Search

The submarine volcano Axial Seamount has exhibited an inflation predictable eruption cycle, which allowed for the successful forecast of its 2015 eruption. However, the exact triggering mechanism of its eruptions remains ambiguous. The inflation predictable eruption pattern suggests a magma reservoir pressure threshold at which eruptions occur, and as such, an overpressure eruption triggering mechanism. However, recent models of volcano unrest suggest that eruptions are triggered when conditions of critical stress are achieved in the host rock surrounding a magma reservoir. We test hypotheses of eruption triggering using 3-dimensional finite element models which track stress evolution and mechanical failure in the host rock surrounding the Axial magma reservoir. In addition, we provide an assessment of model sensitivity to various temperature and non-temperature-dependent rheologies and external tectonic stresses. In this way, we assess the contribution of these conditions to volcanic deformation, crustal stress evolution, and eruption forecasts. We conclude that model rheology significantly impacts the predicted timing of through-going failure and eruption. Models consistently predict eruption at a reservoir pressure threshold of 12–14 MPa regardless of assumed model rheology, lending support to the interpretation that eruptions at Axial Seamount are triggered by reservoir overpressurization.

Near-axis seamounts provide a unique setting to investigate three-dimensional mantle processes associated with the formation of new oceanic crust and lithosphere. Here, we investigate the characteristics and evolution of the 8˚20’N Seamount Chain, a lineament of seamounts that extends ~ 175 km west of the East Pacific Rise (EPR) axis, just north of the fracture zone of the Siqueiros Transform Fault. Shipboard gravity, magnetic, and bathymetric data acquired in 2016 are utilized to constrain models of seamount emplacement and evolution. Geophysical observations indicate that these seamounts formed during four distinct episodes of volcanism coinciding with changes in regional plate motion that are also reflected in the development of intra-transform spreading centers (ITSCs) along the Siqueiros transform fault (Fornari et al. 1989; Pockalny et al. 1997). Although volcanism is divided into distinct segments, the magnetic data indicate continuous volcanic construction over long portions of the chain. Crustal thickness variations along the chain up to 0.75 km increase eastward, inferred from gravity measurements, suggest that plate reorganization has considerably impacted melt distribution in the area surrounding the Siqueiros-EPR ridge transform intersection. This appears to have resulted in increased volcanism and the formation of the 8˚20’N Seamounts. These findings indicate that melting processes in the mantle and subsequently the formation of new oceanic crust and lithosphere are highly sensitive to tectonic stress changes in the vicinity of fast-spreading transform fault offsets.

We use numerical models to study the mechanical stability of magma reservoirs embedded in elastic host rock. We quantify the overpressure required to open tensile fractures (the failure overpressure), as a function of the depth and the size of the reservoir, the loading by the volcanic edifice, and the pore fluid pressure in the crust. We show that the pore fluid pressure is the most important parameter controlling the magnitude of the failure overpressure rather than the reservoir depth and the edifice load. Under lithostatic pore fluid pressure conditions, the failure overpressure is on the order of the rock tensile strength (a few tens of megapascals). Under zero pore fluid pressure conditions, the failure overpressure increases linearly with depth (a few hundreds of megapascals at 5 km depth). We use our models to forecast the failure displacement (the cumulative surface displacement just before an eruption) on volcanoes showing unrest: Sinabung and Agung (Indonesia) and Okmok and Westdahl (Aleutian). By comparison between our forecast and the observation, we provide valuable constraint on the pore fluid pressure conditions on the volcanic system. At Okmok, the occurrence of the 2008 eruption can be explained with a 1,000 m reservoir embedded in high pore fluid pressure, whereas the absence of eruption at Westdahl better suggests that the pore fluid pressure is much lower than lithostatic. Our finding suggests that the pore fluid pressure conditions around the reservoir may play an important role in the triggering of an eruption by encouraging or discouraging the failure of the reservoir.

Highlights: New data reveal for the first time a history of the last ~33.7 ky of uplift of Samosir. Minimum uplift rates were high (4.9 cm/year) for the first 11.2 ky but diminished after that to

High-resolution magnetic data collected along the axis and the south flank of the Puna Ridge, the submarine extension of the East Rift Zone of Kilauea Volcano, Hawaii, aid in the interpretation of magmatic processes. Crustal magnetization varies along the crest of the Puna Ridge. Three prominent magnetization highs are located between the depths of 1500 and 2500 m. Each of the magnetization highs is associated with distinct volcanic features: a flow field on top of fissured terrain, a lava plateau, and a staircase of lava terraces. The crustal magnetization highs can be explained by the presence of these volcanic features, taking into account water depth and the size of the volcanic features and making simple assumptions about the rock magnetization. No measurements of rock magnetization have been made on Puna Ridge samples. We assume two values: 50 and 10 A/m, based on values from mid-ocean ridge basalts and the estimated ages of Puna Ridge rocks [Clague et al., 1995]. The two shallowest magnetization highs are adjacent to a landslide scarp. Lava flows overtop the scarp suggesting that a dike has been emplaced and eruptions have occurred since the landslide formed. A possible scenario is that the collapse of the flank led to dike intrusion and eruption at this location because of a reduction in the horizontal stress within the edifice. Our observations suggest that flows are starting to fill in the gap caused by the landslide to rebuild this section of the north flank.

