mobilization of gold as a polymetallic melt during pelite anatexis at the challenger deposit south australia a metamorphosed archean gold deposit the challenger gold deposit in south australia is hosted by pelitic migmatites that underwent peak metamorphism at 7 5 1 5 kbars and at least 800 c nd model ages suggest a protolith age of 2900 ma zircon u pb and garnet sm nd dating indicates that peak metamorphism occurred at 2440 ma and that garnet was present during this event at this time in situ vapor absent melting affected the pelitic host rock indicating that peak metamorphism occurred without introduction of a foreign fluid or melt thus evidence indicating the presence of ore minerals during peak metamorphism implies a prepeak mineralization event this evidence includes the occurrence of invisible gold in lollingite but not in adjacent arsenopyrite and the presence of spherical gold sulfide inclusions in peak metamorphic garnet and other silicates textures developed between pyrrhotite lollingite arsenopyrite and gold the lack of silicate alteration minerals supports this conclusion experimental evidence is presented to support our interpretation that a gold rich polymetallic melt was mobilized into leucosomes synchronously with silicate melt during peak metamorphism visible gold is restricted to migmatitic leucosomes and to a lesser extent melanosomes large inclusions of gold with coexisting arsenopyrite pyrrhotite and bismuth are hosted within peak metamorphic silicate minerals and at grain boundaries trails of micrometric spherical inclusions of predominantly gold and bismuth propagate along annealed fractures from these larger inclusions we show that these gold sulfide inclusions represent the crystallized products of an immiscible polymetallic melt metamorphism of the challenger deposit resulted in partial melting of the pelitic host rock and formation of this gold rich melt these two melts were synchronously redistributed to accumulate as leucosomes during development of a stromatic migmatite formation of a polymetallic melt thereby enabled extensive mobilization of gold producing leucosomes enriched in gold this occurred because the immiscible metallic melt was physically entrained rather than chemically dissolved within the silicate melt gold rich shoots at challenger parallel the plunge of ptygmatically folded leucosomes that are shown to be parasitic to a larger scale fold geometry which appears to be structurally related to the ore shoots it is interpreted that concurrent migration of polymetallic and silicate melt allowed concentration of gold into a series of dilational structures which developed within the larger scale fold geometry challenger represents a new deposit type other deposits around the world such as renco and griffins find are hosted in granulite facies rocks but challenger is the first reported example of leucosome hosted gold in migmatites

constrained 3d modeling of the mesoproterozoic benagerie volcanics australia abstract the benagerie volcanics form part of a hotspot track that affected large sections of eastern australia during the mesoproterozoic understanding the location and controls on eruption centres of the benagerie volcanics and coeval plutons is difficult because they do not outcrop and are known only from limited drillhole data we have applied various combinations of forward and inversion modeling of magnetic and gravity to well constrained petrophysics and drillholes areas to characterize the 3d geometry and extent of the volcanic province and its crustal architecture thereby adding to a better understanding of the emplacement mechanisms involved we interpret the sw limit of the benagerie shear zone corridor to be a major terrane boundary separating relatively magnetic crust to the sw from crust with low magnetic susceptibility to the ne 2d and 3d models show a pronounced thickening of felsic material coincident with the shear zone that may be interpreted as a plutonic root thereby corroborating interpretations that this shear zone acted as a structural conduit for the ascent and emplacement of magma

1 8 1 5 ga links between the north and south australian cratons and the early middle proterozoic configuration of australia abstract the archaean and early middle proterozoic 1 8 1 5 ga terranes of the north australian craton and the south australian craton are separated by 400 km of ca 1 33 1 10 ga orogenic belts and phanerozoic sediments however there is a diverse range of geological phenomena that correlate between the component terranes of the two cratons and provide evidence for a shared tectonic evolution between approximately 1 8 and 1 5 ga in order to honour these correlations we propose a reconstruction in which the south australian craton is rotated 52 counterclockwise about a pole located at 136 e and 25 s present day coordinates relative to its current position this reconstruction aligns the ca 1 8 1 6 ga orogenic belts preserved in the arunta inlier and the gawler craton and the ca 1 6 1 5 ga orogenic belts preserved in the mount isa block and the curnamona province before 1 5 ga the south australian craton was not a separate entity but part of a greater proto australian continent which was characterised by accretion along a southward migrating convergent margin ca 1 8 1 6 ga followed by convergence along the eastern margin ca 1 6 1 5 ga after 1 5 ga the south australian craton broke away from the north australian craton only to be reattached in its current position during the ca 1 33 1 10 ga albany fraser and musgrave orogenies

domino boudinage under layer parallel simple shear abstract the boudin segments of a torn competent layer experience synthetic rotation in layer parallel simple shear as long as the individual segments in a boudin train are constrained by their neighbors even a highly viscous boudin deforms internally to create the necessary space for rotation the rotation rate is then much smaller compared to the case of an isolated segment hence a small tilt of boudin segments is not indicative of low strain the rotation rate at this stage largely depends on the aspect ratio of the boudin segments and the scaled gap width once the tilted boudins are no longer constrained by their neighbors the rotation rate greatly accelerates in the case of a low viscosity ratio between the boudins and the host the boudin segments develop complex shapes which may give an impression of shear band boudins forming under the opposite shear sense we furthermore investigate the behavior of boudin trains of finite length the terminal segments are displaced out of the shear plane deforming into isoclinal folds and separate into groups of boudin segments that rotate into the shear direction and eventually lead to an overall chaotic appearance of the structure natural examples of domino boudinage from a high shear strain detachment zone in the western cyclades greece show many similarities with the modeled structures suggesting that under simple shear deformation the rotation and separation of boudin segments is an indicator for high shear strain

