bluuebunny/crossref_metadata_2025 huggingface dataset:
Palaeoenvironmental assessment of marine organic-rich sediments using molecular organic geochemistry
Authors: Brassell S. C. , Eglinton G. , Howell V. J.
Published: April 2008

The solvent extracts of contemporary aquatic sediments contain a complex assemblage of compounds derived from living organisms. Among such biological debris many lipid components occur which possess structural and stereochemical characteristics that attest their origin from terrigenous higher plants, unicellular algae such as diatoms and dinoflagellates, or bacteria, including methanogens. Such compounds are markers of the organisms contributing to a given environment, and can provide information not available from micropalaeontological studies, namely evidence of bacterial activity, contributions from calcareous algae to sediments deposited below the calcite compensation depth and possibly sediment inputs from coccolithophorids in their non-coccolith bearing growth stage. In certain circumstances, especially in sedimentary sequences not subjected to elevated temperatures, many source-specific compounds can survive unaltered and thus are found in Mesozoic marine sediments deposited over 165 million years ago. In such immature ancient sediments, unaltered lipids occur together with their early stage diagenetic products, many of which retain molecular features that reflect their biological origins and are therefore also of value as source indicators. As diagenesis proceeds to catagenesis and kerogen breakdown increases, part of the molecular information is gradually lost as defunctionalization occurs and aliphatic and aromatic hydrocarbons come to dominate the sedimentary lipid distributions. Hence, attempts to identify the detailed sources of sedimentary organic matter from geolipid distributions become more difficult and rely more heavily on the occurrence of particular biologically-specific skeletons (e.g. head-to-head isoprenoids). The recognition of the diagenetic products of biolipids does, however, provide the basis for understanding lipid diagenetic pathways and offers the possibility of extending the scope of molecular assessment of sediment inputs to older and more thermally mature sediments. Despite these limitations imposed by the processes of sediment diagenesis and catagenesis, molecular organic geochemistry provides a unique record of past and present environments, with the potential of evaluating both biological inputs and subsequent sedimentary processes.
The use of lipids as facies indicators
Authors: Comet P. A. , Eglinton G.
Published: April 2008

An approach to palaeoenvironmental interpretation of sediments using lipid data, is developed. This approach initially attempts to summarize the major lipid components of plants, bacteria and invertebrate animals and to relate these data to sedimentary lipid distributions. Most of the major lipid groups found in sediments are used in this study. The very large amount of lipid data generated, necessitated the development of a simple system for data processing. The method of data analysis used involved the construction of a simple taxonomic hierarchy based on ‘Ruzicka’s Rules’ i.e. the biosynthetic origins of the major lipid types. By bringing in the concept of taxonomic hierarchy based on the biosynthetic pathways, a palaeontological approach towards lipid geochemistry is being evolved.

Secondly, DSDP lipid data (derived from the O.G.U., Bristol University) are collated and grouped using matrices of lipid data and histograms of the lipid groups, i.e. all compounds of a common structure are ‘summed’. These data are compared with the geological data and the results briefly discussed for each sediment.

In conclusion, the diagenetically stable lipid patterns of three representative environments, i.e. marine planktonic, bacterially-dominated marine, and a lacustrine facies, are presented and discussed. From these data the rules by which environments are described are extended; the parameters so derived have good potential for source correlation.
The carbon cycle—tracking the path of organic particles from sea to sediment
Authors: Degens E. T. , Ittekkot V.
Published: April 2008

Sedimentation of organic matter in the marine habitat represents a major link in the transfer of carbon from the hydrosphere to the geosphere. Evidence is presented that zooplankton and phytoplankton communities are actively involved in the fast removal of small-sized mineral and organic particles from the euphotic zone to the deep sea. For instance, copepods package detritus into faecal pellets which are jetted to the sea floor at velocities of about 500 m per day. Attention is also drawn to coccoliths. They excrete mucus which is instrumental in scavenging detritus and in forming macroflocs which rapidly sink. Time series sediment traps deployed in various shallow and deep-sea environments permit collection of such particles in mid-water. Variations in the distribution pattern of sugars, amino sugars, and amino acids indicate seasonality in the fluxes of ‘fresh’ materials. These data suggest that the bulk of small-sized particles presently suspended in surface waters of the open ocean is carried off by planktonic organisms to the sea floor. Prior to the advent of copepods or mucus-excreting nannoplankton in the stratigraphic record, the ancient sea must have looked different with respect to the content of suspended solids since settling velocities—according to Stoke’s law—are only a few centimetres a day for clay-sized particles. Even at later geological times, environmental events that lead to unbalanced plankton communities should have found their echo in the sediment facies.
Oceanographic controls on the accumulation of organic matter in marine sediments
Authors: Calvert S. E.
Published: April 2008

The concentration of organic matter in marine deposits depends on the relative rates of accumulation of the various sedimentary components and the ease with which these components are preserved after burial. The supply of organic matter to any area of sea floor is controlled by the primary production rate in the surface ocean and the depth through which particulate material must settle. The accumulation of organic matter in sediments depends on the primary setting flux and on the bulk sedimentation rate, more carbon surviving decomposition where it is buried rapidly.

