because of increasing amounts of crystal plasticity and pressure solution of quartz which is typical for the brittle–viscous transition of quartz-bearing rocks. Open fractures and pores were rapidly closed hampering the deposition of new minerals. Graphite enrichment has been found in both paragneiss and metabasite shear zones. There is clear evidence that graphite enrichment results from fluid phases which carried C into the shear zones from external sources suggesting that C was largely mobile during the shearing processes.

P−T conditions during emplacement of the Bay of Islands ophiolite complex

A high-temperature contact is described between the basal pargasite-bearing spinel-lherzolites of the Bay of Islands ophiolite complex and underlying garnet-granulite facies metagabbros of its dynamothermal aureole. Three distinct high-temperature hydrous assemblages occur in the basal mylonites of the peridotite, and spinel- and garnet-bearing corona textures indicative of increase in pressure under constant or increasing temperature conditions are described for the first time from the uppermost part of the aureole. On the basis of garnet-clinopyroxene geothermometry and garnet-forming reactions in metabasic rocks, P−T conditions of 7–11 kbar, 750–850°C are estimated for rocks on both sides of the contact. Steep inverted gradients in both temperature and pressure of equilibration occur in the aureole, which most likely represents a thinned, overturned and metamorphosed section through an ophiolite sequence. It is proposed that the aureole formed in a low-angle shear zone cutting the oceanic crust and upper mantle. Age data shows that the Bay of Islands Complex was 30–40 Ma old and therefore relatively cold at the time of formation of the aureole. Prolonged (> 1 Ma) shear heating must therefore have occurred at high shear stresses and movement rates (≥ 1 kbar, 10 cm/yr) to produce the high contact temperatures. The displacement surface probably initiated as a discrete fault, evolving into a viscous shear zone with time. Downward movement of the locus of shearing into weaker lithologies and finally thrusting of the ophiolite-aureole complex over cold sediments accounts for the preservation of steep metamorphic gradients in the aureole. The observed pressures at the ophiolite-aureole contact are 3–7 kbar in excess of the expected load pressure from the present thickness of the ophiolite. The cause of the pressure excess was removed before formation of lower-grade parts of the aureole. Possible explanations are tectonic thinning of the ophiolite during displacement or more likely emplacement of nappes on top of the ophiolite before formation of the aureole. A model involving detachment of the ophiolite slice from below a subduction zone can account for the high pressures, rapid uplift and erosion during displacement, and the coincidence of K-Ar ages of amphiboles from the aureole and the sheeted dyke complex of the ophiolite.

Ductile shear zones: some aspects of constant slip velocity and constant shear stress models

Summary. The thermomechanical differential equations governing deformation in viscous shear zones have been solved for both constant velocity and constant stress boundary conditions. The solutions show that the inertial term in these equations can be neglected everywhere. The starting condition of the constant velocity model has been shown to be a constant velocity gradient and not a Heaviside function. The temperature anomaly produced by shear heating at the centre of the shear zone is shown to increase gradually and continuously with time, not reaching an asymptotic value. Conclusions for the constant velocity boundary condition are otherwise generally similar to those presented by Yuen et d. and agree with Fleitout & Froidevaux. The temperatures reached by constant velocity shears are sufficient for partial melting. Constant stress boundary condition shear zone models show an initially broad shear zone with uniform shear velocity gradient. Depending on the level of applied shear stress and ambient temperature, localized intense shear heating may develop followed by thermal runaway. At lower ambient temperatures relatively high stresses are required to produce thermal runaway. The broadening of the constant velocity shear zone proceeds more rapidly with increased ambient temperature. This can be used to show that shear zones broaden with depth. The merging of parallel shear zone pairs has been investigated and shear zones separated by distances of less than lOkm coalesce to form a single shear zone within 3 Myr. Only shear zones separated by 50 km or more remain distinct over periods of tens of millions of years.