bluuebunny/crossref_metadata_2025_split huggingface dataset:
Lithospheric and crustal reactivation of an ancient plate boundary: the assembly and disassembly of the Salmon River suture zone, Idaho, USA
Authors: Tikoff B. , Kelso P. , Manduca C. , Markley M. J. , Gillaspy J. 
Published: April 2008

AbstractThe Salmon River suture zone, western Idaho, is a fundamental lithospheric boundary between the North American craton and the accreted terranes of the Cordilleran margin. The initial juxtaposition along this north-south-oriented structure occurred during Early Cretaceous time. This zone was potentially reactivated twice by subsequent tectonism, once during Cretaceous time and once during Miocene time. The Late Cretaceous western Idaho shear zone formed along the Salmon River suture zone, as denoted by a sharp gradient in the isotopic signature of the granitoids that intruded the lithospheric boundary zone. The reconstructed Late Cretaceous orientation of the western Idaho shear zone contains subvertical fabrics (lineation, foliation). The same boundary also acted as a locus for subsequent Miocene Basin and Range extensional deformation. Domino-style normal faulting and deep (2100 m) basin formation accommodated the motion between the extending accreted terranes to the west and the unextended Idaho batholith to the east. Whereas either the mantle boundary or a crustal-scale structuring controls the regional extent of the extensionally reactivated zone, locally crustal basement faults and lithological contacts control the orientation and precise location of faults that accommodate reactivation. The multiple reactivation of the Salmon River suture zone is critical for several reasons. The Early Cretaceous suture zone apparently created a fundamental lithospheric flaw, which was reactivated after terrane accretion. Whether this zone was a fracture or a shear zone, the fabric in the mantle lithosphere was apparently not ‘healed’ during orogenesis. Thus, juxtaposition of mantle lithosphere, which is inferred to occur by faulting in the uppermost mantle, acts as a weakness during later tectonism. Second, the paucity of strike-slip plate boundaries in the geological record makes sense in the context of reactivation. The vertical, lithospheric-scale nature of these structures makes them particularly susceptible to lithospheric-scale reactivation during both transcurrent and/or extensional deformation. These reactivations both overprint the earlier deformation and modify the original geometry. Steeply dipping fabrics, rather than vertical fabrics, may be the general signature of major, ancient strike-slip faults.
Rheological partitioning during multiple reactivation of the Palaeozoic Brevard Fault Zone, Southern Appalachians, USA
Authors: Hatcher Robert D. 
Published: April 2008

AbstractThe Brevard Fault Zone is a linear, NE-trending, gently to moderately SE-dipping fault zone traceable some 750km from Alabama to Virginia in the crystalline southern Appalachians. It ranges from 1 to 3 km wide and contains a mappable lithostratigraphy. The Brevard Fault Zone has been interpreted as a thrust, strike-slip fault (both dextral and sinistral), a suture and terrane boundary, and a fundamental crustal tectonic boundary. Deformation was partitioned in space and time, and motion was both strikeslip (dextral) and dip-slip (thrust). Early strike-slip and thrust movement was coupled to map-scale structures in the deep Inner Piedmont, late Palaeozoic dextral motion was confined to a zone of 1–3 km width, and the latest reactivation consisted of brittle thrusting confined to a zone of 100m width. The fault zone is cut by undeformed NW-trending Mesozoic dolerite dykes. The Brevard Fault Zone is characterized by the presence of a prominent retrograde (chlorite-muscovite stable) S-C fabric that indicates dextral motion. This fabric is related to late Palaeozoic (Alleghanian) dextral reactivation of the fault zone, with an unknown displacement at a time when huge volumes of fluid were fluxed through the zone. The deformation overprints an earlier (Acadian) high-temperature (garnet-staurolite-kyanite) fabric that also yields a dextral motion sense, and involved a component of thrusting. This mid-Palaeozoic deformation was coupled with west-directed, near-metamorphic peak thrusting and flow from the deep Inner Piedmont (to the east) that was buttressed against the primordial Brevard Fault Zone so that the motion became SW directed, and plastic flow became constricted in this narrow 1—3 km zone. Both of these plastic deformations were overprinted by late Alleghanian NW-directed dip-slip brittle deformation confined to the NW side of the Brevard Fault Zone. This last deformation involved at least 10 km of displacement and was related to reactivation of this block of crust as part of the late Alleghanian, NW-directed Blue Ridge-Piedmont megathrust sheet, and formed out of sequence with respect to the megathrust sheet. The Brevard Fault Zone was clearly a zone of crustal weakness that had a suitable mechanical stratigraphy that imparted sufficient anisotropy to localize the initial Acadian fault(s). Early Alleghanian fluid fluxing weakened the already strongly anisotropic fault zone and probably focused ductile reactivation at a shallower crustal depth during the early Alleghanian event. Late Alleghanian reactivation occurred even shallower as an almost discrete boundary in the brittle regime. This is one of the few faults in the Appalachians to have undergone deformational partitioning to permit multiple reactivation during Palaeozoic time.
The role of fault zones and melts as agents of weakening, hardening and differentiation of the continental crust: a synthesis
Authors: Handy M. R. , Mulch A. , Rosenau M. , Rosenberg C. L. 
Published: April 2008

