Pseudotachylyte as field evidence for lower-crustal earthquakes during the intracontinental Petermann Orogeny (Musgrave Block, Central Australia)

Geophysical evidence for lower continental crustal earthquakes in almost all collisional orogens is in con\ufb02ict with the widely accepted notion that rocks, under high grade conditions, should \ufb02ow rather than fracture. Pseudotachylytes are remnants of frictional melts generated during seismic slip and can therefore be used as an indicator of former seismogenic fault zones. The Fregon Subdomain in Central Australia was deformed under dry sub-eclogitic conditions of 600\u2013700 \u25e6 C and 1.0\u20131.2 GPa during the intracontinental Petermann Orogeny (ca. 550 Ma) and contains abundant pseudotachylyte. These pseudotachylytes are commonly foliated, recrystallized, and cross-cut by other pseudotachylytes, re\ufb02ecting repeated generation during ongoing ductile deformation. This interplay is interpreted as evidence for repeated seismic brittle failure and post- to inter-seismic creep under dry lower-crustal conditions. Thermodynamic modelling of the pseudotachylyte bulk composition gives the same PT conditions of shearing as in surrounding mylonites. We conclude that pseudotachylytes in the Fregon Subdomain are a direct analogue of current seismicity in dry lower continental crust

Colombia: Doctoral Excursion, Earth Sciences Department ETH Zurich, August 21 – September 5, 2018

Field guide of Doctoral Excursion to Colombia, co-organized by Luz M. Mejia, R.Ott, L. Echeverria-Pazos, E. Erlanger. Chapter 11: Climate Change Effects on Colombian Biodiversity written by Luz M. Mejia and B. Revels

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

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

Pseudotachylyte formation vs. mylonitization \u2013 repeated cycles of seismic fracture and aseismic creep in the middle crust (Woodroffe Thrust, Central Australia)

The Musgrave Ranges in Central Australia provide excellent exposure of the shallowly south-dipping Woodroffe Thrust, which placed 3c 1200 Ma granulites onto amphibolite facies gneisses. This 3c 400 km long E-W structure developed under mid-crustal conditions during the intracratonic Petermann Orogeny around 550 Ma. From field observations and measurements, the shortening direction is constrained to be N-S and the movement sense top-to-north. Ductile deformation during this process almost entirely localized in the footwall rocks, developing a zone of mylonites, ultramylonites and sheared pseudotachylytes, several hundred metres wide, with pseudotachylyte abundance rapidly decreasing further into the footwall. In contrast, the hanging wall behaved in a predominantly brittle manner, producing significant volumes of pseudotachylyte breccia and isolated veins, but was otherwise mostly unaffected and only weakly foliated. The difference in rheological behaviour is reflected in the pseudotachylyte fabric, which is dominantly sheared in the footwall and largely unsheared in the hanging wall. Low-strain domains in the footwall show that localized shearing initiated along pseudotachylyte veins and that shear zones and mylonitic foliations were in turn exploited by subsequent pseudotachylyte veins. Neither phyllonitization nor synkinematic growth of new muscovite is observed. In contrast to models with a simple brittle-to-viscous transition, these observations show that a continuous cycle of brittle fracturing and shearing is active in dry mid-crustal environments. The products of multiple earthquakes and ductile overprint, repeatedly exploiting the same structural discontinuity, are composite layers of sheared pseudotachylyte. In the Woodroffe Thrust, these layers are numerous and frequently observed parallel to the foliation in the footwall mylonites. The thickest of these sheared pseudotachylyte horizons ( 3c 15 m thick) mark the immediate contact to the hanging wall and almost entirely consist of pseudotachylyte matrix. Particularly in the footwall, but locally also in the hanging wall, shear strain can additionally be concentrated along the margins of dolerite dykes, whose mineral assemblages will be studied to determine the metamorphic conditions that were active during development of the Woodroffe Thrust

Strain localization on different scales and the importance of brittle precursors during deformation in the lower crust (Davenport Shear Zone, Central Australia)

High strain rocks in the Musgrave Ranges (Central Australia) provide a rather unique insight into the development of lower crustal shear zones during the 550 Ma Petermann Orogeny, allowing common models for lower crustal deformation to be critically evaluated. The observed structures in the study area are, from south to north: (1) The Mann Fault, which is poorly exposed but evident on airborne geomagnetic maps. This regional scale fault with a component of dextral shear shows a step-over resulting in the formation of a pull-apart basin. (2) The Davenport Shear Zone, accommodating the horizontal extension in a 7 km wide WNW-ESE-trending mylonitic zone developed under subeclogitic, lower crustal conditions. This high strain zone is bounded to the north by a more than 50 km long, continuous, sheared dolerite dyke. North of this dyke, the 3c 1200 Ma Musgravian fabric is still preserved, only slightly rotated and typically N-S trending. (3) The Woodroffe Thrust, marking the northern boundary of the Musgrave Ranges, brings these lower crustal rocks on top of amphibolite facies units, with a top-to-north sense of movement. Strain in the Davenport Shear Zone is very heterogeneously distributed, with localization and partitioning from the kilometre down to the millimetre scale. Pseudotachylyte is commonly associated with dykes, especially on the boundaries, and is often sheared. The orientation of sheared dykes and localized shear zones is typically at a high angle to either side of the shortening direction, resulting in a variable sense of shear and a major component of flattening, with a nearly horizontal extension direction. Detailed outcrop-scale mapping shows that compositional inhomogeneities, such as quartz veins, are generally not exploited, even when favourably oriented for shear reactivation. Ultramylonitic shear zones are sometimes only a few millimetres wide but extend for several metres and are generally oblique to the background foliation. Pseudotachylyte often predates or is coeval with localized shearing and fracturing clearly played a major role in the nucleation of mesoscale discrete shear zones. In order to constrain the conditions of pseudotachylyte formation, and to establish whether they developed under lower crustal subeclogitic conditions, garnet-bearing sheared pseudotachylytes were sampled for geothermobarometric analysis

