Rift Tip Research Articles

Origin of paired extension-compression during rotational rifting: An early Paleogene example from the northeast Atlantic region and its implications

Abstract The formation of paired extension-compression (PEC) postulated by rotational kinematics of rift propagation is demonstrated by analogue models but rarely observed in nature. In our study of the early Paleogene continental rift in the northeast Atlantic, a PEC is proposed based on the northeastward propagation and the coeval compression at the rift tip. The propagation is deduced from tectono-magmatic trends, including along-axis development of magmatism, and migration of tectonic faulting inward and toward the rift tip. Where this propagation terminated, we documented coeval extension and compression in the form of a horst-and-graben system (H&G) and V-shaped anticline (VA), respectively. Given their structural characteristics and spatiotemporal relationships with the rift, their origin is best illustrated by a three-stage model: (1) Rifting initiated at the site of mantle upwelling and propagated northeastward in the Paleocene. (2) The rift tip was stalled by an elevated mafic-ultramafic body at the Barents Shelf, which led to forward projection of the rift’s driving force to create the H&G and the VA (PEC), dissipating the along-axis force component. (3) Domination of axis-perpendicular components then promoted orthogonal extension and sheared margin development. Our study suggests that PEC has a crucial role in both termination of propagation and rift-mode conversion.

Tracking the Cracking: A Holistic Analysis of Rapid Ice Shelf Fracture Using Seismology, Geodesy, and Satellite Imagery on the Pine Island Glacier Ice Shelf, West Antarctica.

Ice shelves regulate the stability of marine ice sheets. We track fractures on Pine Island Glacier, a quickly accelerating glacier in West Antarctica that contributes more to sea level rise than any other glacier. Using an on‐ice seismic network deployed from 2012 to 2014, we catalog icequakes that dominantly consist of flexural gravity waves. Icequakes occur near the rift tip and in two distinct areas of the shear margin, and TerraSAR‐X imagery shows significant fracture in each source region. Rift‐tip icequakes increase with ice speed, linking rift fracture to glaciological stresses and/or localized thinning. Using a simple flexural gravity wave model, we deconvolve wave propagation effects to estimate icequake source durations of 19.5–50.0 s and transient loads of 3.8–14.0 kPa corresponding to 4.3–15.9 m of crevasse growth per icequake. These long‐source durations suggest that water flow may limit the rate of crevasse opening.

Effects of the Hydrous Domain in the Mantle Wedge on Magma Formation and Mixing at the Northeast Lau Spreading Center, SW Pacific

AbstractAbundant volcanic activity occurs in the back‐arc region of the northern Tofua island arc where the Northeast Lau Spreading Center (NELSC) propagates southwards into older crust causing the formation of numerous seamounts at the propagating rift tip. An off‐axis volcanic diagonal ridge (DR) occurs at the eastern flank of the NELSC, linking the large rear‐arc volcano Niuatahi with the NELSC. New geochemical data from the NELSC, the southern propagator seamounts, and DR reveal that the NELSC lavas are tholeiitic basalts whereas the rear‐arc volcanoes typically erupt lavas with boninitic composition. The sharp geochemical boundary probably reflects the viscosity contrast between off‐axis hydrous harzburgitic mantle and dry fertile mantle beneath the NELSC. The new data do not indicate an inflow of Samoa plume mantle into the NELSC, confirming previously published He isotope data. The NELSC magmas form by mixing of an enriched and a depleted Indian Ocean‐type upper mantle end‐member implying a highly heterogeneous upper mantle composition in this area. Most NELSC lavas are little affected by a slab component implying that melting is adiabatic beneath the spreading center. The DR lavas show the influence of a component from the subducted Louisville Seamount Chain, which was previously thought to be restricted to the nearby arc volcanoes Niuatoputapu and Tafahi. This signature is rarely detected along the NELSC implying little mixing of melts from the low‐viscosity hydrous portion of the mantle wedge beneath the rear‐arc volcanoes into the melting region of the dry mantle beneath the NELSC.

