τeκτoniκa https://tektonika.online/index.php/home <p>Diamond open Acess Journal publishing peer-reviewed research in Structural Geology and Tectonics</p> Aberdeen University en-US τeκτoniκa 2976-548X Overture for the Mandara and Vasuki Plates https://tektonika.online/index.php/home/article/view/61 <h4>Models of past plate motions in the Indian Ocean help map the supercontinent Gondwana and investigate how mantle plumes influence plate tectonics. Reducing confidence in this, however, the range of available models all produce large pre-94 Ma movements, in various kinematic senses, between India and Madagascar. There is no observational evidence for any of these motions, suggesting along with their diversity that they are artefacts stemming from contrasting resolutions of techniques used for reconstructing India and Madagascar to Antarctica. A higher resolution approach to India–Antarctica reconstruction concentrates on geophysical records of relative plate motion azimuths. Applying its results regionally eliminates Indo-Malagasy motions before 94 Ma, and prompts new hypotheses of two small tectonic plates. The early Cretaceous Mandara plate, in the Enderby Basin off East Antarctica, may have initiated and rotated at a mid-ocean ridge that was supplied by excess melt from the Kerguelen plume. The late Cretaceous Vasuki plate may have conveyed Sri Lanka southwards across the western Bay of Bengal. The 85°E and Comorin ridges may have formed at active transform fault zones along Vasuki’s margins that were supplied with excess melt from the Crozet and Marion plumes. The model confidently implies the presence of 500,000 km<sup>2</sup> of continental crust beneath the Kerguelen Plateau, places Sri Lanka 1000 km further east within Gondwana than previous reconstructions, and casts doubt on the existence of plate kinematic signals that have previously been attributed to the arrival and spread of the Marion plume beneath India and Madagascar at ~105 Ma.</h4> Graeme Eagles Copyright (c) 2024 Graeme Eagles 2024-03-26 2024-03-26 2 1 111 131 10.55575/tektonika2024.2.1.61 Effect of Grain-Size and Textural Weakening in Polyphase Crustal and Mantle Lithospheric Shear Zones https://tektonika.online/index.php/home/article/view/68 <p>Strain localization to form narrow plate boundary shear zones in Earth’s lithosphere requires a significant amount of strain weakening. Here we investigate the relative contributions of grain-size-sensitive weakening versus textural weakening in polyphase shear zones in granitoid and peridotitic rocks through two-dimensional centimetre-scale bulk simple shear numerical models. The models deploy both constant grain size (only textural weakening) and dynamic grain-size evolution based on the paleowattmeter. Our results reveal that for granitoid rocks, textural weakening dominates, especially at temperatures around 550 °C, while grain-size-sensitive weakening plays a secondary yet significant role. For peridotitic rocks, intense weakening is evident below temperatures of ~1000 °C due to grain-size reduction, while textural weakening has a minor effect on weakening for experiments above 1000 °C. Two-dimensional experiments are compared to one-dimensional, single-phase models to reveal the effect of geometrical complexities in stress and grain-size evolution. These results are discussed in the context of natural lithospheric shear zones and are compared with established piezometers for individual mineral phases. Our findings underscore the vital role of grain-size-sensitive rheologies, particularly in the mantle lithosphere, for the initial weakening of ductile shear zones. These insights offer quantitative constraints that advance our understanding of the long-term strength of lithospheric plate boundaries.</p> Jonas Ruh Whitney Behr Leif Tokle Copyright (c) 2024 Jonas Ruh, Whitney Behr, Leif Tokle 2024-03-14 2024-03-14 2 1 91 110 10.55575/tektonika2024.2.1.68 Impact of Salt Tectonics on Temperatures Distribution Revealed by RSCM Thermometry in the SW Alps (France) https://tektonika.online/index.php/home/article/view/60 <p>Evaporites have a strong impact on the structural and sedimentary evolution of sedimentary basins and fold-and-thrust belts. They also have a thermal conductivity that can be more important than other sedimentary rocks and are thus able to modify the thermal history of these sedimentary basins and fold-and-thrust belts. Even though this property is known and has been of interest for the oil and gas industry, no field examples have been studied trying to decipher how salt rock impacts temperature distribution in fold-and-thrust belts. In this paper, we use the Raman Spectroscopy on Carbonaceous Material (RSCM) to track the record of the peak thermal event around three salt structures from the southern sub-Alpine fold-and-thrust belt in SE France. These three salt structures are (1) the Astoin allochthonous salt sheet and the associated overturned megaflap, (2) the Rocher de Hongrie and (3) the Daluis diapir. Our results show that the resulting record of peak temperatures around the structures is different depending on the type of salt structure and its kinematic. The Astoin structure shows that salt tectonics during the Jurassic-Cretaceous has impacted the temperature distribution around the allochthonous salt sheet while at Daluis and the Rocher de Hongrie, the temperatures have overprinted an already existing salt-related structure. The impact of the salt structure on temperature distribution is always local but the interpretation of the RSCM temperatures may systematically be difficult without considering early salt tectonics in the structural evolution of the area.