AbstractThe rheology of crustal fault zones containing melts is governed primarily by two strain-dependent mechanical discontinuities: (1) a strength minimum parallel to mylonitic foliation just below the active brittle-viscous (b-v) transition; (2) the anatectic front, which marks the upper depth limit of anatectic flow. The mode of syntectonic melt segregation in fault zones is determined by the scale of strain localization and melt-space connectivity, to an extent dependent on strain, strain rate and melt fraction in the rock. Melt drains from the mylonitic wall rock into dilatant shear surfaces, which propagate sporadically as veins. Anatectic flow at natural strain rates therefore involves melt-assisted creep punctuated by melt-induced veining. On the crustal scale, dilatant shear surfaces and vein networks serve as conduits for the rapid, buoyancy-driven ascent of transiently overpressured melt from melt-source rocks at or just below the anatectic front to sinks higher in the crust. Strength estimates for natural rocks that experienced anatectic flow indicate that melts weaken the continental crust, particularly in depth intervals where they spread laterally beneath low-permeability layers or along active shear zones with a pronounced mylonitic foliation. However, acute weakening associated wit h strength drops of more than an order of magnitude occurs only during short periods (103–105a) of crustal-scale veining. Cooling and crystallization at the end of these veining episodes is fast and hardens the crust to strengths at least as great as, and in some cases greater than, its pre-melting strength. Repeated melt-induced weakening then hardening of fault zones may be linked to other orogenic processes that occur episodically (shifting centres of clastic sedimentation and volcanism) and has implications for stress transmission across orogenic wedges and magmatic arcs.
Propagation of continental break-up in the southwestern South China Sea
Authors: Huchon P. , Nguyen T. N. H. , Chamot-Rooke N. 
Published: April 2008

Abstract

We present new bathymetric, seismic and gravity data on the southwestern tip of the South China Sea oceanic basin, where propagation of continental break-up occurred before
c.
15 Ma. The oceanic domain has a V-shape typical of oceanic propagating rifts. The tectonic fabric of its margins shows that the main stretching direction was slightly oblique to that of the rift axis. A 2D gravity anomaly inversion, corrected for the thermal effect, is used to estimate the crustal structure. At the continent-ocean boundary, the continental crust is stretched by a factor of about four, rapidly decreasing to about two over a few tens of kilometres, a distance corresponding to just over 1 Ma of break-up propagation. Thus, strain localization occurs at the tip of the propagating oceanic crust just before break-up. The along-axis variation in continental crustal stretching is in good agreement with the kinematics of the oceanic crust derived from magnetic anomalies. This analysis suggests that break-up propagates toward the pole of relative rotation and is primarily controlled by the amount of stretching of the continental crust before oceanization.
Nature of the continent-ocean transition on the non-volcanic rifted margin of the central Great Australian Bight
Authors: Sayers Jacques , Symonds Philip A. , Direen Nicholas G. , Bernardel George 
Published: April 2008

