the ivrea zone and lower crustal magnetization abstract the ivrea zone in northern italy is an arcuate belt of rocks whose geological and geophysical properties indicate that they were once deep in the crust it is one of several presumed lower crustal cross sections whose magnetic properties have been well studied in a systematic way the others are the lofoten vesteralen terrane in norway the pikwitonei cross lake subprovince in manitoba the kapuskasing uplift in ontario and to a lesser extent the lewisian terrane in scotland rocks from the ivrea zone have highly variable magnetic properties consistently strong magnetization is associated with serpentinized ultramafics and the main metagabbro sequence many amphibolites and a few metapelites are also strongly magnetic natural remanent magnetization generally with a strong present field overprint has intensities comparable to induced magnetization in the more highly magnetic rocks although the strongly magnetic units have magnetizations high enough about 4 a m to account for the magnetic anomaly associated with the zone they would be required to make up a considerably larger proportion of the subsurface body than they represent in the surface outcrop a common observation in the other crustal cross sections as well like the other cross sections magnetization in the ivrea zone is carried by magnetite with some pyrrhotite in less magnetic metapelitic samples the ivrea zone contains more ultramafic rocks than most of the other sections but shows no direct correlation between magnetization and composition degree of serpentinization has a far greater influence on the magnetization of the ultramafic rocks

collisional snowbird tectonic zone resurrected growth of laurentia during the 1 9 ga accretionary phase of the hudsonian orogeny the 2800 km long snowbird tectonic zone is one of the most controversial tectonic features of the canadian shield metamorphic and in situ geochronologic data reported here reveal that a 1 9 ga medium to high pressure belt extends along most of this tectonic zone in contrast to recent interpretations a collisional origin is indicated by the length of this metamorphic belt tectonic thickening documented in parts of it geological contrasts across it and subduction type microdiamonds within it this collisional event marks a pre 1 865 ga phase of the hudsonian orogeny involving microcontinent accretion that was fundamental to the growth of laurentia

is taiwan the result of arc continent or arc arc collision abstract conventionally it has been accepted that the formation of taiwan results from the collision of the luzon arc with the eurasian continental margin we suggest that taiwan results from the collision of the luzon arc with the former ryukyu subduction zone before the collision the latter extended a few hundred kilometres southwest of its present day termination in the early to middle miocene the subduction became inactive in its southwest portion and the manila trench migrated northeastward giving rise to the formation of the luzon arc the collision started in the late miocene by closing the oceanic domain located between the luzon arc and the former ryukyu arc because the luzon arc moved northwestward with respect to the former ne sw trending ryukyu arc the oblique collision first resulted in the indentation on the south ryukyu arc west of 123 5 e and the narrowing of the corresponding portion of the okinawa trough prior to the uplift of taiwan today the hsuehshan and backbone ranges represent the uplifted portion of the backarc basin and the longitudinal valley corresponds to the suture zone between the former ryukyu and the luzon arcs in this model we propose to link although they are not of the same age two truncated backarc basins the active okinawa trough to the northeast and the former active tainan backarc basin to the southwest the former backarc basin extended from southwest of taiwan to southern japan and could have been initiated near the paleo location of taiwan as early as the eocene oligocene the result of the collision is the truncation of the ancient backarc basin associated with the shortening deformation and uplift of the ancient outer arc and related backarc region which provides the fundamental mechanism of the taiwan mountain building this model is in agreement with a counterclockwise rotation of the portion of taiwan located west of the coastal range belonging to the luzon arc and a clockwise rotation of the south ryukyu arc earthquake focal mechanisms geodetic observations and the distribution of the petrologic and structural features are also consistent with the proposed arc arc collision model