A High Resolution, Three‐Dimensional View of the D‐28 Calving Event From Amery Ice Shelf With ICESat‐2 and Satellite Imagery

AbstractTabular calving events occur from Antarctica's large ice shelves only every few decades, and are preceded by rift propagation. We used high‐resolution imagery and ICESat‐2 data to determine the propagation rates for the three active rifts on Amery Ice Shelf (AIS; T1, T2, and E3) and observe the calving of D‐28 on September 25, 2019 along T1. AIS front advance accelerated downstream of T1 in the years before calving, possibly increasing stress at the rift tip. T1 experienced significant acceleration for 12 days before calving, coinciding with a jump in propagation of E3. ICESat‐2's high resolution and repeat acquisitions every 91 days allowed for analysis of the ice front before and after calving, and rift detection where it was not visible in imagery as a ∼1 m surface depression, suggesting that it propagates as a basal fracture. Our results show that ICESat‐2 can provide process‐scale information about iceberg calving.

Ice Shelf Rift Propagation and the Mechanics of Wave‐Induced Fracture

AbstractDistant storms, tsunamis, and earthquakes generate waves on floating ice shelves. Previous studies, however, have disagreed about whether the resulting wave‐induced stresses may cause ice shelf rift propagation. Most ice shelf rifts show long periods of dormancy suggesting that they have low background stress concentrations and may therefore be susceptible to wave‐induced stresses. Here I quantify wave‐induced stresses on the Ross Ice Shelf Nascent Rift and the Amery Ice Shelf Loose Tooth T2 Rift using passive seismology. I then relate these stresses to a fracture mechanical model of rift propagation that accounts for rift cohesive strength due to refrozen melange, ice inertia, and spatial heterogeneity in fracture toughness due to the presence of high toughness suture zones. I infer wave‐induced stresses using the wave impedance tensor, a rank three tensor that relates seismically observable particle velocities to components of the stress tensor. I find that wave‐induced stresses are an order of magnitude larger on the Ross Ice Shelf as compared to the Amery Ice Shelf. In the absence of additional rift strength, my model predicts that the Nascent Rift should have experienced extensive rift propagation. The observation that no such propagation occurred during this time therefore suggests that the Nascent Rift experiences strengthening from either refrozen melange or rift tip processes zone dynamics. This study illustrates one way in which passive seismology may illuminate glacier calving physics.

Opening of Hess Deep Rift at the Galapagos Triple Junction

AbstractAt the Galapagos triple junction, the westward propagating Cocos‐Nazca (C‐N) Rift breaks into ~0.5 Ma crust accreted at the East Pacific Rise. Rifting transitions to full magmatic seafloor spreading in the wake of the propagating tip. The 25‐km‐long Hess Deep rift is the transitional segment from rifting to spreading. Intrarift ridge (IRR), located within Hess Deep rift, is interpreted as a detachment fault, which exhumes deep‐seated rocks to the seafloor. Although transitional segments must have occurred throughout the westward propagation of C‐N Rift, IRR is the only obvious detachment fault along the base of the Rift scarps in the last ~5 Ma of its propagation. IRR formation may be in response to a decrease in spreading rate (~40 to <20 mm/yr) and presumed lower melt supply, resulting from the formation of the Galapagos microplate ~1.4 Ma, which now controls the opening at the C‐N Rift tip.

Recent rift formation and impact on the structural integrity of the Brunt Ice Shelf, East Antarctica

Abstract. We report on the recent reactivation of a large rift in the Brunt Ice Shelf, East Antarctica, in December 2012 and the formation of a 50 km long new rift in October 2016. Observations from a suite of ground-based and remote sensing instruments between January 2000 and July 2017 were used to track progress of both rifts in unprecedented detail. Results reveal a steady accelerating trend in their width, in combination with alternating episodes of fast ( > 600 m day−1) and slow propagation of the rift tip, controlled by the heterogeneous structure of the ice shelf. A numerical ice flow model and a simple propagation algorithm based on the stress distribution in the ice shelf were successfully used to hindcast the observed trajectories and to simulate future rift progression under different assumptions. Results show a high likelihood of ice loss at the McDonald Ice Rumples, the only pinning point of the ice shelf. The nascent iceberg calving and associated reduction in pinning of the Brunt Ice Shelf may provide a uniquely monitored natural experiment of ice shelf variability and provoke a deeper understanding of similar processes elsewhere in Antarctica.

