Cockade-bearing breccias, cataclasites and gouges in a single fault zone: Microstructures and geochemisty

2020 ◽  
Author(s):  
Alfons Berger ◽  
Marco Herwegh
Keyword(s):  
Mechanical Behavior ◽  
Fault Zone ◽  
Hydrothermal Systems ◽  
Permeability Evolution ◽  
Dynamic Changes ◽  
Fault Rocks ◽  
Local Permeability ◽  
Major Fault ◽  
Deformation Event ◽  
Fault Gouges

<p>The seismic-interseismic cycle strongly relates to the interplay between dilation owing to fracturing and frictional granular flow on one hand side and hydrothermal cementation processes on the other side. This study investigates different fault rocks of a crustal-scale fault zone in the Central Alps (Switzerland). We combine microstructural with geochemical approaches to decipher the interaction of grain size reduction via frictional processes with precipitation and resulting particle size increases. The three major fault rocks, i.e. (1) cockade-bearing breccias, (2) cataclasites, and (3) fault gouges, differ in their microstructure. The chemical data clearly demonstrate a decreasing gain of volume along this group of tectonites. Their different precipitation volumes most likely relate to dynamic changed of the local permeability of these rocks. The fluid pathways control the precipitation at different localities and times, which affect the healing of these fault rocks inducing a gain in rock strength. During the next deformation event, the extent of healing therefore directly controls the mechanical behavior of the rock. The estimated volume gain (~+110%) in cockade-bearing breccias is consistent with the seismic dilatant behavior of these frictional rocks as already proposed from other arguments (Berger and Herwegh 2019). This is in contrast to the fault-gouges with only minor gains in volume and mass resulting in a predominantly non-cohesive deformation style. This example indicates that permeability evolution (and related hydrothermal processes) strongly influences the mechanical behavior of such faults. This shows the highly dynamic behavior with time in long-lived fault systems. These dynamic changes in precipitation and resulting different strengths occur at different timescales from minutes (seismic events) to thousands of years.</p><p>Ref.: Berger, A., Herwegh, M., 2019. Cockade structures as a paleo-earthquake proxy in upper crustal hydrothermal systems. Nature Scientific Reports, 9, 9209.</p>

Across-strike geometry of the Grand Pabos fault zone: evidence for Devonian dextral transpression in the Quebec Appalachians

1993 ◽  
Vol 30 (7) ◽  
pp. 1363-1373 ◽  
Author(s):  
Donna Kirkwood ◽  
Michel Malo
Keyword(s):  
Fault Zone ◽  
High Strain ◽  
Shear Bands ◽  
Ductile Deformation ◽  
Brittle Deformation ◽  
Fault Rocks ◽  
Deformation Structures ◽  
Riedel Shear ◽  
Deformation Features ◽  
Major Fault

The principal faults of southeastern Gaspé Peninsula in Quebec consist of a central high-strain zone that is characterized by mainly ductile deformation structures and bordered by low-strain zones each dominated by brittle deformation structures. The overall geometry of shear fractures within the low-strain zones is quite similar to the expected geometry of Riedel shear fractures. The brittle structures overprint the dominant C–S-type fabric of the high-strain zone, which implies that brittle deformation outlasted ductile deformation. The asymmetry of local micro- to meso-scale deformation features along the fault zones reflects the non-coaxiality of the shear. Other features described within the fault zone (stylolitic cleavage, shear bands, and reverse faults) are evidence for a component of shortening perpendicular or oblique to the fault zone. The geometry of the Grand Pabos fault zone (GPFZ), a major fault of southern Gaspé, indicates that deeper seated fault rocks (high-strain zone) have been brought up to higher crustal levels and are presently in contact with brittlely deformed fault rocks (low-strain zone). The proposed model for the evolution of the GPFZ involves Early to Late Devonian, dextral, transcurrent movement accompanied by relatively minor amounts of vertical slip within a dextral transpressive regime. The main pulse of the Acadian orogeny in Gaspé is restricted to the Devonian and therefore occurred later than elsewhere in the Canadian Appalachians.

DETAIL GRAVITY PROFILE ACROSS SAN ANDREAS FAULT ZONE

Geophysics ◽  
1967 ◽  
Vol 32 (2) ◽  
pp. 297-301 ◽  
Author(s):  
S. N. Domenico
Keyword(s):  
Fault Zone ◽  
Mojave Desert ◽  
San Andreas Fault ◽  
Surface Expression ◽  
Fault Zones ◽  
Rock Masses ◽  
San Andreas ◽  
Major Fault ◽  
Reduced Density ◽  
Gravity Profile

A gravity profile was obtained from closely spaced readings along a traverse approximately nine miles in length across the San Andreas fault zone immediately south of Palmdale, California in the western Mojave Desert. Corrected gravity values show a slight but distinctive minimum associated with the fault zone which may be attributed to the reduced density of the shattered rock masses in the fault zone. The existence of this minimum suggests that major fault zones may be traced across terrain, on which surface expression of the fault does not exist, by successive profiles across the suspected position of the fault zone.