The preferential preservation of organic matter under anoxic conditions, a widely-accepted explanation for the formation of sapropels, black shales and petroleum source beds, is considered to be of secondary importance in governing the accumulation of carbon in marine deposits. The modern sediments of the Black Sea, a widely-used analogue of the environment of black shale formation, do not have exceptionally high carbon contents, although an earlier sapropel is known to have formed during the change from an oxic lake to an anoxic marine basin. Likewise, the available information on modern anoxic fjords shows that they do not have sediments containing more carbon than their oxic counterparts. And finally, the inverse correlation between the carbon content of slope sediments and the oxygen content of the near-bottom waters in areas where the oxygen minimum intersects the sea floor is shown to be one of coincidence and understandable on the basis of the relationship between sedimentary carbon levels and other sediment properties.

The supply of organic matter to marine deposits, controlled by the primary production rate, and the bulk sedimentation rate appear to be the most important factors which determine the organic content of sediments. Recent work on the location of former upwelling centres suggests that variations in palaeoproductivity may be responsible for the formation of many organic-rich rocks and may provide a better basis for predicting the location of petroleum source beds in the geological record.
Coastal upwelling and a history of organic-rich mudstone deposition off Peru
Authors: Suess E. , Kulm L. D. , Killingley J. S.
Published: April 2008

The present-day upwelling circulation off Peru, the regional pattern of organic matter in surface sediments and the stable carbon isotope characteristics of Neogene and Quaternary carbonate lithologies suggest a unique feedback mechanism in continental margin deposition and subsequent alteration after burial. In such a scenario, the high bioproductivity, the position of a poleward flowing undercurrent and the rate of subsidence of margin basins appear to be the principal variables controlling this mechanism.

Transfer of organic matter from the sea surface to the sea floor is particularly efficient in the upwelling ecosystem off Peru. Preservation and burial are enhanced by high bulk sedimentation rates along the upper continental slope (between 11°–15°S) at depths where the subsurface current velocities decrease below those normally associated with the poleward flow. Burial and preservation are diminished, however, where shallow water depths promote continuous reworking of the bottom sediments by onshore flows and alongshore water movement (between 6°–10°S). The resulting sedimentary facies are distinctly different from each other in that the former process yields an organic-rich (> 5 wt % Corg) and the latter process yields a calcareous (> 15 wt % CaCo3) mud facies. The bulk sediment accumulation and individual component fluxes are estimated for both portions of the margin situated between 6° and 15°S latitude and lying in < 500 m of water depth. Furthermore, the chemical environment of organic-matter decomposition in the rapidly accumulating carbonate-poor facies is dominated by microbial fermentation and methanogenesis, whereas, the muds containing lesser amounts of organic matter are dominated by microbial sulphate reduction. These differences in facies composition persist throughout the subsequent stages of compaction and diagenesis. Most prominent among these is the formation of ‘organic’ dolomites with distinctly different isotopic signatures and mineral assemblages. The original upwelling facies (i.e., organic-rich muds or calcareous muds), the extent of reworking by subsurface currents, and the subsidence history of the margin basins may be inferred from these sedimentary signatures.
Palaeo-upwelling and the distribution of organic-rich rocks
Authors: Parrish J. T.
Published: April 2008

As many as half the world’s organic-rich rocks may have been deposited in upwelling zones.

This paper alludes to the way in which ancient upwelling zones may be predicted and the criteria by which such areas may be recognized from the sedimentary record. The predicted distribution of upwelling sites is compared with the distribution of organic-rich rocks, and the other probable indicators of upwelling, notably phosphatic rocks, glauconite and biogenic siliceous sediments. A correspondence is shown. This correspondence is consistent with not all upwelling zones being underlain by organic-rich sediments and with not all organic-rich sediments originating beneath areas of upwelling.
The depositional environment and petroleum geochemistry of the Marl Slate-Kupferschiefer
Authors: Gibbons M. J.
Published: April 2008

The Marl Slate is the basal unit of the Upper Permian (Zechstein) in northeast England. A finely laminated, organic-rich, silty dolomite/dolomitic siltstone, it is equivalent to the Kupferschiefer of the Southern North Sea basin and northern Germany and Poland. Several lines of evidence indicate the Marl Slate/Kupferschiefer accumulated in the anoxic bottom waters of a shallow (< 200 m), stratified, epicontinental sea. The vertical succession of the first Zechstein cycle is explicable in terms of the effect on sedimentation of a stratified water column undergoing progressive evaporitic concentration.