aeromagnetic study of the hengshan wutai fuping region unraveling a crustal profile of the paleoproterozoic trans north china orogen abstract an integrated crustal profile of the intervening trans north china orogen tnco is one of the key issues to understanding the tectonic evolution of the north china craton however the existing geological studies focus only on the surface mapping based petrological geochemical and structural analysis but lack subsurface geophysical evidence and thus make the crustal profile interpretations ambiguous in contrast the current geophysical data covers a very large scale lithospheric mantle and fails to image the detailed structural pattern of the orogenic crust to achieve this goal we present high resolution aeromagnetic data for the hengshan wutai fuping region the largest exposure of the central tnco the reduced to pole magnetic anomaly map firstly verifies the regional tectonic subdivision that the high grade metamorphic terranes i e hengshan and fuping complexes are consistent with high magnetic responses and long wavelength anomalies intervened by a low grade terrane wutai complex characterized by low magnetic responses and short wavelength anomalies 3d euler deconvolution reveals that the tendencies of the clustered solutions show large consistence with the major structural pattern of the region which is characterized by a fan shaped doubly vergent orogenic wedge upward continuation further shows that the northwest part of the orogen yields a thicker crust and is most likely located closer to the paleosubduction zone the new aeromagnetic data combined with structural petrological and metamorphic data indicate that an eastward dipping subduction zone was most possibly active before the collision of the western and eastern blocks leading to the formation of the tnco and the final amalgamation of the north china craton

petrography and rank of the bhangtar coals southeastern bhutan in bhutan a potential coal deposit is exposed at bhangtar in the landslide zone nineteen coal seams are encountered in this area and occur in the lower gondwana supergroup preserved in between the main boundary fault and the thrust the coal is low in moisture 1 76 but the coal cores show moisture values of 3 16 the ash content is up to 48 87 and increases substantially in the younger seams the volatile content on a pure coal basis ranges from 23 38 to 41 02 the sulphur content is less than 0 61 the coals are non coking the amount of trace elements in the coal is quite low the average petrographic composition of the bhangtar coal is vitrinite 31 exinite 2 inertinite 31 and mineral and shaly matter 36 the vitrinite proportion decreases from the older to the younger seams which are shaly vitrinite telinite is derived mainly from cordaitalean wood and is fractured or unfractured showing an oil reflectance rm range of 0 71 0 72 and 0 54 0 60 respectively the oil reflectance variation is governed by a similar variation in the volatiles of the coal exinite is mainly sporinite and cutinite and is constituted of organic matter from upper permian mioflora so that a late permian age can be assigned to the bhangtar coal based on oil reflectance the rank of the coal is metalignitous to hypobituminous the average microlithotype composition of the coal is vitrite 30 clarite 1 vitrinertite v 14 vitrinertite i 11 durite 3 fusite 14 and carbominerite 27 vitrite decreases in proportion towards the younger seams intermediates show a concomitant increase while durite and fusite remain constant carbonaceous shale contains fragmentary inertinite and vitrinite macerals and is interlayered with micro bands of shaly coal which is characterised by abundant fragments of fusinite and vitrinite the coal is very fragile and thus amenable to economic beneficiation the removal of carbonaceous shale in this process increases the vitrinite content thereby reducing the inertinite the coal is used as fuel in electric power plants the petrographic composition of the bhangtar coal has been compared with those of the upper and lower permian coals of the gondwana coalfields of peninsular india the bhangtar coal is characteristically distinct from the gondwana coals of india in petrography and rank but correlates petrographically with the kameng coals of arunachal pradesh india

transpressive duplex and flower structure dent fault system nw england abstract revised mapping along the dent fault northwest england has improved the resolution of folds and faults formed during variscan late carboniferous sinistral transpression a nne trending east down monocline comprising the fell end syncline and taythes anticline was forced in carboniferous cover above a reactivated precursor to the dent fault within the lower palaeozoic basement the taythes anticline is periclinal due to interference with earlier acadian folds the steep limb of the monocline was eventually cut by the west dipping dent fault the hangingwall of the dent fault was dissected by sub vertical or east dipping faults together forming a positive flower structure in cross section and a contractional duplex in plan view the footwall to the dent fault preserves evidence of mostly dip slip displacements whereas strike slip was preferentially partitioned into the hangingwall faults this pattern of displacement partitioning may be typical of transpressive structures in general the faults of the taythes duplex formed in a restraining overlap zone between the dent fault and the rawthey fault to the west the orientations of the duplex faults were a response to kinematic boundary conditions rather than to the regional stress field directly kinematic constraints provided by the dent and neighbouring variscan faults yield a nnw sse regional shortening direction in this part of the variscan foreland