The role of asthenospheric flow during rift propagation and breakup

Continental rifting precedes the breakup of continents, leading to the formation of passive margins and oceanic lithosphere. Although rifting dynamics is classically described in terms of either active rifting caused by active mantle upwelling, or passive rifting caused by far-field extensional stresses, it was proposed that a transition from passive to active rifting can result from changes in buoyancy forces due to localized thinning of the lithosphere. Three-dimensional numerical experiments of rifting near an Euler pole allow the quantification of these buoyancy forces and show that gravitational stresses are strong enough not only to sustain rifting and drive axis-parallel motion in the asthenosphere dome, but also to promote along-axis asthenospheric flow and to drive the propagation of the rift tip toward its rotation pole. We show that gradients of gravitational potential energy due to the presence of the dome of asthenosphere induce time-dependent phases of compressional and transcurrent stress regimes, despite an overall divergent plate setting. Our experiments predict overdeepened bathymetry at the tip of the propagating rift, as well as the variability of focal mechanisms of shallow seismic events similar to those observed in such a setting. We also explain the episodes of basin inversion documented in many rifted continental margins.

Degassing and magma mixing during the eruption of Surtsey Volcano (Iceland, 1963–1967): the signatures of a dynamic and discrete rift propagation event

The eruption of Surtsey (Iceland, 1963–1967) is a rare example of observed volcanism at a propagating rift tip, where Iceland’s Eastern Volcanic Zone is advancing southwesterly toward the Reykjanes Ridge. We use olivine-hosted melt inclusions, embayments, and matrix glasses to investigate major element, trace element, and volatile characteristics of Surtsey’s magmatic system, and to parameterize decompression models that describe degassing of C-H-S-O species during this well-studied eruption. Major elements show that the inclusions represent heterogeneous melts with a range of compositions similar to those of Icelandic lavas. Trace elements discriminate between two groups of inclusions: the majority are compositionally similar to Eastern Volcanic Zone alkalic basalts, and the remainder are notably more depleted in incompatible trace elements and trend toward fields defined by tholeiites on and around Iceland. Strongly correlated Cl and Nb for all inclusions indicates that seawater did not affect Surtsey’s volatile budget, despite having clearly affected its eruption style. Both types of inclusions are volatile rich (≤1.47 wt.% H2O; ≤2534 ppm CO2 as measured or ≤5286 ppm once corrected for vapor bubbles and post-entrapment crystallization). They are more hydrous than primary melts in the onshore Eastern Volcanic Zone and have parental CO2/Nb (≤590) at the upper end of regional estimates. Saturation pressures calculated from corrected CO2 values indicate that melts in both inclusion types initially crystallized at similar depths in the upper mantle (17–21 km), and then partially crystallized during ascent through the lower crust (7–13 km). Historical data show that gases emitted on Surtsey shifted from being relatively reduced in 1964/1965 to more oxidized in 1967, after a protracted period of effusive activity and resurgence of pyroclastic activity from satellite vents. Closed-system degassing models predict the compositions and redox states of the 1964/1965 gases extremely well, but cannot account for the oxidized gases emitted in 1967, which may have been contaminated by ambient air in a system that was opening as the eruption waned. Surtsey’s pyroclastic resurgence can be explained by recharge from ephemeral and compositionally heterogeneous magma bodies, tapped from possibly as deep as the mantle-crust boundary, in a process consistent with the progressively increasing interconnection between magma bodies that is typical at propagating rift tips. The eruption of Surtsey therefore shows that magma system evolution at rift tips can occur in dynamic and discrete events, with influx of new magma having explosive consequences.

Seismicity within a propagating ice shelf rift: The relationship between icequake locations and ice shelf structure

AbstractIceberg calving is a dominant mass loss mechanism for Antarctic ice shelves, second only to basal melting. An important process involved in calving is the initiation and propagation of through‐penetrating fractures called rifts; however, the mechanisms controlling rift propagation remain poorly understood. To investigate the mechanics of ice shelf rifting, we analyzed seismicity associated with a propagating rift tip on the Amery Ice Shelf, using data collected during the austral summers of 2004–2007. We apply a suite of passive seismological techniques including icequake locations, back projection, and moment tensor inversion. We confirm previous results that show ice shelf rifting is characterized by periods of relative quiescence punctuated by swarms of intense seismicity of 1 to 3 h. Even during periods of quiescence, we find significant deformation around the rift tip. Moment tensors, calculated for a subset of the largest icequakes (Mw > −2.0) located near the rift tip, show steeply dipping fault planes, horizontal or shallowly plunging stress orientations, and often have a significant volumetric component. They also reveal that much of the observed seismicity is limited to the upper 50 m of the ice shelf. This suggests a complex system of deformation that involves the propagating rift, the region behind the rift tip, and a system of rift‐transverse crevasses. Small‐scale variations in the mechanical structure of the ice shelf, especially rift‐transverse crevasses and accreted marine ice, play an important role in modulating the rate and location of seismicity associated with the propagating ice shelf rifts.

