bluuebunny/crossref_metadata_2025 huggingface dataset:
Extensional tectonics in the Himalayan orogen, Zanskar, NW India
Authors: Patel R. C. , Singh Sandeep , Asokan A. , Manickavasagam R. M. , Jain A. K.
Published: April 2008

Large-scale nappes of the deformed Higher Himalayan Crystallines (HHC) were thrust southward over the Lesser Himalayan Proterozoic foreland during the Late Cenozoic. Critical evaluation of shear fabrics, reveals that the HHC underwent an earlier phase of ductile shearing with top-to-southwest overthrust-type sense of movement. On which was superposed layer-parallel NE-trending extension. As a consequence, the Zanskar Shear Zone (ZSZ) along the northern boundary of the HHC is re-interpreted as a complex ductile shear zone with an initial top-to-southwest sense of thrusting followed by the later superposition of top to northeast-directed extension. This period of extension is recognized across the whole of the HHC and is the result of the uplift and exhumation of the HHC by southwestward directed thrusting and ramping along numerous thrusts including the Main Central Thrust (MCT).
Annapurna detachment fault in the Greater Himalaya of central Nepal
Authors: Brown Richard L. , Nazarchuk Jeffrey H.
Published: April 2008

Displacement on the Annapurna detachment fault (ADF) has exhumed the Greater Himalayan metamorphic sequence in central Nepal. The fault is most likely a continuation of the South Tibetan detachment system recognized elsewhere in the Himalayan orogen.

The characteristics of detachment faulting and the structural and metamorphic histories of the hanging wall and footwall in the Thakkhola region are described. Two stages of normal-sense shearing on the ADF are recognized: the first stage occurred at mid-crustal depth; the most recent detachment faulting on the system was brittle and may have been accompanied by orogen-parallel extension.

First-phase isoclinal folds in the Tibetan sedimentary sequence above the ADF reflect a southwesterly sense of shear and are refolded by spectacular northeasterly verging second-phase folds that have affected a crustal thickness of over 6 km. Third-phase southwesterly directed thrust faults and folds have disrupted the earlier structures but predate the most recent period of normal-sense shearing on the ADF.

The kinematic significance of the northeasterly verging second-phase folds is not clear; these structures may have formed during early motion on the ADF but, alternatively, may reflect older events associated with compression and crustal thickening of the orogen.
Pressure, temperature and time constraints on metamorphism across the Main Central Thrust zone and High Himalayan Slab in the Garhwal Himalaya
Authors: Metcalfe R. P.
Published: April 2008

Following the early Eocene collision of India and Asia, continental subduction occurred on the northward-dipping Main Central Thrust (MCT). In western Garhwal, N. India, upper amphibolite-facies gneisses on the High Himalayan Slab are thrust southwards over unmetamorphosed to greenschist facies quartzites, carbonates and metabasics of the Lesser Himalaya. In the Bhagirathi valley, the MCT forms a
c.
10 km thick shear zone composed of mylonitic augen gneiss, amphibolite and metasediments. Metamorphic grade increases both northwards and with structural height. The MCT zone is bounded to the north by the Vaikrita (roof) Thrust and to the south by the Munsiari (floor) Thrust. The Vaikrita Thrust is a diffuse high-temperature shear zone, whereas the Munsiari Thrust is a relatively discrete fault formed under brittle-ductile conditions. North of the MCT zone, at the top of the High Himalayan Slab a northward-dipping extensional shear zone, the Jhala normal fault, was responsible for the downthrow of the Tethyan sediments to the north with respect to the uplifting High Himalayan Slab gneisses to the south.

Thermobarometic transects reveal a sudden increase in both pressure and temperature across the Vaikrita Thrust from south to north but with subsequent decreases accompanying structural height in the High Himalayan Slab. Temperatures increase going up-structural section from about 500° C to 770° C across the MCT zone, but then decrease again to the north varying between about 550 and 640° C. Similarly, pressures increase sharply up-structural section across the MCT zone from 6 to 12 kbar, then decrease towards the top of the slab to between 7 and 8.9 kbar. The inverted
P-T
gradient across the MCT zone changes to approximately isothermal and isobaric conditions in the top 9 km (horizontal distance) of the High Himalayan slab.

Cooling rates for the upper MCT zone determined from
40
Ar/
39
Ar (hornblende) and K-Ar (muscovite and biotite) cooling ages suggest a return to erosion-controlled denudation following extension at the top of the High Himalayan Slab. Additional K-Ar (muscovite) cooling ages from a transect through the MCT zone and High Himalayan Slab are progressively younger towards the south, reflecting the southward propagation of the deformation sequence with time. Hornblende
40
Ar/
39
Ar cooling ages from the MCT zone suggest that structurally lower rocks have not been heated above
c
. 500° C since the Precambrian, whilst a 19.8 ± 2.6 Ma hornblende age from the MCT zone dates the latest high-temperature shearing at higher structural levels in the MCT zone and places a minimum age constraint on Himalayan metamorphism in the Garhwal sector of the Himalaya.
Deformation and metamorphism within the Main Central Thrust zone, Arun Tectonic Window, eastern Nepal
Authors: Meier K. , Hiltner E.
Published: April 2008

