bluuebunny/crossref_metadata_2025 huggingface dataset:
A quantitative study of the influence of pre-existing compositional and fabric heterogeneities upon fracture-zone development during basement reactivation
Authors: Beacom L. E. , Holdsworth R. E. , McCaffrey K. J. W. , Anderson T. B.
Published: April 2008

In common with many other regions of exposed continental basement, the Late Archaean to Palaeoproterozoic Lewisian Complex, NW Scotland, preserves numerous examples of faults that appear to reactivate pre-existing compositional and structural heterogeneities in the host gneisses. A regionally recognized set of late Laxfordian sinistral strike-slip faults and fractures are spatially associated with pre-existing NW-SE-trending ductile shear zones of Inverian and Laxfordian age. Field observations suggest that most of the sinistral displacements have been accommodated along laterally persistent faults (here termed principal displacement zones (PDZ)) that lie sub-parallel to the pre-existing foliation in the shear zones. Geometric and orientation data collected during structural logging of the PDZ faults have been used to quantitatively test the influence of lithology and pre-existing structural geometry on the spatial patterns of fault development. Stereographic analysis shows a strong geometrical correspondence between the intensity and form of the pre-existing anisotropy and the alignment of the PDZ brittle faults. Spatial clustering of PDZ faults varies depending on lithology (amphibolite v. acid gneiss v. quartz-mica schist). A close correlation exists between the geometry and intensity of the pre-existing foliation and fault spatial clustering. The results demonstrate that reactivation of pre-existing anisotropies in typical continental basement gneisses exert a significant control on brittle fault development and growth in the upper crust.
Sequential ductile to brittle reactivation of major fault zones along the accretionary margin of Gondwana in Central Argentina
Authors: Simpson Carol , Whitmeyer Steven J. , De Paor Declan G. , Gromet L. Peter , Miro Roberto , Krol Michael A. , Short Heather
Published: April 2008

Metamorphic and plutonic basement rocks and cover sequences of the Eastern Sierras Pampeans, Argentina, have undergone multiple episodes of fault reactivation. Faults take advantage of mid- to late Cambrian, NW-SE-striking, steeply east-dipping foliations in Vendian-aged accretionary prism metasedimentary rocks. Foliations in peraluminous schists, paragneisses and migmatites are deflected into late Cambrian amphibolite-grade high-strain zones. Greenschist-grade mylonite zones and thick retrogressed ultramylonite zones with mainly NNW strikes, easterly dips, and east-over-west movement, affect the metasedimentary rocks and Ordovician-aged intrusive rocks and are presumably related to early Devonian accretion of terranes to the west of Gondwana. pseudotachylyte veins occur in nearly all mylonite zones. Brittle deformation during Carboniferous to Triassic time produced major pull-apart basins located above terrane boundaries. Outcrop patterns of Triassic to Cretaceous sedimentary rocks are consistent with transtensional pull-apart basins followed by Andean transpressional deformation. The theoretical basis for fault reactivation and production of ‘short cuts’ is examined in the context of Tertiary to Recent basin inversion faults. The inversion faults follow the Palaeozoic trends and produce the present-day NNW-oriented, deep sedimentary basins and intervening ranges of basement rocks.
Repeated reactivation in the Apennine-Maghrebide system, Italy: a possible example of fault-zone weakening?
Authors: Tavarnelli Enrico , Decandia Francesco Antonio , Renda Pietro , Tramutoli Mariano , Gueguen Erwan , Alberti Mauro
Published: April 2008

Italy owes its complex geological structure to a double switch in tectonic regime, which involved the opening of the Tethys Ocean during Early Mesozoic time, its closure leading to development of the Apennine-Maghrebide fold-and-thrust belt during the Eocene-Recent interval, and the post-orogenic opening of the Tyrrhenian Sea since Miocene time. This history of tectonic inversion is partly preserved within two major fault zones, the Valnerina Line, in the central Apennines, and the Gratteri-Mount Mufara Line, in central-northern Sicily, which were repeatedly reactivated with different kinematic characters. The relatively long life of these structures indicates that strain was localized along anisotropies inherited from early deformation episodes. However, the progressive widening of both fault zones through time may result from strain-hardening fault-rock behaviour during subsequent deformations, thus suggesting that fault reactivation does not imply fault-zone weakening as is often assumed.
Weak zones in Precambrian Sweden
Authors: Talbot C. J.
Published: April 2008

Examples of weak fault zones in and bordering Precambrian Sweden are reviewed and then analysed in terms of the factors that rendered them weak. The criterion taken here for weak zones is evidence of post-glacial uplift having reactivated old shear zones that are still active now. Strain analysis of the Singö shear zone demonstrates that it was already weak while it was deep and ductile between 1.86 and 1.6 Ga. Thus the orientations of strain ellipsoids indicate pure rather than simple shear across shear strands in which the dissolution of quartz and feldspar indicate high synshear fluid pressures. The characteristics of weak strands of the Baltic-Bothnian shear zones and the Senja-Bothnia shear zones are long histories of superimposed ductile, semi-ductile and brittle structures, indicating repeated reactivations with different kinematics. The shear zones that weakened are those that reactivated in several episodes during their history. Repeated reactivation renders major shear zones in the ‘brittle’ upper crust ‘ductile’ by rounding major angularities along subvertical and subhorizontal zones. This generates continuous seams of cataclasites in which pore fluids can pressurize so that internal strains can remain ductile and almost aseismic by frictional sliding or flow. This review ends by discussing how multiple reactivations weaken major faults and how reactivation remains focused on particular zones.
The role of syn-rift magmatism in the rift-to-drift evolution of the West Iberia continental margin: geophysical observations
Authors: Whitmarsh R.B. , Minshull T.A. , Russell S.M. , Dean S.M. , Louden K.E. , Chian D.
Published: April 2008

