Fault Zone Deformation Research Articles

Deformation mechanisms accommodating faulting of quartzite under upper crustal conditions

Analysis of the deformation microstructures associated with a high-level fault in quartzite (Skiag Bridge, Assynt, NW Scotland) reveals a complex variation in the deformation mechanisms active during faulting. The different mechanisms have been identified using an integrated study involving optical, cathodoluminescence and electron (both SEM and TEM) microscopy. The specific mechanisms identified include: intragranular cleavage fracture (types 1, 2 and 3), brittle intergranular fracture (types 1 and 2), low-temperature ductile fracture, diffusion mass transfer and low-temperature crystal plasticity. Fracturing dominates the deformation (faulting), initially via intragranular extension cleavage fractures due to stress concentrations at grain contacts (although many of these may be healed by quasi-simultaneous diffusive mass transfer processes). These eventually link and are then exploited as shear fractures, leading to the development of microbreccia-cataclasite zones which define a three-dimensional fracture array. Quasi-simultaneous diffusive mass transfer processes may heal these through-going fractures. Continued fault zone deformation involves the development of a damage (‘wake’) zone along the displacement zone borders where low-temperature plasticity and subsequent low-temperature ductile fracture processes aid the expansion of the fault zone. This study emphasizes that the evolution of the Skiag Bridge fault zone has involved three main categories of deformation mechanisms: fracture, crystal plasticity and diffusion mass transfer. The interrelationship between these categories, and the transition between individual fracture mechanisms, are significant aspects of this evolution. The examples presented demonstrate the complex interrelationships which exist between a group of deformation mechanisms and emphasize the potential importance of low-temperature plasticity and low-temperature ductile fracture processes during faulting under upper crustal conditions.

Geometry and kinematics of recumbent folding and low-angle detachment in the Pardailhan nappe (Montagne Noire, Southern French Massif Central)

The Pardailhan nappe is the uppermost unit of the superimposed Variscan recumbent fold nappes in the Southern Montagne Noire thrust belt. Detailed mapping and structural investigation demonstrates the presence of a welldevel-oped upper normal limb, a frontal ENE-WSW oriented major hinge and a predominant overturned limb many kilometres in size in lower Palaeozoic non-metamorphic sediments. Finite strain and microstructures related to recumbent folding show maximum elongation and stretching perpendicular to the fold axes consistent with tangential shortening and general south-southwestward overthrust tectonics. Minor internal deformation throughout the fold structure suggests a rigid overturning mechanism and only small amounts of progressive shear during the Variscan recumbent folding (late Early Carboniferous). Post-shortening deformation in the nappe is accounted for by subsequent heterogeneous deformation in extensional shear and fault zones. The earlier folded and schistose structures were displaced along E-W trending low-angle normal faults. Microstructures of the superimposed brittle and ductile deformation in gently S-dipping detachment zones indicate southwest to southward hanging wall movements in the uppermost units. Throughout the lower limb, the extensional features show a progressive rotation into a W-SW direction and a dextral wrench component along the tectonic and metamorphic discontinuity with underlying metamorphic polyphase fold structures. Mechanisms and timing of recumbent fold development and subsequent detachment tectonics are discussed in relation to Variscan orogenic convergence and late orogenic (Late Carboniferous) extension induced by the uplift of metamorphic basement. The compressional and subsequent extensional tectonics in the Southern Montagne Noire nappe belt are associated with the evolution of a late Variscan metamorphic core complex.