The recent history of the Galapagos triple junction preserved on the Pacific plate

At the Galapagos triple junction, the Cocos and Nazca plates are broken by a succession of transient rifts north and south of the Cocos–Nazca (C–N) Rift. Modeling has suggested that each rift initiated at the East Pacific Rise (EPR), its location controlled by the distance of the C–N Rift tip from the EPR. Evidence on the Pacific plate confirms that each transient rift formed a true RRR triple junction with the EPR and clarifies the history of the region. At ∼1.5Ma the triple junctions began jumping rapidly toward the C–N Rift suggesting that the C–N Rift tip moved closer to the EPR. Pacific abyssal hills became broad and shallow indicating enhanced magma supply to the region. At ∼1.4Ma, the Galapagos microplate developed when extension became fixed on the southern transient rift to form the South scarp of the future Dietz rift basin. Lavas flooded the area and a Galapagos–Nazca magmatic spreading center initiated at the EPR. We suggest that a hotspot was approaching the southern triple junction from the west. The hotspot crossed to the Nazca plate ∼1.25Ma. Dietz seamount formed within the young spreading center, dikes intruded Dietz rift basin, and eruptions built volcanic ridges. Since ∼0.8Ma magmatic spreading has jumped northward twice, most recently to Dietz volcanic ridge. Amagmatic extension to the east has formed the large North scarp of Dietz rift basin. Northward jumping of the southern triple junction has maintained the microplate boundary close to the proposed hotspot.

The formation of a failed continental breakup basin: The Cenozoic development of the Faroe‐Shetland Basin

AbstractUltra‐large rift basins, which may represent palaeo‐propagating rift tips ahead of continental rupture, provide an opportunity to study the processes that cause continental lithosphere thinning and rupture at an intermediate stage. One such rift basin is the Faroe‐Shetland Basin (FSB) on the north‐east Atlantic margin. To determine the mode and timing of thinning of the FSB, we have quantified apparent upper crustal β‐factors (stretching factors) from fault heaves and apparent whole‐lithosphere β‐factors by flexural backstripping and decompaction. These observations are compared with models of rift basin formation to determine the mode and timing of thinning of the FSB. We find that the Late Jurassic to Late Palaeocene (pre‐Atlantic) history of the FSB can be explained by a Jurassic to Cretaceous depth‐uniform lithosphere thinning event with a β‐factor of ~1.3 followed by a Late Palaeocene transient regional uplift of 450–550 m. However, post‐Palaeocene subsidence in the FSB of more than 1.9 km indicates that a Palaeocene rift with a β‐factor of more than 1.4 occurred, but there is only minor Palaeocene or post‐Palaeocene faulting (upper crustal β‐factors of less than 1.1). The subsidence is too localized within the FSB to be caused by a regional mantle anomaly. To resolve the β‐factor discrepancy, we propose that the lithospheric mantle and lower crust experienced a greater degree of thinning than the upper crust. Syn‐breakup volcanism within the FSB suggests that depth‐dependent thinning was synchronous with continental breakup at the adjacent Faroes and Møre margins. We suggest that depth‐dependent continental lithospheric thinning can result from small‐scale convection that thins the lithosphere along multiple offset axes prior to continental rupture, leaving a failed breakup basin once seafloor spreading begins. This study provides insight into the structure and formation of a generic global class of ultra‐large rift basins formed by failed continental breakup.