</p> Naïm Célini Jean-Paul Callot Abdeltif Lahfid Frédéric Mouthereau Copyright (c) 2024 Naïm Célini, Jean-Paul Callot, Abdeltif Lahfid, Frédéric Mouthereau 2024-03-06 2024-03-06 2 1 68 90 10.55575/tektonika2024.2.1.60 Quaternary-Active Faults and the Role of Inherited Structures in the Sacramento-San Joaquin Delta, Western Central Valley, Northern California https://tektonika.online/index.php/home/article/view/46 <p style="font-weight: 400;">Seismic sources and their associated hazards within the Sacramento-San Joaquin Delta region of north-central California are relatively poorly characterized as compared to other, more heavily studied regions of northern California, such as the San Francisco Bay Area. Here we present a synthesis of subsurface, bedrock geology, and geodetic datasets from the Delta and from the Coast Ranges and Diablo Range to the northwest and southwest, respectively. We integrate these data and our own surface geologic and geomorphic observations to present a comprehensive review of faults in the Delta that exhibit Quaternary activity. Structural geologic data from the surrounding region highlight the significant influence that Late Cretaceous-to-Paleogene forearc structures exert on the geometry and kinematics of major Quaternary-active structures within the Delta. These inherited structures — including the Pittsburg-Kirby Hills Fault, Midland Fault, and Great Valley Fault System — exhibit a range of geometries and kinematics. Analysis of geomorphology along these structures suggests that these structures combine to accommodate Quaternary strain across the Delta region. A clearer understanding of subsurface geometries and structural relationships, built upon the regional tectonic history, provides insight into modern deformation accommodated on older structures and helps inform interpretations of seismic hazard within the Delta.</p> Charles Trexler Jack Willard Belle Philibosian Copyright (c) 2024 Charles Trexler, Jack Willard, Belle Philibosian 2024-02-29 2024-02-29 2 1 40 67 10.55575/tektonika2024.2.1.46 Fingerprinting Fluid Source in Calcite Veins: Combining LA-ICP-MS U-Pb Calcite Dating with Trace Elements and Clumped Isotope Palaeothermometry https://tektonika.online/index.php/home/article/view/2 <p class="p1">Application of geochemical proxies to vein minerals - particularly calcite - can fingerprint the source of fluids controlling various important geological processes from seismicity to geothermal systems. Determining fluid source, e.g. meteoric, marine, magmatic or metamorphic waters, can be challenging when using only trace elements and stable isotopes as different fluids can have overlapping geochemical characteristics, such as δ<sup><span class="s1">18</span></sup>O. In this contribution we show that by combining the recently developed LA-ICP-MS U-Pb calcite geochronometer with stable isotopes (including clumped isotope palaeothermometry) and trace element analysis, the fluid source of veins can be more readily determined. Calcite veins hosted in the Devonian Montrose Volcanic Formation at Lunan Bay in the Midland Valley Terrane of Central Scotland were used as a case study. δD values of fluid inclusions in the calcite, and parent fluid δ<sup><span class="s1">18</span></sup>O values reconstructed from clumped isotope palaeothermometry, gave values which could represent a range of fluid sources: metamorphic or magmatic fluids, or surface waters which had undergone much fluid-rock interaction. Trace elements showed no particularly distinctive patterns. LA-ICP-MS U-Pb dating determined the vein calcite precipitation age – 318±30 Ma – indicating a metamorphic or magmatic fluid source was unlikely as there was no metamorphic or magmatic activity was occurring in the area at this time. The vein fluid source was therefore interpreted to be a surface water (meteoric based on paleogeographic reconstruction) which had undergone significant water-rock interaction. This study highlights the importance of combining the recently developed LA-ICP-MS U-Pb calcite geochronometer with stable isotopes and trace elements to help determine fluid sources of veins, and indeed any geological feature where calcite precipitated from a fluid that may have resided in the crust for a period of time (e.g. fault precipitates or cements).</p> John MacDonald Jacob VanderWal Nick Roberts Ian Winkelstern John Faithfull Adrian Boyce Copyright (c) 2024 John MacDonald, Jacob VanderWal, Nick Roberts, Ian Winkelstern, John Faithfull, Adrian Boyce 2024-02-05 2024-02-05 2 1 27 39 10.55575/tektonika2024.2.1.2 Mesoproterozoic Strike-Slip Faulting within the Åland Rapakivi Batholith, Southwestern Finland https://tektonika.online/index.php/home/article/view/51 <p>Paleostress inversion analysis of outcrop data from brittle fault structures within the Mesoproterozoic 1.58 Ga Åland rapakivi granite, southwestern Finland, revealed two separate strike-slip faulting stages. Stage 1 is dominated by dextral slip along E–W-trending faults under WNW–ESE to NNW–SSE compression, whereas Stage 2 displays less prominent faulting localized in an orthogonal network of N–S and E–W trending faults that developed under NE–SW compression. Relative age constraints indicate that faulting occurred between 1.58 and 0.5 Ga, and further correlation with previously published results indicate a 1.55–1.4 Ga age for Stage 1 faulting, while Stage 2 is compatible with previously described fault reactivations between 1.3–1.2 Ga. To place the results of the fault analyses in a wider framework, we conducted a regional structural interpretation using bathymetric, topographic, and geophysical datasets and reviewed previously published results. Based on the above, we attribute the emplacement of the 1.6–1.5 Ga rapakivi granites and the subsequent development of the Mesoproterozoic sedimentary basins to the reactivation of inherited Paleoproterozoic shear zones during Mesoproterozoic crustal extension. As such, this study contributes towards understanding the relationships between magmatism and strain localisation in continental (failed) rift settings.</p> Nicklas Nordbäck Pietari Skyttä Jon Engström Nikolas Ovaskainen Jussi Mattila Ismo Aaltonen Copyright (c) 2024 Nicklas Nordbäck, Pietari Skyttä, Jon Engström, Nikolas Ovaskainen, Jussi Mattila, Ismo Aaltonen 2024-02-01 2024-02-01 2 1 1 26 10.55575/tektonika2024.2.1.51