Total organic contents of the Marl Slate generally fall in the range 5–15%, decreasing upwards. The kerogen is largely amorphous and oil-prone. Original pyrolysis yields approach 100 kg hydrocarbon/tonne rock. It is immature in northeast England but even where mature in the Southern North Sea basin and Northern Europe the unit is unlikely to have sourced commercially significant amounts of oil because of its limited thickness (generally <2 m).
Carbonaceous sediments and palaeoenvironment of the Cretaceous South Atlantic Ocean
Authors: Zimmerman H. B. , Boersma A. , McCoy F. W.
Published: April 2008

The distribution of sapropelic sediments in the early South Atlantic Ocean is used to test palaeocirculation models related to oxygen-deficient conditions. The data presented here demonstrate that preservation of organic matter was widespread and occurred in many phsyiographic settings. Given the silled-basin physiography of the relatively isolated South Atlantic and the benign Cretaceous climate, a preservational model of sapropel accumulation and its corollary of restricted bottom circulation successfully satisfy the constraints of the geological record. Under the environmental conditions outlined for the early and mid-Cretaceous, a generally dysoxic condition for the warm saline bottom-water is appropriate. The prevailing dysoxia, however, was punctuated by episodic bottom anoxia, which often expanded into the upper reaches of the water column. In the Cretaceous South Atlantic, the only excursion from the dysoxic condition occurred during a Cenomanian ‘ventilation event’ (represented by a widespread hiatus in the stratigraphic record) which is tied to an interval of climatic cooling and the establishment of effective circulation between the North and South Atlantic Oceans. The geological record for epicontinental regions, although modified by local physiography and other environmental parameters, suggests a relationship between sea-level positions and sapropel accumulation. No relationship, however, can be established between sea-level and sapropel preservation in the oceanic basins of the South Atlantic.
South Atlantic organic-rich sediments: facies, processes and environments of deposition
Authors: Stow D. A. V.
Published: April 2008

Organic-rich sediments (black shales) occur in parts of the South Atlantic ocean within Jurassic, Cretaceous and Neogene sections. These black shales are represented by various lithologies, including mudstones, marlstones, limestones, sandstones, pebbly mudstones and diatomites. They range from thin to very thick bedded, laminated, fissile or structureless, and show varying degrees and types of bioturbation. Texturally and compositionally they are equally varied. Organic carbon may be of principally marine or terrigenous source and forms from 1% to over 20% of the sediment. In many cases, the black shales are interbedded with organic-poor lithologies in cycles with irregular periodicities that average 20,000 to 140,000 years. The range of depositional processes of these black shales includes sliding, debris flows, high- and low-concentration turbidity currents, pelagic/hemipelagic sedimentation, and winnowing by shelf bottom currents. They occur in various palaeogeographical settings from nearshore shelf to ocean basin, and have been differently influenced by variations in bottom water oxygenation, organic matter supply and rates of sedimentation.
Organic-rich Cretaceous sediments from the North Atlantic
Authors: Summerhayes C. P.
Published: April 2008

Organic-rich sediments are abundant in the early and middle Cretaceous of the deep North Atlantic, in the calcareous Blake-Bahama Formation (Valanginian-Barremian), in the siliceous Hatteras Formation (Aptian-Cenomanian), and, in some places, in the Plantagenet Formation (Turonian-Santonian).

The regional patterns of organic facies and organic enrichment reflect:

(1)

the rate of supply of marine versus terrestrial organic matter,

(2)

the rate of burial of organic matter,

(3)

the oxygenation of the bottom water, and

(4)

the oceanographic history of the North Atlantic basin.

Rates of supply of marine organic matter (most now present as amorphous matter) seem to have been similar in the eastern and western North Atlantic, though they became higher in the east in Cenomanian and younger times, when coastal upwelling intensified there. Rates of supply of terrestrial organic matter were higher in the west, and off Iberia and in Biscay (except in the Cenomanian-Turonian). Organic matter is most abundant where rates of sedimentation were highest, presumably because organic matter was rapidly removed from the zone of intensive recycling near the sediment water interface. Some of the organic rich deposits were introduced by downslope displacement from the oxygen minimum zone on the continental margins.

The evidence suggests that bottom waters were intermittently suboxic (0.5–0.2 ml/l O
2
) or anoxic (<0.2 ml/l O
2
), encouraging preservation of organic matter on the deep sea floor. A more or less permanently anoxic expanded oxygen minimum zone is also implied.

While the global Cretaceous ocean seems to have been generally poorly oxygenated, local events within the North Atlantic give it a somewhat different history of accumulation of organic matter from other ocean basins. Nevertheless, the global organic enrichment event at the Cenomanian-Turonian boundary is clearly visible in the North Atlantic.
The carbon isotope record of the Cenozoic: history of organic carbon burial and of oxygen in the ocean and atmosphere
Authors: Shackleton N. J.
Published: April 2008

Carbon isotope measurements in deep-sea sediments may be used to obtain the history of the mean
13
C content of CO
2
dissolved in the ocean, of
13
C gradients within the ocean, and of the globally averaged
13
C content of the carbonate accumulated on the sea floor. The last has some interest in relation to energy reserves, because it provides a means of monitoring the changing rate of accumulation of organic matter on the globe.

