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Peculiar conditions may be required for the origin of life and/or the evolution of complex organisms. Hence, Earth attributes—such as plate-tectonics, oceans, magnetism and a large moon—may be necessary preconditions, for our own existence, that are rare in the general population of planets. The unknown magnitude of this observational bias undermines understanding of our planet. However the discovery and characterization of exoplanets, along with advances in mathematical modelling of Earth systems, now allow this “anthropic selection” effect to be more thoroughly evaluated than before. This paper looks at a number of properties of our Solar System and our planet. It examines their possible benefits for life, whether these properties might be rare, whether they required fine-tuning and whether they have an associated habitability-lifetime. It also discusses additional data likely to become available in the near future. None of the individual properties considered show convincing evidence for anthropic bias. However, the time-scales associated with habitability— in particular, those associated with solar-warming, with axial stability and with planetary-cooling—are surprisingly similar and this provides tentative support for the view that Earth may be special.

Lower heating of our planet by the young Sun was compensated by higher warming from factors such as greater greenhouse gas concentrations or reduced albedo. Earth's climate history has therefore been one of increasing solar forcing through time roughly cancelled by decreasing forcing due to geological and biological processes. The current generation of coupled carbon-cycle/climate models suggests that decreasing geological forcing-due to falling rates of outgassing, continent growth, and plate spreading-can account for much of Earth's climate history. If Earth-like planets orbiting in the habitable zone of red dwarfs experience a similar history of decreasing geological forcing, their climates will cool at a faster rate than is compensated for by the relatively slow evolution of their smaller stars. As a result, they will become globally glaciated within a few billion years. The results of this paper therefore suggest that coupled carbon-cycle/climate models account, parsimoniously, for both the faint young Sun paradox and the puzzle of why Earth orbits a relatively rare and short-lived star-type.

A wealth of Earth-sized exoplanets will be discovered in the coming years, proving a large pool of candidates from which the targets for the search for life beyond the Solar system will be chosen. The target selection process will require the leveraging of all available information in order to maximise the robustness of the target list and make the most productive use of follow-up resources. Here, we present the results of a suite of $n$-body simulations that demonstrate the degree to which the orbital architecture of the Solar system impacts the variability of Earth's orbital elements. By varying the orbit of Jupiter and keeping the initial orbits of the other planets constant, we demonstrate how subtle changes in Solar system architecture could alter the Earth's orbital evolution -- a key factor in the Milankovitch cycles that alter the amount and distribution of solar insolation, thereby driving periodic climate change on our planet. The amplitudes and frequencies of Earth's modern orbital cycles fall in the middle of the range seen in our runs for all parameters considered -- neither unusually fast nor slow, nor large nor small. This finding runs counter to the `Rare Earth' hypothesis, which suggests that conditions on Earth are so unusual that life elsewhere is essentially impossible. Our results highlight how dynamical simulations of newly discovered exoplanetary systems could be used as an additional means to assess the potential targets of biosignature searches, and thereby help focus the search for life to the most promising targets., Comment: 19 pages; 11 figures; accepted for publication in the Astronomical Journal Version 2 - incorporates typo corrections and minor changes noted at the proofing stage, after acceptance

Astronomically calibrated cyclostratigraphy relies on correct matching of observed sedimentary cycles to predicted astronomical drivers such as eccentricity, obliquity, and climate precession. However, the periods of these astronomical cycles, in the past, are not perfectly known because: (i) they drift through time; (ii) they overlap; and (iii) they are affected by the poorly constrained recession history of the Moon. This paper estimates the resulting uncertainties in ancient Milankovitch cycle periods and shows that they lead to: (i) problems with using Milankovitch cycles for accurate measurement of durations (potential errors are around 25% by the start of the Phanerozoic); (ii) problems with correctly identifying the Milankovitch cycles responsible for observed period ratios (e.g., the ratio for long-eccentricity/short-eccentricity overlaps, within error, with the ratio for short-eccentricity/precession); and (iii) problems with verifying that observed cycles are Milankovitch driven at all (the probability of a random period ratio matching a predicted Milankovitch ratio, within error, is 20–70% in the Phanerozoic). Milankovitch-derived ages and durations should therefore be treated with caution unless supported by additional information such as radiometric constraints.