The Lesser Himalayan Arun Tectonic Window comprises the Upper Precambrian, polymetamorphosed metasediments of the Nawakot Complex. These are tectonically overlain by Precambrian gneisses of the Higher Himalayan Kathmandu Complex. The main phase metamorphism within the Higher Himalayan rocks predated thrusting along the Main Central Thrust. By contrast, the main metamorphism (M2) within the Nawakot Complex commenced synchronously with it being underthrust during D2 along the Main Central Thrust Zone beneath the amphibolite facies rocks of the Kathmandu Complex. During D2 rocks on both sides of the Main Central Thrust Zone underwent intensive mylonitization within a several hundred meter wide zone of non-coaxial shearing. Although the stratigraphic order within the Nawakot Complex is the right way up, rocks within the Arun Tectonic Window show a concentric inverted metamorphic zoning, ranging from lower greenschist facies in the core of the window, up to lower amphibolite facies at the outer edge of the window. This ‘inverted metamorphism’ (M2) of the Nawakot Complex originated as a result of syn- to post-D2 downward heat conduction from the ‘hot’ hanging wall of the Main Central Thrust onto the ‘cold’ footwall. This heat flow also caused the strong retrograde overprinting at the base of the Kathmandu Complex. After a static period, a late- to post-M2 NNE-shortening (D3) commenced along the Main Boundary Thrust. The Arun Tectonic Window is the result of doming of the Main Central Thrust during D4 followed by erosional exhumation.
Evolution of foreland structures: an example from the Sulaiman thrust lobe of Pakistan, southwest of the Himalayas
Authors: Jadoon I. A. K. , Lawrence R. D. , Lillie R. J.
Published: April 2008

Three cross-sections based on integrated surface geology, borehole and seismic reflection data have been used to interpret the style and evolution of the evolving Sulaiman foreland. A 10 km thick stratigraphic section outcrops at the deformation front. Available evidence suggests detachment of nearly all the stratigraphic sections from the crystalline basement. The decollement is believed to be located along the brittle-ductile boundary at the base of the wedge in the Sulaiman lobe of gentler topography. Coherent stratigraphy and structural uplift of 4–8 km is interpreted to be a result of a passive-roof duplex style of deformation. The duplex sequence consists of competent Jurassic and older strata. It has a floor thrust (decollement) in Palaeozoic rocks and a roof thrust (passive-roof backthrust) in thick Cretaceous shales. The roof sequence preserved over several horses along length of the section shows hindward translation.

The structural variation in cross-sections has led to the modification of an earlier interpretation that folding precedes duplexing. The present chronology of structures is the: (1) development of an incipient (
c.
3.5 km half-wavelength) Uch fault-propagation fold at the tip of the decollement; (2) formation of a low amplitude, broad (
c.
20 km half-wavelength) concentric detachment fold (Sul); (3) increase in amplitude of a detachment fold (Loti/Zin) to a critical level for development of ramp and duplex structures.
Braided-channel pattern and palaeohydrology using an index of total sinuosity
Authors: Robertson-Rintoul M. S. E. , Richards K. S.
Published: April 2008

Channel patterns are commonly represented as discrete types (e.g. straight, meandering, braided) separated by threshold values of slope and discharge. However, for gravel-bed river patterns, an index of total sinuosity (∑
P
) can be defined which varies continuously with both stream power per unit channel-belt length (represented as the product of mean annual flood and valley gradient) and median bed material diameter. An empirical relationship between ∑
P
and these controlling variables can be used to provide palaeo-hydrological reconstructions for braided stream traces on terrace fragments, as an alternative to methods based on bed material sizes and criteria for the threshold of grain motion.
Anabranch confluence kinetics and sedimentation processes in gravel-braided streams
Authors: Ashmore Peter
Published: April 2008

Based on observations of channel changes and bar sedimentation in small-scale hydraulic models, it is argued that confluence zones play the key role in sedimentation patterns and processes in gravel-bed braided streams. The control is exerted because confluences are nodes in the channel network and therefore sedimentation and channel pattern downstream of a confluence are mediated by the response of the confluence to changes in discharge, bedload, and planform of the upstream confluent channels. Examples of confluence kinetics and sedimentation occurring over time-scales equivalent to days or weeks of competent flows in the prototype are described for the first time. Renewed braiding is directly associated with flow expansion downstream of confluences and triggered by migration of unit bars or bedload sheets through the confluence. Lateral translation of confluences may lead to the accretion of point bar complexes, while downstream translation often causes erosion or complete removal of existing bars. The effects of a change in position or orientation of one of the upstream channels are similar to those of confluence translation. In addition to the changes caused by progressive shifting of the confluence zone, a second group of adjustments involving changes in the discharge or bedload transport rate in the confluent channels is also common. These include: rotation of the scour axis redirecting flow and sediment movement downstream of the confluence and leading to downstream channel avulsion in some cases; a change in the total discharge of the confluence causing expansion or contraction of the scour zone; and the virtual abandonment of one of the confluent channels resulting in obliteration of the scour hole. Observing a confluence for several hours in the model leaves the visual impression of sediment being distributed downstream of the confluence rather like water spurting from a hose as it plays about. This descriptive synthesis of confluence kinetics provides the basis for analysis and quantitative modelling of confluence dynamics, which requires the acquisition of field data, as well as further laboratory experiments.
Radar facies: recognition of facies patterns and heterogeneities within Pleistocene Rhine gravels, NE Switzerland
Authors: Huggenberger Peter
Published: April 2008