new insights on 3 d plates interaction near taiwan from tomography and tectonic implications abstract recent tomographic results are used to trace the south china sea and philippine sea subducting slabs south and northeast of taiwan along the manila and ryukyu trenches respectively in particular the 3 d plates interaction beneath taiwan is discussed based on a close up view of the tomographic sections together with earthquake hypocenters distribution our study indicates that 1 the east dipping south china sea slab can be followed to the north up to the latitude of hualien 2 the eurasian plate subducts beneath most part of the taiwan island down to the 670 km depth discontinuity 3 the north dipping philippine sea slab can be followed slightly west of the longitude of hualien both plates thus interact beneath northern taiwan where the arc continent collision is paroxysmal 4 slab detachment is envisaged at the northern edge of the subducted eurasian plate beneath the coastal range of taiwan which may facilitate the northwestward motion of the philippine sea plate with respect to eurasia slabs geometries obtained from tomographic sections allow us to reconstruct the late neogene plate kinematics and dynamics in this region our main conclusions are 1 the size of the original south china sea was about twice its present size 2 the subducted part of the west philippine basin i e the largest oceanic basin of the philippine sea plate extends only 400 km north of the ryukyu trench 3 slab detachment might have occurred 3 5 my ago beneath the central and northern ryukyu arc along a weak zone that is aligned with the gagua ridge an ancient plate boundary 4 the ryukyu trench has propagated westward from 126 e of longitude southeast of miyako island to the locus of the present arc continent collision along a major lithospheric tear that cut through the continent ocean boundary first and then through the continental lithosphere as a consequence the southern ryukyu margin developed progressively from east to west as a subduction zone during the last 8 my it has been elaborated onto a passive margin which corresponded to the edge of the eurasian continental shelf bordering the south china sea prior to pliocene 5 the southern okinawa trough rifted during the last 2 1 ma following the propagation of the subduction zone 6 subduction of the eurasian margin probably stopped recently beneath northern taiwan and a flip of subduction from west to east is being initiated near hualien

active continental subduction and crustal exhumation the taiwan orogeny a tectonic model of active continental subduction followed by crustal exhumation is proposed to explain the orogeny in taiwan the subducted crust is represented by a low velocity zone dipping eastwards beneath the major part of taiwan while the exhumed crust is marked by a high velocity bulge high heat flow and absence of seismicity beneath the eastern central range the boundary between the subducted and exhumed crust has been identified from surface geology and analyses of thermal history across the central range the dynamic force that has been driving the exhumed crust is identified by results from focal mechanisms structural geology and geodetic survey in the eastern central range such a tectonic model may provide a good explanation for the evolution of the taiwan orogeny as well as an active case for studying other long extinct systems of continental subduction and exhumation

continental transform tectonics great slave lake shear zone ca 1 9 ga northwest canada great slave lake shear zone gslsz is a northeast trending dextral mylonite zone across which the coeval north trending thelon and taltson magmatic zones on the west margin of the churchill province are offset 300 700 km it is proposed that the gslsz is a continental transform structure related to oblique collisional indentation of the active margin of the churchill province by the microcontinental slave province this accounts for geologic and geophysical differences between the thelon and taltson zones and explains the localization of a tibetan type queen maud uplift north of the gslsz and the abrupt loss of slip on the gslsz east of the uplift it also provides a new model for the athapuscow aulacogen analogous to the modern bay of bengal indo burman ranges sagaing transform andaman sea situation

velocity structure of the saint elias alaska region from local earthquake tomography abstract we use over 8000 regional and local earthquakes and 150 000 seismic arrivals recorded by a dense broadband seismic network in the st elias region southern alaska and nearby regional stations to compute the 3d velocity structure down to a depth of 80 km the tectonics of this region is dominated by a collision between the subducting yakutat microplate and the north american continent this model reveals slower velocities along the northern edge of the yakutat microplate in the st elias coastal range that can be traced as deep as 30 km this velocity anomaly contrasts with the offshore area in the south and the region north of the chugach st elias and bagley faults and corresponds to tectonically off scraped and deformed rocks of the yakutat sedimentary cover the chugach st elias fault serves as a back stop for the deformed yakutat sediments rather than the bagley fault and represents a prominent structural boundary east of the pamplona fault zone the northward dipping yakutat basement is imaged at depth and extends toward the wrangell volcanoes the region between the bagley fault and the wrangell volcanic field is characterized by higher crustal velocities whereas the wrangell volcanic field is characterized by slower velocities both near the surface and at depth a strong low velocity anomaly corresponding to the copper river sedimentary basin can be traced down to 20 km depth

quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation abstract a new method of analysing isothermal remanent magnetisation irm acquisition curves based on cumulative log gaussian analysis robertson and france phys earth planet inter 82 1994 223 234 is proposed it is based on the curve fitting of the irm acquisition curve versus the logarithm of the applied field with i the acquisition curve on a linear scale ii the acquisition curve expressed as a gradient and iii the acquisition curve on a probability scale even when a sample is not saturated its magnetic properties can be defined although with less certainty the number of magnetic components required for an optimal fit to a measured irm acquisition curve is evaluated statistically the method discriminates on the basis of different mineral coercivity therefore additional rock magnetic tests are still required to separate minerals with similar coercivities