Reconstructed temperatures through the Phanerozoic indicate a gradual cooling or, at best, no significant temperature trend, despite a 4.4% increase in solar heating. It is possible that an underlying warming trend has simply been swamped by ‘noise’ due to significant data-errors and/or natural fluctuations. Alternatively, the lack of warming may indicate cooling by biological and/or geological processes, which happen to have the right amplitude to cancel the effects of solar warming. This paper demonstrates that the absence of Phanerozoic warming cannot be explained as a warming trend hidden by noise. It also shows that, given widely accepted estimates of climate sensitivity, it cannot be explained as cancellation by negative feedback in the climate system. The Gaia hypothesis, anthropic selection or some other unconventional mechanism may therefore have to be invoked to explain the absence of long-term warming through the Phanerozoic.

Meter-scale, peritidal carbonate cycles are a common feature of the geological record but debate continues about what processes lead to their formation. Three conceptual models, or a combination thereof, are commonly invoked to explain cycle formation; eustasy, tectonics, or autocyclicity. These three models are tested with a large new dataset from different Early Jurassic plate margins from western Tethys. Study of seven logged sections from Spain, Italy, Greece, Tunisia, Morocco, and Gibraltar enables an analysis of the possible roles of local versus regional patterns and controls on cyclicity within a Sinemurian time slice. Cycle types are diverse and include shallowing-upward cycles (parasequences) but also deepening-upward and diagenetic cycles (high-frequency sequences) and subtidal cycles. Numbers of cycles per section and cycle stacking patterns within this time slice vary from section to section. Statistical tests (runs tests, time series, and bundling) all indicate random stacking of cycles within sections and an absence of any bundling of thicknesses or of facies trends. Assessment of cycle types by their occurrence and stacking patterns indicates little support for either eustasy or autocyclicity being the dominant cycle-forming mechanism. However, the variability in numbers of cycles per section, thickness variations of the sections, cycle type variability, and randomness of stacking patterns all favor a pulsed, tectonic control for the creation and filling of accommodation space. This conclusion is further supported by evidence that has largely arisen during the course of this study of syndepositional extensional tectonics in the Sinemurian on these rifted Tethyan margins. Although tectonics appears to be the dominant control, superimposed eustasy and/or autocyclic processes cannot be discounted.

Analysis of old erosion surfaces and estimates of exhumation from apatite fission track data can be used to infer late Neogene surface uplift of Britain, Greenland, Norway and Svalbard of 1–2 km. Subsidence and sedimentation in adjacent offshore basins can be found from interpretation of seismic and well log data. Various mechanisms for surface uplift have been proposed but the underlying cause remains unexplained. Since the multiple glaciations that took place during the late Neogene were a common factor, a possible glacially-forced tectonic mechanism to thicken the crust and produce surface uplift has been investigated. This could result from the relatively slow accumulation of ice that loads the crust as an ice sheet grows during a glacial period, followed by relatively rapid retreat and unloading around its periphery at the end. Unloading could create transient stresses that induce lateral flow in a ductile lower crust to thicken it onshore and produce surface uplift, with associated thinning beneath adjacent offshore basins, producing subsidence. Simple calculations show that the proposed mechanism is feasible and indicate that crustal thickening and surface uplift accumulated from a number of glacial cycles can account for the observed surface uplift, with an acceptable flow rate in the lower crust at the end of each cycle if the viscosity of ductile flow is sufficiently low.

Thin sheet sandstone beds in a continental succession may be the products of unconfined overbank flow or deposition in lacustrine environments. In the distal parts of fluvial distributary systems these two settings may be intercalated, recording fluctuations in lake level in response to climatic changes. Field studies of the Luna and Huesca fluvial distributary systems in the Miocene of the Ebro Basin, Spain, are here used to characterise sheet sandstone deposited in distal fluvial and lake margin environments. In this study we document the facies and relationships between 322 individual sandbodies deposited in both alluvial settings as sheetfloods and in lacustrine settings as deltaic lobes. The alluvial sandstone sheets were deposited from lateral and frontal sheetflood events when unconfined flow replaced channelised flow as the main transport mechanism. The term sheetflood is used here to refer to sub-aerial, unconfined, turbulent flow events that undergo expansion, thinning and deceleration with increasing radial distance from source. The depositional process is similar to modern “floodouts” and “terminal splays” of the Channel Country and Lake Eyre, central Australia. Such a process was thought to be the dominant depositional mechanism during lake lowstands. The sheets formed in the lacustrine setting are interpreted as deltaic sediments deposited in a shallow-gradient lake similar to the modern day Volga Delta, Caspian Sea. Shallow-water, friction-dominated, deltaic processes are thought to have been the primary depositional mechanism in the distal sectors of the fluvial systems during lake highstands.