Pleistocene braided-river deposits in river valleys constitute a large fraction of natural groundwater reservoirs in Switzerland. A key for estimating the residence times of water and for determining the extent of macrodispersion, which describes large-scale mixing processes in the aquifers, is a knowledge of the distribution of hydraulic conductivities. In many contamination problems, sedimentological information is sparse and drill-core descriptions and pumping-tests only give a limited picture of the geometry of inhomogeneities. Ground-probing radar (GPR) offers the potential to resolve sedimentary structures and lithofacies in gravel deposits. For example, the geometry of characteristic sedimentary structures and the textures of late Pleistocene Rhine gravel are portrayed on GPR reflection images.

Because of the ability of the GPR method to detect changes in water content, the reflection image can be related to small changes in the degree of sediment saturation, which may also reflect a change in sediment composition. This allows a distinction between several characteristic lithofacies that are typical of late Pleistocene braided-river depositional systems. The main limitation of the GPR method is the rapid attenuation of electromagnetic waves in the ground, especially in clay-covered regions.
Avulsive channel systems: characteristics and examples
Authors: Richards Keith , Chandra Shobhit , Friend Peter
Published: April 2008

Fluvial processes in large subsiding sedimentary basins (e.g. tectonic foredeeps) must be adjusted in the long term to distribute sedimentation areally, although in the short term deposition is localized along the river courses. This means that the major long-term characteristic of the river pattern is that it is avulsion-dominated. Individual channels in an avulsive system display one of the conventional pattern types (e.g. meandering, braiding), but these patterns may reflect the behavioural relationships between the ‘dominant’ and ‘secondary’ channels in the system. These relationships change through time as aggradation along the dominant channel encourages avulsion into the course of former secondary channels, and the channels exchange roles. Thus, channel system behaviour is not explained by the variation of stream power used to analyse the behaviour of individual (meandering or braided) rivers, and palaeohydrological reconstructions for individual channels within such a system are of limited significance. Examples of avulsive systems are noted, including the northern Gangetic Plain.
River management and conservation issues on formerly braided river systems; the case of the River Tay, Scotland
Authors: Gilvear David J.
Published: April 2008

Although the present-day River Tay, Scotland is generally confined to a single course and exhibits limited lateral mobility, historical maps (1747–53) reveal that during the eighteenth and nineteenth centuries a braided river channel pattern was more characteristic of a number of reaches on the piedmont valley floor. Possible explanations of the change in channel pattern include flood embankment construction, flow regulation by impoundment and possibly a change in flood magnitude and frequency, and alterations in sediment supply.

Areas of former braiding isolated from the main channel by embankments or incorporated into the floodplain sediments, exhibit a greater susceptibility to erosion and are prone to flood embankment breaches. In contrast, river channel reaches still exhibiting unconfined braiding tendencies have been designated areas of high conservational value due to large expanses of gravel at different stages of vegetation colonization and high plant species diversity.

It is suggested that the identification of areas of former braiding and reaches prone to braiding is useful for land use management both for improved flood protection and floodplain habitat conservation.
Sedimentary structures exposed in bar tops in the Brahmaputra River, Bangladesh
Authors: Bristow C. S.
Published: April 2008

The Brahmaputra River is one of the world’s largest sand-bed braided rivers with a channel belt up to 15 km wide, a mean channel depth of 5 m and maximum scour depths of up to 40 m. The recorded discharge varies by around 60 000 cumecs every year following the annual monsoon, producing dramatic stage fluctuations with the water level falling between 7 and 8 m from bankfull to low flow stage. In the dry season large areas of bar top and channel bed are exposed and low flow channels cut natural sections through bar tops exposing the internal stratification. Elements of upstream, downstream and lateral accretion are identified, although the high width/depth ratio of channels (up to 500:1) results in extremely low depositional dips on bed bounding surfaces. Within the accretionary elements vertical and lateral changes in sedimentary structures are ubiquitous and varied but some patterns can be discerned. Channel deposits are dominated by sinuous-crested dunes and trough cross-stratification. Upper stage plane bed lamination, often with a very low angle depositional dip, has been observed in beds up to 4 m thick and is generally found at the base of the exposed bar top sections. Trough cross-stratification in bar tops occurs in sets from 0.1 m to 3 m thick with rapid vertical and lateral changes in set thickness. Very large sets of trough cross-stratification, >3 m thick, do not occur in the bar top sections. Current ripple lamination is most abundant on the tops of bars with very high rates of climb associated with high rates of sediment deposition. From these observations a generalised vertical sequence of bar top origin might include trough cross-stratification overlain by upper stage plane bed lamination, isolated sets of trough cross-stratification truncated and capped by current ripple lamination. This sequence, which is considered to represent the deposits of a single flood, may occur within the upstream, downstream or lateral accretion elements. However, it should be noted that vertical and lateral changes in facies are extremely abundant with many changes in sedimentary structures and reactivation surfaces within each depositional episode.
Differentiated rocks of the Galapagos hotspot
Authors: McBirney Alexander R.
Published: April 2008