Abstract

A region of 50–120 km width defines the continent-ocean transition (COT) in the central Great Australian Bight. It is characterized by a thin apron of post-break-up sediments overlying complexly deformed sediments and intruded crust bounded landward by a basement ridge complex and oceanward by rough oceanic basement. Recently acquired deep reflection and refraction seismic data have significantly enhanced understanding of the COT and basement ridge. Modelled gravity and magnetic data, and features interpreted from seismic data, are consistent with aspects of extensional and break-up models proposed for the West Iberia margin. Many of the features and relationships observed beneath the outer margin of the central Great Australian Bight can be explained by extension within a lithosphere-scale ‘pure-shear’ environment involving four layers: brittle upper crust and upper mantle, and ductile lower crust and lower lithospheric mantle. The COT is interpreted to be underlain by extended continental lithosphere. Thus, the continent-ocean boundary is unequivocally defined between oceanic crust and the COT and appears to be associated with sea-floor spreading magnetic anomaly 33, indicating that break-up and sea-floor spreading did not commence until
c.
83 Ma (early Campanian time), later than the currently accepted 95 Ma age. The major part of the basement ridge complex is probably a combination of serpentinized peridotites and mafic intrusions or extrusions derived by mantle upwelling and limited partial melting. The magmatic products of this process probably cooled during chron 34 producing a distinctive magnetic anomaly, but one that does not relate to break-up and sea-floor spreading.
Deformation fabrics of faulted rocks, and some syntectonic stress estimates from the active Woodlark Basin detachment zone
Authors: Roller Sybille , Behrmann Jan H. , Kopf Achim 
Published: April 2008

Abstract
Ocean Drilling Program (ODP) Leg 180 investigated, in the western Woodlark Basin off Papua New Guinea, the nature and evolution of continental extension, eventually leading to crustal break-up and sea-floor spreading. At Moresby Seamount, the rift-related extension is localized at a recently active low-angle (30°) detachment fault, partly buried beneath a Pliocene-Pleistocene sedimentary synrift sequence. Data from three drillsites sample the detachment fault itself, secondary faults in its hanging wall and a steep normal fault cutting the footwall. The fault plane itself is manifested as a strongly altered fault gouge. Deformation of turbiditic sediments in several fault zones in the hanging wall is dominated by brittle mechanisms, and accompanied by intensive veining and pervasive diagenetic cementation. The metabasic rocks of the footwall below the detachment show an unusual transition from ductile to brittle deformation fabrics with increasing depth. Many fracture systems show evidence of repeated opening and healing during multistage hydrothermal mineralization. Syn-mylonitic microstructures and vein fill mineralogy suggest exhumation of the detachment footwall from considerable depth in the crust. Two palaeo-piezometers were applied to calcite-filled veins that show evidence of plastic deformation. Differential stress values of similar magnitude and probably close to the rock failure strength are found in both the hanging wall and footwall.
Micropalaeontological and palaeomagnetic approaches to stratigraphic anomalies in rift basins: ODP Site 1109, Woodlark Basin
Authors: Resig Johanna M. , Frost Gina M. , Ishikawa Naoto , Perembo Russell C.B. 
Published: April 2008