The origin and geochemical evolution of the Woodlark Rift of Papua New Guinea

Geochemical, isotopic, and geochronologic data for exhumed rocks in the Woodlark Rift of Papua New Guinea (PNG) allow a tectonic link to be established with the Late Cretaceous Whitsunday Volcanic Province (WVP) of northeastern Australia. Most of the metamorphic rocks in the Woodlark Rift have Nd isotopic compositions (εNd=+1.7 to +6.2) similar to the Nd isotopic compositions of rocks in the WVP (εNd=+1.3 to +6.6; Ewart et al., 1992), and contain inherited zircons with 90 to 100Ma U–Pb ages that overlap the timing of magmatism in the WVP. None of the metamorphic rocks in the Woodlark Rift have the highly evolved Hf and Nd isotopic compositions expected of ancient continental crust. Magmas were erupted in the WVP during the middle Cretaceous as eastern Gondwana was rifted apart. The protoliths of felsic and intermediate metamorphic rocks in the Woodlark Rift are interpreted to be related to the magmatic products produced during this Cretaceous rifting event. Some mafic metamorphic rocks exposed in the western Woodlark Rift (eclogites and amphibolites) are not related to the WVP and instead could have originated as basaltic lavas crystallized from mantle melts at (U)HP depths in the Late Cenozoic, or as fragments of Mesozoic aged oceanic lithosphere.Isotopic and elemental comparisons between basement gneisses and Quaternary felsic volcanic rocks demonstrate that felsic lavas in the D'Entrecasteaux Islands did not form solely from partial melting of metamorphic rocks during exhumation. Instead, the isotopic compositions and geochemistry of Quaternary felsic volcanic rocks indicate a significant contribution from the partial melting of the mantle in this region. When combined with geophysical data for the western Woodlark Rift, this suggests that future seafloor spreading will commence south of Fergusson Island, and west of the present-day active seafloor spreading rift tip.

Sensitivity analysis of rift induced delamination with application to Rwenzori Mountains

SUMMARY Rift induced delamination (RID) was favoured as a geodynamic process explaining the special situation of an extremely elevated old crustal block located in a continental rift system, the Rwenzori Mountains. The concept of RID is an upwelling asthenosphere encircling stiff lithosphere. If temperature- and stress-dependent viscosity of the undermost lower crust is reduced and strength fails, the detachment of cold and dense mantle-lithosphere root is trigged. Surface expressions are propagating rift tips surrounding an uplifting less dense crustal block. This study was performed to test the scope of application of RID hypothesis. Characteristic features of the numerical models are kept constant, whereas parameters controlling the initial temperature anomaly, rheological properties and the background geotherm are varied systematically. The system answer is a bifurcation, either the anomaly cools down or the central mantle lithosphere detaches and sinks down, or, rather an unsuccessful and successful RID. The excess temperature of the prescribed initial anomaly is most sensitive and serves at the transition from cooling to delamination as a critical temperature. Delamination works only in certain ranges. The anomaly full width ranges between 20 and 50km, the offset of the symmetric anomalies lies between 64 and 180km and the top anomaly has to reach into the lower crust. The corresponding geometric characteristics of the Rwenzori region always lies inside the found parameter ranges for the delamination regime, even near the optimum. Quantitative parameters concerning rheology are uncertain but, qualitatively, conform to the available findings. Thus, we establish RID as a possible mechanism coinciding roughly with the local observations, such as, an old crustal horst inside a young rift system, a missing mountain root, the high seismicity, low seismic velocities in the lower crust, minor volcanism and extreme topography. If RID is valid for the very special situation of the Rwenzoris and by now the sole example, one ought to generalize it and the parameter regimes explored can be used as a pattern for searching similar geological situations.

Transient rifting north of the Galápagos Triple Junction

Seafloor bathymetry north of the Galápagos microplate in the Eastern Pacific Ocean contains evidence for a sequence of short-lived rifts cross-cutting abyssal hills adjacent to the East Pacific Rise. These secondary rifts are sub-parallel to the Incipient Rift that marks the current triple junction at 2°40′N and are nearly perpendicular to the direction of opening between the Cocos and Nazca plates. Secondary rifts likely initiated from the EPR by lithospheric cracking. Eventually, their activity stopped and they were carried away from the triple junction as part of the Cocos plate. We developed and analyzed numerical models of rift interaction to understand the evolution of rift configuration in this area. By varying the geometry and locations of rifts, we constrain the factors that have led to the location and orientation of secondary rifts at the northern Galápagos Triple Junction. Interaction between secondary rifts and the Cocos-Nazca Rift results in ~ 10° clockwise rotation of the secondary rift propagation direction, as observed. Furthermore, if a rift has become detached from the East Pacific Rise, a zone of reduced tension is present ahead of the rift tip, prohibiting its connection to the East Pacific Rise. Two zones of tensile stress enhancement develop along the East Pacific Rise next to the detached rift and control where new cracks are likely to form. Although the magnitude of tensile stress enhancement is controlled by the gap between the detached rift and the East Pacific Rise, whether the new rift forms north or south of the detached rift is controlled by position of the Cocos-Nazca Rift. Therefore, the sequence of ancient rifts found northeast of the current triple junction at 2°40′N represents the natural consequence of rift disconnection events and their position records the kinematic history of the gap between the East Pacific Rise and the Cocos-Nazca Rift.