Virtually all the MORB-like tholeiites and differentiated rocks of the Galapagos Islands are concentrated along the central axis of the archipelago within 100 km of its leading island. Both have a remarkable symmetry. Those close to the centre are strongly tholeiitic, but towards the north and south they become increasingly alkaline.

Most of the differentiated suites are readily explained by crystal fractionation at shallow depths, but voluminous, aphyric rhyolites erupted from a large active volcano immediately downstream from the hotspot are more difficult to explain. Although the geochemical criteria are consistent with crystal fractionation, these extreme differentiates are not accompanied by expected amounts of intermediate compositions, such as those found on older islands; instead, they have been erupted concurrently with basalts that have undergone only minor amounts of differentiation.

The central region in which tholeiitic rocks and their differentiates have been erupted lies west of a transform fault that separates 10 million-year-old lithosphere on the west from thinner 5 million-year-old lithosphere on the east. It is also the region of thickest crust: the MOHO reaches a maximum depth of about 18 km below sea-level directly underneath the same region.

Three possible mechanisms of differentiation have been considered for the voluminous rhyolites: (a) inward crystallization of a large reservoir; (b) side-wall crystallization with convective segregation of a light felsic liquid; and (c) melting of the base of the crust. Although the first two are consistent with the geochemical nature of the rhyolites, they are not compatible with the geological relations. Moreover, they do not explain why such rocks are found only in very restricted settings. In the case of the Galapagos, these extreme differentiates are found only in the region where the crust reaches a maximum thickness near the centre of the Galapagos Platform and directly downstream from the hotspot. On a global scale, oceanic rhyolites are found close to spreading ridges and nowhere else.

These relations indicate that oceanic rhyolites are generated only where the lithospheric mantle is thin enough to permit the thermal effect of hotspots to reach the base of unusually thick crust. Remelting of pockets of felsic differentiates produced by earlier differentiation when the gabbroic layer was close to the ridge would result in trace element compositions consistent with crystal fractionation.
Palaeozoic and Cenozoic lithoprobes and the loss of >120 km of Archaean lithosphere, Sino-Korean craton, China
Authors: Menzies Martin A. , Fan Weiming , Zhang Ming
Published: April 2008

In eastern China Palaeozoic kimberlites and Cenozoic basalts have been erupted through the same Archaean crust, thus providing deep probes of the cratonic lower lithosphere over a period of 400 Ma. While Palaeozoic diamondiferous kimberlites point to the existence of thick, refractory lower lithosphere in the east, Cenozoic basalt-borne xenoliths reveal the presence of hot, thin, less refractory lower lithosphere. Remnants of the Archaean lithosphere may have survived as harzburgites which are chemically similar to those from the Kaapvaal craton but very different from recently accreted lherzolites. In the absence of convincing evidence for supra-subduction or intraplate processes it is believed that the dramatic change of lithosphere architecture in the Phanerozoic was caused by indentor tectonics resulting from the collision of India and Eurasia. Passive reactivation and remobilization of the Archaean lower lithosphere, in particular metasome horizons, contributed to Cenozoic magmatism aligned along major lithospheric faults.
Spreading-induced faulting and fracturing of oceanic crust: examples from the Sheeted Dyke Complex of the Troodos ophiolite, Cyprus
Authors: Dietrich Dorothee , Spencer Sara
Published: April 2008

Detailed mapping and structural analysis of the Sheeted Dyke Complex of the Troodos ophiolite, Cyprus, has provided valuable insight into the extensional processes active during the formation of oceanic crust. Oceanic extension is essentially accommodated by: (1) a dense network of small-scale normal faults (metric to several metres spacing of the fault planes) which migrate upwards in time and space through the Sheeted Dyke Complex; and (2) larger-scale normal faults (decametric spacing of the fault planes) which detach into sub-horizontal shear zones and which cause rotation of overlying dyke packets. Geometric models are derived from these structures which differ markedly from the known models of continental extension. In particular, a new ramp/flat model of normal faulting is proposed for the most frequently occurring type of small-scale faults.
Troodos revisited: the Mount Olympus gravity anomaly
Authors: Shelton A. W.
Published: April 2008

Eighty new gravity stations have been established to define the intense gravity low reported by Gass & Masson-Smith in 1963. The anomaly is confirmed as circular; the implications of this, against a background 200 mGal positive with an E-W trend, are explored.

The residual anomaly has been isolated and an anomalous mass determined. From this, permissible volumes have been calculated assuming serpentinization of mantle material to be solely responsible for the anomaly. Within these constraints, modelling of the residual reveals a solution with a northerly dip consistent with the underlying subduction polarity. The modelling excludes causative bodies other than those near-surface, equidimensional in plan and having a high density contrast.

