Neil S. Mancktelow

Nonlithostatic pressure during sediment subduction and the development and exhumation of high pressure metamorphic rocks

A subduction shear zone can be modeled as a long narrow channel, with the flow of subducted metasedimentary rocks in the channel driven by two sets of forces: the downward shearing force exerted by the subducting slab and the gradient in the hydraulic potential, which combines the effect of both pressure and buoyancy. If the channel walls are effectively rigid, very slight narrowing or broadening of the channel (convergence angles 2 GPa in the channel at only 40 km depth. The model is consistent with a horizontal balance of forces across the plates and with a reasonable value for the thickness of subducted sediment (∼650 m). The practical limit for overpressures attainable in subduction zones is determined by the strength and permeability of the channel walls. At 40 km depth the channel is effectively confined on both sides by cold lithospheric mantle, which should be strong enough to support a significant tectonic overpressure. Episodic failure of the upper plate to produce great earthquakes at 30–40 km focal depth could vent overpressured fluid from the channel, allowing a cyclical buildup and release of both rock and fluid pressure. Topography on the subducting plate (e.g., seamounts and thinned continental crust) may lead to an anvil-like jamming of the channel and local high overpressures. Tectonic erosion by topography on the lower plate of slivers from overlying continental crust and the compression of these slivers between the topography and the narrowing channel walls could produce high overpressures in continental rocks. A decrease in the convergence rate or cessation of subduction, with a consequent general warming within the channel and associated viscosity decrease, promotes exhumation by buoyant reverse flow. The most rapid reverse flow occurs in the region of previously greatest overpressure. Since the exhumation distance is shorter than for a simple lithostatic pressure distribution and any increase in temperature is coupled with a strong increase in the rate of exhumation, preservation of high-pressure assemblages at the surface in fossil subduction zones is promoted for such a model.

Rb–Sr microchrons of synkinematic mica in mylonites: an example from the DAV fault of the Eastern Alps

Abstract A technique for texturally controlled microsampling from rock thick sections and subsequent Rb–Sr dating of μg-sized samples has been developed. It allows direct dating of minerals unequivocally related to deformation. The technique has been applied to strictly synkinematically grown white mica prepared from mylonites related to the Defereggen–Antholz–Vals (DAV) fault south of the Tauern window (Eastern Alps). Control of deformation ages is provided by field relationships with independently dated plutons and dykes dated here by single mineral Rb–Sr. The field observations and isotopic ages are fully consistent and demonstrate that this is a reliable method for sub-mm-scale geochronology. The ages obtained establish that sinistral strike slip shearing associated with the DAV fault lasted from at least 33 Ma to 30 Ma, followed by an abrupt change to dextral transpression related to the adjacent Periadriatic fault.

Ultrafine-grained quartz mylonites from high-grade shear zones: Evidence for strong dry middle to lower crust

Creep strength of the crust depends upon the rheology of the most common mineral, usually quartz. Recrystallized quartz grains in many high-grade shear zones from the middle to lower crust are typically large (millimeter sized), implying active grain boundary migration, but equivalents from old polymetamorphic and water-deficient basement sheared at similar crustal depths can be very small. For the latter, strong crystallographic preferred orientation (CPO) of quartz, with c axes aligned close to Y, progressive misorientation of crystals, subgrain and dislocation development, and core-mantle structures with recrystallized grains of 2‐8 m size, all point to dislocation glide dominantly on the prism a system with recrystallization by subgrain rotation. Recrystallized grain size piezometry of such quartz indicates high flow stress in fine-grained shear zones, while the synkinematic metamorphic mineral assemblage and the CPO are typical of amphibolite facies conditions. This is evidence that middle to lower crust is not inevitably weak due to its high temperature: water content also has an important influence.

Two‐ and three‐dimensional thermal modeling of a low‐angle detachment: Exhumation history of the Simplon Fault Zone, central Alps

[1]xa0Two alternative models have been proposed to explain footwall exhumation along major low-angle detachments: (1) crustal-scale exhumation along a detachment fault that maintained a low dip angle or (2) exhumation along a high-angle fault passively rotated by isostatic rebound (“rolling hinge model”). These proposed models were tested against a well-documented example of a low-angle detachment fault in the European central Alps, the Simplon Fault Zone (SFZ). An extensive thermochronological data set provides the basis for 2- and 3-D thermokinematic models (Pecube), coupled with a stochastic inversion algorithm (the Neighbourhood Algorithm). Model results establish that the thermochronological pattern is better reproduced by a low-angle detachment that maintained a 30° dip, rather than by a rolling hinge model. Although a range of histories involving either steady state or variable exhumation rates is possible, the preferred model of highest probability is for a variable rate, with the fault zone initiated at 18.5 ± 2.5 Ma and active until the present day. Footwall exhumation was relatively fast until 14.5 ± 1.5 Ma (∼1.4 mm yr−1). This enhanced SFZ footwall exhumation is similar in timing and kinematics to orogen-parallel extension reported throughout the Alpine orogen. After 14.5 Ma, SFZ footwall exhumation continued at a reduced rate (∼0.7 mm yr−1) until 4 Ma. The subsequent increase (to ∼1 mm yr−1) reflects enhanced regional erosion rates across both footwall and hanging wall after circa 4 Ma (from 0.35 ± 0.15 mm yr−1 to 0.70 ± 0.15 mm yr−1), probably in response to climate changes during the Pliocene.

Ultrahigh-resolution seismic reflection imaging of the Alpine Fault, New Zealand

[1]xa0High-resolution seismic reflection surveys across active fault zones are capable of supplying key structural information required for assessments of seismic hazard and risk. We have recorded a 360 m long ultrahigh-resolution seismic reflection profile across the Alpine Fault in New Zealand. The Alpine Fault, a continental transform that juxtaposes major tectonic plates, is capable of generating large (M > 7.8) damaging earthquakes. Our seismic profile across a northern section of the fault targets fault zone structures in Holocene to late Pleistocene sediments and underlying Triassic and Paleozoic basement units from 3.5 to 150 m depth. Since ultrashallow seismic data are strongly influenced by near-surface heterogeneity and source-generated noise, an innovative processing sequence and nonstandard processing parameters are required to produce detailed information on the complex alluvial, glaciofluvial and glaciolacustrine sediments and shallow to steep dipping fault-related features. We present high-quality images of structures and deformation within the fault zone that extend and complement interpretations based on shallow paleoseismic and ground-penetrating radar studies. Our images demonstrate that the Alpine Fault dips 75°–80° to the southeast through the Quaternary sediments, and there is evidence that it continues to dip steeply between the shallow basement units. We interpret characteristic curved basement surfaces on either side of the Alpine Fault and deformation in the footwall as consequences of normal drag generated by the reverse-slip components of displacement on the fault. The fault dip and apparent ∼35 m vertical offset of the late Pleistocene erosional basement surface across the Alpine Fault yield a provisional dip-slip rate of 2.0 ± 0.6 mm/yr. The more significant dextral-slip rate cannot be determined from our seismic profile.

Why calcite can be stronger than quartz

[1]xa0In the Neves area (Eastern Alps), calcite forms asymmetric centimeter-scale single-crystal porphyroclasts in quartz mylonites developed during hydrous amphibolite facies metamorphism at ∼550°C. 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 ∼ 3–4) and coarse calcite (n ≥ 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.