Data are shown for the
13
C composition of Cenozoic marine carbonates, based on a suite of sites from DSDP (Deep Sea Drilling Project) Leg 74 in the South Atlantic. If the published estimates of global accumulation rates for marine carbonate are recalculated on the basis of recent timescales for the Cenozoic it is found that changes were rather subtle, contrary to the conclusion of Davies and Worsley (1981), so that a model of constant carbon input may be used to estimate changes in organic carbon accumulation from the
13
C data. A cumulation of changes in the global organic carbon reservoir is also obtained and is used to infer changes in the amount of oxygen in the atmosphere, which was 20% higher than today during most of the Cenozoic. However, the solubility of oxygen in the warm early Cenozoic oceans was about 20% lower than today’s value, so that the early Cenozoic ocean had a similar dissolved oxygen content to today’s ocean.
The Lewisian complex: a typical Precambrian high-grade terrain?
Authors: Park R. G. , Tarney J.
Published: April 2008

The main structural, lithological and geochemical features of the Lewisian complex are summarized in relation to available geochronology and an attempt is made to integrate this information into an evolutionary model for the Lewisian from the Archaean through the Proterozoic. The phase of extensive crustal generation which formed the early Lewisian (Scourian) complex occurred at about 2900 Ma and was marked by the production of vast volumes of tonalite which, together with supracrustal components, were affected by strong horizontal thrusting and deformation. The deeper parts of the Lewisian crust were subject to (Badcallian) granulite facies metamorphism culminating at
c.
2700 Ma. Major NW-SE shear zones developed shortly after this and resulted in the segmentation of the Lewisian crust and juxtaposition of different crustal levels. This marked a period of quite extensive retrogression of the high-grade gneisses, which continued (with further local shearing) during and after the emplacement of the Scourie dyke suite at
c.
2400 Ma. Extension of the Lewisian crust occurred in the southern (Gairloch) area around 2000 Ma and extensive outpourings of mafic volcanics, with associated exhalative mineralization, took place accompanied by sedimentation in the developing basin to form the Loch Maree Group. Igneous activity related to the South Harris igneous complex may have occurred at this period on the Outer Hebrides. Laxfordian deformation and metamorphism began before 1900 Ma and resulted in major reactivation of the earlier shear zones between and within the juxtaposed crustal blocks. More pervasive deformation accompanied the closure of the Loch Maree volcano-sedimentary basin, with major overthrusting and isoclinal folding. Culmination of Laxfordian metamorphic activity occurred at
c.
1900 Ma with migmatization and granite injection at Laxford and on Harris, and locally in the southern area. Shear deformation, increasingly more brittle and localized, continued to affect the Lewisian complex until 1400 Ma and probably until 1000 Ma. Interestingly, the main shear zones established at the end of the Archaean crust-forming episode at 2600 Ma continued to be the focus of tectonic, metamorphic and magmatic activity over the following 1500 million years.
Geochemistry of the Scourian complex: petrogenesis and tectonic models
Authors: Tarney J. , Weaver B. L.
Published: April 2008

The segmentation of the Lewisian complex which occurred at the end of the Archaean resulted in the juxtaposition of zones representing different crustal levels, including deep crustal granulites. Critical aspects of the geochemistry of Lewisian gneisses are considered in relation to their petrogenesis and tectonic models. Most of the Lewisian is made up of a bimodal suite of tonalitic-trondhjemitic gneisses enclosing mafic-ultramafic enclaves and layered complexes which are often associated with metasediments. These mafic components have a low-pressure petrogenesis and represent tectonic inclusions incorporated into the complex at the time of gneiss generation. They are more abundant in the granulites. The silicic gneisses have a higher pressure petrogenesis and their geochemistry is consistent with hydrous partial melting of amphibolites in a shallow-dipping subducting slab with hornblende as a residual phase.

Granulite metamorphism occurred or peaked some 200 Ma or so after the phase of gneiss generation at 2.9 Ga and terminated about 2.5 Ga. Concentrations of U, Th and Rb are at least an order of magnitude lower in the granulites than in the equivalent amphibolite-facies gneiss zones, and those of K and Pb about 50% lower. Concentrations of other elements are not significantly different. Currently popular models for the removal of these elements are not significantly different. Currently popular models for the removal of these elements from the deep crust through partial melting or transport in a carbonic fluid phase are examined, but each has problems in accounting for all the available data. An alternative model, considering the low contents of heat-producing elements as a primary feature of silicic magmas emplaced into the deep crust, is explored. This relies more on fluid processes removing heat-producing elements from the ocean crust during subduction, followed by hydrous melting of the amphibolite to produce water-saturated magmas which cannot rise to high crustal levels. In contrast to modern subduction zone magmas, which are largely derived from the mantle wedge that has been modified by addition of a fluid-transported, LIL element, ‘subduction component’, Archaean sodic tonalite-trondhjemite magmas result from hydrous melting of amphibolite which has lost that component. Granulite-facies metamorphism is an independent process that is commonly superimposed on deep crustal rocks which are already low in heat-producing elements, and during which further loss of these elements may occur, but granulite-metamorphism may also be superimposed on more normal crustal rocks without significant chemical changes.
Internal evolution of the major Precambrian shear belt at Torridon, NW Scotland
Authors: Wheeler J. , Windley B. F. , Davies F. B.
Published: April 2008