The anomalous mass allows an estimate of the uplift forces acting beneath the summit of Mt Olympus. These are shown to be three times the mass of the topography above the level of 1000 m. Implications for the rate of uplift are discussed.
MORB peridotite-sea water interaction: experimental constraints on the behaviour of trace elements,
87
Sr/
86
Sr and
143
Nd/
144
Nd ratios
Authors: Menzies Martin A. , Long Andrew , Ingram Gerry , Tatnell Matthew , Janecky David
Published: April 2008

The geochemical changes associated with the experimental serpentinization of peridotite are due to reaction with sea water and to the growth of new mineral phases. Since the rare earth elements (REE) and Sr are primarily ensconced in clinopyroxene, it is the unreactive nature of this phase in the presence of sea water that determines the REE content, the
87
Sr/
86
Sr and
143
Nd/
144
Nd ratio of the experimentally produced serpentinites. Serpentinites (after lherzolite and dunite) have chondrite-normalized REE abundance patterns similar to the initial peridotite indicating that the light REE (LREE) are not selectively mobilized by peridotite-sea water interaction at 300°C. However, the serpentinites produced as a result of harzburgite-sea water experiments show an increase in LREE content. Despite the sporadic behaviour of the LREE, the
143
Nd/
144
Nd ratios in experimentally produced serpentinites (after lherzolite and harzburgite) are identical to primary clinopyroxene in the unaltered peridotite. In contrast, a marked change in the strontium isotopic composition of the peridotites occurs during experimental serpentinization due to the growth of hydrous and Ca-rich phases which facilitates the uptake of sea water Sr. Whereas harzburgite and dunite alter to produce serpentinites with high Sr contents and
87
Sr/
86
Sr > 0.709, lherzolites tend to alter to serpentinites with
87
Sr/
86
Sr < 0.709. This behaviour invalidates the use of Rb-Sr data in understanding the origin of oceanic and ophiolitic peridotites, but the relative immobility of the light REE (in clinopyroxene-bearing peridotites) and the low REE content of sea water encourages the careful use of the REE and neodymium isotopes as a petrogenetic indicator for elucidating the origin of serpentinized abyssal peridotites.
The growth of continental crust during the Late Proterozoic: geochemical evidence from the Arabian Shield
Authors: Harris N. B. W. , Hawkesworth C. J. , Tindle A. G.
Published: April 2008

Proterozoic magmatic provinces that result from active subduction provide a critical link between contrasting styles of crustal growth during the Archæan and Phanerozoic. The Arabian Shield is an excellent example of Late Proterozoic (860–560 Ma ago) felsic magmatism that is largely calc-alkaline in composition and displays the selective enrichment of LIL elements indicative of subduction. Juvenile Nd and Sr isotope ratios confirm that the Arabian Shield represents a collage of island arcs that were responsible for a significant proportion of global crustal growth (as measured by volume of magma extracted from the mantle per unit time) during the Pan-African tectonothermal event.

The high Rb/Sr ratios and low Sr concentrations of the majority of granitoids from the Arabian Shield suggest that they may be modelled as the products of intra-crustal feldspar fractionation from a basaltic source. However, about 250f Arabian granitoids, mostly formed between 860 and 820 Ma ago, are characterized by high Sr and depleted Y abundances, indicative of melts which have not undergone fractional crystallization of feldspar, derived from a garnet-rich source. In the absence of tectonic thickening this suggests that these magmas were generated by melting the subducted slab, a process observed in contemporary arcs where unusually young, hot crust has been subducted.

The average composition of the Arabian crust at its present level of exposure is highly silica rich (SiO
2
= 66%), and mass balance constraints on the composition of the continental crust of the Arabian Shield together with seismic evidence for a lower crust of gabbroic composition, require that either delamination of a mafic lower crust has occurred or the average composition of the mantle-to-crust flux was intermediate rather than basic in composition. The latter explanation is preferred since crustal delamination implies unrealistically high crustal growth rates during the Pan-African event. A flux of intermediate composition is indicative of slab melting during subduction.

Thus trace element and mass balance considerations support the hypothesis that a significant proportion of Arabian granitoids formed during the early stages of the Pan-African event were derived from partial melting of the subducted slab, indicating that subduction of young, hot oceanic lithosphere was widespread at that time.
Evaluating Late Quaternary uplift in Greece and Cyprus
Authors: Vita-Finzi Claudio
Published: April 2008

Uplift data based on radiometric and archaeological dating are being used to trace the tectonic history of the eastern Mediterranean. In the Gulf of Corinth new U-series and
14
C ages suggest that there is an upper limit to coseismic uplift on both main and antithetic normal faults and that distributed extension develops serially. U-series and
14
C dates from southwestern Cyprus support geomorphological and archaeological evidence for Late Quaternary southward tilting of the island.
Basic and intermediate volcanism of the Mogollon-Datil volcanic field: implications for mid-Tertiary tectonic transitions in southwestern New Mexico, USA
Authors: Davis Jon M. , Elston Wolfgang E. , Hawkesworth Chris J.
Published: April 2008