The presence of a well-defined ocean-continent transition (OCT) and the absence of large volumes of extrusive or intrusive rocks on the West Iberia margin make it a good place to investigate how the largely amagmatic rifting and break-up of continental lithosphere evolves into oceanic crust produced by magmatic sea-floor spreading. In the southern Iberia Abyssal Plain there is a broad OCT with a characteristic seismic and magnetic character, distinct from both thinned continental crust and normal oceanic crust, which supports the notion that it consists predominantly of exhumed and serpentinized mantle. Interpretations of magnetic and seismic data indicate that on average only small amounts of syn-rift melt exist within the OCT. Isolated, probably margin-parallel, intrusive melt bodies are scattered within the eastern part of the OCT well beneath the top of acoustic basement. Within the western part of the OCT, closer to unambiguous sea-floor spreading magnetic anomalies, such bodies were later(?) emplaced at higher levels and more closely together in the basement until eventually sea-floor spreading began. The evidence does not support the hypothesis that ultraslow sea-floor spreading can explain the magnetic anomalies observed within the wider parts of the West Iberia OCT, where the OCT evolution is best resolved.
Petrochemistry of serpentinized peridotite from the Iberia Abyssal Plain (ODP Leg 173): its character intermediate between suboceanic and sub-continental upper mantle
Authors: Abe Natsue
Published: April 2008

Highly serpentinized peridotites derived from the upper mantle beneath a non-volcanic continental margin were sampled from the southern Iberia Abyssal Plain (Ocean Drilling Program Leg 173), and the primary mantle minerals were analysed for major and trace elements to petrologically characterize the upper mantle. The
Cr
-number (= Cr/(Cr + Al) atomic ratio) of the peridotite spinels varies widely from 0.1 to 0.6. The Na
2
O content of clinopyroxene is rather constant, 0.5–0.8 wt %. The chondrite-normalized rare earth element (REE) patterns have light REE (LREE) depleted convex-upward shapes. The LREE/HREE (heavy REE) ratio in clinopyroxene is constant irrespective of the degree of melt extraction of the sample as measured by the
Cr
-number of spinel. The trend of the peridotites’ mineral chemistry is different from both the simple melt extraction and the general mantle metasomatic trends. The geochemical character of the Iberia Abyssal Plain peridotite is intermediate between those of abyssal peridotites and peridotite xenoliths from continental regions. These geochemical features, especially for the trace elements in clinopyroxene, are rather similar to those in peridotite xenoliths from arcs. This chemical trend is probably the result of ‘open-system melting’, which involves melting simultaneously with enrichment of trace elements by the influx agent.
The evolution of amphibolites from Site 1067, ODP Leg 173 (Iberia Abyssal Plain): Jurassic rifting to the Pyrenean compression
Authors: Gardien V. , Poupeau G. , Muceku B. , Hébert R. , Beaudoin G. , Labrin E.
Published: April 2008

During ODP Leg 173 (April–May 1997) five new sites (1065, 1067–1070) were drilled at the ocean-continent transition (OCT) zone off the West Iberia margin in the Iberia Abyssal Plain. At Site 1067 the 92 m of cored amphibolites were subdivided into three units based on textural criteria. Unit 1 consists of highly foliated and folded amphibolites and acidic gneiss concordant to and folded along with the foliation of the amphibolite. Unit 2, in the middle part of the section, consists of brecciated amphibolites. Unit 3, at the bottom of the hole, is a weakly deformed zone where magmatic textures are observed in the amphibolites and in associated anorthosites. The amphibolites contain tschermakitic to magnesio-hornblende amphibole, plagioclase, zircon, apatite ± titanite ± Fe-oxide ± quartz. The acidic gneiss consists of garnet, plagioclase, alkali-feldspar, quartz, biotite and zircon. Chlorite, sericite and ilmenite occur as secondary phases. The metamorphic evolution of the amphibolite and acidic gneiss started under amphibolitefacies conditions (Stage I: 670 ± 40°C and 7 ± 1 kbar). Further exhumation took place through low-grade amphibolite-facies (Stage II: 550 ± 50°C and 5.5 ± 1 kbar) to green-schist-facies (Stage III: < 500°C and < 3 kbar) conditions contemporaneous with the development of ductile structures. The late metamorphic evolution of the amphibolite ended under ocean-floor conditions. Oxygen isotope ratios and studies of fluid inclusion indicate that a magmatic water-rich fluid in equilibrium with the igneous protolith predominated during Stage I. During Stages II and IIII low-temperature water-rich fluids of metamorphic origin predominated, with a probable contribution of sea water. Apatite fission-track dating indicates that the amphibolites record two thermal excursions below 120°C. The first took place atc.113–100 Ma and could be related to Cretaceous rifting. The second was brief, and so did not anneal older tracks; it occurred between 75 and 55 Ma and could be related to the Pyrenean orogeny.
The role of lower crust and continental upper mantle during formation of non-volcanic passive margins: evidence from the Alps
Authors: Müntener Othmar , Hermann Jörg
Published: April 2008