The Torridon shear belt is a NW-striking region of heterogeneous reworking within the Lewisian complex. Mapping of the deformation fabric intensity reveals a geometry of low-strain, lozenge-shaped blocks surrounded by an anastomosing set of shear zones which form a linked system of flats and lateral ramps. These various orientations of shear zones share a common movement direction but show both sinistral-reverse and dextral-normal shear sense. This is true both in the Inverian, when the linked system was established, and in the Laxfordian, when shearing followed the established network but was localized along the ductile Scourie dykes. Possible explanations for the occurrence of opposed shear senses include local differential shear, and a modification of the Lister & Williams model of deformation. In the latter, shear zones generally show a sense of shear opposite to the overall displacement on the belt, and this underlines the need for a comprehensive analysis of the geometry of such shear belts.

The Inverian structure controlled emplacement of the Scourie dyke swarm, which in turn controlled the localization and orientation of the Laxfordian structures. Given the ductility of the dykes, this leads to a simple explanation for the coaxiality of Inverian and Laxfordian structures. A tentative model for the evolution of the Torridon shear belt is given, in which emphasis is placed on the importance of dykes as easy-slip horizons.
The structural history of the Canisp Shear Zone
Authors: Attfield P.
Published: April 2008

The Canisp Shear Zone is a major structure in the central region of the mainland Lewisian. Evidence is presented to show that there have been two major phases of movement. In the first phase, during the Inverian, a broad belt of steeply-inclined foliation was formed bounded to the south by the north limb of the Lochinver antiform. The associated lineation plunges steeply to the SE, indicating a dip-slip movement with downthrow to the north, and a small dextral strike-slip component.

Quartz-epidiorite and ultrabasic dykes were then emplaced sub-parallel to the shear plane, their emplacement was controlled by pre-existing planes of weakness.

The second, Laxfordian, phase of movement reactivated the Inverian structure. Deformation was concentrated in narrow zones of high strain, with an associated strong, penetrative lineation plunging shallowly ESE. The shear sense is dominantly dextral strike-slip with a small downthrow to the north.

This change from dominantly dip-slip to dominantly strike-slip movement agrees with models for other areas of the Lewisian (Coward & Park, this vol.) and appears to be associated with a change in style towards more localized zones of deformation.
The Outer Hebrides Fault Zone: evidence for normal movements
Authors: White S. H. , Glasser J.
Published: April 2008

The Outer Hebrides Fault Zone is a major structural feature within the Lewisian of the Outer Hebrides. It is well exposed in the Uists where it consists of high-strain strands of ultracataclasite, cataclasite and phyllonite in a crush melange. Microstructural studies of the fault rocks indicate that the sequence crush melange-cataclasite-phyllonite is typical of that for
c
-type mylonite development and reflects increasing retrogression. This, plus the anastomosing pattern of the high-strain zones, suggests that the fault rocks in the Uists mark the edge of a major fault zone. Fold and shear band asymmetries indicate a major phase of normal movement. It is not possible to assign definite ages to either, but it is clear that the formation of the crush melange preceded that of the high-strain strands, which appear to be immediately post-Caledonian.
The palaeomagnetic record in the Lewisian terrain
Authors: Piper J. D. A.
Published: April 2008

Palaeomagnetic investigations of the Lewisian rocks identify a diverse remanence record which varies from zone to zone. The characteristic (A) remanence in the central mainland zone is a strong and stable NW downward-directed and magnetite-held component. This occurs sporadically elsewhere but is largely displaced in the northern mainland zone and in Lewis and Harris by lower blocking temperature/coercivity components with more variable directions; in addition hematite overprints occur sporadically in the central zone and in the Loch Torridon area of the southern mainland zone. Geological relationships and palaeomagnetic field tests suggest that all of these components were acquired during post-Laxfordian uplift and cooling of this terrain but a steep E downward-directed component is a feature of pre-Laxfordian belts in the south and may be considerably older. The collective data define migration of the geomagnetic field axis during uplift and cooling of the terrain and are linked in general terms to mineral ages in the range
c.
1720–1500 Ma; further rock magnetic and
39
Ar/
40
Ar investigations are in progress to characterize the areal significance of the components of remanence and to date them more precisely.
Mid-Archaean basic magmatism of southern West Greenland
Authors: Hall R. P. , Hughes D. J. , Friend C. R. L.
Published: April 2008