Basic to intermediate volcanism of the Tertiary Mogollon-Datil volcanic field can be divided petrologically and geochemically into three temporal groups; Pre-30 Ma, 30–20, Ma and Post-20 Ma. The Pre-30 Ma and 30–20 Ma groups are dominated by high-K calc-alkaline andesites and mildly alkaline basaltic andesites respectively. These both have major and trace element characteristics typical of an orogenic origin. In contrast, the late Tertiary, Post-20 Ma lavas are typically alkaline basalts and have geochemical characteristics more consistent with a within-plate setting. The 30–20 Ma basic lavas have high Ba and Sr, and low Nb contents, resulting in high LIL/HFS element ratios (Ba/Nb
c.
80).
87
Sr/
86
Sr ratios are > 0.7065. These features are inferred to have been derived from continental mantle lithosphere modified by subduction-related processes in the Proterozoic. The Pre-30 Ma lavas have many similar characteristics but with Rb and Th enriched relative to Ba and Sr, lower
87
Sr/
86
Sr, and more subalkalic parental magmas. In contrast, the Post-20 Ma lavas show a tendency to higher Nb contents (low LIL/HFS ratios), and lower
87
Sr/
86
Sr ratios similar to OIB-like magmas derived from partial melting of the convecting asthenospheric mantle. The overall shift from predominantly lithosphere to asthenosphere-derived magmas with time in the evolution of the Mogollon-Datil volcanic field is consistent with models in which magmatism was triggered by lithosphere extension. It is concluded that calc-alkaline magmas with minor and trace element features similar to those from destructive plate margins were generated in an extensional tectonic setting. In detail, the marked change to within-plate style magmatism took place
c.
10 Ma after the period of maximum extension.
Terrestrially sourced oils: where do they exist and what are our limits of knowledge? — a geochemical perspective
Authors: Powell T. G. , Boreham C. J.
Published: April 2008

There are relatively few well-documented cases where significant amounts of oil (> 500 million barrels) have their origin from coals or their associated carbonaceous shales. The best-documented cases are from Australia and Indonesia and possibly the North Sea. The extensive debate as to whether coals as such or their associated carbonaceous shales are the source for oil has been essentially sterile: it has failed to take into account the heterogeneous nature of coals, the variety of their floral origins and the continuity of mineral dilution from pure coal to carbonaceous shale. Each case must be considered on its merits.

The overall petrographic composition of coal is a poor guide to its petroleum potential. Sub-microscopic lipids of bacterial and also plant origin occur in vitrinite and inertinite and contribute significantly to petroleum potential. Suberinite and cutinite are the major source of waxy hydrocarbons.

Hydrogen Indices measured on the immature versions of documented terrigenous source rocks are surprisingly low falling in the range 200–350 and occasionally lower. There is not a simple relationship between the elemental composition of terrigenous kerogen, the gross pyrolysis yield by Rock-Eval and the yield of normal hydrocarbons in pyrolysis gaschromatography. Our assessment of liquid potential in marginal source rocks, wherein most terrigenous sediments lie, is far from perfect.

A combination of the mass balance approach in conjunction with pyrolysis studies has been shown to be the most effective way of demonstrating source potential and hydrocarbon products (gas vs. oil) expected from these marginal source rocks. There is clear evidence that significant amounts of gas are generated in the conventional oil window, and that high gas to oil ratios facilitate migration from the leaner source rocks. Comparative maturation studies on oils and gas-condensates show that they can be generated at similar maturation levels.

The majority of terrigenous oils and condensates are paraffinic and the oils may have a high wax content. Exceptions occur when resin is a significant source in which case there is an increased proportion of naphthenic and aromatic hydrocarbons. Oils derived from coal-bearing sequences have high pristane to phytane ratios (>4.0); they have a high proportion of iso- and anteiso-alkanes derived from bacterial sources; hopanes predominate over steranes which are dominated by the C
29
member. A variety of diterpenoid hydrocarbons may be found which relate to the nature of the parent flora and increasingly can be used for oil-source correlation in non-marine sequences.
The oil-generating potential of plants from coal and coal-bearing strata through time: a review with new evidence from Carboniferous plants
Authors: Collinson Margaret E. , Van Bergen Pim F. , Scott Andrew C. , De Leeuw Jan W.
Published: April 2008