The remnants of a Mesozoic passive continental margin and of the Tethyan ocean floor are preserved in the Austroalpine and Upper Penninic nappes in eastern Switzerland and northern Italy. Reconstructions of the continent-ocean transition indicate that large areas of subcontinental mantle rocks, but only limited areas of lower-crustal rocks were exposed on the Tethyan sea floor. Microstructures, large shear zones, and the retrograde metamorphic evolution of peridotite and gabbro from Malenco (northern Italy) are investigated to evaluate the role of lower crust and upper mantle during formation of non-volcanic passive continental margins. The combination of petrological constraints and microstructures suggests two contrasting stages: (1) high-temperature (> 650 °C) shearing and annealing of microstructures are attributed to pre-rift tectonics; (2) localized mylonitic shear zones cut the high-temperature structures and developed during nearly isothermal decompression (
T
<600°C), followed by cooling and hydration of the rocks. These shear zones formed during exhumation of the lower crust and upper mantle and are related to early rifting of the Adriatic passive continental margin. The microstructures of the hydrous mylonites display drastic grain-size reduction, which results from a combination of dynamic recrystallization and metamorphic hydration reactions at temperatures <650°C. Strain softening facilitated the formation of crustal-scale shear zones along which the lower crust and upper mantle were exhumed to shallow crustal levels of
c.
10–15 km. Such large shear zones excised 10–20 km of mostly intermediate and lower crust, and are linked to and contemporaneous with the formation of rift-related basins in the upper crust. Boudinage of the lower crust during early rifting is proposed as a major process to explain the scarcity of exposed lower crust along non-volcanic passive margins. The compilation of pressure-temperature data and rift-related structures in the deep crust and upper continental mantle from the Alps suggests that most peridotites preserve a high-temperature evolution that is not related to Mesozoic rifting. Granulite-facies rocks occur in pre-rift lower and middle continental crust. Exhumed granulites along passive continental margins preserve much of a history that is not related to the exhumation itself, but to tectonic processes predating rifting.
A tale of two kinds of normal fault: the importance of strain weakening in fault development
Authors: Buck W. Roger , Lavier Luc L.
Published: April 2008

We search for a description of fault formation consistent with the main features of two very different types of extensional faults: (1) large-offset, low-angle normal faults; (2) small-offset, high-angle normal faults. We use an advanced numerical model to predict how the pattern of faulting varies as a function of the imposed magnitude and rate of weakening of an extending Mohr-Coulomb layer. We assume that fault weakeing is due to reduction of cohesion with fault offset. Faults initiate and slip at high dip angles. When the fault offset is large (i.e. comparable with layer thickness) then the inactive footwall fault surface can be rotated to a flat orientation. We find two requirements for development of a large-offset fault. The magnitude of cohesion loss must be greater than
c.
20% of the initial total extensional yield strength. Also, the rate of weakening with fault offset has to be moderate: the fault offset to lose cohesion has to be less than
c.
2 km and more than
c.
100 m, with the lower bound being harder to define. Using the same cohesion and rate of offset weakening, extension of a thick layer can lead to development of multiple, small-offset, high-angle faults rather than a single ‘low-angle’ fault. For cohesion reduction of 20 MPa a brittle lithosphere thicker than 20 km leads to multiple faults. Finally, we show that inclusion of thermal advection weakening can shift the transition to thinner layers for the same magnitude and rate of cohesion weakening.
Evidence for seismogenic normal faults at shallow dips in continental rifts
Authors: Abers Geoffrey A.
Published: April 2008

Several recent observations indicate that normal faulting earthquakes occasionally occur on faults dipping <35°, dips often considered shallow. Most of these occur in the Woodlark and Aegean rifts. These two rifts are found to generate significantly more earthquakes than others and are the most rapidly extending, and so display the widest variety of fault behaviour. Even within the Woodlark Rift system extension rates vary along strike, with the shallowest-dipping faults confined to the most rapidly rifting segment. Here, several events (M
w
6.0–6.8) feature nodal planes dipping 23–35°. These planes are subparallel to shear zones bounding nearby metamorphic core complexes, including one imaged to 8–9 km depth by seismic reflection profiling. In the western Gulf of Corinth at least one large event (M
w
6.4) occurred on a fault dipping
c.
33°. Similarly to the Woodlark example, this rift segment exhibits a high opening rate (10–20 mm a
−1
). Several other cases elsewhere, based on older historical data, microseismicity, or geological inference suggest seismic slip at similar or shallower dips. However, no documented large earthquake exhibits seismic slip on subhorizontal surfaces (dip <10–15°). Stress rotation may explain the 23–35° dips, but thus far no realistic mechanism has been found. More likely, these faults represent surfaces somewhat weaker than surrounding rock, through some combination of modest cohesion of the surrounding rock and slightly lower frictional coefficients on the fault. Such weakening may be a consequence of high slip rates, which rapidly generate large offsets, and of mature fault systems.
Evolution of the Miocene-Recent Woodlark Rift Basin, SW Pacific, inferred from sediments drilled during Ocean Drilling Program Leg 180
Authors: Robertson A.H.F. , Awadallah S.A.M. , Gerbaudo S. , Lackschewitz K.S. , Monteleone B.D. , Sharp T.R. ,   other members of The Shipboard Scientific Party
Published: April 2008