blind orogen integrated appraisal of multiple episodes of mesoproterozoic deformation and reworking in the fowler domain western gawler craton australia abstract the fowler domain in the western gawler craton in southern australia is a poorly exposed region that can only presented be explored using a combination of information from drilling and potential field geophysics regional maps of the total magnetic intensity tmi field of the fowler domain highlight an anastomosing system of terrain scale shear zones that bound four crustal scale tectonic blocks from west to east the colona barton central and nundroo blocks integrated thermobarometry and electron microprobe chemical dating of metamorphic monazites from drillholes in the fowler domain suggest that the colona block in the west underwent two mid crustal amphibolite grade metamorphic events at ca 1643 and 1600 ma the younger age corresponds to the timing of regional high grade metamorphism in the barton block together the age data suggest that the western fowler domain underwent a major tectonothermal event at ca 1600 ma in contrast regional lower crustal metamorphism in the nundroo block which forms the eastern fowler domain occurred at ca 1545 ma in both the barton and nundroo blocks petrological relationships mineral zoning and pressure temperature p t modelling suggest the terrains cooled in the mid to lower crust rather than undergoing exhumation immediately following peak metamorphism age data from the geophysically defined shear zone systems that bound the blocks suggest that exhumation of these lower crustal domains occurred between ca 1470 and 1450 ma and was associated with transpressional reactivation of the terrain during the coorabie orogeny a key finding of this study is that the tectonic evolution of the crustal blocks in the fowler domain was not in concert until at least 1500 ma thus the evolution of the individual blocks is unlikely to be related to the macroscopic character of the terrain defined by the regional scale shear zone systems which are one of the youngest tectonic imprints on the fowler domain coincident gravity and magnetic forward modelling of significant bounding faults suggests the shear zones form a steeply dipping transpressional array consistent with the observed metamorphic field character of the different blocks this study provides a demonstration of the integration of geophysical and petrological approaches to investigate the time integrated tectonic evolution of poorly exposed basement terrains

why do continents break up parallel to ancient orogenic belts the frequently observed parallelism between rifts and the pre existing orogenic fabric of continents suggests that the inherited tectonic fabric of the lithosphere influences the rupture of continents we propose that the existence of a pervasive fabric in the lithospheric mantle induces an anisotropie strength in the lithosphere that guides the propagation of continental rifts subcrustal mantle mechanical anisotropy is supported by i the anisotropie strength of olivine ii an ubiquitous tectonic fabric in exposed mantle rocks and iii measurements of seismic and electrical anisotropy during major episodes of continent rifting parallel to orogenic belts ocean opening through rifting and continent break up is frequently related to the occurrence ofhotspots there is howevcr a discrepancy between hot spots acting as pin point sources of heat and the linear extent of rifts over thou sands ofkilometres moreover rifts tend to parallel pre existing orogenic fab

multi scale analysis of proterozoic shear zones an integrated structural and geophysical study abstract structural mapping of poorly exposed shear zone outcrops is integrated with the analysis of aeromagnetic and bouguer gravity data to develop a multi scale kinematic and relative overprinting chronology for the palaeoproterozoic tallacootra shear zone australia d 2 mylonitic fabrics at outcrop record kimban aged ca 1730 1690 ma n s shortening and correlate with sz 1 movements overprinting d 3 sinistral shear zones record the partitioning of near ideal simple shear and initiated riedel to regional scale sz 2 strike slip on the tallacootra shear zone sz 2 previously undocumented ne sw extension led to the emplacement of aplite dykes into the shear zone and can be correlated to the ca 1595 1575 ma hiltaba magmatic event d 4 dextral transpression during the ca 1470 1450 ma coorabie orogeny reactivated the tallacootra shear zone sz 2 r4 exhuming lower crust of the northwestern fowler domain within a positive flower structure this latest shear zone movement is related to a system of west dipping shear zones that penetrate the crust and sole into a lithospheric detachment indicating wholesale crustal shortening these methods demonstrate the value of integrating multi scale structural analyses for the study of shear zones with limited exposure

architecture of proterozoic shear zones in the christie domain western gawler craton australia geophysical appraisal of a poorly exposed orogenic terrane abstract large scale mesoproterozoic crustal shear zones are partially exposed from beneath cover sequences in the christie domain of the northwestern gawler craton southern australia these structures are associated with gravity and magnetic anomalies that allow them to be mapped under cover and their three dimensional geometry and kinematics to be evaluated gravity and magnetic forward modelling indicate that these shear zones form an imbricate oblique thrust stack with combined top to the southeast left lateral transport the longest shear zones in the stack penetrate the crust to at least 15 km depth and dip to the northwest at 70 their inferred sense of motion are consistent with kinematic indicators from sparse outcrops from northwest to southeast the stack includes crustal slices bound by the karari fault zone tallacootra and blowout shear zones colona and coorabie fault zones and the muckanippie shear zone these structures separate discrete tectonometamorphic packages within the previously undifferentiated christie gneiss of the archean mulgathing complex and imply several generations of transport of material from the lower and middle crust three dimensional inversion of gravity data indicates that the muckanippie shear zone a shallowly rooted second order splay of the coorabie fault zone has an antithetic dip to the southeast and is part of a positive flower structure cross cutting relationships coupled with recent reconnaissance geochronology suggest that the western gawler craton part of the mawson continent was under a long lasting oblique slip deformational regime after ca 1590 ma this tectonic reworking is correlated with episodic pervasive metamorphism and deformation recorded elsewhere in the gawler craton between ca 1550 and 1450 coorabie orogeny pointing to a complex history of stabilization in proterozoic rodinia the tallacootra colona and coorabie structures are all cut at a high angle to their strikes by mesozoic rift faults of the southern rift system and may form piercing points in reconstructions of gondwana and earlier rodinia configurations

