John J. Armitage

The importance of rift history for volcanic margin formation

Rifting and magmatism are fundamental geological processes that shape the surface of our planet. A relationship between the two is widely acknowledged but its precise nature has eluded geoscientists and remained controversial. Largely on the basis of detailed observations from the North Atlantic Ocean, mantle temperature was identified as the primary factor controlling magmatic production, with most authors seeking to explain observed variations in volcanic activity at rifted margins in terms of the mantle temperature at the time of break-up. However, as more detailed observations have been made at other rifted margins worldwide, the validity of this interpretation and the importance of other factors in controlling break-up style have been much debated. One such observation is from the northwest Indian Ocean, where, despite an unequivocal link between an onshore flood basalt province, continental break-up and a hot-spot track leading to an active ocean island volcano, the associated continental margins show little magmatism. Here we reconcile these observations by applying a numerical model that accounts explicitly for the effects of earlier episodes of extension. Our approach allows us to directly compare break-up magmatism generated at different locations and so isolate the key controlling factors. We show that the volume of rift-related magmatism generated, both in the northwest Indian Ocean and at the better-known North Atlantic margins, depends not only on the mantle temperature but, to a similar degree, on the rift history. The inherited extensional history can either suppress or enhance melt generation, which can explain previously enigmatic observations.

Abrupt landscape change post–6 Ma on the central Great Plains, USA

The principal control on landscape evolution in the central Great Plains of the United States over the past 10 m.y. is a contentious subject. New sedimentary data collected from Late Miocene Ogallala Group and Pliocene Broadwater Formation of the Nebraskan Great Plains demonstrates a twofold increase in the median grain size (from 20 mm to >40 mm) exported from the Rocky Mountains across the Miocene-Pliocene boundary. Paleoslope reconstructions derived from these data support the tilting of the Miocene Ogallala Group after 6 Ma, but demonstrate that the transport slope of the lower part of the unconformably overlying Pliocene succession is identical to the present-day slope. These data allow us to constrain the timing of differential uplift in the Great Plains to between 6 and 3.7 m.y.; the wavelength and short duration of this tilting are best explained by the initiation of localized dynamic topography. Our results also suggest a threefold to fourfold increase in specific stream power at this time, meaning that Pliocene rivers draining the central Rockies were considerably more competent than their Miocene predecessors. Incision during this period was not continuous. A significant episode of aggradation from 3.7 to 2.5 Ma is best explained by high rates of sediment supply relating to the warm, wet mid-Pliocene climate optimum. The modern pattern of incision on the Great Plains occurred from 2.5 Ma, and not from the end of the Miocene as is sometimes supposed, reflecting the onset of major Northern Hemisphere glaciation.

Low seismic velocities below mid‐ocean ridges: Attenuation versus melt retention

The first comprehensive seismic experiment sampling subridge mantle revealed a pronounced low-velocity zone between 40 and 100 km depth below the East Pacific Rise (EPR) that has been attributed to substantial retained melt fractions of 0.3–2%. Such high melt fractions are at odds with low melt productivity and high melt mobility inferred from petrology and geochemistry. Here, we evaluate whether seismic attenuation can reconcile subridge seismic structure with low melt fractions. We start from a dynamic spreading model which includes melt generation and migration and is converted into seismic structure, accounting for temperature-, pressure-, composition-, phase-, and melt-dependent anharmonicity, and temperature-, pressure-, frequency- and hydration-dependent anelasticity. Our models predict a double low-velocity zone: a shallow—approximately triangular—region due to dry melting, and a low-velocity channel between 60 and 150 km depth dominantly controlled by solid state high-temperature seismic attenuation in a damp mantle, with only a minor contribution of (<0.1%) melt. We test how tomographic inversion influences the imaging of our modeled shear velocity features. The EPR experiment revealed a double low-velocity zone, but most tomographic studies would only resolve the deeper velocity minimum. Experimentally constrained anelasticity formulations produce VSas low as observed and can explain lateral variations in near-ridge asthenospheric VS with ±100 K temperature variations and/or zero to high water content. Furthermore, such QS formulations also reproduce low asthenospheric VS below older oceans and continents from basic lithospheric cooling models. Although these structures are compatible with global QS images, they are more attenuating than permitted by EPR data.

