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The drivers of Lower Crustal Earthquakes Along Magma-poor Portions of the East African Rift

Luke Wedmore — 2025-07-27

Deep earthquakes along magma-poor sections of the East African Rift System (EARS) challenge our understanding of the controls on seismogenic thickness because they occur at greater depths and higher temperatures than the frictional-viscous transition zone in typical continental crust. Using a recently published catalogue of relocated earthquakes in southeastern Africa, we demonstrate that seismicity occurs down to the ~40 km deep Moho throughout the magma-poor southern EARS. We then explore the mechanisms that can account for this deep seismicity by combining 1D lithospheric strength profiles with available regional measurements of Moho thickness, the ratio of the crust's P- and S-wave velocities (V_P/V_S), and heat flow. As suggested by previous studies, we find that a mafic lower crustal composition, lower geothermal gradient, and/or high pore fluid pressure can locally facilitate the observed deep seismicity. However, there are sections of the southern EARS where the lower crust is felsic, dry, and warm, and in these cases, we propose that the embrittlement of the lower crust is best explained by strain localisation in space and time. This strain localisation could occur because fault and shear zones in magma-poor sections of the EARS are unusually narrow, or because strain is localised in space and time following large magnitude earthquakes.

Zircon Inheritance Refines the Cambrian Orogenic Architecture of Southeast Australia

Jacob Mulder — 2025-08-11

The Selwyn Block is one of the few accreted terranes identified in the vast Paleozoic Tasmanides of eastern Australia and its incorporation into this orogen marks a first-order event in the tectonic evolution of the Pacific margin of Gondwana. However, the age, composition, and paleogeography of the Selwyn Block are poorly understood because it is almost completely concealed in the middle and lower crust. The prevailing hypothesis suggests the Selwyn Block is a northern continuation of the Proterozoic Western Tasmania Terrane. We test this hypothesis by comparing inherited zircon U-Pb ages (n = 881) from early Paleozoic granitoids intruding the Selwyn Block and the Western Tasmania Terrane. Phase equilibria modelling confirms that typical Western Tasmania Terrane lithologies are melt-fertile and would have contributed inherited zircon grains to local granitoids. The inherited zircon age signature of granitoids in the Western Tasmania Terrane mirrors detrital zircon ages from local Proterozoic strata with age populations at ca. 1430 Ma and 1800--1600 Ma. In comparison, granitoids intruding the Selwyn Block have ca. 600--500 Ma and ca. 1200--900 Ma inherited zircon age populations, likely derived from local Paleozoic strata. Previously published wholerock radiogenic Sr isotopic data and new zircon radiogenic hafnium isotope data also imply distinct melt sources with granitoids intruding the Selwyn Block granitoids having lower initial ⁸⁷Sr/⁸⁶Sr and higher initial

Local Strain Reorientation Explains Deformation of Rift-oblique Tectonic Lineaments Along the Main Ethiopian Rift

Frank Zwaan — 2025-08-22

The interaction between the NE-SW striking Main Ethiopian Rift (MER) and the E-W oriented Yerer-Tullu Wellel Volcano-tectonic lineament (YTVL) represents one of the least understood tectonic problems in the East African Rift System. Despite the numerous studies that have been conducted in the region, the following questions still remain to be answered: did the MER and YTVL evolve simultaneously? Was there a change in plate motion direction to allow a diachronous evolution of the rift and the lineament? How does the E-W oriented YTVL remain active under ca. E-W oriented regional extension? Previous studies propose a two-phase tectonic evolution, involving a change in the direction of plate motion at around 6 Ma causing deformation to focus in the MER. However, this interpretation contradicts plate-tectonic reconstruction data suggesting a constant plate divergence direction since ca. 16 Ma. We use analogue models to study how deformation may occur along the YTVL. We find that the activation of lineaments oriented (near-)parallel to the plate divergence direction is in fact possible, if (1) the lineament sufficiently reduces the strength of the crust and (2) the main rift trend is sufficiently oblique to the overall plate divergence direction. We interpret this to be the result of a local reorientation of the regional extensional strain caused by the obliquity of the main rift, which allows for deformation along the otherwise unfavourably oriented lineament. As such, no multiphase scenario is required to explain the development of the YTVL, and a single-phase scenario that is in line with plate tectonic reconstructions can be adopted instead. Moreover, our model results suggest that other lineaments associated with the MER could also be active in the current tectonic regime. These insights may be of relevance to the interpretation of deformation along similar oblique structures associated with the MER, and with other past and present rift systems as well.