Shallow seismicity and tectonics of the central and northern Lau Basin

The Lau basin is an active backarc comprising several spreading centers and microplates rapidly evolving in time. As such, crustal earthquakes within the Lau Basin derive from intrabasin tectonic, volcanic, and hydrothermal sources. Because of high mantle attenuation, small earthquakes from the basin are seldom recorded on land seismographs and it has been difficult to study the pattern of crustal seismicity. However, the 1994 LABATTS ocean bottom seismograph experiment in the Lau basin recorded more than 100,000 local T- and body phases allowing for a detailed examination of basin seismotectonics. Nearly 1000 events are located within the basin proper, with another 2000 associated with the forearc and the aseismic front. We identify two previously undocumented tectonic features, a triple junction in the northwestern basin and a nanoplate in the central Lau basin. The single most seismically active feature within the Lau basin is the Lau extensional transform zone (LETZ) just north of the Central Lau Spreading Center. Significant seismicity also occurs near the northern extension of the Eastern Lau Spreading Center and along a southeast extension of the LETZ delineating the boundary of a previously unknown nanoplate. Swarm activity is common in the backarc basin and dominates regions associated with actively reorganizing tectonics. We see no evidence of deformation along the southern boundary of the Niuafo'ou microplate, where 4.5 cm/yr of strain is predicted. It is possible that the plate boundary extends eastward from the Fonualei rift (FR) tip towards the trench creating a Niuatoputapu–Tonga plate division rather than westward implied by the currently accepted Niuafo'ou–Tonga plate system, but more data are necessary from the FR to test that hypothesis. Instead, we suggest that the Niuafo'ou–Tonga pole is ~ 1.5° further north than previously proposed, reducing the predicted strain in this region to < 2 cm/yr.

Formation and stability of magmatic segments in the Main Ethiopian and Afar rifts

As rifting progresses to seafloor spreading, extension within the continental crust commonly is accommodated by a combination of fault slip and dike intrusion. Consistent patterns in the spatial arrangement of long-lived magma intrusion zones in the Ethiopian and Afar rift sectors, East Africa, suggest that the magma intrusions help to localize strain during repeated rifting episodes. Within the broad Main Ethiopian Rift, extensional deformation has localized since ∼3m.y. in narrow magmatic segments, that are oriented oblique to the orientation of the Miocene border faults, but (sub-) orthogonal to the extension direction. Numerical models combined with geophysical and geological observations from East Africa are used to examine the viability of self-sustaining magmatic segmentation. Initiation of the magmatic segments is shown to result from magma injections, which focus strain in narrow elongated zones. During magmatic phases of segment evolution the segments are weak, and extensional stresses localize at the rift tips, promoting along-axis lengthening. During amagmatic phases of extension, the numerical models predict strain localization within the magmatic segments and, to a lesser extent, broadly distributed extension within the rift zone. This promotes segment stability; the segments remain the preferred location for magma intrusion during new magmatic phases. These results are applied to the formation and maintenance of MER segmentation. The Fentale–Dofen segment is currently in a non-magmatic phase of extension; the Dabbahu segment in the Red Sea Rift is currently experiencing a rifting episode and therefore is in a transient magmatic cycle. The observed patterns of instantaneous localized deformation, seismicity, and dike intrusions sometimes propagating beyond the tip of the magmatic segments occur as predicted by the models.

Rift induced delamination of mantle lithosphere and crustal uplift: a new mechanism for explaining Rwenzori Mountains’ extreme elevation?