timing of mesoproterozoic tectonic activity in the northwestern gawler craton constrained by 40ar 39ar geochronology abstract an array of northeast trending shear zones in the north west gawler craton of south australia has been implicated in widely disparate proterozoic continental reconstructions these shear zones are interpreted to have formed in response to sinistral transpression and dissect the north west gawler craton into several geological domains each with contrasting metamorphic histories new 40ar 39ar data provide age constraints for movement along these shear zones and are interpreted to indicate that the karari tallacootra and coorabie shear zones were last active at 1450 ma despite distinct differences in metamorphic grade the various geological domains bounded by shear zones exhibit evidence for commonality in event histories prior to deformation at 1450 ma albeit at different crustal levels this is interpreted to indicate that deformation at 1450 ma was responsible for reshuffling of crustal blocks that were already adjacent rather than amalgamation of exotic terranes the new data also provide additional evidence for a widespread metamorphic and deformational event at 1530 1550 ma across the north western gawler craton the timing of deformation at 1450 is 100 ma younger than suggested in several published tectonic reconstructions and importantly is also significantly older than grenvillian age tectonism in adjacent provinces to the west and north

evolution of the mount woods inlier northern gawler craton southern australia an integrated structural and aeromagnetic analysis structural mapping integrated with interpretation and forward modelling of aeromagnetic data form complimentary and powerful tools for regional structural analysis because both techniques focus on architecture and overprinting relationships this approach is used to constrain the geometry and evolution of the sparsely exposed mount woods inlier in the northern gawler craton the mount woods inlier records a history of poly phase deformation high temperature metamorphism and syn and post orogenic magmatism between ca 1736 and 1584 ma the earliest deformation involved isoclinal folding and the development of bedding parallel and axial planar gneissic foliation s1 this was accompanied by high temperature upper amphibolite to granulite facies metamorphism at ca 1736 ma during subsequent north south shortening d2 open to isoclinal south southeast oriented f2 folds developed as the palaeoproterozoic successions of the inlier were thrust over the archaean nuclei of the gawler craton the syn d2 engenina adamellite was emplaced at ca 1692 ma the post d2 history involved shear zone development and localised folding exhumation of metamorphic rocks and deposition of clastic sediments prior to the emplacement of the ca 1584 ma granite balta suite the mount woods inlier is interpreted as the northern continuation of the kimban orogen

multi scale integrated structural and aeromagnetic analysis to guide tectonic models an example from the eastern musgrave province central australia abstract in many polydeformed precambrian provinces the understanding of their tectonic history is limited by insufficient exposure which leads to difficulty in developing links between local and regional architecture and results in poorly constrained models of their regional scale architecture high resolution aeromagnetic data allows interpretation and modelling at a scale comparable to structural mapping and by integrating these methods these links can be developed with high confidence as an example we integrate structural mapping aeromagnetic data interpretation and 3d magnetic inversions in a multi scale structural analysis of the polydeformed eastern musgrave province in central australia deformation during the ca 1200 ma musgravian orogeny is characterised by an originally northeast trending structural grain defined by shear zones and tight to isoclinal folds at all scales 3d magnetic inversion models of syn to post musgravian granites indicate that this structural grain is predominantly west dipping this architecture suggests that a northeast oriented fold and thrust belt developed as a result of northwest southeast compression in central australia during the musgravian orogeny the next deformation event is characterised by an array of originally east southeast trending shear zones that truncate and offset the musgravian structural grain with apparent dextral displacements of up to 1 km these shear zones are commonly co located with mafic and granitic dykes suggesting a dilational origin and they are interpreted to have initiated under northeast southwest extension during the ca 1080 ma giles event the architecture of these two mesoproterozoic events was later extensively overprinted by two deformation events during the pan african aged ca 550 ma petermann orogeny the first is characterised by the widespread reactivation of mesoproterozoic structures resulting in a network of shear zones of variable orientation and the second is characterised by the development of crustal scale transpressional shear zones motion on which has caused the rotation of lithospheric blocks and the reorientation of prior structural trends by up to 90 collectively the architecture of these two events suggests broadly north south crustal shortening in central australia as an intraplate response to gondwana assembly this example demonstrates that the integration of structural geology with aeromagnetic interpretation and modelling can be used to develop high confidence links between local and regional structures these links permit the derivation of better constrained models of major tectonic events particularly in regions where outcrop is poor and the structural architecture is complex

does thermal weakening explain basin inversion stochastic modelling of the thermal structure beneath sedimentary basins abstract the fact that basin inversion is possible long after the cessation of rifting indicates that at least some sedimentary basins are characterized by permanent relative weakness this paper investigates whether the steady state thermal structure in itself can provide the necessary pattern of rheological weakening to facilitate compressional inversion of a sedimentary basin by monte carlo simulation of a two dimensional thermal finite element model we find that generally the maximum moho temperature and therefore also the weakest upper mantle is encountered beneath the flanks of the basin the reason for this is heat refraction deviating away from the central parts of the sedimentary cover and into the adjacent basement highs the weakest crust however is located in the center of the basin due to sediment thermal blanketing lower confining pressure and the presence of relatively weak sediments

