Conference Agenda

Kinematic analysis (shear sense and vorticity quantification) is important in structural geological/tectonic investigations that helps relate field structures to large scale tectonics (e.g., thrusting in mountain belts such as the Himalaya and Alps). Mesoscale and microscale shear structures are commonly analysed for this. However, in order to work on kinematics, it is the norm to look at structures in the XZ section of the strain ellipsoid, i.e., a section parallel to the stretching lineation and perpendicular to the foliation. This poses a challenge in many deformed rocks, including those from shear zones, because not all rocks develop a visible foliation and/or stretching lineation. Through this presentation, the author highlights the importance of carrying out Anisotropy of Magnetic Susceptibility (AMS) analysis of such tectonically deformed rocks. This analysis provides structural geologists with orientations of three principal axes of the AMS ellipsoid (a second order tensor; K1>K2>K3), which are equated with three principal axes of the strain ellipsoid (X>Y>Z). This helps identify the K1K3 section (=XZ of strain ellipsoid), which is the reference frame for any kinematic analysis. Examples are presented from quartzite of the Eastern Ghats region (eastern India) and metapelites from Hutti region (Dharwar Craton, southern India), where AMS and quartz CPO (SEM-EBSD) data have been integrated to decipher kinematics. Further, the author discusses a case of a BIF from Norway, in which TEM analysis was done in a thin film of magnetite extracted parallel to the K1K3 plane to study nanostructures in the kinematic reference frame.

Dauphiné twin boundaries (DTBs) represent special coincident site lattice (CSL) boundaries characterized by low interfacial energy and high structural coherency. This study integrates Atomic Force Microscopy (AFM), Electron Backscatter Diffraction (EBSD), and full-field numerical simulatons to investigate the role of DTBs in strain accommodation and deformation mechanism. AFM imaging of chemically polished thin-section surfaces reveals distinct morphological contrasts between random high-angle grain boundaries (RHAGBs) and DTBs, where RHAGBs develop nanometric grooves, whereas DTBs remain topographically coherent. RHAGB–DTB junctions display “bridge-like” nanoscale features indicating local reorganization of boundary geometry, consistent with CSL-controlled stabilization. Misorientation relationships further suggest transformation of RHAGB segments into low-energy twin boundaries, reducing the overall surface energy of the aggregate. Crystallographic preferred orientation (CPO) analyses indicate activation of rhomb<a> slip systems only within recrystallized grains, suggesting strain partitioning resulting from stiffness differences between the positive and negative rhomb faces of quartz. To evaluate the influence of DTBs on slip-system activity, numerical simulations were performed in the full-field ELLE-EVPFFT package - marking its first application to quartz deformation studies. Simulations were conducted under varying CRSS ratios for basal<a>, rhomb<a>, and prism[c] slip systems. Simulations indicate dominance of rhomb<a> slip in conditions favouring basal<a> slip when DTBs are present in abundance. It can be postulated that the high frequency of DTBs is expected to lower rhomb<a> CRSS due to rhomb-plane rotation within twins. These observations suggest that DTBs act as atomically controlled, low-energy interfaces that modulate slip activity, strain partitioning, and potentially fluid transport in quartz-rich crustal rocks.

Stylolithe roughness paleopiezometry estimates paleostress magnitudes from the crossover length (Lc) separating two power-law regimes in the roughness power spectral density (PSD). However, standard inversion workflows require continuous and well-preserved profiles, whereas natural stylolithes are commonly interrupted by fractures, weathering and/or sampling damage. This study develops and tests a damage-tolerant inversion framework for recovering Lc from incomplete stylolithe profiles. A synthetic benchmark of 400 continuous profiles, spanning Lc = 0.5–2.0 mm and profile lengths of 10–100 mm, was submitted to controlled gap damage from 5% to 40%, generating 9600 damaged profiles with known ground truth. Four gap-treatment strategies were evaluated. Concatenation, zero-filling, and autocorrelation-based imputation systematically failed to preserve the two-slope spectral structure required for reliable Lc detection. In contrast, linear bridging (“blank bridge”) preserved the PSD structure within defined limits, although it introduced a predictable positive Lc bias that increased with damage level and with the dimensionless ratio L/Lc. Profiles with L/Lc < 15 remained reliable up to 30% damage, whereas profiles with 15 ≤ L/Lc < 40 were reliable up to 20% damage. Finally, profiles with L/Lc ≥ 40 should not be processed using full-profile blank bridging. Validation on 11 natural stylolithe profiles from the Toca Formation, Lower Congo basin, confirms that the synthetic error trends transfer to real samples. Because the positive Lc bias propagates into conservative lower-bound paleostress estimates, damaged stylolithes can provide useful stress constraints when appropriate quality-control criteria are applied.