Sheetfloods are typically invoked as the mechanism responsible for the kilometre-scale transport of sand-sized sediment grains in shallow-gradient fluvial systems. This concept is based on the lateral extent of ancient thin, sheet sandstone deposits rather than on fluid dynamics, which has resulted in a loosely constrained model for sheetfloods. This study tested the conceptual mechanism by developing a depth-averaged, 2D computational fluid dynamics model. The model results compare well against observations from modern deposits at Lake Eyre to provide a quantitative, physically sound basis for sheetfloods that can be applied in ancient and modern settings to constrain otherwise qualitative interpretations.

A fundamental control on the evolution of carbonate platforms is accommodation, the space available for sediment deposition. Here we adopt a sensitivity-analysis approach, using the process-based forward model CARB3D+, to quantitatively investigate the sedimentary response of isolated carbonate platforms to individual accommodation drivers of eustasy, subsidence, and surface dissolution. Sediment production, the mechanism invoked to fill accommodation, is also investigated. Fourth-order Milankovich-driven sea-level cycles dominate icehouse carbonate sedimentology, with 400 ky cycles modulating the effect of 100 ky cycles of comparable magnitude when both cycles are present. Both overwhelm higher-frequency, lower-amplitude fifth-order cycles, which are rarely preserved on the platform top. Subsidence is a fundamental control on sedimentary stacking patterns, sequence-boundary frequency (SBF), and platform geometry, with rapid subsidence increasing the preservation of platform-top sequences. Most icehouse platforms are subject to extended periods of platform-top exposure. "Missed beats," when the platform top remains emergent at a sea-level maximum, are common and create non-Milankovich SBFs. Furthermore, during platform emergence, surface lowering via surface dissolution also creates accommodation for the subsequent platform flooding event, with surface dissolution rates in humid climates comparable to moderate rates of subsidence. However, accommodation generation by surface dissolution is self-limiting, in as much as accommodation generated by dissolution is rapidly refilled on flooding. Sediment production is a key control on unfilled accommodation, which our simulations suggest is considerable on icehouse platforms. Increased shallow-water (< 3 m depth) sediment production enhances intra-platform bathymetric relief between the margin and interior; this is driven in large part by a higher sedimentation rate at the margin than in the interior. Platforms can be flooded to depths up to 45 m during marine transgressions, and accommodation often remains unfilled. There are two distinct phases of platform-top sedimentation. During the transgression and early highstand, platform margins accumulate at a greater rate than interior sediments, generating differential intra-platform bathymetry. Subsequently, accommodation in the interior fills by forced regression and progradation from the margins into the platform interior. Numerical forward models can augment our understanding of icehouse carbonate-platform evolution by identifying the nature of the sedimentary response to individual controls. They can thus contribute to improving characterization of carbonate reservoirs in icehouse platforms, such as the large hydrocarbon fields in the North Caspian and S.E. Asia.