location structure and seismicity of the seattle fault zone washington evidence from aeromagnetic anomalies geologic mapping and seismic reflection data a high resolution aeromagnetic survey of the puget lowland shows details of the seattle fault zone an active but largely concealed east trending zone of reverse faulting at the southern margin of the seattle basin three elongate east trending magnetic anomalies are associated with north dipping tertiary strata exposed in the hanging wall the magnetic anomalies indicate where these strata continue beneath glacial deposits the northernmost anomaly a narrow elongate magnetic high precisely correlates with magnetic miocene volcanic conglomerate the middle anomaly a broad magnetic low correlates with thick nonmagnetic eocene and oligocene marine and fluvial strata the southern anomaly a broad complex magnetic high correlates with eocene volcanic and sedimentary rocks this tripartite package of anomalies is especially clear over bainbridge island west of seattle and over the region east of lake washington although attenuated in the intervening region the pattern can be correlated with the mapped strike of beds following a northwest striking anticline beneath seattle the aeromagnetic and geologic data define three main strands of the seattle fault zone identified in marine seismic reflection profiles to be subparallel to mapped bedrock trends over a distance of 50 km the locus of faulting coincides with a diffuse zone of shallow crustal seismicity and the region of uplift produced by the m 7 seattle earthquake of a d 900 930

slab interactions in the taiwan region based on the p and s velocity distributions in the upper mantle abstract we present a new model of p and s velocity anomalies in the upper mantle beneath the taiwan region based on the inversion of travel time data from the global international seismological center isc catalog we clearly observed the anomalies of high p and s velocities associated with the subducting plates beneath the ryukyu and luzon arcs at depths in the range of 100 200 km the anomalies related to both slabs seem to be connected which might be evidence of the lithosphere collision of the two oppositely oriented subduction zones this model has been carefully verified using different synthetic tests based on the derived seismic model we propose a model of recent plate reconstructions in the region around taiwan initially we presume the existence of two oppositely oriented subduction zones underneath the luzon island and the ryukyu arc separated with a transform fault the nw movement of the philippine sea plate led to first a shortening and then a disappearance of this transform fault as a result the edge of the luzon arc collided with the edge of the ryukyu arc simple simulations indicate that in this edge area very strong stresses and deformation might take place that result in significant shortening of the earth surface we believe that the origin of the taiwan island was caused by the collisional processes due to the interaction of the two subduction zones note that in this case no other conditions such as the existence of arcs and or buoyant continental crustal blocks are required to explain the origin of the thick strongly shortened crust in taiwan

geochronological constraints on the legs lake shear zone with implications for regional exhumation of lower continental crust western churchill province canadian shield the legs lake shear zone marks the southeastern boundary of an extensive region 20 000 km2 of high pressure 0 8 1 5 gpa granulite facies rocks in the western churchill province canada the shear zone is one of the largest exhumation related structures in the canadian shield and coincides with the central segment of the 2 800 km long snowbird tectonic zone the movement history of this shear zone is critical for the development of models for the exhumation history of the high pressure region we used electron microprobe u th pb dating of monazite with supplemental id tims u pb geochronology to place constraints on the timing of shear zone activity combining these and other data we suggest that regional exhumation occurred during at least three distinct phases over an 150 million year period the first phase involved high temperature decompression from 1 0 to 0 8 0 7 gpa shortly following 1 9 ga peak metamorphism possibly under an extensional regime the second phase involved rock uplift and decompression of the hanging wall to 0 5 0 4 gpa during east vergent thrusting across the legs lake shear zone at ca 1 85 ga this phase was likely driven by early collision related convergence in the trans hudson orogen the final phase of regional exhumation involving the removal of 15 20 km of overburden from both footwall and hanging wall likely occurred after 1 78 ga and may have been related to regional extensional faulting

the cora lake shear zone athabasca granulite terrane an intraplate response to far field orogenic processes during the amalgamation of laurentia the cora lake shear zone clsz is a 4 6 km wide localized high strain zone that bisects the polydeformed athabasca granulite terrane northern saskatchewan it also coincides with the geophysical trace of the snowbird tectonic zone the clsz represents a major lithotectonic and thermobarometric discontinuity within the exposure of 20 000 km2 of high pressure granulites most rocks have a strong mineral lineation plunging moderately to the southwest the northwestern subdomain hangingwall is characterized by ca 2 6 ga plutonic rocks that contain an early subhorizontal gneissic layering ca 2 57 ga that was overprinted by large amplitude folds and a partitioned but pervasive axial planar dextral shear fabric at ca 1 9 ga thermobarometry suggests metamorphic conditions of 0 9 gpa and 750 c during both of the phases of tectonism the footwall is predominantly underlain by the ca 3 3 3 0 ga chipman tonalite layers of intercalated mafic and felsic granulite and the widespread 1 9 ga chipman