The results of drilling during Ocean Drilling Program Leg 180 provide insights into fundamental processes of continental break-up, because rifting can be related to westward propagation of a spreading centre into continental crust. A generally north-south transect of holes was drilled across the Woodlark Rift on the uplifted northern rift margin on the Moresby Seamount (Sites 1114 and 1116), on the hanging wall of the low-angle (25–30°) extensional Moresby Detachment Fault (Sites 1108, 1110–1113 and 1117) and across the downflexed northern rift margin (Sites 1118, 1109 and 1115). The results, when placed in the regional tectonic context, document a history of Palaeogene ophiolite emplacement, followed by Miocene arc-related sedimentation. Regional uplift and emergence of the forearc area took place in Late Miocene time. Submergence to form the Woodlark Rift began in latest Miocene time, marked by widespread marine transgression and shallow-water deposition, accompanied by input of air-fall tephra and volcaniclastic sediments. During Pliocene time, deposition within the rift basin was dominated by deep-water turbidites, including high-density turbidites in the south. Strong extension along the north-dipping Moresby Detachment Fault was active during Pleistocene time, associated with uplift of the Moresby Seamount and shedding of fault-derived talus, mainly of meta-ophiolitic origin. During Pleistocene time, a carbonate platform was constructed to the NW, trapping clastic sediment and resulting in a switch to slower, more pelagic and hemipelagic deposition within the Woodlark Rift Basin. The marked change in rift basin configuration during Pleistocene time may relate to westward propagation of the Woodlark oceanic spreading centre at
c.
2 Ma.
40
Ar/
39
Ar ages of fallout tephra layers and volcaniclastic deposits in the sedimentary succession of the western Woodlark Basin, Papua New Guinea: the marine record of Miocene-Pleistocene volcanism
Authors: Lackschewitz K.S. , Bogaard P.V.D. , Mertz D.F.
Published: April 2008

Five fallout tephra layers and 13 heterolithological volcaniclastic deposits drilled at Holes 1115A, 1115B, 1115C, 1109C, 1109D and 1118A, during Leg 180 on the downflexed northern margin of the western Woodlark Basin, have been dated by single-crystal laser
40
Ar/
39
Ar analyses. The fallout tephra layers range in age from 0.135 ± 0.008 Ma to 2.84 ± 0.03 Ma. Sedimentation ages determined for the volcaniclastic deposits range from 1.75 ± 0.29 Ma to 3.79 ± 0.01 Ma, closely matching the nannofossil, planktonic foraminifer and palaeomagnetic chronostratigraphies of the holes. However, two volcaniclastic deposits from 516.91 m below seafloor (mbsf) and 632.5 mbsf in Hole 1118A are significantly older than indicated by biostratigraphic and palaeomagnetic data, probably because of the presence of older reworked volcanic crystals. The youngest ash layer is derived from explosive eruptions in the Dawson Strait area of the D’Entrecasteaux Islands, whereas the four older tephra layers are attributed to explosive eruptions in the Moresby Strait area of the D’Entrecasteaux Islands. The
40
Ar/
39
Ar ages of volcaniclastic sand layers in Holes 1115C and 1118A indicate a transition from a shallow-water succession (<150m) to a deeper-water succession (150–500m) with rapid deposition of volcaniclastic sands, mainly by turbiditic currents, at 3.8 Ma. This transition is related to the subsidence of the margin during rifting of the Woodlark Basin. Two volcaniclastic deposits with ages of 13.84 ± 0.02 Ma and 14.04 ± 0.03 Ma, respectively, provide important time markers in the middle Miocene sedimentary sequence at Hole 1115C, where biostratigraphic ages are scarce. Our
40
Ar/
39
Ar ages represent the first marine record of Miocene to Pleistocene volcanism in the area of eastern Papua.
Tectonic evolution of the NW Red Sea-Gulf of Suez rift system
Authors: Khalil S.M. , McClay K.R.
Published: April 2008

The NW Red Sea-Gulf of Suez rift system was initiated during Late Oligocene time and underwent extension in a N65°E direction, almost orthogonal to pre-existing WNW-trending Pan African shear-zone fabrics in the crystalline basement of the Sinai-African plate. Earliest syn-rift sediments are Upper Oligocene continental clastic deposits with minor syn-rift basalts. Early Miocene sedimentation was dominated by shallow marine clastic deposits, which developed variable stratigraphic architectures as a response to the interaction of extensional faulting, sea-level changes, sediment supply and dispersal. Analysis of fault geometries, fault kinematics and sedimentation patterns indicates that rift-normal extension predominated throughout the Late Oligocene-Early Mid-Miocene evolution of the rift. Reactivation of the Precambrian basement fabrics was the main factor controlling the fault architecture, fault linkage and evolution of the NW Red Sea-Gulf of Suez rift. Individual faults were initially strongly segmented and offset across ‘soft-linked’ relay structures. With increased extension these faults became linked by breaking down relay structures with the development of local ‘hard-linked’ transfer faults, thus giving rise to the rhomboidal fault pattern of the rift system. In Mid-Miocene time, the Levant-Gulf of Aqaba transform boundary was established, linking the Red Sea rift plate boundary to the convergent Bitlis-Zagros plate boundary. This resulted in a dramatic decrease in extension rates within the Gulf of Suez whereas the northern Red Sea continued to extend, with significant syn-rift sediments deposited in Late Miocene-Pliocene time in offshore fault-bounded basins.
Transfer zones normal and oblique to rift trend: examples from the Perth Basin, Western Australia
Authors: Song Tingguang , Cawood Peter A. , Middleton Mike
Published: April 2008