proterozoic extensional tectonics of the central fennoscandian shield results from the baltic and bothnian echoes from the lithosphere experiment a three dimensional crustal structure of the bothnian sea is deduced from the near vertical reflection data of the crosscutting lines 1 6 7 and c of the babel baltic and bothnian echoes from the lithosphere experiment for this study the common depth point stacked sections have been migrated with a velocity 6 5 km s and displayed as stacked envelope sections to show their enhanced large scale crustal structures the seismic sections show listric shear zones which dip toward se and flatten at major detachment zones 35 40 km and 48 km in depth the detachment zones are associated with changes in p wave velocity from 6 8 km s to 7 0 km s and from 7 4 km s to 8 2 km s and coincide with the middle and lower crustal boundary and the moho boundary respectively the geometry and crosscutting relationship of the detachment zone are well developed on the e w directed line 7 from where the moho detachment zone can be interpreted as to be younger different shear zone systems operated in the parallel n s directed lines 1 and 6 and therefore it is not easy to correlate crustal structures between lines 1 and 6 in the thinned southern part of study area the lower crust has a highly reflective lamellar structure and a strongly reflective subhorizontal moho increased reflectivity of the lower crust is attributed to stretching and mafic underplating of the crust associated with the subjotnian diabase dykes and rapakivi magmatism in the upper crust the rapakivi granite and diabase association is characterized by a rather transparent area within which strong reflections mimic graben and horst structures the distribution of the bimodal association seems to be controlled by the low angle listric shear zones the spatial distribution of outcropping lithologies combined with deeply penetrating shear zones suggests a simple shear extensional deformation the first stage of deformation which concentrated in the upper and middle crust records the collapse of the overthickened svecofennian crust the second stage involving the entire crust may record either the continuation of the postcollisional collapse or a separate anorogenic extensional period the seismic structures predict major post svecofennian deformation faulting and tilting in the central fennoscandian shield the extensional tectonics operating in the fennoscandian shield in the proterozoic is comparable to modern extensional tectonics

new insights into magnetic derivatives for structural mapping magnetic and gravity derivatives can be likened to seismic attributes in that they can help define estimate the physical properties of the source structure causing the anomaly this contribution looks at the tilt derivative first reported in 1994 and more recently used to derive the local wavenumber 1997 we will show that the combination of the tilt derivative and its total horizontal derivative are highly suitable for mapping shallow basement structure and mineral exploration targets and that they have distinct advantages over many conventional derivatives we provide the simple theory behind the derivatives use a range of simple 2d models to illustrate their response and apply them to mapping a mineral target in namibia

comparing 1800 1600 ma accretionary and basin processes in australia and laurentia possible geographic connections in columbia abstract we focus on potential pinning points on the southern and eastern margins of australia and the southern and western margins of laurentia and interpret a convergent plate boundary along the southern margin of the proto australian continent during the palaeo to mesoproterozoic 1800 1600 ma specific geological events within the southern australian margin can be correlated with events preserved along the medicine bow yavapai mazatzal belt in southwestern laurentia these belts were characterised by southward continental growth with varying degrees of crustal reworking and accretion of juvenile material in australia crustal reworking appears to have been the dominant process from 1800 to 1670 ma with more juvenile material accreted at 1620 ma in contrast the southern laurentian terranes were dominated by accretion of juvenile material from 1800 to 1620 ma although reworked continental crust was also accreted to the margin in the interior of both continents basins developed with comparable histories from 1800 to 1670 ma thereafter the evolution of the interior basins diverged with sag phase processes dominant in northern australia and a protracted depositional hiatus in laurentia the geologic evolution is consistent with palaeomagnetic data and provides further evidence for geographic links between eastern australia and western laurentia from 1800 to 1600 ma

refined proterozoic evolution of the gawler craton south australia through u pb zircon geochronology abstract through the application of both conventional u pb zircon analyses and small sample u pb isotopic analyses the nature and timing of tectonic events leading to the formation of the gawler craton have been defined more precisely constraints on deposition of early proterozoic iron formation bearing sediments have been narrowed down to the period 1960 1847 ma deformed acid volcanics including the economically important moonta porphyry have zircon ages of 1790 and 1740 ma the voluminous acid gawler range volcanics and correlatives to the east were erupted over a short interval at 1592 2 ma and were intruded by anorogenic granites at 1575 ma small sample zircon analyses proved to be an extremely valuable adjunct to conventional analyses generally yielding more concordant data which forced a curved discordia through an upper intercept slightly younger than from a conventional straight line discordia