This paper describes a new 3-D forward numerical model (CARBONATE 3D) that simulates the stratigraphic and sedimentological development of carbonate platforms and mixed carbonate-siliciclastic shelves by simulating the following sedimentary processes: (l) Carbonate shallow, open-marine production, dependent on water depth, restriction and sediment input; (2) Carbonate shallow, restricted-marine production, dependent on water restriction; (3) Pelagic sediment production and deposition; (4) Coarse and fine siliciclastic input; (5) Erosion, transport and redeposition of sediment, dependent on currents, slope, depth and restriction as well as sediment grain-size and composition; (6) Dissolution of subaerially exposed carbonate. In this paper the model is used to investigate the controlling mechanisms on the sequence stratigraphy of isolated carbonate platforms and atolls and to predict distinctive architectural signatures from different drowning mechanisms. Investigation of the mechanisms controlling atoll strata shows that although relative sea-level is the major control, antecedent topography, environmental setting and early diagenesis have profound influence on what stratigraphic geometries and facies develop. Hence care must be taken if sea-level curves are interpreted from real stratigraphies. Atoll drowning by fast sea-level rise, by lowered production and by repeated exposure and fast subsequent sea-level rises are investigated and different stratigraphic signatures for the respective mechanisms predicted. A fast relative sea-level rise results in a bucket-shaped morphology developed prior to drowning and a sharp transition from the platform margin facies to a pelagic cover. Drowning caused by lowered platform margin production is predicted to result in the development of a dome-shaped, shallow-water shoal over the whole platform top prior to drowning. Fourth order amplitudes of several tens of metres, typical of \\\'icehouse\\\' settings, cause atoll drowning at subsidence rates where atolls subject to fourth order amplitude of only a few metres, typical of \\\'greenhouse\\\' settings, can keep up with the rising sea-level In the resultant strata, vertical facies belts are less well developed but horizontally extensive facies bands are more prominent. High fourth order amplitudes (up to 80 m) without sufficient third order scale subsidence will not lead to drowning, however, as the platform can recover in each fourth order lowstand. These results suggest that atolls might be easier to drown in \\\'icehouse\\\' rather than in \\\'greenhouse\\\' conditions but only in situations with suitably high rates of longer-term relative sea-level rise or sufficient lag times.

This paper concerns mechanisms of salt (and ductile shale) movement. It investigates salt flow due to differential loading, folding of the overburden during compression and drag by a moving overburden. The approach is to compare the salt flux caused by these processes to that generated by buoyancy. It is demonstrated that overburden folding and drag by the overburden can, under commonly encountered conditions, result in greater amounts of salt movement than that produced by buoyancy or differential loading. These conclusions apply during the early stages of salt anticline, salt pillow and salt roller formation but not during the later stages of salt diapir and salt wall growth when buoyancy dominates. The quantitative significance of these alternatives to buoyancy is determined by considering an elastic plate overlying a viscous fluid. This is the simplest mathematical model that can reproduce the processes considered. The model shows that: (1) Under certain conditions, these mechanisms produce more salt movement than buoyancy. Differential loading dominates when the surface slopes become more than a small fraction of the slope of the salt top. Overburden buckling dominates if the in-plane stress exceeds a critical value. Drag dominates when the salt layer is thinner than a few hundred metres. (2) The strength of the overburden inhibits formation of salt diapirs, even those due to buoyancy, on wavelengths less than about 12 km.

SUMMARY The summation of Gaussian beams in homogeneous layered media has been implemented for 3-D seismic forward modelling in this paper. The Gaussian beam method, which considers a bundle of rays in the vicinity of a central ray with a bell-shaped amplitude, is known as an extended ray method for handling caustics and diffractions. However, owing to the restriction of the paraxial assumption and the lack of a universal beam phase, this method has not been accurately applied to 3-D problems. These difficulties are overcome in this paper by providing (i) the optimal determination of the beam orientation and the beam shape by minimizing their beam widths; (ii) the ranges of validity, such as the beam conditions and the paraxial condition; and (iii) the diffractions of Gaussian beams from edges and tips. Comparisons with other solutions for caustics and diffractions demonstrate the accuracy of this extended method even in singular regions.

There are several advantages in using velocities to describe the deformation of the crust and lithosphere induced by extension, compression, isostasy and other processes. (1) The method is very general. Any deformation style can be described in this way. Simple examples are: pure shear, inclined or vertical simple shear, bulk rotation; Airy or flexural isostasy and compaction. (2) Velocity fields can be combined to model more complex deformation. (3) The syn-tectonic evolution of parameters such as temperature, pressure, hydrocarbon maturity and porosity are easily modelled once the deformation velocities are specified. (4) The syn-tectonic evolution of sedimentary architectures can be modelled using a general tectono-sedimentary forward modelling equation which incorporates the deformation velocities. These advantages are illustrated by a computer model which simulates clastic sediments prograding into an active half-graben formed by extension above a listric fault. Processes included in this model are compaction of the sediments, hanging wall deformation by inclined simple shear and simultaneous footwall deformation. To avoid space problems, the velocity field for the hanging wall block above the non-rigid footwall must obey a simple contact condition.