Abstract

The stratigraphic succession of the western Woodlark Basin is examined in detail relative to ODP Site 1109, where
c.
55 m of Pliocene sediment lies anomalously shallow owing to slumping between
c.
1.10 and 0.65 Ma in the tectonically active rift basin. Palaeomagnetic reversals and varying percentages of the characteristic microfossils,
Globigerinoides fistulosus
and discoasters, within the deformed sediment defining the slump indicate that the Pliocene sediment was periodically introduced by a number of slumps rather than being emplaced by a single slump event. Palaeomagnetic and biostratigraphic events in the remainder of the hemipelagic section of Site 1109 to
c.
4 Ma appear undisturbed and consistent with those at adjoining sites.
Globorotalia truncatulinoides
first appeared between 2.65 and 2.71 Ma at these low-latitude sites, which extends the previously reported area of evolution of the species in the southwestern Pacific toward the Equator. Seven of the nine coiling changes in
Pulleniatina
through time are recognized and dated using sedimentation rates. Along with palaeomagnetic events and microfossil species datum levels, these coiling changes can be used to correlate between sites in the area.
The role of detachment faulting in the formation of an ocean-continent transition: insights from the Iberia Abyssal Plain
Authors: Manatschal G. , Froitzheim N. , Rubenach M. , Turrin B.D. 
Published: April 2008

Abstract

The Iberia Abyssal Plain segment of the West Iberia margin was drilled during Ocean Drilling Program Legs 149 and 173 and has been extensively studied geophysically. We present new microstructural investigations and new age data. These, together with observed distribution of upper- and lower-crustal and mantle rocks along the ocean-continent transition suggest the existence of three detachment faults, one of which was previously unrecognized. This information, together with a simple kinematic inversion of the reinterpreted seismic section Lusigal 12, allows discussion of the kinematic evolution of detachment faulting in terms of the temporal sequence of faulting, offset along individual faults, and thinning of the crust during faulting. Our study shows that the detachment structures recognized in the seismic profile became active only during a final stage of rifting when the crust was already considerably thinned to
c.
12 km. The total amount of extension accommodated by the detachment faults is of the order of 32.6 km corresponding to a β factor of about two. During rifting, the mode of deformation changed oceanwards. Initial listric faulting led to asymmetric basins, accommodating low amounts of extension, and was followed by a situation in which the footwall was pulled out from underneath a relatively stable hanging wall accommodating high amounts of extension. Deformation along the latter faults resulted in a conveyor-belt type sediment accumulation in which the exhumed footwall rocks were exposed, eroded and redeposited along the same active fault system.
Geodynamic models of continental extension and the formation of non-volcanic rifted continental margins
Authors: Bowling Jerry C. , Harry Dennis L. 
Published: April 2008

AbstractFinite-element models of continental rifting show that formation of non-volcanic rifted margins may be the result of extension of a rheologically homogeneous crust. In such circumstances lithosphere necking does not become well developed until late in the rift history, delaying the onset of decompression melting in the asthenosphere until the last 10% of the rifting episode. This result is robust over a broad range of mantle temperatures, margin geometries, and extension rates. A cool mantle is not required, so the models are able to account for the production of oceanic crust at the end of amagmatic rifting episodes. The duration of the syn-rift melting episode is most sensitive to changes in extension rate, with higher extension rates leading to shorter periods of melt production. The duration of the rifting episode is controlled by extension rate and initial crustal thickness, and the geometry of the margin after continental break-up is controlled by initial crustal thickness and the distribution of pre-existing rheological heterogeneity in the crust. The model results are generally compatible with the dimensions and extension rates of rifted continental margins across the globe, and provide a particularly good fit to the evolution of the Iberia Abyssal Plain margin.
Palaeogroundwater of glacial origin in the Cambrian-Vendian aquifer of northern Estonia
Authors: Vaikmäe R. , Vallner L. , Loosli H. H. , Blaser P. C. , Juillard-Tardent M. 
Published: April 2008