The Perth Basin is a major tectonic province along the western margin of the Australian continent. Basin morphology is controlled by north-striking faults formed during Permian rifting and reactivated during later tectonic events, notably during continental break-up in Late Jurassic-Early Cretaceous time. Transfer structures, including those normal and oblique to the major faults, compartmentalized the basin into segments of distinctive character. East-west transfer faults, perpendicular to the basin trend, were active throughout the rift stage of basin development and are recognized only in the northernmost onshore part of the Perth Basin, corresponding to the depocentre for Permian sediment accumulation. Northerly trending normal faults change in character and/or terminate at these east-west structures. The NW-striking transfer zones influenced deformational features formed during the Late Jurassic-Early Cretaceous break-up. No continuous fault plane has been identified with these zones in the sedimentary sequences. They are characterized by the termination and/or swing of major normal faults at the transfer zones. Sinistral strike-slip movement of at least 16 km is recognized across the Abrolhos Transfer Zone on the basis of offset in the trend of the Beagle Fault system. The orientation, age of activation, and position of these zones are similar to those of transform faults in the adjoining Indian Ocean, suggesting that the two structures are contiguous.
Patterns of extension and magmatism along the continent-ocean boundary, South China margin
Authors: Clift Peter D. , Lin Jian ,   Odp Leg 184 Scientific Party
Published: April 2008

Early Oligocene sea-floor spreading in the South China Sea was preceded by at least two episodes (in Maastrichtian and Mid-Eocene time) of continental extension that generated a series of rift basins on the South China margin, which are separated from the continent-ocean boundary (COB) by an outer structural high. Regional multichannel seismic profiles showing faulting of the pre-rift basement allow the amount of extension in the upper crust to be measured. The total subsidence across the South China margin is far in excess of that predicted using a forward flexural-cantilever model of extension and the degree of faulting measured seismically in the upper crust. This mismatch suggests preferential extension of the lower crust, increasing towards the COB to account for the subsidence. The same feature is seen in the Nam Con Som Basin, which is located close to the southwest end of an extinct propagating spreading ridge offshore from Vietnam. However, in the Beibu Gulf Basin, which is not adjacent to the COB, subsidence is approximately compatible with uniform extension in the upper and lower crust across the entire basin, if not at all locations. We predict that extension of the lower crust exceeds that in the lithospheric mantle along the COB. Heat-flow measurements at Ocean Drilling Program (ODP) sites on the Chinese continental slope and on the conjugate Dangerous Grounds margin yield values consistent with, or slightly higher than, those predicted by models of uniform extension in the lithosphere. Although there is no magmatism comparable with the seaward-dipping volcanic rocks of rifted volcanic margins, there is seismic evidence of rift-related volcanic rocks spanning a width of
c.
25 km landward of the COB. Simple adiabatic melting models do not predict magmatism, and we suggest that the presence of water in the mantle lithosphere, together with residual pre-rift heat, may instead be responsible for increasing melting here. Deep-water syn-rift sediments recovered by the ODP near the COB indicate that volcanism was submarine and that rifting culminated in a mass wasting event that marks a break-up unconformity. The average extension in South China Sea is much less than that seen in the extreme ‘non-volcanic’ Iberian margin. The South China margin may represent an intermediary form of continental extension between the end member extremes of the Iberia-type non-volcanic and the Greenland-type volcanic margin.
Anomalous melt production after continental break-up in the southern Iberia Abyssal Plain
Authors: Minshull T.A. , Dean S.M. , White R.S. , Whitmarsh R.B.
Published: April 2008

Recent geophysical work and Ocean Drilling Program drilling in the southern Iberia Abyssal Plain have indicated that, in a transition zone up to 170 km wide between thinned continental crust and oceanic crust, the basement consists of serpentinized peridotite mantle with sparse mafic intrusive or extrusive rocks. There is no evidence for the addition of significant magmatic material to the stretched continental crust landward of this zone during the last phase of rifting, whereas seaward of this zone, where the half-spreading rate is about 10 mm a
−1
, the crust rapidly reaches a thickness of
c.
6 km, which is normal for Atlantic oceanic crust. Models of melt generation during pure shear, finite-duration continental rifting can successfully reproduce the observed absence of significant syn-rift magmatism on, within and beneath the thinned continental crust if the rifting episode is longer than 10–20 Ma. However, for normal mantle potential temperatures, such models predict significant melt generation in the transition zone seaward of the thinned continental crust even for rift durations longer than 20 Ma. Restricted melting beneath the transition zone might be explained partly by lateral heat loss to the adjacent continental lithosphere, by anomalously low mantle potential temperatures at break-up time, or by depth-dependent stretching such that the observed infinite stretching factor for the crust is not representative of the lithosphere as a whole. An additional mechanism for restricted melt production involves a transitional state between the end of continental extension and the onset of steady-state sea-floor spreading, during which mantle upwelling is less focused than at normal oceanic spreading centres.
Palaeowater in coastal aquifers of Spain
Authors: Manzano M. , Custodio E. , Loosli H. , Cabrera M. C. , Riera X. , Custodio J.
Published: April 2008