Can Pseudotachylytes Form via Fracture-Induced Decompression Melting Under Hydrous Conditions?

Mattia Pistone — 2025-08-28

Frictional rock sliding and resultant shear heating along a fault plane are proposed as the necessary conditions to generate earthquake-related pseudotachylytes. However, frictional melting alone is energy expensive, requiring large temperature increases of several hundreds to thousands of degrees. Using the example of the pseudotachylyte structures of the Balmuccia Peridotite Massif (Ivrea-Verbano Zone, Alps, Italy), a minimum temperature increase of up to ~540°C for frictional melting has been proposed under isobaric and anhydrous conditions. Such conditions are however inconsistent with the moderate temperature increases, of up to~400°C, and diminishing pressures, less than 0.7 GPa, required to form the observed A-type pseudotachylyte structures, with ultramafic composition corresponding to the melting of specific mineral phases as a function of Carboniferous Period pressure and temperature conditions. Here we show that pseudotachylytes could be produced by fracture-induced decompression melting under hydrous conditions, that favor the formation of immiscible liquids derived from melting Al-Cr spinel and orthopyroxene with suspended clinopyroxene minerals in this immiscible melt. We propose thermodynamic calculations that constrain phase stability in the pressure-temperature space of the Balmuccia lherzolite under both anhydrous and hydrous conditions (0.5 to 1 wt.% H₂O). We illustrate that the Al-Cr spinel+orthopyroxene composition of the pseudotachylyte is consistent with lower pressure conditions than those of the initial peridotite prior to fracturing. These thermodynamic calculations help determine the pressure-temperature path of pseudotachylyte formation, not only favored by frictional heating but also by pressure drop (0.3--0.9 GPa) following dilation related to rock fracturing. Our results call for a reassessment of the origin of many pseudotachylytes formed in the lower crust and upper mantle. We show that fracture-induced decompression melting under hydrous conditions can be a viable mechanism that assists frictional melting by reducing the temperature rise from ambient temperature to melting temperature by 18% to 74%. A similar process may be significant in producing other pseudotachylytes during tectonic movement of lithospheric blocks in the deep crust and upper mantle.

Is there a Cretan Supradetachment Basin? Insights From Detailed Mapping on Northwestern Crete (Greece)

Willem Jan Zachariasse — 2025-09-28

The island of Crete in the South Aegean forearc exposes a fragmented and dismembered non-metamorphic nappe stack that is separated from underlying, exhumed high-pressure, low-temperature metamorphic rocks by an extensional detachment system. Exhumation and nappe thinning is thought to result from large-scale extension, which occurred mostly between ~20 and 13 Ma according to cooling ages. Such major extension normally forms surface depressions, but surprisingly, sedimentary basin sediments on Crete post-date the bulk of exhumation and are younger than ~11 Ma. Because the oldest sedimentary rocks do not rework metamorphic rocks, they were interpreted as a (late-stage) supra-detachment basin, although the tectonic reconstruction of the oldest sediments is challenging. Here, we provide a new detailed geological map of northwestern Crete where the oldest sediments are best exposed. We show that the stratigraphy contains several hiatuses because of tectonic reorganizations that separate superimposed basin systems that were bounded by different major faults accommodating first N-S and later E-W extension. We find that even the oldest of these fault systems, starting at 10.9 Ma and governing the oldest sedimentary units of the Topolia conglomerates, must already have cut through the Cretan Detachment. The sedimentary basins of northwestern Crete thus entirely post-date activity of this detachment. Final exhumation of Crete's HP-LT complex was instead likely related to erosion in uplifted footwalls of normal faults. Our results highlight a paradox that during Crete's crustal thinning and HP-LT rock exhumation, it maintained a high topography, and that all basin formation occurred during later fore-arc extension.