scholarly journals Stress fields of ancient seismicity recorded in the dynamic geometry of pseudotachylyte in the Outer Hebrides Fault Zone, UK

2020 ◽  
Vol 178 (1) ◽  
pp. jgs2020-101
Author(s):  
L.R. Campbell ◽  
G.E. Lloyd ◽  
R.J. Phillips ◽  
R.C. Walcott ◽  
R.E. Holdsworth
Keyword(s):  
Fault Zone ◽  
Dynamic Geometry ◽  
Shear Zones ◽  
Stress Fields ◽  
Tectonic Activity ◽  
Sensitivity Testing ◽  
Fault Rocks ◽  
Outer Hebrides ◽  
Supplementary Material

Heterogeneous sequences of exhumed fault rocks preserve a record of the long-term evolution of fault strength and deformation behaviour during prolonged tectonic activity. Along the Outer Hebrides Fault Zone (OHFZ) in NW Scotland, numerous pseudotachylytes record palaeoseismic slip events within sequences of mylonites, cataclasites and phyllonites. To date, the kinematics and controls on seismicity within the long active history of the OHFZ have been poorly constrained. Additional uncertainties over the relative location of a meteorite impact and possible pre-OHFZ brittle faulting also complicate interpretation of the diffuse seismic record. We present kinematic analyses of seismicity in the OHFZ, combining observations of offset markers, en echelon injection veins and injection vein geometry to reconstruct slip directions and stress fields. This new dataset indicates that a range of fault orientations, slip directions and slip senses hosted seismicity in the OHFZ. Such complexity requires several stress field orientations, in contrast with the NW–SE Caledonian compression traditionally attributed to frictional melting along the OHFZ, indicating that seismicity had a long-term presence across the fault zone. Persistence of strong frictional failure alongside the simultaneous development of weak fault rocks and phyllonitic shear zones in parts of the OHFZ has significant implications for understanding seismic hazard along mature continental faults.Supplementary material: Tables listing analysed orientation measurements plus further information and sensitivity testing of palaeostress analysis parameters are available at https://doi.org/10.6084/m9.figshare.c.5134797

scholarly journals The Hydrothermal Breccia of Berglia-Glassberget, Trøndelag, Norway: Snapshot of a Triassic Earthquake

Minerals ◽  
2018 ◽  
Vol 8 (5) ◽  
pp. 175 ◽  
Author(s):  
Axel Müller ◽  
Morgan Ganerød ◽  
Michael Wiedenbeck ◽  
Skule Svendsen Spjelkavik ◽  
Rune Selbekk
Keyword(s):  
Fluid Inclusion ◽  
Fault Zone ◽  
Open Space ◽  
Single Pulse ◽  
Structural Features ◽  
Late Triassic ◽  
Fault Movement ◽  
Trace Element Chemistry ◽  
Major Fault ◽  
Crystal Quartz

The quartz-K-feldspar-cemented breccia of Berglia-Glassberget in the Lierne municipality in central Norway forms an ellipsoid structure 250 m × 500 m in size. The hydrothermal breccia is barren in terms of economic commodities but famous among mineral collectors for being a large and rich site of crystal quartz of various colours and habits. Despite being a famous collector site, the mineralization is rather unique in respect to its geological setting. It occurs within Late Palaeoproterozoic metarhyolites of the Lower Allochthon of the Norwegian Caledonides regionally isolated from any other contemporaneous hydrothermal or magmatic event. In order to understand better the formation of the Berglia-Glassberget breccia, the chemistry, fluid inclusion petrography and age of the breccia cement were determined. Structural features indicate that the Berglia-Glassberget is a fault-related, fluid-assisted, hydraulic breccia which formed by single pulse stress released by a seismic event. 40Ar-39Ar dating of K-feldspar cement revealed a middle Triassic age (240.3 ± 0.4 Ma) for this event. The influx into the fault zone of an aqueous CO2-bearing fluid triggered the sudden fault movement. The high percentage of open space in the breccia fractures with cavities up 3 m × 3 m × 4 m in size, fluid inclusion microthermometry, and trace element chemistry of quartz suggests that the breccia was formed at depths between 4 and 0.5 km (1.1 to 0.1 kbar). The origin of the breccia-cementing, CO2-bearing Na-HCO3-SO4 fluid may have been predominantly of metamorphic origin due to decarbonation reactions (T > 200 °C) of limestones of the underlying Olden Nappe. The decarbonation reactions were initiated by deeply derived, hot fluids channelled to sub-surface levels by a major fault zone, implying that the breccia is situated on a deep-seated structure. Regionally, the Berglia-Glassberget occurs at a supposed triple junction of long-lived fault zones belonging to the Møre-Trøndelag, Lærdal-Gjende and the Kollstraumen fault complexes. These fault systems and the associated Berglia-Glassberget earthquake are the expression of rifting and faulting in northern Europe during the middle/late Triassic.

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