Thin oceanic crust and flood basalts: India‐Seychelles breakup

Recent seismic experiments showed that separation of India from the Seychelles occurred in two phases of rifting. The first brief phase of rifting between India and the Laxmi Ridge formed the Gop Rift, which is characterized by thick oceanic crust and underplating of the adjacent continental margins. The age of the Gop Rift is uncertain, initiation of seafloor spreading being some time between 71 and 66 Ma. This was then followed by rifting and seafloor spreading between the Laxmi Ridge and the Seychelles, the onset of which is well dated by magnetic anomalies at 63.4 Ma and characterized by thin oceanic crust. Both of these rift events occurred within 1000 km of the center of the Deccan flood basalts, which formed at 65 ± 1 Ma. To constrain the age of the Gop Rift and to explore the reasons for the change in crustal structure between the Gop Rift and Seychelles-Laxmi Ridge margins, we employ a geodynamic model of rift evolution in which melt volumes, seismic velocity, and rare earth element (REE) chemistry of the melt are estimated. We explore the consequences of different thermal structures, hydration, and depletion on the melt production during the India-Seychelles breakup to understand the reasons behind the thin oceanic crust observed. Magmatism at the Gop Rift is consistent with a model in which the seafloor spreading began at 71 Ma, ca. 6 Myr prior to the Deccan. The opening occurred above a hot mantle layer (temperature of 200°C, thickness of 50 km) that we interpret as incubated Deccan material, which had spread laterally beneath the lithosphere. This scenario is consistent with observed lower crustal seismic velocities of 7.4 km s?1 and 12 km igneous crustal thickness. The model indicates that when the seafloor spreading migrated to the Seychelles-Laxmi Ridge at 63 Ma, the thermal anomaly was reduced significantly but not sufficient to explain the observed reduction in breakup magmatism. From observations here of 5.2 km oceanic crust, lower crustal seismic velocities of 6.9 km s?1 and a flat REE profile, we infer that breakup occurred in a region of mantle that became depleted by prior extension related to the Gop Rift.

Lithospheric controls on melt production during continental breakup at slow rates of extension: Application to the North Atlantic

Rifted margins form from extension and breakup of the continental lithosphere. If this extension is coeval with a region of hotter lithosphere, then it is generally assumed that a volcanic margin would follow. Here we present the results of numerical simulations of rift margin evolution by extending continental lithosphere above a thermal anomaly. We find that unless the lithosphere is thinned prior to the arrival of the thermal anomaly or half spreading rates are more than ? 50mmyr?1, the lithosphere acts as a lid to the hot material. The thermal anomaly cools significantly by conduction before having an effect on decompression melt production. If the lithosphere is thinned by the formation of extensional basins then the thermal anomaly advects into the thinned region and leads to enhanced decompression melting. In the North Atlantic a series of extensional basins off the coast of northwest Europe and Greenland provide the required thinning. This observation suggests that volcanic margins that show slow rates of extension, only occur where there is the combination of a thermal anomaly and previous regional thinning of the lithosphere.

Fragmentation Model of the Grain Size Mix of Sediment Supplied to Basins

A key factor in the downstream dispersal and fractionation of sediment is the grain size distribution of sediment supplied by upstream catchments. Modeling of the grain size distribution of modern bedload in the main trunk channels of tectonically uplifting catchments, including the sediment at their outlets, and the weathering products of a range of bedrock lithologies in southern Italy and Sicily reveals fractal dimensions of 2.3–2.7, similar to the fractal dimension of many natural materials undergoing fragmentation. We examine the impact of changing statistical properties of the grain size distribution of the sediment supply in simulating grain size trends in sedimentary basins. Model simulations show a marked movement of the gravel front and patterns of progradation and retrogradation in basin stratigraphy. These grain size trends and sedimentary architectures are generated simply by variations in the grain size mix of the sediment supply, without variations in base level or sediment discharge. Variation in the grain size distribution of the sediment supply may therefore act as a first-order control on sequence stratigraphic architectures in sedimentary basins.

The influence of long-wavelength tilting and climatic change on sediment accumulation

The elevation of continental interiors over time is demonstrably variable. A major part of change in elevation within the continental interior is likely driven by density changes within the upper mantle and by global mantle convection. For example, upper-mantle flow has been invoked as the cause of Neogene uplift of the interior Rocky Mountains and Colorado Plateau, warping and tilting sediment transport slopes that link to the widespread deposition of gravel units within the Great Plains. These geomorphic and sedimentologic features, however, can also be generated by an increase in runoff, since erosion will promote change in elevation due to isostatic compensation and the loading of the lithosphere by the deposition of sediment. To explore the consequences of change in topography and climate, we use a general length-dependent diffusive sediment transport law to model both erosion and deposition that includes the concentrative effects of river systems. The simplicity of the approach means that we can collapse sediment transport to one dimension and couple erosion and deposition with plate flexure. We find that for a landscape that is gently tilted (slope of order of 10 –3 ), a change in runoff has a minor effect on transport gradient, as sediment transport and associated flexural response maintain topography at a similar elevation. However, there can be a significant change in depositional style when the degree of tilt is altered by, for example, a local change in upper-mantle density. An increase in buoyancy within the upper mantle, which increases slopes, leads to a transient reduction in grain sizes deposited at a fixed location. This behavior is due to a temporary retreat of the zone of erosion into the catchment and a transient increase in accommodation space relative to sediment supply. A reduction in tilt has the opposite effect, the older deposits are eroded, and the erosion-deposition transition rapidly moves down system. There is convincing evidence that the formation of thin and laterally extensive conglomeratic units of the Great Plains was due to a reduced rate of subsidence. Based on the results of our model, we suggest that the deposition of widespread conglomeratic units within continental interiors is generally a consequence of a reduction in slope, as the dynamic support for regions of high topography is reduced.