The results are presented from two-dimensional, transient numerical models which incorporate temperature, rheology, sedimentation and isostasy for finite rift durations. These models are used to address the role of fault growth and resultant basin geometry in the presence of rheological heterogeneity, generated during continental lithospheric extension. The transition between brittle and plastic rheologies, within the yield strength envelope, is used as a constraint on fault development and is found to be sensitive to the temperature perturbations initiated by the rifting process. Numerical results show that during the early stages of extension, cooling across the fault surface by hanging wall advection induces hardening directly below the fault tip. As a consequence the fault surface becomes detached from the brittle/plastic transition in the upper crust. As rifting continues, major basin-forming faults must grow downwards under the tectonic stress field to maintain contact with the transition surface. This increases the surface area of the fault, allowing the basin to cool further, resulting in an increase in flanl uplift and a deepening of the basin. For longer rift durations, thermal advection from lower lithospheric thinning heats up the basin, cancelling out the earlier cooling effect. The architecture of the rift and its sedimentary infill are therefore controlled by the dynamic response of the lithosphere to varying strength controls. The strength of the lithosphere itself is strongly dependent on temperature, pore pressure and strain rate. This type of thermal feedback process is greatest in areas of rapid extension, where greater discontinuities are set up by the rifting process.

Mathematical models of various sedimentary basin processes including hanging wall deformation, evaporite deposition, carbonate platform sedimentation and sequence development on active domino-style fault blocks are presented. Many of these processes may be formulated using the equation of continuity of an open system as a starting point. Sedimentary and tectonic processes may be combined into a single mathematical formulation, provided that an Eulerian coordinate system is used, and this ensures that tectonics and sedimentation are modelled as simultaneous rather than sequential processes. The resulting algorithms are fast, robust and applicable to many, geologically very different, situations, but may be limited to relatively simple examples. The conditions under which the models are numerically stable are also easily found. The abstract nature of the mathematical models requires that simply determined properties such as palaeo-water depth and palaeo-slope must be used as proxy facies markers. The main limitation of the method is that it is restricted to modelling processes which change slowly with time and space and the method can therefore not cope properly with episodic or chaotic processes.

Abstract Computer models have improved dramatically the mathematical understanding of gravity currents; however, these models cannot replace the analysis by experimental work. The use of scaled analogue models, or physical models, proved to be essential in validating depth average velocity equation for turbidity currents. In order to reduce the level of complexity to solve this equation numerically and to create an efficient computer model to simulate these currents, some mathematical approximations were applied during the development of the velocity equation (Waltham & Davison, 2001). Therefore, willing to prove that these approximations would not compromise the numerical results, many experiments were performed to acquire a spatio-temporal velocity evolution database for both unconfined particle free and particulate turbidity flows. Comparing the results from the numerical and physical simulations, it was concluded that, unfortunately, the approximations have influenced the numerical results. Nevertheless, the data and visual comparisons between the simulations also revealed some encouraging results, which will stimulate some future research to improve the model accuracy. Introduction Particulate gravity currents have been discussed in many scientific studies, especially in sedimentary geology. Their importance is due to the fact that these currents have a substantial influence on deep-water depositional systems. However, these currents can occur not only in submarine but also in subaerial environments, for example, hot clouds of volcanic ash going down slope after an eruption. In a submarine environment, turbidity currents are generally triggered by slope failures (Partson et al., 2000). These failures provide a large volume of sediment and water mixture, which has normally a greater density than the surrounding water. The difference in density between these two fluids is the ignition of the current. A difference of only a few percent is enough to raise the fluid pressure force that together with the fluid weight component, if on a slope, induce the current to propagate (Waltham, 2004). Many studies have been published about turbidity currents behavior and how to reproduce them in fluid mechanics laboratory. Some of these studies concern simple time series measurements of flow parameters (Kneller et al., 1997; Best et al., 2001), while others concern spatio-temporal evolution of flow parameters (McCafrey et al., 2003 & 2005). Although such studies may yield valuable knowledge to this research, none of them provide information about the spatio-temporal evolution of velocity, or any other parameter, for unconfined turbidity currents. In other words, it is not possible to analyze how velocity varys over time at different points in these currents. Since an empirical methodology is being used to validate a numerical simulation, a database about the spatio-temporal evolution of velocity for unconfined turbidity currents became essential to achieve the foremost objective of this work. Thus, a physical model was created in order to run experiments simulating turbidity currents and measuring their velocities to finally get the required database. Meanwhile, the numerical simulation was created by developing a computer program, using finite difference to solve the partial diferential equations applied to model the phenomenon. Once both the numerical model and the database are ready, they will be compared against each other to check how precise the applied equations are.

