Richard C. Ghail

A 400-km-scale strike-slip zone near the boundary of Thetis Regio, Venus

We present a structural analysis of an area of Venus’ surface centred on 3°S, 116°E. The area is dominated by a wide zone of deformation striking 050°N, distinguished from the terrain on either side of it by markedly different structural and morphological characteristics. The deformation zone contains two principal fault sets defined by form, position and orientation. Members of the first set are present along the length of the deformation zone. They are discontinuous, consistently right-stepping, and are interpreted as Riedel shears in a sinistral strike-slip regime. Members of the second fault set are interpreted as normal faults and are often seen in pairs forming extensional grabens. These faults are most prominent in the central region of the deformation zone, and coincide with a deflection in strike of both the first fault set and the boundaries of the deformation zone. A detailed kinematic analysis of all fault orientations and the boundaries of the deformation zone fit the predictions of transtensional theory, supporting the hypothesis that this sinistral strike-slip zone contains an extensional jog. This interpretation is further supported by analysis of the interaction between faults belonging to the two sets. Variation in intercept geometry, kinematic linking of the two fault sets, and rotation of fault strike between the two fault sets all strongly suggest simultaneous development of these structures, and therefore a common tectonic origin. A driving mechanism for such large-scale horizontal deformation on Venus has yet to be established.

A Pyroclastic Flow Deposit on Venus

Abstract Explosive volcanism on Venus is severely inhibited by its high atmospheric pressure and lack of water. This paper shows that a deposit located near 16°S, 145°E, here referred to as Scathach Fluctus, displays a number of morphological characteristics consistent with a pyroclastic flow deposit. These characteristics, particularly the lack of channelization and evidence for momentum- rather than cooling-limited flow length, contrast with fissure-fed lava flow deposits. The total erupted volume is estimated to have been between 225 and 875 km3 but this may have been emplaced in more than one event. Interaction between Scathach Fluctus and a small volcanic cone constrains the flow velocity to 48 m s−1, and plausible volatile concentrations to at least 1.8 wt% H2O, 4.3 wt% CO2 or 6.1 wt% SO2, the latter two values implying that magma was sourced directly from the mantle. The deposit has radar characteristics, particularly an exponential backscatter function, that are similar to those of nearly half the planetary surface, implying that pyroclastic deposits may be much more common on Venus than has been recognized to date, and suggesting both a relatively volatile-rich mantle and a volcanic source for atmospheric SO2.

Deoxygenated gas occurrences in the Lambeth Group of central London, UK

Deoxygenated air poses a serious, life-threatening hazard (confined space hypoxia) for engineering projects in London, particularly within the Upnor Formation of the Lambeth Group. This paper reviews its causes and postulates that it was induced by regional-scale dewatering and drawdown of the Lower Aquifer during the industrial growth of London, bringing air into contact with these sediments, which became oxidized. Ensuing post-industrial recharge and resaturation resulted in accumulations of often compressed deoxygenated air, trapped beneath overlying impermeable clay strata. Historically, glauconite has been considered to be the mineral most likely to remove oxygen from the trapped air, but it lacks potency as a reducing agent and remains unaltered in many oxidized sediments. Pyrite and organic carbon are both plausible, but rare. Green rust, a mixed Fe(II) and Fe(III) layered double hydroxide, is more likely, as it rapidly oxidizes on contact with air and is only briefly observed in fresh core samples. Two key ground engineering hazards are, therefore, identified: the misidentification of risk by reliance on observations of glauconite; and the likelihood of encountering pressurized deoxygenated air within Lambeth Group sediments in underground projects following short- or long-term changes in the groundwater level.

Drift-filled hollows in Battersea: investigation of the structure and geology along the route of the Northern Line Extension, London

Drift-filled hollows (DFHs) are a major subsurface hazard for engineering in London. They are characterized by a steeply inclined cone-shaped hollow into (sometimes through) the London Clay Formation, filled with unconsolidated fine- to coarse-grained drift and often covered by terrace gravels, making them difficult to identify at the surface. Their origin remains uncertain but most probably formed towards the end of glacial epochs by meltwater scouring, perhaps of collapsed pingos. Usually associated with tributaries to the Thames, DFHs are particularly prevalent in the Battersea area, through which the Northern Line Extension (NLE) is to be built. This study uses 283 public borehole records and site reports to build a 3D geological ground model of two known DFHs in the Battersea area to develop a more complete understanding of their origin. We show that DFHs are probably older than previously assumed, dating from the end-Anglian (Marine Isotope Stage (MIS) 12), c. 300 kyr ago, before the deposition of the River Terrace Deposits. The two DFHs modelled fall into distinct types: a small shallow DFH that is probably a purely scour feature in origin, and a larger, deeper DFH that probably formed by the scouring of a perhaps fault-controlled pingo. It is unclear whether the faults controlled pingo formation passively by acting as a conduit for water, or in a more active sense by driving ground movements. Both DFHs represent a significant hazard for the NLE and require more detailed investigations to properly constrain their extent.

