Journal of the Geological Society

Two calc-alkaline plutonic complexes. Bazman and Natanz, intruded through the south-eastern active and south-western ancient continental margins of Central Iran, Have been dated by the Rb-Sr whole-rock isochron method at 74 ± 2 and 24 ± 4.5 Ma, respectively. Detailed trace element studies together with low ⁸⁷ Sr/ ⁸⁶ Sr initial ratios (<0.706) indicate that these complexes represent parts of an Andean-type magmatic are formed in response to subduction of Tethyan oceanic crust beneath Central Iran. Geochemical data on the igneous rocks at Bazman suggest that subduction of the Oman oceanic crust was well established by the late Cretaceous. On the evidence of the Natanz rocks, the Arabian-Central Iranian collision did not occur during the late Cretaceous, but took place during late Paleogene or early Neogene time. The Natanz low-Rb diorites and gabbros cannot be comagmatic with the more salic rocks, for they have distinctly lower ⁸⁷ Sr/ ⁸⁶ Sr ratios and the gabbros, at least, were intruded some 10 Ma earlier.

A seismically active low-angle normal fault is recognized at depth in the Northern Apennines, Italy, where recent exhumation has also exposed ancient examples at the surface, notably the Zuccale fault on Elba. Field-based and microstructural studies of the Zuccale fault reveal that an initial phase of pervasive cataclasis increased fault zone permeability, promoting influx of CO ₂ -rich hydrous fluids. This triggered low-grade alteration and the onset of stress-induced dissolution–precipitation processes (e.g. pressure solution) as the dominant grain-scale deformation process in the pre-existing cataclasites leading to shear localization and the formation of a narrow foliated fault core dominated by fine-grained hydrous mineral phases. These rocks exhibit ductile deformation textures very similar to those formed during pressure-solution-accommodated ‘frictional–viscous’ creep in experimental fault rock analogues. The presence of multiple hydrofracture sets also points to the local attainment of fluid overpressures following development of the foliated fault core, which significantly enhanced the sealing capacity of the fault zone. A slip model for low-angle normal faults in the Apennines is proposed in which aseismic frictional–viscous creep occurs on a weak, slow-moving (slip rate <1 mm a ⁻¹ ) fault, interspersed with small seismic ruptures caused by cyclic hydrofracturing events. Our findings are potentially applicable to other examples of low-angle normal faults in many tectonic settings.

Controversy exists over the presence of normal faults that initiate and slip at low angles (<30°). Evidence from the Zuccale Fault, which dips c . 15° east on the island of Elba, Italy, indicates that the early stages of low-angle faulting were synchronous with development of subsidiary high-angle footwall faults. These footwall structures controlled fault rock distribution and caused the development of flow folds similar to those produced in analogue studies of synchronous high- and low-angle faulting. The current configuration of footwall structures indicates that the Zuccale Fault cannot have originally dipped greater than c . 20° east, and represents a primary low-angle normal fault.

A composite Tethyan Late Jurassic–Early Cretaceous carbon and oxygen isotope curve is presented. C-isotope data provide information on the evolution and perturbation of the global carbon cycle. O-isotope data are used as a palaeotemperature proxy in combination with palaeontological information. The resulting trends in climate and in palaeoceanography are compared with biocalcification trends and oceanographic conditions favouring or inhibiting biocalcification. Positive C-isotope anomalies in the Valanginian and Aptian correlate with episodes of increased volcanic activity regarded as a source of excess atmospheric carbon dioxide. A major warming pulse accompanies the Aptian but not the Valanginian C-isotope event. The observed change in Early Aptian temperatures could have triggered the destabilization of sedimentary gas hydrates and the sudden release of methane to the biosphere as recorded as a distinct negative carbon isotope pulse preceding the positive excursion. Both C-isotope anomalies are accompanied by biocalcification crises that may have been triggered by p CO ₂ -induced changes in climate and in surface water chemistry. Elevated nutrient levels in river-influenced coastal waters and in upwelling regions further weakened marine calcification. These conditions contrast with ‘normal’ trophic conditions prevailing in the latest Jurassic and favouring biocalcification. The C- and O-isotope curves record a stable mode of carbon cycling and stable temperatures. We conclude that biocalcification is mostly triggered (and inhibited) by CO ₂ conditions in the atmosphere–ocean system.

Strombolian explosions at Heimaey and Stromboli are described. Two main components of activity within a typical strombolian explosion are distinguished: an initial, high velocity, gas thrust part due to gas decompression and a subsequent convective part. Initial gas velocities at Heimaey averaged 157 m/s (standard deviation 28 m/s from 15 observations) and at Stromboli 31 m/s (standard deviation 12 m/s from 8 observations) for one vent and 56 m/s for a second vent. Velocities decreased approximately exponentially with height, and decelerations of up to 50 gravities were observed during the gas thrust events. A model of the gas thrust process is developed and values are deduced for the gas/solid mass ratio in the ejected material. Evidence is resented for the several-foId concentration of gas into that part f the magma expelled explosively, and a model in which large bursting gas bubbles are responsible for the explosions is shown to be compatible with the observations. Excess pressure within such bubbles is found to be of order 2.5 × 10 ⁴ N/m (0.25 atmospheres) at Heimaey and 600N/m ² (0.006 atmospheres) at Stromboli. Pressures inside bubbles of a few metres diameter are found to be of comparable magnitudes. Average gas release rates of 3 to 6 × 10 ³ kgm/s at Heimaey and at least 0.13 kgm/s at Stromboli are indicated.