three dimensional vp and vs structural models associated with the active subduction and collision tectonics in the taiwan region summary 3 d vp and vs models for the crust and upper mantle beneath the taiwan area have been determined using selected high resolution earthquake data from an island wide seismic network and two local seismic arrays lateral structural variations in the upper crust as also evident from surface geology are responsible for the observed large traveltime residuals or station corrections prior shallow velocity information inferred from traveltime residuals and joint hypocentral determination jhd station corrections for the uppermost crust is essential to facilitate a reliable tomographic inversion a finite difference method that is efficient and accurate for a highly heterogeneous velocity structure is applied to calculate p and s wave traveltimes from the source to receiving stations all earthquakes in the taiwan central weather bureau s catalogue are then relocated using the resultant 3 d vp and vs models the depth of the moho varies significantly especially along the east west direction in the western coastal plain and western foothills the depth of the moho is around 35 km which deepens gradually eastward reaches a maximum depth of 55 km beneath the eastern central mountain range shallows up rapidly beneath the longitudinal valley and coastal range and merges with the thin philippine sea plate offshore of eastern taiwan in central taiwan the central mountain range is bounded to the east and west by two steeply westward dipping active faults from the upper crust to a depth of about 30 km therefore the uplifted and thickened central mountain range serves as a backstop for the converging eurasian and philippine sea plates the crust beneath the central mountain range is characterized by a brittle high velocity and seismically active upper crust 15 km and a ductile low velocity and aseismic mid to lower crust below 15 km most probably due to the high geothermal activity from the excess heat supplied from the hot upper mantle beneath the thin oceanic crust to the east from the surrounding hotter upper mantle beneath the thickened continental crust and from shear heating during active collision the collision zone in eastern taiwan is characterized by an active and steeply eastward dipping seismic zone along a region of low vp and high vp vs ratio near the taitung region in southeastern taiwan it transforms into an active westward steeply dipping seismic zone along a transition zone between the high vp and vs oceanic crust and the low vp and vs continental crust near hualien region in central eastern taiwan there is no apparent seismicity within many sedimentary basins imaged from the tomographic inversion however af ew basins are either bounded on one side by an active fault or are characterized by blind faults beneath the geometry of the subduction zone in northeastern taiwan can be clearly imaged from the relocated earthquake locations behind the subduction a region of low vp and high vp vs ratio at depths of 5 to 10 km can be identified beneath the tatun chilung volcano group indicating a potential magma reservoir two steeply dipping linear seismic zones in the volcano region may mark the upward escape paths of the magmatic materials in the region

crustal structure and deformation at the northern manila trench between taiwan and luzon islands abstract the philippine sea plate overrides the eurasian plate along the east dipping manila trench between taiwan and luzon islands from south to north the plate convergence gradually evolves from normal subduction of the south china sea lithosphere to initial collision of the taiwan orogen the subduction related earthquakes become diffusive close to taiwan the accretionary prism is dramatically wider toward taiwan to understand the plate convergent features of this subduction collision transition zone we have analyzed twelve seismic reflection profiles across the manila trench between luzon and taiwan the results show that the basement of northern south china sea basin generally dips toward east and south the northern manila trench accumulates more trench fill sediments in the south than in the north a sequence boundary t0 is suggested to distinguish the hemipelagic and trench fill sediments probably due to the collision in taiwan the sequence boundary t0 displays more gentle slope or has been uplifted in the north structural analysis shows that the subducting crust in the northern manila trench area can be characterized by three distinctive zones a normal fault zone nfz a proto thrust zone ptz and a thrust zone tz the nfz is defined by the distribution of numerous normal faults in the top or upper portion of the subducting crust the normal faults are gradually buried by trench fill sediments when they are closer to the deep trench it is suggested that normal fault may take place at the location where the crust starts to bend and induces gravity sliding of the upper sedimentary layers some buried normal faults could be reactivated to blind thrust faults because of stronger plate convergence near the accretionary prism the ptz is located between the nfz and the frontal thrust of the accretionary prism it contains blind thrust faults or folds instead of thrust faults the successive distribution of the crustal structures of normal faults blind thrust and thrust faults at the trench area suggests that the blind thrust faults develop along the location of pre existing normal faults the brittle deformation occurs at the lower part of the sedimentary layers probably because of stronger compaction and less water content eventually blind thrust faults may propagate upward and become thrust faults at the seabed