The catastrophic collapse of oceanic volcanoes is observed worldwide and poses a hazard to populations living in coastal areas as tsunami waves can form. Despite the hazard associated with the collapse of oceanic volcanoes, the preceding processes and conditions are poorly understood. Prime examples to study edifice stability include Kilauea volcano (Hawai’i, USA) as remnants of massive landslides are found offshore and a future collapse cannot be excluded, and Anak Krakatau (Sunda Strait, Indonesia), which collapsed catastrophically in December 2018. An important factor for edifice stability is the mechanical behavior of the rocks building the edifice. We conducted direct shear experiments on samples from Anak Krakatau and Kilauea and used the experimental results to inform finite-element models. We find that volcanic rocks are generally strong (µ > 0.6). The composition, mineralogy and rate-and-state friction are highly variable between all samples. Thus, whether a volcanic edifice can fail catastrophically depends on the rocks in which the failure surface forms. The finite-element models show that major internal structures, e.g., faults or material boundaries, and topography affect the deformation behavior of the edifice and hence their knowledge is important when studying edifice stability. However, internal structures and topography are not sufficient to cause failure and external factors (e.g., eruption, seismicity) are needed to initiate failure. Moreover, it is evident that failure along internal structures is likely retrogressive, initiating in the submarine portion of the edifice. Thus, studying the submarine portion of the volcanic edifice when studying edifice stability at oceanic volcanoes is essential.

The Trans-Mexican Volcanic Belt (TMVB) stretches over 1000 km from the W-Pacific-coast to the E-Gulf-coast. The oblique angle of the volcanic arc respect to the trench and the associated slip partitioning (Ego and Ansan, 2002), the consequent slab tears of the subducting Cocos plate in several locations (e.g. Blatter et al., 2007; Castellanos et al., 2018) and resulting compositional variability of the volcanic products including intraplate (OIB) lavas and subduction related products (Gomez-Tuena et al., 2007) have already been recognized. Nevertheless, other more isolated centers of active volcanism in Mexico, including the Los Tuxtlas Volcanic Field (LTVF), can be found between the TMVB and the Central America-Arc, despite the continuous subduction of the Cocos plate from the Gulf of California to Panama (Ferrari et al., 2012). The complex tectonic setting of the LTVF bears review because of the prominent role of volcanotectonic interaction within the field. The considerable distance to the trench, and the predominantly alkaline character of products of LTVF volcanic products originally led to the suggestion that the LTVF lies in a divergent tectonic environment (intracontinental fracture zone/rift zone associated with extensional tectonics) (e.g. Pichler and Weyl, 1976; Robin, 1976; Thorpe, 1977), without a genetic relationship with the TMVB (e.g. Robin and Tournon, 1978; Cantagrel and Robin, 1976; Robin, 1982). Later, more geochemical data suggested the existence of clearly subduction-derived material for the LTVF (Nelson et al. 1995). In this study, the available geophysical and geochemical data will be used to shed light upon this complex tectonic situation.

Divergent triple junctions are paramount in our understanding of plate tectonics. Their evolution is widely attributed to mantle plume upwelling. Recent analogue modelling studies explored the structural effects in static model crust caused by the upwelling of model plumes. Such experiments ignore the dynamic interaction between plume upwelling and far-field tectonic stresses causing a divergence in plate motion near plumes, a fundamental aspect in the evolution of divergent triple junctions. Our MultiBox analogue apparatus allows us to model an evolving plume, simulated by a balloon inflated with water and placed beneath ductile (polydimethylsiloxane-corundum mixture) and brittle (G23T sands) layers simulating, respectively, the lower and upper crusts. Three experimental series were conducted: plume-only (PO), plume with symmetric extension (PSE), and plume with asymmetric extension (PAE), to systematically investigate the structural effects of plume upwelling within divergent strain fields. Results show that PO experiments produce axisymmetric doming and radial-to-polygonal faulting, indicating a point-symmetric lithospheric response to pure plume-driven uplift. During the PSE and PAE experiments, radial faults reorganized into three principal rift segments, with progressive deformation leading to a directional bias in the triple-rift evolution. Both symmetric and asymmetric extensions favored the growth of two dominant rift arms perpendicular to the direction of extensions, while the third rift, parallel to the extension direction, developed less well. Our experiments offer clearer insight into natural examples, such as the Benue Trough in Nigeria and other divergent triple junctions, into why some rift arms open into oceans while others fail.