Gas generation and accumulation by aquifer drawdown and recharge in the London Basin

Air at high pressure has recently been encountered during ground investigation borehole drilling for the Thames Tideway project in London. The air was released from within the Upnor Formation (at the base of the Lambeth Group beds), which is at the top of the Lower Aquifer. Cavitation within the granular beds of the Lower Aquifer would have occurred as a consequence of drawdown from the extensive water extraction that began at the start of the nineteenth century, resulting in the water table dropping by almost 70 m. Estimated historical pore pressure profiles are postulated and the cavitation process that occurred, along with the subsequent pressurization of the air during recharge that has occurred in recent years, are discussed using behavioural concepts from unsaturated soil mechanics. The hypotheses are corroborated by a series of simple column experiments where the processes are modelled using analogous soils. Engineering implications of the presence of the pressurized air are discussed: besides the potential for borehole blow-out, another serious consideration is the depletion of oxygen from the trapped air by chemical reaction with the soil, increasing the risk of confined space hypoxia.

The distribution of volcanism in the Beta‐Atla‐Themis region of Venus: Its relationship to rifting and implications for global tectonic regimes

A new analysis of the spatial relationships between volcanic features and rifts on Venus provides new constraints on models of planetary evolution. We developed a new database of volcanic features for the Beta-Atla-Themis (BAT) region, and used nearest neighbour measurements to determine relationships between different types of volcanic features and the rifts. Nearest neighbour analysis shows that all the dome-type and corona-type sub-populations tend to cluster. Rift associations were inferred from the deviation of a features population distribution (as a function of distance from rift) from that of a random population. Dome-type features in general have no discernible relationship with rifts. Most corona type features have a strong association with rifts, with intermediate and large volcanoes also tending to occur close to or on rifts. Shield fields, on the other hand, tend to occur away from rifts. Our new evidence supports classifications of rifts on Venus into different types, possibly by age, with a shift from globally dispersed (more uniform) volcanism towards the more rift-focused distribution, which suggests a shift in tectonic regime. Our observations are consistent with recent models proposing the evolution of Venus from a stagnant lid regime to a subcrustal spreading regime. We also present evidence for a failed rift on Venus and note that this process may be analogous, albeit on a larger scale, to a proposed model for the evolution of the East African Rift system.

Identification of Ground Engineering Hazards in London Through the Use of Predictive 4D Geomodelling Tools

Unexpected ground conditions are responsible for a number of engineering problems and hazards across London. A major reason for the unexpected nature of these events is the historical proprietary approach to site investigation, which has restricted knowledge transfer and inhibited the development of a London-wide geological context. Recent changes mean that much of these data are now accessible in the public domain for site investigation and useful in predicting the engineering hazards resulting from complex and variable geology. We advocate applying predictive 4D modelling to better constrain geological structures, facies, fissures and groundwater flow, and adopting an iterative approach that both informs local site models and updates the regional context. Reverse normal faulting, indicative of basin inversion, pull-apart structures and periglacial scour/pingo features are identified in a pilot study regional model. Applying this model to a small site investigation predicted otherwise unexpected faulting, generating horizontal fissuring that causes water to flow along the length of the fault system. Widely adopting this approach in London and other urban areas may substantially reduce the risk of unexpected ground hazards.

Engineering geology and tunnelling in the Limmo Peninsula, East London

The Limmo Peninsula site has some of the most complex geology of Londons Crossrail project and was the launching point for four tunnel boring machines (TBMs) to allow construction of Crossrails eastern running tunnels. It is located in East London, c. 2 km east of the Canary Wharf business district, adjacent to the River Lea. It consists of a ventilation shaft, an auxiliary shaft, two sprayed concrete lining (SCL) tunnels interconnecting the shafts and four SCL adits for assisting in the launching of the TBMs. As part of the design requirements, some geological formations had to be depressurized from surface wells. The site is geologically complex: it is in the vicinity of a drift-filled hollow and it is located within the area of influence of several tectonic features. A geological ground model developed from important new information obtained during the design stage ground investigations and from direct observations conducted during construction stages reveals an inverted transtensional flower structure (i.e. it is now a transpressional restraining bend). Of special interest are the unusually low values of undrained shear strength of the London Clay associated with the tectonic setting.