Structural Control of Flank Volcanism in Continental Rifts

Sensitivity of rift tectonics to global variability in the efficiency of river erosion

The efficiency of erosion in leveling relief mainly depends on climate and strength of exposed rocks. However, whether erosion is sufficiently efficient to influence the architecture of a tectonic plate boundary remains a topic of debate. Here, we analyze continental rift landscapes reworked by river incision to assess a globally representative range of fluvial erosion efficiency. We then simulate crustal extension exposed to surface processes acting within this documented range. We find that more efficient erosion favors the growth of half-grabens over horsts, which can explain contrasting tectonic styles across the Basin and Range province and the East African Rift. This suggests that variability in Earth’s geological structures partly reflects variability in hydrological conditions and associated surface processes.

Erosion and sedimentation constantly rework topography created by tectonics but also modulate stresses in the underlying crust by redistributing surficial loads. Decades of numerical modeling further suggest that surface processes help focus deformation onto fewer, longer-lived faults at tectonic plate boundaries. However, because the surface evolution parameters used in these models are not quantitatively calibrated against real landscapes and because the history of fault activity can be difficult to infer from the geological record, the sensitivity of tectonic deformation to a realistic range of erosional efficiency remains unknown. Here, we model the growth of half-grabens, where slip on a master normal fault shapes an adjacent mountain range as it accommodates crustal stretching. We subject our simulations to fluvial incision acting at rates assessed by morphometric analysis of rivers draining natural rift systems. Increasing erosional efficiency within the geologically documented range alleviates the energy cost of topographic growth and increases the total extension that can be accommodated by half-graben master faults by as much as ∼50%. Efficient erosion favors an eventual basin-ward relocalization of strain, preventing the development of horst structures. This behavior is consistent with structural and morphometric observations across 12 normal fault-bounded ranges, suggesting that surface erodibility and climatic conditions have a measurable impact on the tectonic makeup of Earth’s plate boundaries.

Fault-controlled deep hydrothermal flow in a back-arc tectonic setting, SE Tyrrhenian Sea

Understanding magmatic systems and deep hydrothermal circulation beneath arc-volcanoes provides insights into deep processes associated with slab-subduction and mantle-wedge partial melting. Here we analyze hydrothermal flow below a structural high (Capo Vaticano Ridge, CVR) located offshore Capo Vaticano (western Calabria) and affected by magmatic intrusions generated from above the Ionian subducting-slab. In order to explain observations, we combine geophysical and numerical modelling results. Fluid-flow modelling shows that temperature distribution and geothermal gradient are controlled mainly by hydrothermal circulation, in turn affected by heat source, fault pattern, rock permeability, basement topography and sediment thickness. Two main faults, shaping the structural high and fracturing intensely the continental crust, enable deep hydrothermal circulation and shallow fluid discharge. Distribution of seismicity at depth supports the hypothesis of a slab below Capo Vaticano, deep enough to enable mantle-wedge partial melting above the subduction zone. Melt migration at shallow levels forms the magmatic intrusions inferred by magnetic anomalies and by δ³He enrichment in the discharged fluids at the CVR summit. Our results add new insights on the southern Tyrrhenian Sea arc-related magmatism and on the Calabrian inner-arc tectonic setting dissected by seismogenic faults able to trigger high-destructive earthquakes.

A new volcanic province: an inventory of subglacial volcanoes in West Antarctica

The West Antarctic Ice Sheet overlies the West Antarctic Rift System about which, due to the comprehensive ice cover, we have only limited and sporadic knowledge of volcanic activity and its extent. Improving our understanding of subglacial volcanic activity across the province is important both for helping to constrain how volcanism and rifting may have influenced ice-sheet growth and decay over previous glacial cycles, and in light of concerns over whether enhanced geothermal heat fluxes and subglacial melting may contribute to instability of the West Antarctic Ice Sheet. Here, we use ice-sheet bed-elevation data to locate individual conical edifices protruding upwards into the ice across West Antarctica, and we propose that these edifices represent subglacial volcanoes. We used aeromagnetic, aerogravity, satellite imagery and databases of confirmed volcanoes to support this interpretation. The overall result presented here constitutes a first inventory of West Antarctica's subglacial volcanism. We identified 138 volcanoes, 91 of which have not previously been identified, and which are widely distributed throughout the deep basins of West Antarctica, but are especially concentrated and orientated along the >3000 km central axis of the West Antarctic Rift System.

Off-axis magmatism along a subaerial back-arc rift: Observations from the Taupo Volcanic Zone, New Zealand

Continental rifting and seafloor spreading play a fundamental role in the generation of new crust. However, the distribution of magma and its relationship with tectonics and volcanism remain poorly understood, particularly in back-arc settings. We show evidence for a large, long-lived, off-axis magmatic intrusion located on the margin of the Taupo Volcanic Zone, New Zealand. Geodetic data acquired since the 1950s show evidence for uplift outside of the region of active extension, consistent with the inflation of a magmatic body at a depth of ~9.5 km. Satellite radar interferometry and Global Positioning System data suggest that there was an increase in the inflation rate from 2003 to 2011, which correlates with intense earthquake activity in the region. Our results suggest that the continued growth of a large magmatic body may represent the birth of a new magma chamber on the margins of a back-arc rift system.

Structural Processes at Slow-Spreading Ridges

Slow-spreading (<35 millimeters per year) mid-ocean ridges are dominated by segmented, asymmetric, rifted depressions like continental rifts. Fast-spreading ridges display symmetric, elevated volcanic edifices that vary in shape and size along axis. Deep earthquakes, major normal faults, and exposures of lower crustal rocks are common only along slow-spreading ridges. These contrasting features suggest that mechanical deformation is far more important in crustal formation at slow-spreading ridges than at fast-spreading ridges. New seismic images suggest that the nature and scale of segmentation of slow-spreading ridges is integral to the deformational process and not to magmatic processes that may control segmentation on fast-spreading ridges.