The finite difference method, for the modelling of hangingwall deformation above faults, is applied to a wider range of problems. Close agreement has been achieved with the overall geometry of hangingwalls in sandbox analogue models of extensional faulting. These results give insights into the dynamics involved and zones of faulting can be indicated using stretch rate and shear rate maps. The controlling equations are then presented in a more general form which allows the effects of compaction to be included. Finally, the method is shown to be applicable to vertical and horizontal velocities (rather than speed and direction) and this has been used to model the crust deforming by brittle failure at the top, pure shear at the bottom and with a brittle-ductile transition in between. This system therefore allows modelling of crustal extension without the use of a detachment.

An aim of seismic reflection profiling of the present-day deep continental crust is to infer the geologic structure and hence to infer the tectonic processes that led to the formation of that structure. An important test of the validity of such interpretations lies in comparison with exposed geologic sections that once lay at deep crustal levels. The Massiccio dei Laghi (Lakes Massif) of Northern Italy provides such a section, on a scale comparable with that of contemporary deep seismic profiles, yet illustrates heterogeneity on a wide range of scales. On account of its importance as providing a substantial composite cross section through continental crustal rocks, coupled with its relative accessibility in the inner arc of the Western Alps, the region has been subjected to an extraordinary degree of structural, petrological, geochemical and petrophysical study by geoscientists during the past few decades. The rocks of the region record Paleozoic accretion, metamorphic and magmatic processes, the effects of the Hercynian orogeny,post-orogenic magmatic underplating and associated lithospheric stretching and thinning, Mesozoic extension and effects associated with the position of the region in Alpine tectonism (see review by Handy et al., 1999). The assembly of the rock units probably dates from Permo-Triassic time onwards.Their relative positions in outcrop are likely close to how they that might be taken as a model for a magmatically underplated and extended crustal section. Such a section can be compared with interpretations drawn from present-day seismic reflection profiles taken from regions of extended lithosphere that have not had the misfortune(good fortune?) to be upended and exposed during a subsequent phase of collisional orogeny.

This paper describes a computer model based on algorithms that simulate processes of carbonate sediment production, deposition, erosion and redeposition, and sequence stratigraphy of carbonate platforms. We use the exceptionally well-exposed cliff sections through the reef-rimmed Miocene carbonate platform of Mallorca, Spain, to test the ability of our program to simulate an outcropping stratigraphy. When an outcrop-derived sea level curve and the average Holocene rates of production and erosion are used in our model, we can precisely match the natural and the synthetic stratigraphies. In addition, we can reconstruct the nonoutcropping parts of the platform. This modeling indicates the ability of our program to simulate the sequence stratigraphy, stratal geometries, and facies of a real carbonate platform. In addition, this modeling strongly suggests that the processes for which we have no field evidence and that we are not modeling, i.e., compaction, differential subsidence, and three-dimensional sedimentary processes, are unimportant factors in the development of the Llucmajor Platform of Mallorca. The model is then used to explore and bracket some of the rates of different processes considered to be important in controlling the development of this late Miocene platform. Carbonate production and a decrease in production from the reefal margin into lagoonal areas have important controls on slope progradation rates and drowning of lagoons. When model-matching stratal geometries and sea level curves, one can establish a precise relationship between rates of sea level rise and rates of production. However, the solution is not unique in that production rates may be doubled and the sea level curve periodicity halved, and the same result is achieved. Both outcrop and modeling data indicate that rates of sea level rise never exceeded production rates. Therefore, production and accommoda ion were limited by sea level. One consequence of this is that erosion rates are very important in controlling the carbonate stratigraphy. With a constrained sea level curve, erosion rates are refined from the modeling of the Mallorca outcrops by the levels of erosion surfaces generated during sea level falls. The outcrops and the modeling also indicate that the bulk of the carbonate production and platform progradation occurs during the transgressive and highstand periods rather than during periods of falling sea level. Condensed sections characterize lowstands and not transgressions. This and other features of the Mallorca platform illustrate the inapplicability of many published carbonate sequence stratigraphic models. Modeling has contributed to three advances in our analysis of this carbonate platform. First, this method is probably the best way to study the many effects that the different controlling parameters have on platform evolution. Second, if some parameters can be quantified from field analysis, then other parameters can be bracketed from modeling. Third, this method strongly suggests that the parameters we are not modeling have no important control on platform evolution. Accurate forward modeling of carbonate stratigraphies allows petroleum geologists to independently test sequence stratigraphic interpretations, reconstruct partially exposed or imaged carbonate stratigraphies, locate and quantify the cross sections of likely reservoir facies, illustrate the development and likely interconnections of reservoir facies, and predict stratigraphies around a bas n margin. End_Page 247

Terence P Kee, Mark J Burchell and David A Waltham outline the network at the heart of this young but rapidly growing field in the UK.