Estimates of the rates of microstructural changes in mylonites

Abstract Microstructures developed during dislocation creep such as the dynamically recrystallized grain-size and the subgrain-size are modified in response to increasing differential stresses and shear-strain rates adjacent to rigid minerals in mylonites. In two specimens variations in quartz subgrain-size around rigid minerals has been quantified from orientation contrast SEM images. In a deformed granite from the Pogallo ductile fault zone, quartz two-dimensional subgrain-size is reduced from background values of about 30 μm to 8 μm and then 4 μm in progressively narrower channels between rigid feldspars. In pelitic mylonites from the Alpine fault zone, New Zealand, quartz bands are wrapped around rigid garnets and the subgrain-size is reduced from background values of about 20 μm to 14 μm within 2000 μm of the garnet to 11 μm adjacent to the garnet. In both specimens examined the microstructure responds to both increases and decreases in differential stresses and coupled strain-rates. The analysis of the distribution of subgrain sizes provides information for the assessment of microstructural stability and the estimation of the rates of microstructural changes. The New Zealand specimens have good regional shear strain-rate controls and minimum subgrain boundary migration rates (during reduction in differential stresses and strain-rates) of 1.2 × 10 −9 μm s −1 to 1.2 × 10 −11 μm s −1 are estimated, for bulk shear strain rates of 10 −10 s −1 and 10 12 s −1 respectively. Palaeopiezometry coupled with rheological considerations suggests that these microstructural changes correspond to minimum differential stress-rates of 5 × 10 −9 MPa s −1 to 5 × 10 −11 MPa s −1 . Similar or slower regional stress-rates during fault zone evolution, will allow continuous re-equilibration of the microstructure. Thus the time at which any given microstructure is frozen in will be critically dependent upon fault zone deformation history. In the Alpine fault zone rapid uplift and high near surface temperatures combine to generate rapid stress drops as mylonitic deformation gives way to cataclastic faulting during uplift. Mylonite microstructure is frozen in during this rapid stress drop and represents the shallowest level of major crystal-plastic deformation.

Surface slip subsequent to the 24 November 1987 Superstition Hills, California, earthquake monitored by digital creepmeters

Three creepmeters of new design were operated on the Superstition Hills fault following the 24 November 1987 earthquake. Each creepmeter consists of a digital displacement sensor interrogated periodically by a computer. At the end of each hour, the computer determines a recording rate appropriate to the velocity of the slip on the fault in that hour. No data are recorded if the fault is inactive. Data are recorded at a maximum rate of one sample per minute if slip on the fault is detected. The resolution and range of the creepmeters are 10 μm and 15 cm, respectively. Fault activity is observed to be confined to periods of episodic slip (creep events) with mean amplitudes 3 to 6 mm and durations less than 4 hr. A total of 80 creep events were recorded. During the first week following the earthquake, creep events occurred several times each day. After 6 months, events occurred every 3 weeks. This increase in the mean time interval between creep events results in the observed decay in afterslip rate. Between creep events, the fault is inactive although more than 200 microstrain of fault-zone deformation may accumulate before each event. Although creep events were observed to propagate unilaterally in both directions along the fault, the most impulsive events appeared to propagate bilaterally from a dilatant offset separating the northern and southern halves of the 24-km-long fault zone. Maximum propagation velocities of 30 m/sec were observed. Aftershocks (2 < M < 4.7) were observed to occur near the creepmeters following 6 to 9 cm of surface slip on the fault. A future aftershock in this region is anticipated in mid-1989 based on afterslip rates in the year following the main shock.

Earthquakes and Rock Deformation in Crustal Fault Zones

The physical origin of earthquakes lies ultimately in the geological structure of fault zones and the deformation processes that occur therein in response to tectonic stress. Although the possibility now exists that the nucleation regions for large earthquakes at depths of,..., 10 km may shortly become directly accessible by deep drilling, our present knowledge of fault structure and the shallow earthquake source is derived largely from a variety of indirect sources. These include seismological studies, surface studies of fault zones and earthquake ruptures, geodetic information on modes of fault slip, ·geophysical constraints on fault zone structure and rheology, and information garnered from materials science and experi­ mental rock deformation. Studies of fault zone structure and the rock products of faulting provide complementary information on deformation processes at depth in fault zones. Although descriptions of fault rocks are widespread in the geological literature (e.g. Spry 1969, Higgins 1971), it is only in the past decade that they have begun to be interpreted in the context of the physical conditions and processes prevalent in seismically active fault zones at different crustal depths (Sibs on 1977, Watts & Williams 1979, Anderson et a11983, Wise et aI 1984). Such interpretations are still at an early stage, but the deformation textures and structural associations of fault rocks have already been shown to have the potential to yield information on such diverse topics as shear stress levels, power dissipation, and seismic efficiency during earthquake faulting; on fluid pressure levels and episodic fluid flow accompanying