In the framework of the PALAEAUX project, three coastal aquifers have been studied in Spain, looking at the possible existence of Pleistocene or Early Holocene fresh groundwater, in addition to the formerly studied Llobregat Delta. They constitute representative situations of the most common coastal aquifers. The main characteristics of these four areas are:

The Llobregat Delta system, studied since 1965, is placed to the southwest of Barcelona (NE Mediterranean coast of the Iberian Peninsula). It is a sedimentary fluvial formation of Pleistocene to recent age, which is representative of small size coastal alluvial formations with the deep confined aquifer open to the sea offshore. Marine sea water penetrated the deep Pleistocene layer during the Flandrian sea-level rise but afterwards relatively high freshwater heads upstream allowed its progressive flushing to the sea through the submarine outlet since 6000–8000 a ago. Freshwater turnover time is shorter than the Holocene duration and it has been renovated due to natural discharge of the aquifer to the sea. The renovation process has recently been accelerated due to intensive groundwater exploitation during the past century.
Nowadays, only freshwater a few centuries old remains in low permeability areas of the
confined Pleistocene aquifer.

The Doñana sedimentary aquifer system corresponds to the ancient estuary of the Guadalquivir
River (SW Spain). It contains old saline groundwater that has not been flushed away due to the
low freshwater head. In this aquifer
39
At,
85
Kr,
3
H,
14
C and
13
C measurements have been used
to ascertain the age structure of several mixed groundwater samples. After correction, the
oldest fresh groundwater
14
C age is
c.
12–15 ka but
39
Ar ages point to somewhat younger
water. Neither the stable isotope contents nor the recharge temperature calculated from noble
gases show a climatic signature for these waters.

The Inca-Sa Pobla carbonate system is the northeast sector of the island of Mallorca. Brackish
and salt groundwater in the coastal area has been found along the coastal strip of S’Albufera,
displaying what seems a convective-like flow pattern inside the up to 200 m thick aquifer. The
most consistent estimated average
14
C ages of the mixed water vary between 9 and 13 ka, i.e.
Early Holocene-Late Pleistocene. Water
18
O and
2
H contents do not show a clear climatic
signature.

The Amurga volcanic Massif is on the southeast of the island of Gran Canaria. Brackish
groundwater is found in the thick phonolite rock sequence. The unsaturated zone can be up to
200 m thick. Groundwater salinity is due to recharge under arid conditions and the influence
of marine airborne salts. Groundwater
14
C age is possibly
c.
11 ka, indicating a Late
Pleistocene age water mixture. The small recharge due to aridity and the thick unsaturated zone
explains this. Stable-water isotopes show no clear climatic signature
Palaeowaters from the Glatt Valley, Switzerland
Authors: Purtschert R. , Beyerle U. , Aeschbach-Hertig W. , Kipfer R. , Loosli H. H.
Published: April 2008

Isotopic and noble gas data from a confined gravel aquifer in the Swiss Plateau has been investigated. From glaciomorphological studies it is well known that the Glatt Valley was repeatedly ice covered during the last glacial period. Corrected radiocarbon ages range from 0 to >28 ka
bp
and reveal a gap of between 25 and 17 ka
bp
, indicating an interruption of groundwater recharge during the last glacial maximum. Based on
39
Ar measurements, a contribution of younger water components with residence times of a few hundred years have been identified in some waters. Recharge temperatures, estimated by analysis of noble gas contents, suggest a temperature difference of
c.
5°C at the Holocene-Pleistocene transition at 12 ka
bp
. The long-term temporal δ
18
O-recharge temperature relation over the last 30 ka has a slope of
c.
0.49‰ °C
−1
, consistent with the modern seasonal relation.
Weichselian palaeoclimate and palaeoenvironment in Europe: background for palaeogroundwater formation
Authors: Vaikmäe R. , Edmunds W. M. , Manzano M.
Published: April 2008

