The heavily faulted Martian terrains of Ceraunius Fossae and Tractus Fossae, south of the Alba Mons volcano, have previously only been considered as parts of larger tectonic studies of Alba Mons, and the complexity of the faulting remains consequently unclear. As these terrains are in midst of the large Tharsis’ volcanoes, the study of their surface deformation has the potential to help unravel the volcano-tectonic deformation history associated with the growth of Tharsis, as well as decipher details of the responsible magma-tectonic processes. In this study, we distinguish between faults and collapse structures based on image and topographic evidence of pit-crater chains. We mapped ~12,000 faults, which we grouped into 3 distinct fault groups based on orientation, morphology, and relative ages. These show a temporal evolution in the mapped fault orientations from NE to NS to NW, with associated perpendicular stress orientations. Collapse features were also mapped and categorized into 4 different groups: pit-crater chains, catenae, u-shaped troughs and chasma. Examining the 4 collapse structures reveals that they are likely 4 different steps in the erosional evolution of pit-crater chains. Together this revealed a structural history heavily influenced by both local (radial to Alba Mons, Pavonis Mons and Ascraeus Mons) and regional (Tharsis radial) lateral diking, and vertical diking from a proposed Ceraunius Fossae centred magma source. This, along with an updated crater size-frequency distribution analysis of the unit ages, reveals a highly active tectonic and magmatic environment south of Alba Mons, in the Late Amazonian.

Numerous graben features transect the Tempe Terra plateau in the northeastern Tharsis Rise, making it one of the most heavily structured regions of Tharsis. The origin of the complex fault geometries, generated over three distinct stages of tectonic activity, is still poorly understood. This work distinguishes between locally-sourced and regionally-sourced structures within Tempe Terra, to isolate regional deformation patterns related to the general development of the Tharsis Rise from the effects of local mechanisms. Comparison of structural observations to predicted deformation patterns from different sources of graben formation in the Martian crust demonstrates the important role of magmatic activity at a variety of scales in driving tectonism in Tempe Terra. Noachian (Stage 1) faulting was the result of local magmatic underplating and associated heating and uplift, which formed part of an incipient stage of widespread Tharsis volcanism that predated development of the main Tharsis Rise. Early Hesperian (Stage 2) faults reflect the interaction of regional stresses from growth of the Tharsis Rise with magmatic activity highly localised along the Tharsis Montes Axial Trend – a linear volcanotectonic trendline including the alignment of the Tharsis Montes volcanoes. Early–Late Hesperian (Stage 3) faulting resulted from a series of dyke swarms from a Tharsis-centred plume, which propagated in a regional stress field generated by growth of the Tharsis Rise. As only Stage 2 NNE faults and Stage 3 ENE faults are linked to regional, Tharsis-related stresses, other observed Tempe Terra fault trends can be excluded when evaluating models of Tharsis’s tectonic evolution.

Distorted crystals carry useful information on processes involved in their formation, deformation and growth. The distortions are accommodated by geometrically necessary dislocations, and therefore characterising those dislocations is an informative task, to assist in, for example, deducing the slip systems that produced the dislocations. Electron Backscatter Diffraction (EBSD) allows detailed quantification of distorted crystal orientations and we summarise here a method for extracting information on dislocations from such data. The Weighted Burgers Vector (WBV) method calculates a vector at each point on an EBSD map, or an average over a region. The vector is a weighted average of the Burgers vectors of dislocation lines intersecting the map surface. It is weighted towards dislocation lines at a high angle to the map but that can be accounted for in interpretation. The method is fast and does not involve specific assumptions about dislocation types. It can be used, with care, to analyse subgrain walls (sharp orientation changes) as well as gradational orientation changes within individual grains. We describe new and published examples of the use of the technique to illustrate its potential; case studies to date mainly use the WBV direction not the magnitude. EBSD orientation data have angular errors, and so does the WBV. We present an analysis of these angular errors, showing there is a trade-off between directional accuracy and area sampled. In summary the technique is fast, free from assumptions, and errors can be taken into account to allow testing of hypotheses about dislocation types.

The structurally complex region of Tempe Terra, located in the northeast of the Tharsis Rise on Mars, preserves deformation related to the growth of Tharsis and lies along the trendline formed by the Tharsis Montes volcanoes. We characterise the spatiotemporal tectonic evolution of Tempe Terra based on comprehensive structural mapping. From this mapping, we identified 16 cross-cutting fault sets and placed these in relative time order, based on a hybrid approach using cross-cutting relationships and buffered crater counting. We are thus able to provide a broad framework for understanding the timing of development for the Tharsis Rise and Tharsis Montes axial trend. Our work shows that Tempe Terra has experienced three distinct stages of tectonic activity from the Middle Noachian to the Late Hesperian. Stage 1 involved E--W extension followed by localised NE--SW extension, which produced local zones of N and NW faulting through the centre and west of Tempe Terra in the Noachian. Stage 2 produced intense NE-oriented faulting concentrated along the Tharsis Montes axial trend in the Early Hesperian as a result of a discrete period of NW--SE extension and local volcanism. Stage 3 involved NW--SE extension coinciding with Tharsis volcanic activity, which generated a regional fabric of ENE-trending graben distributed across Tempe Terra from the Early to Late Hesperian. We observe an overall peak in tectonic activity in the Early Hesperian and find that Tharsis-related extensional deformation in the form of NE-oriented radial faulting did not start in Tempe Terra until this time.