Three groups of metamorphosed basic igneous rocks post-date the Amîtoq gneisses (
c.
3700 Ma) and pre-date the Nûk gneisses (
c.
2900 Ma) in the Archaean craton of southern West Greenland. These are the Malene metavolcanic amphibolites, the Ameralik dykes within the Amîtsoq gneisses and a series of large metagabbro-anorthosite complexes. The Malene rocks range from komatiitic to tholeiitic in chemical affinities and occur as extensive intermixed horizons throughout the craton. Metavolcanic amphibolites which are intercalated with the older Amîtsoq gneisses are chemically similar to those which are isolated from them. They probably all represent lavas erupted in an oceanic environment, some of which were subsequently tectonically intercalated with the older sialic crust. Significant geochemical differences between the Malene metavolcanic rocks and the Ameralik basic dykes, which intrude the Amîtsoq gneisses, suggest that these dykes could not have acted as feeders to the Malene lavas. The petrogenesis of the Malene rocks is considered in terms of normative behaviour and of the system CaO-MgO-Al
2
O
3
-SiO
2
(CMAS). Both demonstrate the importance of olivine and clinopyroxene in the evolution of the Malene magmas and suggest that they were derived from relatively garnet-depleted and clinopyroxene-enriched mantle compared to average modern mantle. Evidence for a complementary aluminous portion of the Archaean mantle is provided by the abundance of metagabbro-anorthosite rocks and associated aluminous tholeiitic metavolcanic amphibolites.
The Archaean high grade gneiss terrain in E Hebei Province, NE China: geological framework and conditions of metamorphism
Authors: Sills J. D. , Wang K. , Yan Y. , Windley B. F.
Published: April 2008

The Archaean high-grade gneiss terrain of eastern Hebei Province, N.E. China, is unusual in that it contains abundant banded iron-formations at amphibolite or granulite facies. The Archaean terrain is divisible into three: an area between Taipingzhai and Malanyu comprising granulite-facies tonalitic gneisses formed about 2.5 Ga (the Qianxi Gneisses); farther south is an amphibolite-granulite-facies terrain consisting of enclaves of iron-formations and metasediments (Shuichang supracrustal association) within orthogneisses (Qianan Gneisses) which are in general more potassic; in the extreme south of the Archaean outcrop the oldest rocks, the Caozhuang group (c.3.5 Ga in age) consist of a series of amphibolites, iron-formations and metasediments enclosed within granodioritic and granitic gneisses. The Qianan Gneisses comprise ‘charnockites’ ofc.2.7 Ga age, tonalitic to granodioritic gneisses and a variety of highly deformed granodioritic and granitic gneisses of uncertain age.P-Tconditions range from 7–8 kb andc.700°C in the north near Taipingzhai to 5–6.5 kb and 650–750°C near Shuichang and 4.5–6.0 kb and 600–650°C in the southernmost area. Thus deeper crustal levels are exposed in the north, but it is not known whether the transition is gradual or whether different crustal levels have been brought together by thrusting or by vertical movements along shear zones.
Active normal faulting and crustal extension
Authors: Jackson J. A.
Published: April 2008

A world-wide review of fault-plane solutions and focal depths for large normal-faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6–15 km on faults dipping between 30 and 60°. In the few cases where levelling or seismic data are good enough, these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust. There is evidence that, in some cases, as a result of the enormous transitory increase in strain rate during large earthquakes, rupture continues into the normally ‘ductile’ lower crust on surfaces with substantially gentler dips. These low-angle surfaces may be analogous to some of the ‘detachments’ seen in metamorphic core complexes of the western USA, but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the ‘detachments’ and the blocks on either side. Such contrasts, however, are unlikely to introduce substantial curvature to an originally planar shear zone.

If the observed spread in active normal-fault dips is caused by rotation of the faults and the blocks they bound during extension, then a maximum β value of 1.7 can be accommodated byseismicactivity on a single generation of normal faults. With continued extension, either a new generation of steeper faults, cutting the rotated first faults, is likely to form, or the deformation will continue aseismically on faults dipping at less than 30°.