magnetization of lower crust and interpretation of regional magnetic anomalies example from lofoten and vesteralen norway the interpretation of regional magnetic anomalies commonly observed over continental crust in both satellite and near surface magnetic data has remained enigmatic in its requirement for laterally extensive thick highly magnetic and deep seated sources presented here are the results of a detailed field analytical and experimental study of the magnetic mineralogy and magnetic properties of rocks from lofoten and vesteralen norway a province of deep seated origin with associated regional magnetic and gravity anomalies these rocks have a bulk magnetic character of surprising uniformity with susceptibilities hysteresis and curie temperatures defined solely by coarse grained magnetite stable remanence is observed in samples from sw lofoten but is not of significant magnitude the thermal enhancement of magnetic susceptibility the hopkinson effect is not important rocks from the deepest part of the section have the largest average susceptibilities 6 10 2 si and magnitudes of natural remanent magnetization nrm 5 10 4 a m2 kg these values of susceptibility together with the oft observed viscous character of the nrm suggest that these granulite facies lithologies particularly those of sw lofoten are perfect examples of lower crustal sources of typical regional magnetic anomalies furthermore these intermediate pressure granulites define a unique magnetic petrology especially for mafic compositions at higher grades of metamorphism high pressure granulites and eclogites or lower grades greenschist and amphibolite facies such magnetite bearing assemblages will be unstable and their high or low grade equivalents will be only weakly magnetic this evidence and these conclusions may be used to constrain models of lower crustal sources of regional magnetic anomalies over stable cratons it portrays the lower crust as a highly magnetic unit defined by metamorphic grade with a curie isotherm of 575 c magnetite and magnetization parallel to the ambient geomagnetic field induced and viscous components

the moho as a magnetic boundary revisited we have re examined the question of whether it is generally true that in continental regions a relatively magnetic crust overlies a relatively non magnetic mantle whereas the original conclusion was based on limited data the present study is based on about 400 globally distributed xenolith samples while it is now clear that the moho represents a maximum velocity gradient in a complex transition from crust to mantle and while crustal magnetization is highly variable the original concept of a magnetic crust overlying a non magnetic mantle is strongly supported by the new data a large upper mantle xenolith suite of ultramafic composition is characterized by non magnetic chrome spinels and magnesian ilmenites such rocks are magnetic only where demonstrably altered the most significantly magnetic rocks within the crust are mafic types prograded to granulite grade or mafic melts of basaltic composition which crystallized deep in the crust at high pressure and temperature prograded rocks have characteristic magnetite curie points and are strongly magnetic especially the more mafic compositions while the mafic rocks formed by crystallization at depth have curie points between 400 570 c and are strongly magnetic induced magnetizations are commonly several a m and can readily account for long wavelength magnetic anomalies measured by satellite and aircraft thus there is no need to search for exotic sources for missing crustal magnetization no metal or other peculiar magnetic species have been observed by us in any of our unaltered samples mineral assemblages including magnetic components are consistent with oxidizing conditions close to the fmq buffer

paleomagnetism and structural history of the ghost range intrusive complex central abitibi belt ontario further evidence for the late archean geomagnetic field of north america on the basis of their remanence properties the lithologies of the late archean 2710 2703 ma ghost range complex an east west trending layered mafic ultramafic extrusive sequence in the central

taiwan orogeny thin skinned or lithospheric collision abstract the taiwan orogeny is young and presently very active it provides an excellent environment for studying ongoing orogenic processes especially since the region is monitored intensively with dense seismological and geodetic networks and new studies aiming at deciphering shallow and deep structures in and around taiwan have been recently conducted or are being planned the available data can be used continually to test critically hypotheses of the taiwan orogeny hypotheses dealing with the mechanics of mountain building are basic to the understanding of taiwan orogeny and are particularly amenable to testing the widely cited thin skinned tectonics hypothesis was formulated to explain mainly the geologic and relatively shallow

contractional uplift of deep crustal rocks along the legs lake shear zone western churchill province canadian shield the legs lake shear zone juxtaposes high pressure 1 0 gpa granulite and eclogite facies rocks with low pressure 0 5 gpa amphibolite to granulite facies rocks and thus may represent an important crustal scale exhumation related structure in the western canadian shield field mapping and structural and petrologic analysis document the deformation and metamorphic history of rocks within and adjacent to the shear zone at least two important phases of deformation are recorded 1 early oblique thrusting d2 that placed high grade rocks over lower grade rocks and 2 more discrete and lower grade brittle ductile normal faulting d3 that may represent the later part of the exhumation history the northwest dipping shear zone consists of 5 8 km of mylonite in map view which bounds the southeast margin of the east athabasca mylonite triangle high pressure granulite facies metamorphism 750 850 c 1 0 1 2 gpa occurred in the east athabasca mylonite triangle at ca 1900 ma prior to d2 juxtaposit