With heights of 4–5 km, the topography of Rwenzori Mountains, a large horst of old crustal rocks located inside a young passive rift system, poses the question “Why are the Rwenzori Mountains so high?”. The Cenozoic Western Rift branch of the East African Rift System is situated within the Late Proterozoic mobile belts between the Archean Tanzania Craton and Congo Craton. The special geological setting of the massif at a rift node encircled by the ends of the northern Western Rift segments of Lake Albert and Lake Edward suggests that the mechanism responsible for the high elevation of the Rwenzoris is related to the rifting process. Our hypothesis is based on the propagation of the rift tips, surrounding the stiff old lithosphere at Rwenzori region, thereby triggering the delamination of the cold and dense mantle lithosphere (ML) root by reducing viscosity and strength of the undermost lower crust. As a result, this unloading induces fast isostatic pop-up of the less dense crustal Rwenzori block. We term this RID—“rift induced delamination of Mantle Lithosphere”. The physical consistency of the RID hypothesis is tested numerically. Viscous flow of 2D models is approximated by a Finite Difference Method with markers in an Eulerian formulation. The equations of conservation of mass, momentum and energy are solved for a multi-component system. Based on laboratory data of appropriate rock samples, a temperature-, pressure- and stress-dependent rheology is assumed. Assuming a simple starting model with a locally heated ML, the ML block between the weakened zones becomes unstable and sinks into the asthenosphere, while the overlying continental crust rises up. Thus, RID seems to be a viable mechanism to explain geodynamically the extreme uplift. Important conditions are a thermal anomaly within the ML, a ductile lower crust with visco-plastic rheology allowing significant strength reduction and lateral density variations. The special situation of a two-sided rifting or offset rift segments to decouple the ML laterally from the surrounding continental lithosphere seems to be most decisive. Further support for the RID mechanism may come from additional crustal thickness and an extensive stress field. Some parameters, such as the excess temperature and yield stress, are very sensitive, small changes determine whether delamination takes place or not.

Horizontal Strain Rate Distribution on an Active Ice Shelf Rift from in-situ GPS Data

Mass loss from the Antarctic ice sheet predominantly occurs at the fringing ice shelves via iceberg calving, which is controlled by the initiation and propagation of large rifts that precede iceberg detachment and can lead to ice shelf break-up. This paper reports on the analysis of Global Positioning System (GPS) data collected at an active rift system on the Amery Ice Shelf, East Antarctica, over two field seasons. Horizontal strain rates are determined for a network of 11 sites observed over three weeks during the 2004/05 Antarctic summer period, and the results are combined with, and compared to, strain rates obtained in the 2002/03 season. Maximum principal strain rates across the network vary between 6 and 32 [x 10-3/yr], while minimum principal strain rates are generally about 1-17 [x 10-3/yr]. Changes evident in the strain distribution can mostly be attributed to existing fractures passing through the GPS network and the episodic movement of the rift tip. It is confirmed that rift propagation in 2005/06 was slowing down. Opening rates are inferred from baselines situated normal to the rift. Analysis of the network using a cumulative sum approach is found to be an effective method to detect small baseline length changes associated with rift propagation.

A geochemical study of off-axis seamount lavas at the Valu Fa Ridge: Constraints on magma genesis and slab contributions in the southern Tonga subduction zone

New geochemical data are presented for fresh lavas from off-axis seamounts east and west of the Valu Fa Ridge and from seamounts south of the propagating rift tip of the Valu Fa Ridge. Most seamount lavas are basaltic and thus more primitive than the mostly andesitic to dacitic lavas from the adjacent Valu Fa Ridge. Slight differences in fluid-mobile element ratios and in Sr isotope ratios suggest small-scale heterogeneities of the slab component in this region with an apparently higher sedimentary input into the off-axis seamount lavas than into the island arc magmas. Off-axis volcanoes near back-arc basin spreading centres thus resemble off-axis seamounts adjacent to mid-oceanic ridges which also show more extreme compositions than the well-homogenized mid-ocean ridge basalts. The geochemical variations support a diapiric ascent of the hydrated or partially molten mantle leading to separate magma supply systems for the different volcanic structures. The mantle wedge between the Valu Fa Ridge and the island arc appears to be uniformly depleted in fluid-immobile incompatible elements compared to average depleted mid-ocean ridge basalts whereas lavas from seamounts west of the Valu Fa Ridge show less depletion in agreement with a depletion event of the whole mantle wedge in the backarc melting zone. Seamount lavas south of the Valu Fa Ridge propagating rift resemble the Valu Fa lavas indicating an increased magma production caused by the influx of water into the mantle.