A review is given of palaeoclimatic and palaeoenvironmental evidence across Europe for the Weichselian period relevant to interpreting the emplacement and circulation of groundwaters. In addition, this provides the background against which the evidence of past climates and environments contained in groundwaters in coastal areas of Europe, from the Baltic to the Atlantic Ocean may be compared. For much of the Weichselian, although significantly colder than at present, conditions were favourable for the recharge of groundwater, as shown, for example, by periods of speleothem growth. During the last glacial maximum (LGM) recharge is likely to have ceased over much of permafrost-covered Europe, although shallow groundwater recharge from meltwater (generated by the geothermal gradients) could have taken place beneath the ice where pressure relief through tunnel valleys may have occurred. Modern recharge could have started as early as 13
14
C ka
BP
, but probably interrupted by the Younger Dryas between 11 and 10
14
C ka
BP
. In the Baltic areas, ice-dammed lakes inhibited the start of the modern hydrogeological regimes until
c.
10.3
14
C ka
BP
. Tundra conditions prevailed over most of ice-free southern Europe at the time of the LGM. At this time the area south of the Portuguese-Spanish border retained a generally warm and relatively humid climate due to the maintenance of warmer sea-surface temperatures derived from Atlantic Ocean circulation. For most of coastal Europe, however, the most significant impact on groundwater circulation is likely to have been the lowering of sea levels that drained large areas of the shelf, such as the North Sea and the English Channel, and also had a significant impact on the Atlantic coast of the Iberian Peninsula where the maximum lowering of up to 130 m would have been experienced. This, together with the general changes in climate, would also have reorganized the atmospheric chemistry over sites in Europe that is likely to be recorded in the groundwater’s chemical and isotopic signatures.
Modelling scenarios for the emplacement of palaeowaters in aquifer systems
Authors: Harrar W. G. , Williams A. T. , Barker J. A. , Van Camp M.
Published: April 2008

The response of coastal aquifer systems to global sea-level rise, the presence of permafrost and glaciation has been analysed using analytical and numerical models. The hydraulic connectivity between confined coastal aquifers and the sea largely controls their response to global sea-level rise. Open aquifer systems have direct hydraulic contact with the sea where they sub crop along the continental slope and base-level history for the aquifer is defined by sea-level history. In these systems, hydraulic heads equilibrate quickly to sea-level change at rates controlled by the aquifer hydraulic diffusivity. Interface movement lags behind the equilibration of the hydraulic heads and is controlled largely by the rate at which freshwater can be flushed from the aquifer through overlying semi-confining units. Interface movement occurs over time periods of tens of thousands of years. In contrast, closed aquifer systems lack direct hydraulic connectivity with the sea, which is controlled by the thickness and permeability of overlying semi-confining layers. During the Late Pleistocene-Early Holocene the base-level history for closed aquifer systems underlying the North Sea Basin was defined by the location of rivers and lakes in areas that are now offshore. These aquifers became coastal aquifers during the Holocene when rivers and lakes were inundated by rising sea levels. Salinization of closed aquifer systems may occur due to the downward diffusion of salts through overlying semi-confining layers in the presence of upward freshwater seepage. Aquifers overridden by glaciers respond in a manner that is largely controlled by aquifer transmissivity and geometry. The presence of permafrost inhibits recharge resulting in lower hydraulic heads and a reduction in aquifer fluxes.
Application of geophysical borehole logging techniques to examine coastal aquifer palaeohydrogeology
Authors: Buckley David K. , Hinsby Klaus , Manzano Marisol
Published: April 2008

Geophysical logging is shown to be a useful technique to support investigations of coastal aquifer hydrogeology. Formation logging can identify the geological layering and fluid logging can characterize the salinity distribution. The measurements also reveal wellbore flow to be common in coastal boreholes, which can mask the salinity stratification in the aquifer matrix. Geophysical logging can be used to guide water sampling and to provide information on the palaeohydrogeology. In combination with water sampling and age determinations, it has shown modern groundwaters overlying Holocene age groundwaters, in turn overlying groundwaters of Pleistocene age, within 150 m of the surface in some of the aquifers studied. Sea-level change in response to Pleistocene glaciations and deglaciations is recognized as a major control on the salinity of groundwaters and on the development of permeable flow routes in coastal aquifers. The permeable routes that developed by groundwater circulation to older and deeper base levels are now partly or wholly occupied by groundwaters of modern flow systems, and can be the focus for saline intrusion. The effects of Pleistocene sea-level change on aquifer development appear to be worldwide and are being increasingly recognized. Examples are described to illustrate the development of flow horizons in relation to rock layering, structure and base levels.
The modern water interface: recognition, protection and development — advance of modern waters in European aquifer systems
Authors: Hinsby K. , Edmunds W. M. , Loosli H. H. , Manzano M. , Condesso De Melo M. T. , Barbecot F.
Published: April 2008

Modern groundwater that has recharged aquifers within the past 50 a shows the influence of humans globally, either by the presence of small concentrations of environmental tracers or in some cases by severe pollution. This study describes important environmental tracers (e.g.
3
H,
85
Kr, chlorofluorocarbons, SF
6
) and contaminants (e.g. NO
3
−
}, pesticides, chlorinated solvents) for modern groundwater dating and recognition of human impacts. Some applications of the described tracers in aquifers investigated in the PALAEAUX study are presented in order to illustrate the advance of modern waters in European aquifer systems. The study shows that the location of the modern water interface varies within a range of between
c
. 10 and
c
. 100 m in the investigated aquifers due to variations in hydrogeological setting, climate and exploitation of the groundwater resource. The subsurface distribution of the modern water indicators and contaminants demonstrate that the advance of modern groundwaters and the fate of harmful substances in them have important implications for protection and development of the water resources. Contaminants that do not degrade or degrade only very slowly will advance further into the aquifers and may eventually contaminate even deep groundwater systems.
Evolution of groundwater systems at the European coastline
Authors: Edmunds W. M. , Hinsby K. , Marlin C. , Condesso de Melo M. T. , Manzano M. , Vaikmae R. , Travi Y.
Published: April 2008