petrophysical constraints on magnetic anomalies associated with metamorphic reactions in northern saskatchewan canada1 the western chipman domain of the east athabasca mylonite triangle in northern saskatchewan canada displays a large positive aeromagnetic anomaly that is the result of retrograde magnetite produc

reconstruction of a large deep crustal terrane implications for the snowbird tectonic zone and early growth of laurentia the 2800 km long snowbird tectonic zone is a well recognized but still enigmatic feature in the western canadian shield it has been interpreted as a paleoproterozoic continental suture or an archean strike slip fault system but here we suggest that the distinctive geometry of the central snowbird tectonic zone is primarily due to the interaction of crosscutting paleoproterozoic intracontinental thrust and strike slip shear zones having a length of hundreds of kilometers first a major zone of thrust sense shearing coeval with early continent continent collision between the superior and western churchill provinces accommodated uplift of a large exposure of granulite facies lower continental crust younger strike slip shear zones perhaps analogous to asian fault systems behind the himalayan orogen offset the thrust zone thus the current geometry and distribution of deep crustal rocks in this region represent a relatively late stage in the tectonic evolution of the western churchill province rather than an accretionary one earlier structures oriented at a high angle to the snowbird tectonic zone may record the fundamental accretionary history in this part of laurentia

mountain building in taiwan insights from 3 d geodynamic models taiwan is widely considered to be a typical example of an arc continent collision surrounded by two opposite dipping subduction zones the manner by which the interaction of the two neighboring slabs caused plate collision and mountain building is insufficiently understood various hypotheses have been proposed but the geodynamic feasibility of those remains to be tested here we present 3 d thermomechanical models to study the geodynamic evolution process of a taiwan like setting after an initial transform fault was consumed in our model setup the boundary between the eurasian plate and the south china sea is northeast trending the results show that all simulations result in toroidal mantle flow around the slab edges and that slab breakoff as well as a small scale mountain belt with high topography and crustal exhumation occurs in most cases the eurasian continental crust is exhumed in a dome like manner exposing higher grade metamorphic rocks facilitated by high erosion rates and a weak continental lower crust rheology but inhibited by the presence of a weak arc a high topography within the orogen as well as continental slab detachment can develop for the convergence direction of n307 degrees and large convergence rates our modeling results are thus generally consistent with the eurasian slab tearing model proposed for taiwan based on seismic tomographic studies and we suggest that the main characteristic features in taiwan can be explained by the combined effects of fast erosion a weak lower crust fast convergence and a small convergence azimuth

multistage exhumation and juxtaposition of lower continental crust in the western canadian shield linking high resolution u pb and 40ar 39ar thermochronometry with pressure temperature deformation paths 1 new u pb titanite apatite rutile and 40ar 39ar hornblende muscovite biotite data are linked with existing pressure temperature deformation paths to impose detailed temporal constraints on the juxtaposition and unroofing of domains in an extensive 20 000 km2 region of exhumed lower crust in the east lake athabasca region of the western canadian shield in the 200 m y between circa 1 9 ga high pressure granulite facies metamorphism and circa 1 7 ga unconformable deposition of athabasca basin sediments on the exhumed rocks our analysis reveals at least three distinct phases of unroofing at different rates specifically we distinguish 1 an early phase of extensional unroofing in the chipman domain at rates of 1 5 2 0 km m y from 1 0 gpa to 0 8 gpa associated with mafic magmatism and metamorphism 2 an episode of regional contractional uplift and associated unroofing along the legs lake shear zone at circa 1850 ma from 0 7 0 8 gpa to 0 4 0 5 gpa and 3 a final period of extensional unroofing at rates of 0 2 0 3 km m y from 0 4 to 0 5 gpa that culminated in transport of current exposures to near surface conditions the cooling patterns and retrograde assemblages are consistent with pauses between exhumational phases the apparent convergence of disparate higher temperature histories in several deep crustal domains at circa 1 89 1 88 ga implies their juxtaposition at 0 7 0 8 gpa conditions regional east directed thrusting of the deep crustal domains as a coherent unit at 1 85 ga juxtaposed the granulites with middle crustal hearne domain rocks at 0 4 0 5 gpa conditions the detailed exhumational history allows correlation with changing regional tectonic regimes associated with the amalgamation of laurentia the temporal and spatial heterogeneity of exhumation patterns in the east lake athabasca region may be a common feature of the unroofing histories of lower crustal rocks