Fracturing and crystal plastic behaviour of garnet under seismic stress in the dry lower continental crust (Musgrave Ranges, Central Australia)

Garnet is a high-strength mineral compared to other common minerals such as quartz and feldspar in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade crystal plastic deformation, except under relatively high-temperature conditions. The microstructure of granulite facies garnet in felsic lower-crustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal plastic deformation. Granulite facies metamorphism at ∼1200 Ma generally dehydrated the rocks and produced millimetre-sized garnets in peraluminous gneisses. A later ∼550 Ma overprint under sub-eclogitic conditions (600–700 ∘C, 1.1–1.3 GPa) developed mylonitic shear zones and abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. In the mylonites, granulite facies garnet porphyroclasts are enriched in calcium along rims and fractures. However, these rims are locally narrower than otherwise comparable rims along original grain boundaries, indicating the contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal plastic deformation, which coincides with areas of enhanced diffusion, usually along zones of crystal lattice distortion and dislocation walls associated with subgrain rotation recrystallization. The fracturing of garnet under dry lower-crustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte.ISSN:1869-9510ISSN:1869-952

Inverted distribution of ductile deformation in the relatively "dry" middle crust across the Woodroffe Thrust, central Australia

Thrust fault systems typically distribute shear strain preferentially into the hanging wall rather than the footwall. The Woodroffe Thrust in the Musgrave Block of central Australia is a regional-scale example that does not \ufb01t this model. It developed due to intracontinental shortening during the Petermann Orogeny (ca. 560\u2013520 Ma) and is interpreted to be at least 600 km long in its E\u2013W strike direction, with an approximate top-to-north minimum displacement of 60\u2013100 km. The associated mylonite zone is most broadly developed in the footwall. The immediate hanging wall was only marginally involved in the mylonitization process, as can be demonstrated from the contrasting thorium signatures of mylonites derived from the upper amphibolite facies footwall and the granulite facies hanging wall protoliths. Thermal weakening cannot account for such an inverse deformation gradient, as syn-deformational P \u2013T estimates for the Petermann Orogeny in the hanging wall and footwall from the same locality are very similar. The distribution of pseudotachylytes, which acted as preferred nucleation sites for shear deformation, also cannot provide an explanation, since these fault rocks are especially prevalent in the immediate hanging wall. The most likely reason for the inverted deformation gradient across the Woodroffe Thrust is water-assisted weakening due to the increased, but still limited, presence of aqueous \ufb02uids in the footwall. We also establish a qualitative increase in the abundance of \ufb02uids in the footwall along an approx. 60 km long section in the direction of thrusting, together with a slight decrease in the temperature of mylonitization (ca. 100 \u25e6 C). These changes in ambient conditions are accompanied by a 6-fold decrease in thickness (from ca. 600 to 100 m) of the Woodroffe Thrust mylonitic zone

Non-steady state microstructures recording transient stresses during repeated earthquake cycles in the lower crust (Musgrave Block, Central Australia)

The Musgrave Block in Central Australia provides generally excellent exposures of water-de\ufb01cient felsic lower crustal rocks that were little affected during exhumation. During the Petermann Orogeny (550 Ma), several largescale shear zones were developed in an overall transpressional setting. The Davenport Shear Zone is a ca. 5 kmwide mylonite zone, showing an intimate interplay between fracture and \ufb02ow, expressed by multiple generations of pseudotachylyte that may crosscut the mylonitic fabric but are themselves subsequently sheared and foliated. Conditions of pseudotachylyte formation and shearing are constrained to be around 650 \u25e6 C and 1.2 GPa, corresponding to lower crustal depths. This interplay between brittle and ductile deformation in the lower crust is interpreted to represent cycles of seismic fracture and aseismic creep. The grain size of quartz in the mylonites is generally relatively coarse, usually between 50 and 100 \ub5m. The crystallographic preferred orientation (CPO) of the quartz grains shows a Y-maximum for the c-axis, indicating dominant prism slip and consistent with the estimated upper amphibolite to sub-eclogitic metamorphic conditions. However, almost all grains also host subgrains that are usually smaller than 5 \ub5m, with misorientation angles across the subgrain boundary on the order of 5 \u25e6 , showing the same CPO. We interpret these subgrains to record transient stress increases prior to the fracturing that produced pseudotachylyte. The grain size of quartz is commonly used as a paleo-piezometer, but application is usually limited to steady-state microstructures. However, from laboratory experiments it is now established that pulses of high stress can lead to the local formation of small subgrains, which might be preserved. The natural quartz microstructures described here are interpreted to document stress \ufb02uctuations on the order of 10 to at least 500 MPa. It follows that although the long-term stresses during aseismic creep in this example of water-de\ufb01cient lower crust were relatively low (a few 10\u2019s of MPa), transient high stresses ( 65 500 MPa), as would be required for brittle fracturing and pseudotachylyte formation in effectively \u201cdry\u201d rock at 1.2 GPa, may also be recorded and still preserved in the quartz microstructure of lower crustal rocks