Interplay Between Brittle and Ductile Deformation in the Lower Crust (Musgrave Ranges, Central Australia)

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Author
Date
2018Type
- Doctoral Thesis
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Abstract
The localization of deformation in dry lower continental crust and the implied rheology
have been investigated on the basis of field and laboratory observations from a well
exposed and preserved section in Central Australia. The strength of the lower crust is the
deciding factor determining whether deformation of crust and mantle are closely linked
or decoupled, which is important for understanding the overall response to strain of
continental plates and, in particular, the development of fault zones at depth. The usual
assumption is that the upper part of the crust is relatively strong and deforms in a brittle
manner, with deformation accommodated by stable frictional sliding and episodic seismic
rupture. Consequently, fault rocks developed in this part of the crust are dominantly
cataclasites and pseudotachylytes. As temperature rises with depth, the rocks begin to
weaken and deform in a viscous manner, with an area of mutual overprinting cataclasites,
pseudotachylytes and mylonites, termed the brittle-ductile (or frictional-viscous)
transition zone. The depth of this zone is usually around 10-15 km, depending on the
geothermal gradient, which is supported by the observation that most large continental
earthquakes are located within this depth range. As this zone is marked by the onset of
ductility in quartz, deeper parts of the crust are expected to flow rather than fracture.
However, geophysical records show earthquakes even in the lower parts of the continental
crust, at depths down to 40-50 km. Rocks from these depths are seldom exhumed to the
surface and therefore rarely preserved.
The central Musgrave Ranges in Central Australia provide a unique insight into the
architecture of the continental lower crust, as several lithospheric scale shear zones that
developed during the ca. 550 Ma Petermann Orogeny are preserved with outstanding
exposure. The Petermann Orogeny localizes deformation in an intraplate position
between the cratons of Australia, which amalgamated during the ca. 1200 Ma Musgravian
Orogeny. This event reached granulite facies metamorphism in the core of the orogen,
with partial melting and breakdown of water-bearing minerals producing effectively dry
rocks. Metamorphic conditions during the later Petermann Orogeny reached sub-eclogitic
facies (650-700 °C, 1.2 GPa) in the Fregon Subdomain, which was thrusted over the lower
grade Mulga Park Subdomain in the north by the Woodroffe Thrust. In the hanging wall,
the Davenport Shear Zone accommodated strike-slip movement in an overall
transpressional setting. Strain is distributed heterogeneously in this 5 km wide mylonite
zone and it encompasses several low strain domains where the initial stages of shear zone
development are still preserved. The shear zone foliation varies from moderately to
steeply dipping towards the SSW, with a sub-horizontal stretching lineation plunging
towards the ESE and WNW. In low strain domains, fine grained dolerite dykes localize
deformation, while quartz-rich pegmatites and mafic enclaves remain largely
undeformed, even if these supposedly weaker layers are oriented in a favorable
orientation for shearing. Instead, narrow shear zones (a few centimeters wide and several
tens of meters long) crosscut lithological boundaries and many of those exhibit parallel
fractures or features characteristic of pseudotachylytes, such as injection veins or
breccias, demonstrating that most narrow shear zones nucleate along brittle precursors.
Sheared pseudotachylyte is also found as clasts in a second generation of pseudotachylyte,
illustrating a cyclical interplay of brittle fracturing and viscous shearing. The
recrystallized paragenesis in the pseudotachylyte therefore gives information not only on
the pressure-temperature conditions during shearing but also during pseudotachylyte
emplacement. Thermodynamic modeling on the basis of local bulk compositions of sheared
pseudotachylytes, derived from quantified X-ray maps using XMapTools, give P-T
estimates similar to those derived from the mylonites (600-700 °C, 1.1-1.3 GPa). While
differential stresses necessary to produce pseudotachylyte must be high (~ 1 GPa) in dry
lower crustal rocks without significant pore fluid pressure, the grain size of quartz in
adjacent mylonites is relatively coarse, in the range 50-100 microns, which indicates longterm
flow stress on the order of 10 MPa or less. However, subgrains are evident in almost
all quartz grains, with sizes below 5 and down to 1 micron. This can be interpreted as a
non-steady state recrystallized grain size, resulting from transient high stresses. Another
indicator for high stress events is the occurrence of fractured and plastically deformed
garnet in a matrix of comparatively weak quartz and feldspar. Multiple generations of
fractures show overprinting relationships and often induced lattice distortions in the
garnet. Crystal plastic behavior is further manifested by subgrain rotation
recrystallization. Dislocations are visible in TEM-images (transmission electron
microscopy) and are mostly organized in dislocation walls, indicating recovery.
Transient high stresses in the mid- to lower crust have been attributed to the downward
propagation of the seismogenic zone during large earthquakes. However, the large
amounts of pseudotachylyte throughout the Fregon Subdomain would require a
tremendous amount of seismicity in the upper crust. In the case of the heterogeneously
deformed Davenport Shear Zone, temporal and spatial variations in stress might instead
be explained by the local interaction of stronger, relatively undeformed blocks within a
network of narrow shear zones. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000265008Publication status
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Contributors
Examiner: Mancktelow, Neil S.
Examiner: Burg, Jean-Pierre
Examiner: Pennacchioni, Giorgio
Examiner: Camacho, Alfredo
Examiner: Menegon, Luca
Publisher
ETH ZurichSubject
STRUCTURAL GEOLOGY; STRUKTURGEOLOGIE; STRUKTUR + TEXTUR + GEFÜGE (PETROGRAPHIE); STRUCTURE + TEXTURE + FABRICS (PETROGRAPHY); Earthquakes; ERDBEBEN + SEEBEBEN (GEOPHYSIK); PSEUDOTACHYLIT (GEOLOGIE); PSEUDOTACHYLITE (GEOLOGY); Lower crustal earthquakes; lower crust; Quartz; Garnet; GRANULITE (PETROGRAPHY); DISLOCATIONS (CRYSTALLOGRAPHY); Recrystallization; ECLOGITE (PETROGRAPHY); Thermodynamic modeling; Australia; Musgrave BlockOrganisational unit
03392 - Burg, Jean-Pierre (emeritus)
Funding
146745 - Interplay between Fracture and Flow during Localization of Deformation in the Middle and Lower Crust (SNF)
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