A combination of microscopy and chemistry is used to characterize high molecular weight components of plant material which contributed to Carboniferous coals and coal-bearing strata. Material was selected from coals, paper coals, coal balls, oil-shales and shales and included cuticles from stems and leaves, periderm from stems and rooting systems, spore walls, algal cell walls and ‘resin’ rodlets. Chemical analyses were undertaken using Curie-point pyrolysis-gas chromatography (-mass spectrometry) (Py-GC-(MS)) and
13
C solid state Nuclear Magnetic Resonance (NMR). Light microscopy and scanning electron microscopy were used to ensure a detailed understanding of the plant material which was analysed chemically. This study has emphasized those plants, plant tissues and organs which are known to have been dominant or major contributors to Carboniferous coals and coal-bearing sequences: arborescent lycophyte periderm (
Diaphorodendron
stems and
Stigmaria
rhizomorphs); pteridosperm cuticles (medullosan (e.g.
Alethopteris
) and
Karinopteris
), lycophyte stem cuticle (
Eskdalia
) and arborescent lycophyte spores (megaspores and microspores). Several algal cell walls (
Tetraedron, Tasmanites
and
Gloeocapsomorpha
) and the ‘resin’ rodlets derived from medullosan pteridosperm petioles have also been analysed. Results show that all of these elements (except resins) contain (or are dominated by) highly resistant, highly aliphatic macromolecules which have the potential to yield, upon catagenesis,
n
-alkanes which are found in crude oils. Resins potentially contribute cyclic hydrocarbons to crude oils. Combining all this evidence it is concluded that Carboniferous coals are oil prone and that explanations for the absence of oil-pools derived from such coals must be sought in geological or exploration factors and not in the nature of the coals themselves. A review of coal-forming floras through time, and of suggested terrestrially sourced oils and their parent coals, leads us to conclude that high molecular weight components of higher plant materials have potentially made a major contribution to oils sourced from coals and coal-bearing strata over a long period of geological time (Devonian onwards). Cell walls of freshwater algae have made a comparable contribution since the Ordovician. The oil-generating potential of each coal or organic-rich sediment will depend upon a combination of depositional environment and the floristic and chemical composition of the source vegetation and its component plants and plant parts.
Microstructural and microchemical consequences of fluid flow in deforming rocks
Authors: Knipe R.J. , McCaig A.M.
Published: April 2008

The different deformation mechanisms possible in rocks impact differently on fluid flow processes because of the range of induced volume changes associated with different deformation histories. Microstructural analysis of deformed rocks can provide crucial information for the identification of fluid flow pathways, determination of the physico-chemical properties of the fluid, quantification of the amount of fluid involved and an assessment of the variation in the open/closed nature of the fluid flow system. A critical factor in the efficiency of fluid flow in deformed rocks is the competition between the processes which maintain the connectivity of the high permeability pathways and those which close such pathways. The range of deformation processes which are involved in this competition will be different depending on the tectonic settings, the deformation conditions and the rock types involved. A brief review of the processes which interact to control fluid flow during deformation in sedimentary basins, crystalline basement under low to moderate grade metamorphism and during prograde metamorphism at moderate to high grades is given.
Fluid-flow processes and diagenesis in sedimentary basins
Authors: Bjørlykke Knut
Published: April 2008

Flow of fluids in sedimentary basins causes transport of heat and dissolved mass and is therefore potentially important in relation to diagenetic reactions. Rather large fluid fluxes are required, however, for this type of transport to be significant in terms of dissolution and precipitation of minerals. Before diagenetic processes are attributed to pore water flow, semi-quantitative calculation of flow rates and duration of flow should be attempted.

Heat flow in sedimentary basins is normally dominated by conduction, except on a local scale. The upwards pore water flux due to compaction is on average smaller than the subsidence rate, and the thermal anomalies caused by compaction are moderate in modern basins. Dissolution and precipitation of minerals which are in equilibrium with the pore water occur when the flow is oblique relative to the isotherms. Due to the low solubility/temperature gradient of the common silicate and carbonate minerals, very large fluxes of pore water are required to transport significant volumes of mass in solution. The greatest potential for transporting solids and creating secondary porosity is during meteoric water flow, because the flow rate then may be several orders of magnitude higher than what is typical for compaction-driven flow. Quartz overgrowth corresponding to 3% of the rock volume requires a total flow of 10
8
cm
3
cm
−2
if the silica is introduced by vertical compaction-driven flow from a source outside the sandstone. Pore water flow precipitating quartz due to cooling will dissolve carbonate cement at a much higher rate. An external import of silica through the flow of water would be expected to cement up the most permeable pathways, such as fractures and well-sorted permeable sand beds. In the case of carbonate cement the solubility gradient is negative and upwards flowing and cooling pore water would cause dissolution rather than precipitation. Also in the case of carbonate rocks, large-scale mass transfer such as dolomitization is easier to explain as occurring at relatively shallow depths rather than during deeper burial.

Ore minerals like galena have a steep solubility/temperature gradient, but concentrated precipitation requires rapid cooling of hot water. Such conditions are most likely to be met when hot fluids are cooled near the surface.
Hydrocarbons and other fluids: paragenesis, interactions and exploration potential inferred from petrographic studies
Authors: Parnell John
Published: April 2008

Residues of hydrocarbons (bitumens
sensu lato
) are found in a wide range of settings in addition to reservoir porosity, including microfractures, megafractures, and several types of vein porosity. Paragenetic relationships between hydrocarbons and inorganic minerals provide information on the relative timing of hydrocarbon migration and the migration of other fluids. Hydrocarbons in reservoir rocks and mineralized fractures are paragenetically late as they are in pre-existing fluid pathways. Some other types of hydrocarbon occurrence are paragenetically earlier, especially proximal to the source rock. Different types of ore mineralization (notably base metal and uranium) in which hydrocarbons occur exhibit varying parageneses which represent distinct burial/thermal histories responsible for the ore types. Co-migration of hydrocarbons and silicon-bearing fluids is evinced by the occurrence of large quantities of authigenic silicate minerals within hydrocarbon residues.