Thermal Regime and Evolution of the Congo Basin as an Intracratonic Basin

The Congo Basin (CB) lies over a thick (200–250 km) and cold lithosphere: we estimate the present-day surface heat-flow to 40 ± 5 mW m−2, from the BHT temperatures, lithology and porosity recorded in two oil wells and in agreement with the only measurement in this area (44 mW m−2). This value is consistent with the thickness of the lithosphere inferred from seismic tomography, assuming stationary conditions. The paleo-thermal regimes can be constrained by additional information, such as the pressures and temperatures derived from kimberlites studies, the variations of vitrinite reflectance with burial or the reconstructed subsidence history. The pressures and the temperatures derived from kimberlites xenocrysts suggest that the conditions were similar for at least 120 Myr. The long-term subsidence can be interpreted by the thermal relaxation of a thick lithosphere after a Neo-Proterozoic rifting stage (ca. 700–635 Ma) with a thinning factor β = 1.4 and a possible reactivation during the Karoo period (ca. 320 Ma). Because the magnitude of the crustal thinning is small, the past thermal conditions throughout the Phanerozoic were probably not very different from the present-day. Additional short term variations (~20–40 Myr) of the subsidence are interpreted by dynamic subsidence or uplift caused by sublithospheric mantle instabilities at the transition between litho-spheres of different thicknesses. These short term variations should not be associated with significant thermal changes. In order to explain the observed maturation of the vitrinite as well as angular uncomformities on seismic lines, one should assume one or two stages of erosion in the basin, representing at least 4 km of removed material. Heat advection by hydrothermal or volcanic fluids can conversely reduce the magnitude of this erosion.

Reconstructing the timescale of a catastrophic fan-forming event on Earth using a Mars model

The calculation of formation timescales of alluvial fans and deltas on Mars is important as it has direct implications for understanding the planets hydrologic history. The robustness of sediment transport models is not in doubt but validation of the broad approach using a terrestrial example of similar scale and likely origin, where hydraulic parameters and timescales are known, is useful. Using a catastrophically formed terrestrial fan, where abundant sedimentological information is available, we find that the modeled hydraulic parameters and formation timescales are in very close agreement with the known values of the event. This supports the general modeling approach as applied to Mars fans but also highlights the added value of detailed sedimentary information when reconstructing hydraulics and timescales on Earth and Mars, which cannot be confidently gleaned from the final snapshot of surface geomorphology alone.

Quantifying Sediment Transport Dynamics on Alluvial Fans From Spatial and Temporal Changes in Grain Size, Death Valley, California

How information about sediment transport processes is transmitted to the sedimentary record remains a complex problem for the interpretation of fluvial stratigraphy. Alluvial fan deposits represent the condensed archive of sediment transport, which is at least partly controlled by tectonics and climate. For three coupled catchment-fan systems in northern Death Valley, California, we measure grain size across 12 well-preserved Holocene and late-Pleistocene surfaces, mapped in detail from field observations and remote sensing. Our results show that fan surfaces correlated to the late Pleistocene are, on average, 30-50% coarser than active or Holocene fan surfaces. We adopt a self-similar form of grain size distribution based on the observed stability of the ratio between mean grain size and standard deviation downstream. Using statistical analysis, we show that fan surface grain size distributions are self-similar. We derive a relative mobility function using our self-similar grain size distributions, which describes the relative probability of a given grain size being transported. We show that the largest mobile grain sizes are between 20 and 35mm, a value that varies over time and is clearly lower in the Holocene than in the Pleistocene; a change we suggest is due to a drier climate in the Holocene. These results support recent findings that alluvial fan sedimentology can record past environmental change and that these landscapes are potentially sensitive to climatic change over a glacial-interglacial cycle. We demonstrate that the self-similarity methodology offers a means to explore changes in relative mobility of grain sizes from preserved fluvial deposits.