[1] We examine mantle melting, fractional crystallization, and melt extraction beneath fast slipping, segmented oceanic transform fault systems. Three-dimensional mantle flow and thermal structures are calculated using a temperature-dependent rheology that incorporates a viscoplastic approximation for brittle deformation in the lithosphere. Thermal solutions are combined with the near-fractional, polybaric melting model of Kinzler and Grove (1992a, 1992b, 1993) to determine extents of melting, the shape of the melting regime, and major element melt composition. We investigate the mantle source region of intratransform spreading centers (ITSCs) using the melt migration approach of Sparks and Parmentier (1991) for two end-member pooling models: (1) a wide pooling region that incorporates all of the melt focused to the ITSC and (2) a narrow pooling region that assumes melt will not migrate across a transform fault or fracture zone. Assuming wide melt pooling, our model predictions can explain both the systematic crustal thickness excesses observed at intermediate and fast slipping transform faults as well as the deeper and lower extents of melting observed in the vicinity of several transform systems. Applying these techniques to the Siqueiros transform on the East Pacific Rise we find that both the viscoplastic rheology and wide melt pooling are required to explain the observed variations in gravity inferred crustal thickness. Finally, we show that mantle potential temperature Tp = 1350°C and fractional crystallization at depths of 9–15.5 km fit the majority of the major element geochemical data from the Siqueiros transform fault system.

Gravity and bathymetry data from transform fault systems associated with mid-ocean ridges that are spreading at different rates indicates that crustal thickening may occur under transform faults associated with intermediate- and fast-spreading ridges. If confirmed, this observation would require a change in our understanding of how magma accretion occurs along rapidly spreading mid-ocean ridges. Mid-ocean ridge morphology and crustal accretion are known to depend on the spreading rate of the ridge. Slow-spreading mid-ocean-ridge segments exhibit significant crustal thinning towards transform and non-transform offsets1,2,3,4,5,6,7,8,9,10,11,12, which is thought to arise from a three-dimensional process of buoyant mantle upwelling and melt migration focused beneath the centres of ridge segments1,2,4,5,6,7,9,10,12. In contrast, fast-spreading mid-ocean ridges are characterized by smaller, segment-scale variations in crustal thickness, which reflect more uniform mantle upwelling beneath the ridge axis13,14,15. Here we present a systematic study of the residual mantle Bouguer gravity anomaly of 19 oceanic transform faults that reveals a strong correlation between gravity signature and spreading rate. Previous studies have shown that slow-slipping transform faults are marked by more positive gravity anomalies than their adjacent ridge segments1,2,4,6, but our analysis reveals that intermediate and fast-slipping transform faults exhibit more negative gravity anomalies than their adjacent ridge segments. This finding indicates that there is a mass deficit at intermediate- and fast-slipping transform faults, which could reflect increased rock porosity, serpentinization of mantle peridotite, and/or crustal thickening. The most negative anomalies correspond to topographic highs flanking the transform faults, rather than to transform troughs (where deformation is probably focused and porosity and alteration are expected to be greatest), indicating that crustal thickening could be an important contributor to the negative gravity anomalies observed. This finding in turn suggests that three-dimensional magma accretion may occur near intermediate- and fast-slipping transform faults.

Seven of the eight transform systems along the equatorial East Pacific Rise from 12° N to 15° S have undergone extension due to reorientation of plate motions and have been segmented into two or more strike-slip fault strands offset by intratransform spreading centers (ITSCs). Earthquakes recorded along these transform systems both teleseismically and hydroacoustically suggest that segmentation geometry plays an important role in how slip is accommodated at oceanic transforms. Results of thermal calculations suggest that the thickness of the brittle layer of a segmented transform fault could be significantly reduced by the thermal effect of ITSCs. Consequently, the potential rupture area, and thus maximum seismic moment, is decreased. Using Coulomb static stress models, we illustrate that long ITSCs will prohibit static stress interaction between transform segments and limit the maximum possible magnitude of earthquakes on a given transform system. Furthermore, transform earthquakes may have the potential to trigger seismicity on normal faults flanking ITSCs.