We present new, detailed carbon-isotope records for bulk carbonate, total organic carbon (TOC) and phytane from three key sections spanning the Cenomanian–Turonian boundary interval (Eastbourne, England; Gubbio, Italy; Tarfaya, Morocco), with the purpose of establishing a common chemostratigraphic framework for Oceanic Anoxic Event (OAE) 2. Isotope curves from all localities are characterized by a positive carbon-isotope excursion of c . 4‰ for TOC and phytane and c . 2.5‰ for carbonate, although diagenetic overprinting appears to have obliterated the primary carbonate carbon-isotope signal in at least part of the Tarfaya section. Stratigraphically, peak δ ¹³ C values for all components are followed by intervals of high, near-constant δ ¹³ C in the form of an isotopic plateau. Recognition of an unambiguous return to background δ ¹³ C values above the plateau is, however, contentious in all sections, hence no firm chemostratigraphic marker for the end-point of the positive isotopic excursion can be established. The stratigraphically consistent first appearance of the calcareous nannofossil Quadrum gartneri at or near the Cenomanian–Turonian boundary as established by ammonite stratigraphy, in conjunction with the end of the δ ¹³ C maximum characteristic of the isotopic plateau, provides a potentially powerful tool for delimiting the stratigraphic extent and duration of OAE 2. This Oceanic Anoxic Event is demonstrated to be largely, if not wholly, confined to the latest part of the Cenomanian stage.

The Cenomanian–Turonian boundary (90.4 Ma) represents a major period of worldwide environmental disturbance. The physical manifestations of this are: elevated atmospheric and oceanic temperatures; a significant sea-level transgression; and a period of widespread anoxia, leading to the formation of oceanic black shales, and the extinction of 26% of all genera. Elevated δ ¹³ C values and enrichment of trace elements in Cenomanian–Turonian boundary sediments, combined with a reduction in ⁸⁷ Sr/ ⁸⁶ Sr, also imply a severe environmental perturbation. At this time oceanic crustal production rates reached their highest level of the last 100 million years. This was principally caused by extensive melting of hot mantle plumes at the base of the oceanic lithosphere, and the development of vast areas (up to 1×10 ⁶ km ² ) of thickened oceanic crust in the Pacific and Indian Oceans. The anomalous volcanism associated with the formation of these oceanic plateaux may have been responsible for the environmental disturbances c. 90 Ma. These eruptions would also have resulted in the emission of large quantities of CO ₂ into the atmosphere, leading to global warming. Additionally, the emission of SO ₂ , H ₂ S, CO ₂ and halogens into the oceans would have made seawater more acidic resulting in the dissolution of carbonate, and further release of CO ₂ This run-away greenhouse effect was probably put into reverse, by the decline of the anomalous volcanic activity, and by increased (CO ₂ -driven) productivity in oceanic surface waters,leading to increased organic carbon burial, black shale deposition, anoxia and mass extinction in the ocean basins

The late Cenomanian carbon isotope excursion can be shown to be synchronous over most areas of the globe where micropalaeontological data are adequate. The sedimentological variations seen in these different areas are thought to be due to palaeobathymetry. The relationship between this ‘anoxic’ event and the major faunal change in the late Cenomanian is described.

The timing and extent of unconformities in the Welsh Basin are investigated using ‘rock preservation curves’ derived from outcrop stratigraphic logs. Four basin-wide unconformities occur, focussed in late Precambrian, late Tremadoc, Pusgillian (early Ashgill) and mid-Devonian times. These bound three megasequences, equating with newly defined lithostratigraphic units, the Dyfed, Gwynedd and Powys Supergroups. Less extensive unconformities bound 18 component sequences.

The majority of the sequence boundaries reflect a component of tectonic or volcanotectonic activity rather than a pure eustatic sea-level change. The megasequence boundaries are attributed to late Precambrian to early Cambrian onset of rifting to form a passive margin, Tremadoc onset of subduction with intra-arc then back-arc extension, late Caradoc end to subduction, and late Early Devonian collisional deformation. The megasequences and controlling events can be tentatively matched with other basins on the Avalonian margin.

More generally, this study shows that sequence analysis is feasible in onshore basins lacking well and seismic data, and that the global eustatic interpretation of sequence stratigraphy is only partially applicable to active margin basins.

The agreement in the pattern of major biomes with that of climatic zonation of the Earth gives a strong indication that climate is the overriding influence controlling the distribution of plant communities. Three aspects of plants which may be preserved in the fossil state, give a signal of the climatic conditions under which they grew: (1) the present climatic association of their 'nearest living relative'; (2) leaf physiognomy of arborescent plants; (3) the character of their secondary xylem ('wood' of ordinary usage) reflecting, by the presence or absence of growth rings, the seasonality (or lack of it) in their environment and the potential for tree growth that it offered. The significance and limitations of these 'palaeoclimatic signals' as they may be read from the fossil plant record are reviewed and evaluated. The recent demonstration that stomatal frequency of leaves is responsive to changes in ambient carbon dioxide partial pressure offers promise for direct palaeobotanical evidence for past changes in the level of this climatically significant atmospheric constituent.