structural inheritance in mountain belts an alpine apennine perspective abstract the geological structure of continental lithosphere shows complex variety that is inherited into orogenic belts and influences the localization and amplification of contractional structures during mountain building in the alpine apennine sector together with other sectors of the tethyan orogenic system the pre orogenic crustal template can include arrays of extensional faults other faults can form adjacent to the evolving mountain belt and subsequently become incorporated as the thrust belts migrate into their forelands while in some areas these inherited features may simply reactivate under inversion more commonly faults show complex partial reactivation structures in volumes of distributed strain faults may serve to nucleate large scale buckle folds for example along basement cover interfaces these different patterns of basement reactivation may reflect spatially varying strength depth profiles in continental lithosphere that are themselves inherited from spatially distinct geological histories even when not themselves reactivating basement faults can control deformation in the overlying sedimentary cover by offsetting otherwise regionally extensive detachment horizons the 3d form of thrust systems can be strongly compartmentalized by pre existing cross faults such as the oblique lineaments of the apennines on a large scale the distribution of pre existing faults and other weaknesses may affect the propensity for orogenic contraction in basement and therefore directly control larger scale tectonic processes in the central mediterranean the evolution of slab roll back and the related growth of overlying extensional basins e g tyrrhenian sea may be strongly modulated by the distribution of rift related weak zones in the adjacent continental crust the subduction of continental crust will strongly depend on the inherited structure of this crust specifically the distribution of deep crust of basic composition this develops relatively higher densities associated with eclogite metamorphism which act in turn to reduce the buoyancy of thickened continental crust that otherwise serves to inhibit further shortening investigating all these aspects from the scale of bulk crustal compositions to the geometry timing and strength of earlier fault zones preserved in orogenic belts requires the integration of substantial multidisciplinary geological data sets the extent to which continental orogenic belts represent the amplification of inherited geological heterogeneities as opposed to self ordered phenomena modulated by the syntectonic environment remains unclear

implications for proterozoic plate margin evolution from geophysical analysis and crustal scale modeling within the western gawler craton australia abstract complexly deformed late archean to mesoproterozoic rocks of the western gawler craton are poorly exposed thus reducing the ability to extract meaningful geological structural and tectonic information this study focuses on shear zone analysis via a constrained integration of qualitative aeromagnetic and bouguer gravity data interpretation with quantitative two dimensional forward modeling this integration provides a means to interpret the crustal architecture as well as constrain a sequence of events which aid the interpretation of a tectonic history for the western gawler craton results indicate a polyphase shearing history dominated by a crustal scale array of predominantly west dipping shear zones the initiation of east west trending sz1 structures was likely coincident with ca 1750 1720 ma crustal extension high heat flow and deposition on thinned areas of archean crust with the development of a dense possibly underplated lower crustal component sz1 r2 dextral transpressional movements reactivated sz1 during the ca 1720 1670 ma kimban orogeny which is interpreted to record collision between the gawler craton and the north australian craton during this event basins marginal to and in the interior of the archean crustal block of the western gawler craton were inverted and the dense lower crust was vertically offset causing some of the long wavelength 30 50 km bouguer gravity anomalies presently observed shear zone activity is constrained until ca 1680 ma during which time subduction rollback established a new arc position at the southern margin of the archean crust with magmatism persisting until ca 1670 ma sz3 structures overprint and offset the interpreted axis of the kimban orogeny within the archean crust these structures record a major phase of crustal scale sinistral strike slip movement associated with north south shortening between ca 1630 and 1540 ma prior to their reactivation at ca 1450 ma this sz3 r4 reactivation is interpreted to cause southwest directed crustal transport which was focused as west side up movements internal to the archean continent whereas dextral strike slip was focused at a major rheological boundary between thick archean crust to the south and thick paleoproterozoic basins to the north

locating a major proterozoic crustal boundary beneath the eastern officer basin australia abstract the archaean to mesoproterozoic basement of northern south australia is almost completely overlain by thick neoproterozoic and younger basins 1 outcrop yet is likely to preserve an important record of the interactions between the archean proterozoic gawler craton and the proterozoic musgrave province during the amalgamation of australia in the proterozoic however constraints on the location and geometry of the boundary between these provinces are poor we use potential field data to determine the 3d basement architecture and so constrain where this palaeo mesoproterozoic boundary may be located beneath the eastern officer basin we establish the geometry and properties of the overlying basins and explicitly include them during forward and inverse modelling of potential field data to highlight the structure of the underlying basement our analysis identifies three crustal domains 1 southeast of the steep northeast southwest middle bore fault positive gravity and magnetic anomalies are co located and sourced from bodies in the upper crust these bodies overlie middle to lower crust that is apparently uniform 2 between the middle bore fault and the southern edge of the munyarai trough the highest amplitude gravity and magnetic anomalies are not co located and are sourced from large northwest dipping bodies 3 to the northwest the crust underlying the munyarai trough has similar properties to the musgrave province suggesting that the musgrave province extends at least 50 km beneath the eastern officer basin although details of the geology in the second central domain are poorly constrained the domain preserves large crustal scale precambrian structures and is interpreted to mark the boundary between the gawler craton and the musgrave province in particular the multiply reactivated middle bore fault forms a major crustal boundary and is interpreted to mark the northern limit of the gawler craton the middle bore fault may have formed as early as the kimban orogeny 1 7 ga although substantial reactivation and modification of the crustal architecture could have occurred between the kimban orogeny and intrusion of the cross cutting gairdner dolerite dykes 827 ma