Large earthquakes have not been observed to nucleate on very low-angle (<20°) normal faults within the continental crust anywhere in the world. Such faults can of course move aseismically, but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active. Thus extensional models that require concentrated simple shear on large sub-horizontal faults within the brittle upper crust will also require a spatial separation between aseismic, very low-angle faulting and seismic high-angle faulting. Since seismogenic high-angle faults dominate the topography of extending regions, upper-crustal very-low-angle faulting, if it occurs on a large scale, presumably does so in areas that are relatively flat.
The tensile strength of the lithosphere and the localization of extension
Authors: Lynch H. David , Morgan Paul
Published: April 2008

Continents appear to rift in preference to oceans. Furthermore, some areas of continents appear to be more susceptible to rifting than others. Experimental rock mechanics data are used to estimate lithospheric strength for lithospheres of different structure, thereby to investigate the possible causes of rift localization. Using optimum creep parameters for silicic, mafic and ultramafic rocks, we find that lithospheric strength is inversely related to both crustal thickness and heat flow. By virtue of its thinner crust, oceanic lithosphere is inherently stronger than continental lithosphere. We find that oceanic lithosphere older than about 10 Ma should be able to withstand the lithospheric forces exerted on it by gravity sliding and plate interactions. Rifting or ridge-jumps are therefore only likely to occur in very young oceanic lithosphere. Low heat flow continental shields should also be able to withstand likely lithospheric forces without significant deformation. As the heat flow increases, however, the lithosphere is weakened dramatically. A smaller amount of weakening is associated with crustal thickening. Thus, unless rifting is localized by a strongly heterogeneous stress field, it will be by anomalously weak lithosphere. This lithosphere is likely to have anomalously high heat flow and/or crustal thickness prior to extension.
Some important consequences of lithospheric extension
Authors: Keen C. E.
Published: April 2008

This paper briefly summarizes two of the important by-products of lithospheric extension during rifting. These are partial melting and the segregation and migration of basaltic melt to crustal levels, and the dynamical consequences of viscous flow in the lower lithosphere and asthenosphere. In both cases, the rift-stage subsidence may be significantly different from that predicted by a simple stretching model and may allow relative uplift of the rift zone, as well as subsidence. The dynamical model also predicts significant uplift of the rift shoulders, doming over the entire rift zone, and asymmetrical continental breakup at rifted continental margins. The relative importance of these predictions depends on the thermal and mechanical properties of the lithosphere. The model predictions help explain some of the discrepancies between the observed behaviour of rifts and results obtained from the simple stretching models.
Tectonomagmatic evolution of Cenozoic extension in the North American Cordillera
Authors: Wernicke Brian P. , England Philip C. , Sonder Leslie J. , Christiansen Robert L.
Published: April 2008

The spatial and temporal distributions of Cenozoic extension and magmatism in the Cordillera suggest that the onset of major crustal extension at a particular latitude was confined to a relatively narrow belt (<100 km, pre-extension) and followed the onset of intermediate and silicic magmatism by no more than a few million years. Extension began in early Eocene time in southern British Columbia, northern Washington, Idaho and Montana. Farther S, extension began at about the Eocene-Oligocene boundary in the Great Basin and slightly later in the Mojave-Sonora Desert region. The intervening area, at the latitude of Las Vegas, remained quiescent until mid-Miocene time. Compositional and isotopic characteristics of most pre-Miocene magmas are consistent with their containing major components of melted continental crust.

In mid-Miocene time, two major changes occurred: widening of the area of extension and the widespread appearance of basaltic magmas. The area affected by extension, from southwestern Montana to the Lake Mead region, widened to several hundred kilometres. By this time extension in southern British Columbia, northern Washington and northern Montana had ceased (probably before the end of the Eocene), and extension S of Lake Mead (except in the Gulf of California) had waned. Regions affected by the broader belt of extension during late Miocene, Pliocene, and Quaternary time experienced basaltic magmatism, which began along a central rift zone in the northern part of the region, and which had within a few million years spread to include most of the region; later basaltic activity has tended to concentrate in restricted zones, especially near the margins of the extended area.

We recognize a correlation between the amount of earlier crustal thickening and Cenozoic extension, and between the length of time after shortening but before extension and the degree to which a given region was intruded by Late Cretaceous plutons. The localization of extension in areas of previous crustal thickening and the dependence of the timing of extension on the thermal state of the overthickened crust is consistent with a simple thermal-mechanical model developed in a companion paper (Sonder
et al.
). This raises the possibility that stresses inherent in the North American Plate dominated over plate-interaction forces as controls of the Cenozoic tectonomagmatic evolution of the North American Cordillera, especially in its earlier stages.
A shear-zone model for the structural evolution of metamorphic core complexes in southeastern Arizona
Authors: Davis George H.
Published: April 2008