new results from physical modelling of arc continent collision in taiwan evolutionary model abstract both the analysis of the geological data and the results from 2d and 3d experimental modelling of arc continent collision in taiwan reveal the following major stages in this process 1 failure and thickening of the subducting crust of the chinese continental margin under the frontal part accretionary prism of the philippine sea plate psp since ca 6 7 ma 2 uplift squeezing and exhumation of the subducted crustal slice of the chinese margin and initiation of the central range about 5 ma ago 3 flexural buckling subsidence of the luzon arc fore arc under strong horizontal compression and sedimentary infilling of this area by the material shed from the growing and exhuming central range 4 failure of the psp along the west vergent longitudinal valley fault dipping beneath the arc and closure of the fore arc 5 subduction reversal and stoppage of continental subduction which has already occurred in northern taiwan and propagates to the south the southward propagation of the collision is not stationary currently occurring initial collision in southern taiwan and offshore to the south does not follow the same described above evolution as in central and northern taiwan the evolution of the collision in time and space should be considered in the framework of a 3d model where the psp rupture is preparing in the central and northern taiwan during a long time passing through a number of evolutionary stages once it has occurred the rupture longitudinal valley fault rapidly propagates to the south into the domain which did not undergo the pre rupture evolution the rupture front is currently located near 21 30 n

horizontal gravity gradient an aid to the definition of crustal structure in north america a map of the magnitude of the horizontal bouguer gravity gradient over the north american continent is used to delineate lateral discontinuities in upper crustal density and or thickness associated with such processes as suturing and rifting the usefulness of gradient trends in mapping major structural boundaries which are sometimes poorly exposed or completely buried is demonstrated by examples such as the buried southward extension of the grenville front and buried boundaries of the superior province gradient trends also draw attention to poorly known structures which may have major tectonic significance and to a continent wide structural fabric which may provide a record of the tectonic growth of the north american continent

velocity field of gps stations in the taiwan area abstract the 131 stations of the taiwan gps network were surveyed 4 6 times from 1990 to 1995 with dual frequency geodetic receivers the standard deviation of an observed baseline length with its linear trend removed is in the range of 6 10 mm for a 3 120 km long baseline the average rates of length change for all baselines of the network and those from nine continuously monitoring permanent stations are used in a least squares adjustment to estimate the velocities of the gps stations relative to paisha penghu situated at the chinese continental margin to the south of fengping in the northern coastal range the velocity vectors of stations in lanhsu lutao and the coastal range trend in the directions of 306 322 with rates of 56 82 mm yr in contrast there is a dramatic decrease in the rates to the north of fengping this may be caused by the motion along the ne sw trending thrusts which obliquely cut the northern coastal range a discontinuity of about 30 mm yr in the rates along with a remarkable change in the directions of station velocity is observed across the longitudinal valley then the moving directions gradually shift to the west for the stations in the western foothills in the kaohsiung pingtung coastal area the station velocities are even directed toward the southwest to the north of the peikang high the velocity vectors of the stations change direction from the west gradually to the north and finally to the east and southeast significant nw se extensional deformation is found in the ilan plain and northern taiwan in general the pattern of the velocity field for gps stations in the taiwan area is quite consistent with the directions of present day tectonic stress

magnetic properties of ultrahigh pressure eclogites controlled by retrograde metamorphism a case study from the zk703 drillhole in donghai eastern china parallel studies on rock magnetic properties petrology and mineralogy were conducted on 16 eclogite samples from the zk703 hole and magnetic susceptibilities and densities of 41 eclogite samples with different degree of retrograde metamorphism from fresh eclogite to fully retrograded eclogite from the chinese continental scientific drilling ccsd near the zk703 hole located at donghai southern sulu ultrahigh pressure metamorphic belt eastern china results show 1 that the high field slopes obtained from the hysteresis loops the paramagnetic fraction para and density have a positive correlation with the volume concentration of garnet omphacite a typical mineral assemblage used to semi quantify the degree of retrograde metamorphism the low field slopes obtained from hysteresis loops the ferrimagnetic susceptibility fraction ferri saturation isothermal remanent magnetization mrs and saturation magnetization ms have a positive correlation with the volume concentration of symplectite a mineral related to retrograded metamorphism therefore they could be potential indicators for quickly semi quantifying the degree of retrograde metamorphism of the eclogite units 2 the dominant magnetic carriers in retrograded eclogites are magnetite particles in pseudo single domain grain size region 3 the p t conditions during the retrograde decompressional process could first increase the concentration of magnetite which can reach up to 3 for extensively retrograded eclogite and then was dissolved for fully retrograded eclogite therefore change in the magnetite contents during the retrograde process is the major factor controlling the magnetism of retrograde eclogites