orogen perpendicular structures in the central tasmanides and implications for the paleozoic tectonic evolution of eastern australia the curvilinear e w structures of the southern thomson orogen are approximately orthogonal to the general n s structural trend of the tasmanides of eastern australia the origin of these orogen perpendicular structures and their implications to tectonic reconstructions of eastern gondwana are not fully understood here we use geophysical data to unravel the geometry kinematics and possible timing of major structures along the boundary between the thomson orogen and the southern tasmanides delamerian and lachlan orogens aeromagnetic data from the southern thomson orogen show wnw e w and or ene trending structural grains corresponding to relatively long wavelength linear geophysical anomalies kinematic analyses indicate strike slip and transpressional deformation along these geophysically defined faults structural relationships indicate that faulting took place during the benambran late ordovician to middle silurian and tabberabberan late early to middle devonian orogenies however some of the described crustal scale structures may have developed in the cambrian during the delamerian orogeny interpretation of deep seismic data shows that the crust of the southern thomson orogen is substantially thicker than the lachlan orogen crust which is separated from the thomson orogen by the north dipping olepoloko fault a major lithospheric scale change across this boundary is also indicated by a contrast in seismic velocities together with evidence for the occurrence of delamerian deformation in both the koonenberry belt and northeastern thomson orogen and a significant contrast in the width of the northern tasmanides versus the southern tasmanides it appears that the southern thomson orogen may represent the locus of orogen perpendicular segmentation which may have occurred in response to along strike plate boundary variations

the role of the east asian active margin in widespread extensional and strike slip deformation in east asia east asia is a region of widespread deformation dominated by normal and strike slip faults deformation has been interpreted to result from extrusion tectonics related to the india eurasia collision which started in the early eocene in east and se china however deformation started earlier than the collision latest cretaceous to palaeocene suggesting that extrusion tectonics is not the only driving mechanism for east asia deformation it is suggested that the east asian active margin has influenced deformation in east asia significantly along the margin cenozoic back arc extension took place behind several adjoining arcs implying eastward rollback of the subducting slab and collapse of the overriding plate towards the retreating hinge line we show that extension took place along a c 7400 km long stretch of the east asian margin during most of the cenozoic physical models are presented simulating overriding plate collapse and back arc extension the models reproduce important aspects of the strain field in east asia for geometrical and rheological conditions scaled to represent east asia modelling shows that the active margin can be held responsible for deformation in east asia as far west as the baikal rift zone located c 3300 km from the margin

tectonic framework and evolution of the gawler craton southern australia the gawler craton is the major crustal province in the southern australian proterozoic and is pivotal in models seeking to describe the evolution of proterozoic australia the craton is host to the olympic dam iron oxide cu au u ree deposit as well as a number of other iron oxide copper gold iocg au and iron ore deposits the evolution of the gawler craton is dominated by two major phases of tectonic activity late archean and late paleoproterozoic early mesoproterozoic in total spanning ca 1 billion years the late archean 2560 2500 ma basin development was coeval with arclike felsic magmatism and mafic ultramafic magmas including komatiites collisional deformation between 2480 and 2420 ma led to ca 400 m y of tectonic quiescence conceivably within an early paleoproterozoic continental interior the second major phase of tectonic activity was in the middle to late paleoproterozoic and early mesoproterozoic 2000 1450 ma paleoproterozoic tectonism was initially dominated by rift related events which produced a series of basins over the interval ca 1900 to 1730 ma some of which contain significant iron ore reserves transient contractional deformation at ca 1850 ma led to batholith scale granitic magmatism in part derived from melting of the late archean basement major basin development was finally terminated by the 1730 to 1690 ma kimban orogeny the effects of which are widespread and include the formation of crustal scale shear systems granitic magmatism and low to high grade metamorphism the kimban orogeny was followed by a renewed period of extension between ca 1680 and 1640 ma leading to local magmatism and sedimentation which appears in part to have been coeval with the high grade ca 1650 ma ooldean event in the western gawler craton at ca 1620 to 1615 ma the generation of the arc related st peter suite in the southern gawler craton implies that craton components were located at an active plate margin the arc environment was superseded by a transition to a continental interior regime which coincided with the voluminous gawler range volcanics 1595 1590 ma and hiltaba suite granitoids 1595 1575 ma the hiltaba suite is dominated by high t fractionated felsic rocks with coeval mafic magmatism confirming a mantle involvement emplacement of the hiltaba suite coincided with the major mineralizing interval in the gawler craton regionally two distinct mineral systems have been recognized the olympic iocg province in the eastern part of the craton and the gold dominated systems within the central gawler gold province the spatial distribution of iocg versus au dominated mineral systems appears to reflect regional variations in crustal composition and hiltaba suite petrogeneses hiltaba aged granites in the olympic iocg province are isotopically more evolved richer in u and th and oxidized compared to similar aged granites in the au dominated province modern day heat flow in the iocg province is significantly higher 90 10 mwm 2 compared to the au dominated province 54 5 mwm 2 suggesting there are important lithospheric compositional differences between the two metallogenic provinces which may reflect an older phase of craton assembly widespread northwest southeast contractional deformation coeval with the emplacement of the hiltaba suite is expressed by the formation and or reactivation of shear zones ranging up to crustal scale within this regime northwest trending structures such as those in the vicinity of the olympic dam deposit are likely to have accommodated dilation associated with strike slip movements intersection between these structures and northeast trending contractional faults may have formed suitable structural traps for 1590 to 1580 ma mineralization the timing of syn hiltaba deformation overlaps with transpression and medium to high grade metamorphism associated with the ca 1570 to 1540 ma kararan orogeny which dominates the geophysical architecture of the central northern part of the craton the youngest phase of deformation in the craton is expressed by reactivation of shear zones between ca 1470 and 1450 ma and regional cooling