Some of the world’s best exposed and most thickly developed mylonites, ultramylonites, and cataclasites are found in parts of a number of mountain ranges in southeastern Arizona. These fault rocks and associated structures occupy metamorphic core complexes, which I view as mountain-size geological exposures of regional, upward-tapering ductile-brittle shear zones. The fundamental characteristics of the core complexes were fashioned by regional crustal extension in the Tertiary. Normal-slip shearing served to ‘telescope’ fault rocks and structures which had formed originally at different depth levels in the crust. The result of normal-slip simple shear within the shear zones was the translation of hanging wall crust by tens of kilometres. The shear zones and linked fault zones are interpreted to have dipped at 45° or more initially, rotating progressively to more gentle inclinations and shallower depths as the crust thinned and stretched. Progressive simple-shear under conditions of a steadily decreasing confining pressure and temperature is dramatically recorded in suites of structures and fabrics: for example mylonite gneiss is converted to microbrecciated mylonite gneiss, which in turn is transformed to cataclasite and ultracataclasite. The physical nature of this progressive deformation is beautifully exposed in the gently dipping Catalina-Rincon shear zone, whose once-deep and once-shallow parts are now exposed to view over broad expanses E and NE of Tucson, Arizona.
Ductile strain and metamorphism in an extensional tectonic setting: a case study from the northern Snake Range, Nevada, USA
Authors: Lee Jeffrey , Miller Elizabeth L. , Sutter John F.
Published: April 2008

In recent years considerable attention has focused on metamorphic core complexes of the Basin and Range Province of the western US Cordillera. These highly extended areas are characterized by an upper plate that has been brittlely attenuated by normal faults separated by a sub-horizontal detachment surface from a lower plate that has been ductilely thinned and stretched. A study of mesoscopic structures, finite strain, microstructures, quartzc-axis fabrics and40Ar/39Ar geochronology was undertaken in order to characterize the nature, geometry, kinematic history and timing of ductile extension in the lower plate of the northern Snake Range metamorphic core complex in E-central Nevada. These data provide new insights into the processes of deep-seated ductile strain beneath supracrustal normal fault mosaics in highly extended regions. Mesoscopic structures and finite strain analyses indicate that the lower plate underwent plane strain with a sub-vertical Z-axis and a sub-horizontal WNW-ESEX-axis. The magnitude of strain increases dramatically from W-to-E, and is constant vertically. Bedding and foliation are everywhere parallel and bedding is thinned 30–90%. The nature and geometry of microstructures andc-axis fabrics changes progressively from W-to-E. Lower strain rocks on the W flank of the range are characterized by a single bedding-parallel foliation, defined by flattened detrital grains, globular grains which have theirc-axes parallel to theZ-axis and symmetrical cross-girdlec-axis fabrics. Further E to slightly higher strains, the rocks are characterized by asymmetrical cross-girdlec-axis fabrics, although globular grains with theirc-axes parallel toZare still present. The high-strain rocks on the E flank of the range are characterized byC-planes, asymmetrical mica ‘fish’ and oblique quartz foliations, and asymmetrical single-girdlec-axis fabrics. These data suggest that the lower plate has not deformed entirely by either coaxial or non-coaxial strain, and we propose an evolutionary model whereby the lower plate deformed during an early period of coaxial strain followed by a later component of non-coaxial strain accompanied by coaxial strain on the E flank of the range.40Ar/39Ar geochronology on lower-plate hornblendes and micas indicates that temperatures of deformation were >280°C, but <530°C, and increased with depth. Geochronological data suggest that lower-plate ductile strain is post-latest Cretaceous, and biotite and muscovite data indicate that ductile deformation was probably ongoing 22–26 Ma. Our data and regional geological relations suggest that ductile extensional deformation in the northern Snake Range occurred because of localized heat input to shallow levels of the crust, and together with seismic reflection data, indicate that lower-plate rocks probably represent the top of a regional metamorphic terrane of Tertiary age.
Geometry and evolution of a mid-crustal extensional fault system: Chemehuevi Mountains, southeastern California
Authors: John Barbara E.
Published: April 2008

The extensional fault system exposed in the Chemehuevi Mountains area of the southern Cordillera provides data on the mode of mid-crustal accommodation to continental extension. A stacked sequence of three mid-Tertiary low-angle normal faults cut gently down-section through deformed Proterozoic and Mesozoic crystalline basement below Cenozoic strata. Hanging wall blocks are consistently displaced relatively NE across these three detachment faults, recording unidirectional extension of quartzofeldspathic crust at palaeodepths of 6–10 km. The two structurally deepest faults in the sequence are exposed over 22 km in a down-dip direction, across a total area in excess of 350 km
2
, and were initiated with a regional dip of less than 15° NE. Both of the structurally deepest faults are corrugated parallel to the direction of transport; wavelengths of the corrugations range between 200 m and 10 km, and amplitudes range from 30 to 400 m. These undulations are broad mullion structures that developed coeval with fault slip. Amplitude and wavelength vary with footwall rock type and pre-existing structural grain. Slip on the faults at the present level of exposure was accomplished by brittle deformation, with the generation of gouge, breccias, rocks of the cataclasite series, and rare pseudotachylites. Major mylonite zones in the Chemehuevi Mountains are older and unrelated to the extensional faulting. These data support the conclusion that mid-crustal extension in the Chemehuevi Mountains area was accommodated by an asymmetrical normal-slip shear system. Extension occurred along seismically active, gently NE-dipping, undulating surfaces. During their evolution they rose from middle- to upper-crustal depths.