As seismic data from the lower crust becomes more readily available, it is import ant to link seismic properties to the ongoing processes within lower crustal evolution. It includes high temperature, pre- and post-migmatisation solid state deformation as well as melt-present deformation condition. We selected two tonalitic migmatites with variable former melt content (one metatexite and one diatexite) from the lower crustal Daqingshan area, northern North China Craton (NCC) to assess the link between seismic properties and rock structure and rheology. Field observation along with microstructural features suggest that the character of hornblende and plagioclase within the residuum of the metatexite can be used to derive information regarding the pre-migmatisation deformation. Residuum’s plagioclase CPO (crystallographic preferred orientation) is consistent with high temperature dislocation creep as the main deformation mechanism; similarly, hornblende shows a strong CPO related to dislocation creep. During syn-melt (melt present) conditions, phenocrysts of plagioclase in the metatexite’s neosome and K-feldspar and peritectic hornblende in the diatexite’s neosome are present. The rheology of the rock was dominated by melt; hence is inferred to follow Newtonian flow. After melt crystallization deformation is minor but again dominated by dislocation creep. For seismic properties (seismic velocity, anisotropy, Vp/Vs ratio, etc.), in pre- and post-melt conditions, migmatites have normal values. While in syn-melt condition, seismic velocities have a greater decrease, Vp/Vs ratios have a great increase, seismic anisotropies are unusually high.

Pressure and temperature change simultaneously in the Earth’s crust from surface to depth. Joint pressure and temperature changes influence many different physical properties. There are many studies on samples at elevated pressure, where the influence of open cracks, fractures, voids and pores have been studied. Applying confining pressure has a direct influence on crack closure, and this influence on dynamic properties (density and elastic modulus, bulk, shear and young’s) of rocks above 200 MPa is assumed linear with the linear increase in wave speed. This is because it is generally assumed that most cracks are closed above 200 MPa, which in nature would correspond to a depth of ~7-8 km. However, from the KTB deep drilling well in Germany, it is known that fluid-filled fractures and pores can remain open until 8 to 9 km depth. Applying temperature can affect the dynamic properties of rock by thermal expansion, possibly reopening cracks that were closed at pressures >200 MPa, and thermally expanding grains. This influence is also assumed to be linear at a temperature below partial melting, and in the absence of phase transitions. A similar effect has been observed by a number of research groups during laboratory experiments and calculating seismic velocity results under 600 MPa confining pressure and 600oC temperature. In this work, an effort has been made to mathematically investigate the influence of temperature and pressure on the seismic properties (velocity of pressure and shear waves, density and Poisson’s ratio) of crystalline rocks, measured during laboratory experiments. Elastic wave speeds, moduli and density are increasing as a function of pressure and decreasing as a function of temperature. However, these pressure and temperature-related changes are shown to be nonlinear from room conditions up to 600oC and 600 MPa. In this presentation, we focus on non-linear changes mainly in the high-pressure portion of the velocity as a function of pressure (>200 MPa). When confining pressure is applied, measured P- and S- waves show an increase in velocity and decrease in anisotropy. However, the effect of temperature on measured P- and S- waves show a decrease in velocity and increases in anisotropy. These changes are not very different from linear, but it is not possible to fit velocity as a function of pressure or temperature with linear mathematical functions. The implications of non-linear relationships between pressure, temperature and elastic wave speeds are discussed in this presentation.

Deformation is a near ubiquitous process that is observed within nearly all naturally forming rocks, terrestrial and extra-terrestrial. Large area electron backscatter diffraction (EBSD) is a technique that enables slip-systems (a form of plastic deformation) to be inferred at a comparable scale to representative texture analysis (≥100 crystals). Extensive laboratory and studies on naturally occurring samples have identified preferential extrinsic parameters for specific slip-system signatures within olivine and clinopyroxene for mantle conditions. Slip-systems in both olivine and augite (high Ca-clinopyroxene) for 21 large area EBSD datasets sourced from 16 different Martian nakhlite meteorites were analysed and assessed against these parameters. When investigating the high and low deformation regions within the samples 10 of the 21 sections exhibited a shift in the slip-system patterns between the low and high deformation regions. The secondary signatures identified within the low deformation regions are inferred to relate to emplacement deformation. Thus, these samples exhibit both shock and emplacement signatures. The observed variations in deformation patterns for the two main regimes of deformation indicate heterogeneous sampling of the nakhlite ejecta crater. Our findings indicate that shock deformation is prevalent throughout the nakhlites, and that great care needs to be taken when interpreting slip-deformation of crystals within apparent lower deformation regions.

The Martian nakhlite meteorites, ormed from a single magma source region, and emplaced during multiple events spanning at least 93 ± 12 Ma, represent a key opportunity to study the evolution of Martian volcanic petrogenesis. Here 16 of the 26 identified nakhlite specimens are studied using coupled large area electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS) mapping to determine shape preferred orientation (SPO) textures of contained augite (high Ca-clinopyroxene) phenocrysts by considering crystallographic preferred orientation (CPO). Textural parameters derived from EBSD and EDS analyses were used to calculate maximum and minimum magma body crystallization thicknesses via three endmember emplacement scenarios: thermal diffusion, crystal settling, and crystal convection. Results from CPO textural analyses indicate weak to moderate fabric textures that are comparable to those in terrestrial clinopyroxenites. In all samples, a consistent foliation within the {001} axis of augite is observed. In all but two of the studied nakhlites this {001} foliation is typically coupled with a weaker lineation fabric in one or more of the {100}, {010}, {001} axes. Results from the calculated magma body thicknesses are consistent with an emplacement mechanism for the nakhlites driven by crystal settling. These crystal settling results infer magmatic body thicknesses ranging from <1 m to several 10’s m, forming two distinguishable groups that appear random when assessed against observed texture, geochemical, and age parameters. Coupled textural and modelling results therefore suggest that the nakhlite source volcano varied in thickness over time yet consistently solidified via the mechanism of crystal settling.