Abstract

A strongly depleted stable isotope composition, absence of
3
H and a low radiocarbon concentration are the main indicators of glacial origin of groundwater in the Cambrian-Vendian aquifer in northern Estonia. It is concluded from noble gas analyses that palaeorecharge occurred at temperatures
c.
0°C. In some wells unexpectedly high gas concentrations have been found. Excess air, up to
c.
50 %, is common but two-five times oversaturation is very unusual, requiring special processes and explanations, e.g. oversaturation may indicate recharge under highpressure conditions, perhaps by subglacial meltwater recharge through the aquifers. Analyses of the gas composition in some groundwater samples also showed a rather high concentration of CH
4
, indicating the influence of biogenic reactions in the subsurface that could cause the rather negative δ
13
C values. Results of δ
13
C analyses in two CH
4
samples also show that the CH
4
is most likely of a biogenic origin. Based on the isotope data, the results of noble gas analyses, and considering the palaeoclimatic and palaeoenvironmental situation in Estonia during the late Weichselian time, it is concluded that palaeorecharge of Cambrian-Vendian aquifer most probably occurred during the last glaciation, probably by subglacial drainage through the tunnel valleys.
The Ribe Formation in western Denmark — Holocene and Pleistocene groundwaters in a coastal Miocene sand aquifer
Authors: Hinsby K. , Harrar W. G. , Nyegaard P. , Konradi P. B. , Rasmussen E. S. , Bidstrup T. , Gregersen U. , Boaretto E. 
Published: April 2008

Abstract

The Ribe Formation is a regionally extensive Miocene sand aquifer that is present in western Denmark at depths ranging from 100 to 300 m below ground surface. Groundwater chemistry and isotope data collected from more than 40 wells show that the Ribe Formation mainly contains high quality Cabi-carbonate type groundwater of Holocene age (100–10 000 a
bp
). Pleistocene age groundwaters, identified on the basis of stable isotopes, noble gases and corrected
14
C values, are present below the island of Rømø, in discharge areas near the coast, and in hydraulically isolated inland areas. The groundwater age distribution in the Ribe Formation was successfully simulated with a numerical groundwater flow model and particle tracking only when the
14
C content in groundwater was corrected for both geochemical reactions and diffusion. The results indicate that geochemical and physical processes significantly influence the
14
C content of groundwater and that the correction factors required for the two processes are of the same magnitude. Flow modelling results indicate that Pleistocene groundwaters were emplaced at depth within the Ribe Fromation under low base-level conditions that prevailed throughout the late Pleistocene — near the coast these waters are essentially isolated from the present flow system, and Pleistocene freshwater may be present offshore. Seismic surveys show that conditions offshore are favourable for the presence of Pleistocene freshwater within the Ribe Formation and other aquifers.
Palaeowaters in the aquifers of the coastal regions of southern and eastern England
Authors: Edmunds W. M. , Buckley D. K. , Darling W. G. , Milne C. J. , Smedley P. L. , Williams A. T. 
Published: April 2008

Abstract
The marked climatic changes that occurred during the Late Quaternary and Holocene have had a significant impact on the evolution of the groundwater systems at and near the English coastline. Lowered sea levels and the emergence of a much larger landmass over most of the past 100 ka have also ensured deeper groundwater circulation in the vicinity of the modern coastline. The impacts on the Chalk and Lower Greensand (Albian) aquifers along the English Channel and North Sea coasts are examined, using mainly geochemical and isotopic evidence, especially from borehole depth profiles and interstitial waters.

Along the south coast, fresh groundwaters are found to depths of 250–300 m below OD (ordnance datum) in the Brighton-Worthing area, as well as beneath Poole Harbour, which are related to deeper circulation during lowered sea levels, controlled by the central palaeovalley of the English Channel. In contrast, pockets of saline groundwater are found, protected in east-west structures, which are considered to be little-modified Chalk formation waters. In the Albian sands, near Worthing, freshwaters dating to 7 ka are found at a depth of −450 m OD, suggesting that movement of groundwater towards the shoreline and possibly beyond is still occurring. The east-west structures also influence groundwater migration in north Kent, where fresh palaeowaters can be identified beneath saline water which invaded during the Holocene sea-level rise. In the East Midlands Sherwood Sandstone aquifer, freshwater is found to a depth of −500 m OD, showing continuous geochemical evolution probably over a period of 100 ka, although an ‘age gap’ of between
c.
20 and 10 ka
c.
corresponds to permafrost cover. These palaeowaters in coastal and near-coastal areas remain effectively isolated from the active present-day meteoric flow system but represent high-value resources that may, in some cases, extend offshore.