Interactions between metals and hydrocarbons may be exploited in the exploration for both metals and hydrocarbons. Metal-rich fossil fuels can preserve a signature of anomalous metal concentrations in the vicinity of ore deposits. Hydrocarbons precipitated by irradiation-solidification processes about radioelement-bearing phases help to identify hydrocarbon migration pathways and in some circumstances allow isotopic dating of migration.
Fluid-rock interactions during continental red bed diagenesis: implications for theoretical models of mineralization in sedimentary basins
Authors: Metcalfe R. , Rochelle C.A. , Savage D. , Higgo J.W.
Published: April 2008

Continental red beds are first-cycle, immature sediments which are deposited in oxidizing conditions and owe their red colouration to the early diagenetic development of hematite. Previously published work on the fluid/rock interactions which occur during the diagenesis of such sediments highlights that pH and redox are critical fluid parameters which control diagenesis and ore formation. However, except in modern surface water and shallow groundwater systems, these parameters cannot be measured directly, and must be estimated from theoretical considerations. These suggest that mixing between fluids of different redox states is likely to be a critical control on heavy metal mobility. Many diagenetic features can be explained largely by the chemical heterogeneity of red beds, and by the diagenetic ranges of redox conditions and pH which are among the greatest for any type of sediment. Such features include: the formation of red bed-hosted ore deposits; the extensive development of hematite; and the development of early diagenetic non-ferroan carbonate and late diagenetic ferroan carbonate cements. In order to develop theoretical models of fluid/rock interactions during such diagenesis, it is important to consider the interrelationships between fluid flow, mineral dissolution and precipitation, and sorption. At the present time such models are at an early stage of development.
Recognition of the thermal effects of fluid flow in sedimentary basins
Authors: Duddy I.R. , Green P.F. , Bray R.J. , Hegarty K.A.
Published: April 2008

Moving fluids are capable of transporting a large amount of heat over long distances in sedimentary basins, but the effects are often ignored when modelling the thermal evolution of sedimentary basins. If significant, the passage of these heated fluids through the sediment pile will leave a thermal signature which can be measured at the present-day using palaeotemperature determinants such as vitrinite reflectance and apatite fission track analysis (AFTA™). The interpretation of palaeotemperature-depth profiles, particularly the slope of a palaeotemperature-depth profile, i.e. the palaeogeothermal gradient, allows fluid-induced temperature profiles to be distinguished from those due to simple conduction of basal heat flow.

Steady-state systems, typified by large-scale lateral fluid flow in foreland basins or where low-temperature hydrothermal circulation systems occur associated with intrusions and thick volcanic piles, are characterized by dog-length geothermal gradients, with high-interval gradients near the surface, above the shallowest aquifer, and lower interval gradients beneath an aquifer. It is argued that the classic occurrence of near vertical vitrinite reflectance-depth profiles observed in many ancient foreland basins can result from this mechanism.

Thermal effects of transient fluid flow are most easily observed at the present-day, where they are characterized by low or negative geothermal gradients beneath aquifers in active geothermal systems, and the same criterion enables their recognition in ancient situations. Similarly, shallow-level igneous intrusion into porous and permeable sediments can produce observable thermal signatures further from the intrusion than will result from simple conduction, but which can be readily explained by movement of fluids heated by the intrusion. Complex steady-state profiles that may be difficult to distinguish from transient profiles without additional geological data may arise where multiple aquifers are separated by aquitards.

The thermal consequences of fluid flow in two of these situations, a foreland basin and igneous intrusion into porous sediments, are illustrated by examples of thermal history reconstruction from the Pliocene Papua New Guinea Fold Belt and the Canning Basin of Australia, respectively.
Geostatistical estimation of manganese oxide resources at the Nsuta Mine
Authors: Al-Hassan S. , Annels A. E.
Published: April 2008

The Nsuta manganese deposit is located about 6.5 km south of Tarkwa in the southwestern region of Ghana. It is associated with a carbonate horizon within a thick series of interbedded grey tuffs and thin argillaceous horizons of Precambrian, Upper Birimian age. The manganese oxide mineralization, produced by supergene enrichment of the stratiform carbonate horizon, occurs as massive, patchy or bedded accumulations of psilomelane, with numerous veinlets and cavity fillings of pyrolusite. Due to the impact of folding and faulting and past mining activity (open-pit), the remaining resources have a complex spatial distribution. The study described is based on a portion of this deposit referred to as Hill D (South Crest). Initial statistical studies revealed the existence of two assay populations which appear to correspond to two geologically identifiable ore types that cannot be differentiated at the present scale of mining. Two geostatistical methods (ordinary kriging and indicator kriging) were used to evaluate the resource and the estimates so produced were then compared. A wireframe model was used to constrain the three dimensional geostatistical block model.