age constraints on terrane scale shear zones in the gawler craton southern australia abstract electron probe micro analysis epma chemical dating of monazite from crustal scale shear zones in the late archaean to mesoproterozoic gawler craton southern australia highlights a long history of shear zone development associated with assembly and reworking of the craton in the southeast gawler craton dextral transpression was focussed into the 250 km long kalinjala shear zone which dominates the structural character of that part of the craton monazite in upper amphibolite facies shear fabrics within the kalinjala shear zone give a mean age of 1682 10 ma this is indistinguishable from an age of 1693 16 ma obtained from monazite inclusions within garnet porphyroclasts within the shear fabric in the western gawler craton monazite populations from two separate locations along the 400 km long tallacootra shear zone give ages of 1680 37 and 1679 39 ma respectively suggesting that both the kalinjala and tallacootra shear zone systems record regional scale strain partitioning toward the end of the ca 1730 1700 ma kimban orogeny in the northwest gawler craton the karari fault zone can be traced for 700 km and separates late archaean rocks from an assembly of tectonic terranes that comprise the northern and northwest gawler craton monazites from amphibolite grade mylonites associated with the karari fault zone have a mean age of 1631 12 ma this age is identical to the timing of regional arc like magmatism represented by the st peter suite in the central gawler craton and suggests that exhumation of the ca 1650 1640 ma ultra high grade granulites of the nawa domain in the northwest gawler craton was linked to the development of the st peter suite magmatic margin in the central gawler craton monazite in the 200 km long yerda shear zone which sheared granites belonging to the ca 1580 ma hiltaba suite have a mean age of 1503 16 ma similar ages of 1516 18 and 1508 14 ma are obtained from monazite inclusions in garnet porphyroclasts within mylonites along the 300 km long coorabie fault zone in the western gawler craton younger ages of 1468 12 and 1457 22 ma from the amphibolite grade mylonitic fabrics in the coorabie fault zone are similar to 40ar 39ar ages from the western gawler craton and suggest that reactivation of the coorabie shear zone in part was associated with regional cooling and stabilisation of the gawler craton at ca 1450 ma the history of deformation on the terrane scale shear zones in the gawler craton therefore does not reflect a single period of terrane assembly or reworking rather it points to a complex pattern of strain partitioning during tectonic events in the late palaeoproterozoic to early mesoproterozoic

the historical development of the magnetic method in exploration the magnetic method perhaps the oldest of geophysical exploration techniques blossomed after the advent of airborne surveys in world war ii with improvements in instrumentation navigation and platform compensation it is now possible to map the entire crustal section at a variety of scales from strongly magnetic basement at regional scale to weakly magnetic sedimentary contacts at local scale methods of data filtering display and interpretation have also advanced especially with the availability of low cost high performance personal computers and color raster graphics the magnetic method is the primary exploration tool in the search for minerals in other arenas the magnetic method has evolved from its sole use for mapping basement structure to include a wide range of new applications such as locating intrasedimentary faults defining subtle lithologic contacts mapping salt domes in weakly magnetic sediments and better defining targets through 3d inversion these new applications have increased the method s utility in all realms of exploration in the search for minerals oil and gas geothermal resources and groundwater and for a variety of other purposes such as natural hazards assessment mapping impact structures and engineering and environmental studies

delineation of a large ultramafic massif embedded within a major sw pacific suture using gravity methods abstract gravity studies have delineated the largest ultramafic massif in new zealand embedded within a buried major sw pacific crustal suture zone this suture records terrane collision onto the gondwana margin during the mesozoic and separates a forearc terrane from an outboard accretionary prism terrane it can be traced throughout the length of new zealand as the junction magnetic anomaly and contains the permian dun mountain ophiolite belt which in the south island of new zealand is characterized by a string of isolated ultramafic massifs in a sheared matrix of serpentinite and sediment our analysis reveals a steep gravity gradient at the suture boundary which is attributed to a newly recognised density contrast 0 1 mg m 3 between terranes of the forearc and the accretionary prism the massif itself is marked by the occurrence of a strong elongate residual gravity anomaly 120 g u extending 50 km along the suture and coincident with the junction magnetic anomaly it is modelled at its southern end as a dense 15 km wide source body extending to at least 6 km in depth in conjunction with detailed aeromagnetic data this modeling indicates the presence of a spindle shaped ultramafic massif analogous to but larger than similar bodies found within the dun mountain ophiolite belt elsewhere this fabric of sheared serpentinites enclosing ultramafic massifs therefore extends at least the length of new zealand and probably beyond in part it may result from accretion of asperities in the subducting plate but it is also due to disruption of larger ultramafic bodies during subsequent strike slip motion which caused the remarkable linearity of the dun mountain belt given the common occurrence of the plate tectonic processes involved it is likely that such structures can be found in other regions around the world using similar geophysical potential field methods