Waltham and Grocke (2006) introduced a new methodology for analysis of the seawater strontium-isotope curve, which showed that changes to a source-flux (e.g., the continental weathering flux) or a source-isotope ratio (e.g., the hydrothermal flux isotope ratio) could be estimated simply by scaling and shifting the time-derivative of the isotopic curve. We used this methodology to analyse the Lower Jurassic and in particular the Toarcian oceanic anoxic event (T-OAE). The power of this approach is nicely illustrated by the fact that, as a consequence, the scale of the T-OAE strontium-isotope excursion is shown to be controlled almost entirely by the time-duration of this event. However, the comment by McArthur and Wignall (M&W) argues that the change in the Toarcian portion of the curve is solely controlled by sedimentation rate and that the weathering event as produced by Waltham and Grocke (2006) is an artefact of this factor. No criticisms in the comment were associated with our mathematical model itself, and thus this shows our approach’s usefulness for understanding the strontium-isotope cycle on geological time scales. There is unequivocal evidence for an environmental perturbation during the T-OAE that resulted in global warming, an intensified hydrological cycle and increased weathering (see [Hesselbo et al., 2000], [Hesselbo et al., 2007], [Cohen et al., 2004], Kemp et al., 2005 D.B. Kemp, A.L. Coe, A.S. Cohen and L. Schwark, Astronomical pacing of methane release in the Early Jurassic period, Nature 437 (2005), pp. 396–399. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (60)[Kemp et al., 2005] and [McElwain et al., 2005]). Many independent lines of evidence support the Toarcian weathering event and suggest that the degree of condensation proposed by M&W is extreme with limited sedimentological evidence and constraint, especially considering the uncertainties of chronostratigraphic constraint for the Jurassic. The exact magnitudes of all the above require additional, concentrated research efforts. We eagerly await the outcome of such research endeavours and the information they will provide to us for understanding the global perturbation during the Early Jurassic, T-OAE.

A geologic cross section, restored to its original horizontal orientation in Permian-Triassic time, has been constructed for the middle and lower continental crustal rocks of the Ivrea-Verbano zone and the adjacent Serie dei Laghi of northwestern Italy. Seismic P-wave velocities of a representative suite of rock samples were measured to high-pressure and high-temperature conditions. A synthetic seismic reflection profile, ∼76 km long and 30 km thick, was computed to compare what can be deduced from the seismic profile with what is known in much more detail from geologic mapping. Imaged features correspond closely to those seen on many present-day profiles, and the broad features of the tectonic evolution would be correctly interpreted, but important recumbent fold structures would be missed, and relationships between intrusive bodies and their country rocks would be unclear.

We have used a new coupled two-dimensional diagenetic and sedimentological model to investigate the evolution of a simple aggrading carbonate platform subject to glacio-eustatic sea-level fluctuations. The model employs variable hydrological zones, internally defined from platform exposure and climate, to provide a framework within which diagenetic processes are simulated at specified rates. Simulations show a clear stacked sequence of diagenetic zones, which are readily recognizable because of their lateral continuity and distinct trends in diagenetic evolution from platform interior to margin. However, spatial associations between unconformity surfaces and underlying diagenetic zones that suggest causality are misleading. The interaction between high amplitude sea-level fluctuations and subsidence results in substantial overprinting and a complex diagenetic history that could not be unraveled by traditional stratigraphic and sedimentological methods.

SUMMARY Ray tracing through models consisting of constant-velocity layers separated by curved interfaces, may be implemented by finding ray-paths whose traveltimes are stationary with respect to changes in ray/interface intersection positions. The amplitude of the resulting event is related to the second derivative of the traveltime with respect to changes in rayhnterface intersection position. For complex multilayered models the traveltime functions can be multivalued. Stationary points are then found by decomposition of the function into a number of single valued functions.