An overview is given of the status and origin of fresh and saline groundwaters in the sedimentary aquifers at or near the present European coastline. Results are presented as six regional maps summarizing, as far as possible, the conditions likely to have existed at the end of the Pleistocene, after the impact of glaciation, when groundwaters might be expected to have reached their maximum offshore evolution prior to the encroachment of sea water during the Holocene marine transgression. In the eastern Baltic, the groundwater evolution was different to other European regions in that freshwater heads were
higher
than the present day, promoting recharge during much of the Late Pleistocene. Near the North Sea coasts, there is generally evidence of freshwater movement to depths of 100–150 m but the absence of deeper freshwater (palaeowater) storage may relate to the low hydraulic gradients in the North Sea Basin. In the southeastern part of the North Sea brackish palaeowater is found between Tertiary marine sediments
c
. 300 m below the island of Rømø, 10 km off the Danish west coast. Freshwater of Pleistocene and Holocene ages is found in aquifers at the English Channel coast in several areas, to depths in excess of −300 m; offshore flow in the Late Pleistocene took place towards the central palaeovalley and some of this groundwater may be preserved off the present coastline. In the two Atlantic coastal areas of France and Portugal-Spain a contrast exists due to the proximity of the continental margin and different hydraulic gradients. In Portugal, freshwaters are found at the coastline, and probably offshore, that contain evidence of recharge during the lowered sea levels. In most of the southwestern Atlantic coast of Spain, fresh recent groundwater discharges along and beyond the coastline, while in the old estuary of the Guadalquivir River, saline Holocene water still encroaches the sediments. On the Mediterranean coast of France and Spain some salinity encroachment took place during sea-level rise. In most of the Spanish aquifers fresh recent groundwater has substituted for palaeowater, except for the low-lying areas (Ebro Delta, Inca-Sa Pobla Plain) where brackish Holocene water is still present.
Celebrating the age of the Earth
Authors: Knell Simon J. , Lewis Cherry L. E.
Published: April 2008

The age of the Earth has been a subject of intellectual interest for many centuries, even millennia. Of the early estimates, Archbishop Ussher’s famous calculation of 4004
bc
for the date of Creation represents one of the shortest time periods ever assigned to the Earth’s age, but by the seventeenth century many naturalists were sceptical of such chronologies. In the eighteenth century it was Nature that provided the record for Hutton and others.

But not all observers of geology enquired about time. Many, like William Smith, simply earned a living from their practical knowledge of it, although his nephew, John Phillips, was one of the first geologists to attempt a numerical age for the Earth from the depositional rates of sediments. For more than fifty years variations of that method prevailed as geology’s main tool for dating the Earth, while the physicists constrained requirements for a long timescale with ever more rigorous, and declining, estimates of a cooling Sun and Earth.

In 1896 the advent of radioactivity provided the means by which the Earth’s age would at last be accurately documented, although it took another sixty years. Since that time ever more sophisticated chronological techniques have contributed to a search for the oldest rocks, the start of life, and human evolution. In the attempt to identify those landmarks, and others, we have greatly progressed our understanding about the processes that shape our planet and the Universe, although in doing so we discover that the now-accepted age of the Earth is but a ‘geochemical accident’ which remains a contentious issue.
Genesis and geochronology: the case of John Phillips (1800–1874)
Authors: Morrell Jack
Published: April 2008

In 1841 John Phillips proposed that there were three great periods of past life on the Earth, namely the Palaeozoic, the Mesozoic and the Cainozoic, terms which are still used today. This was by no means Phillips’ sole contribution to geochronology and this paper examines his evolving views on it over a span of forty years. In the 1820s he adopted the Deluge as a notion which reconciled Genesis and geology. From the 1830s he adopted a liberal Christian position, which saw attempts at such reconciliation as futile and dangerous, and incurred the wrath of so-called scriptural geologists. From 1853 to his death, Phillips was a public figure as successively deputy reader, reader, and professor of geology in the University of Oxford. He was also president of the Geological Society from 1858 to 1860. The publication of Darwin’s
Origin of Species
(1859) not only provoked him to reaffirm his liberal Christian beliefs but also induced him to give greater attention to geochronology as a weapon to be used against Darwinian evolution.
John Joly (1857–1933) and his determinations of the age of the Earth
Authors: Wyse Jackson Patrick N.
Published: April 2008

John Joly (1857–1933) was one of Ireland’s most eminent scientists of the late nineteenth and early twentieth centuries who made important discoveries in physics, geology and photography. He was also a respected and influential diplomat for Trinity College, Dublin, and various Irish organizations, including the Royal Dublin Society. Measuring the age of the Earth occupied his mind for some considerable time — a problem he was to address using a diverse range of methods. His sodium method of 1899, for which he is best known, was hailed by many as revolutionary, but it was later superseded by other techniques, including the utilization of radiometric dating methodologies. Although Joly himself carried out much research in this area, he never fully accepted the large age estimates that radioactivity yielded. Nevertheless, Joly’s work in geochronology was innovative and important, for it challenged earlier methods of arriving at the Earth’s age, particularly those of Lord Kelvin. Although his findings and conclusions were later discredited, he should be remembered for his valuable contribution to this important and fundamental debate in the geological sciences.