aeromagnetic interpretation of the kirkland lake larder lake portion of the abitibi greenstone belt ontario high resolution aeromagnetic data from the larder lake kirkland lake region have been analyzed to develop optimum maps for geologic interpretation the use of the apparent susceptibility technique has led to a map that shows clearly the major lithologies in a major synclinorium in the abitibi greenstone belt calc alkaline volcanics of the blake river group are weakly magnetic but show ring like anomalies associated with intrusives where magnetite has been generated in the country rock alternating bands of mg rich and fe rich tholeiites of the kinojevis group show a strong magnetization associated with the fe rich phases gabbroic intrusions underlain by serpentine and peridotite give strong levels of magnetization these various units have been sampled and it is shown that the susceptibility is not high enough to account for the anomalies rather the anomalies are associated with the mean natural remanent magnetization in the formations sampled these studies have led to a revised geological map of this region

origin of the southern okinawa trough volcanism from detailed seismic tomography 1 magmatism associated with subducting plate edges or slab tears has been suggested in the southern okinawa trough the cross back arc volcanic trail which consists of a cluster of about 70 seamounts is located above a ryukyu slab tear lying along the 123 3 e meridian in november 2003 more than 3300 earthquakes recorded in this area by 15 ocean bottom seismometers and surrounding land stations during a period of 12 days were used to determine the three dimensional vp and vs velocity structures and vp vs ratios a mantle inflow characterized by low vp and vs and high vp vs passing through the slab tear is imaged the fluid and or melt component is rising obliquely from the slab tear in the directions of the cross back arc volcanic trail the northern slope of the southern okinawa trough and to north of iriomote island the asthenospheric intake is also imaged by an inclined chip like high vp vs and low vp and vs body dipping northerly which might be linked to the slab retreat west of the slab tear most of the earthquakes are located around low vp and vs and high vp vs bodies which suggests that the seismicity is related to magmatic and or fluid activities east of it earthquakes are concentrated in an area characterized by high vp and vs velocities and low vp vs suggesting that the magma chamber is absent beneath the axial part of the trough and that normal faulting is the main factor controlling the seismicity

philippine sea and east asian plate tectonics since 52 ma constrained by new subducted slab reconstruction methods we reconstructed philippine sea and east asian plate tectonics since 52 ma from 28 slabs mapped in 3 d from global tomography with a subducted area of 25 of present day global oceanic lithosphere slab constraints include subducted parts of existing pacific indian and philippine sea oceans plus wholly subducted proto south china sea and newly discovered east asian sea mapped slabs were unfolded and restored to the earth surface using three methodologies and input to globally consistent plate reconstructions important constraints include the following 1 the ryukyu slab is 1000 km n s too short to account for 20 philippine sea northward motion from paleolatitudes 2 the marianas pacific subduction zone was at its present location 200 km since 48 10 ma based on a 1000 km deep slab wall 3 the 8000 2500 km east asian sea existed between the pacific and indian oceans at 52 ma based on lower mantle flat slabs 4 the caroline back arc basin moved with the pacific based on the overlapping coeval caroline hot spot track these new constraints allow two classes of philippine sea plate models which we compared to paleomagnetic and geologic data our preferred model involves philippine sea nucleation above the manus plume 0 150 e near the pacific east asian sea plate boundary large philippine sea westward motion and post 40 ma maximum 80 clockwise rotation accompanied late eocene oligocene collision with the caroline pacific plate the philippine sea moved northward post 25 ma over the northern east asian sea forming a northern philippine sea arc that collided with the sw japan ryukyu margin in the miocene 20 14 ma

magnetic properties of exposed deep crustal rocks from the superior province of manitoba abstract a traverse across precambrian rocks of the superior province in central manitoba provided 56 samples for magnetic study from the pikwitonei granulite domain and the adjacent cross lake subprovince these rocks represent a cross section of the continental crust presently exposed at the surface thirty one samples are dominantly paramagnetic and three contain pyrrhotite twenty two samples from silicic plutonic rocks which make up about 80 of the section by volume contain nearly pure magnetite and have susceptibilities that are almost constant between room temperature and 540 c averaging about 1 10 2 si units the mafic supracrustal rocks with much higher iron content are about an order of magnitude less magnetic because they contain little or no magnetite susceptibility is generally independent of metamorphic grade or apparent depth in the crust the discrepancy between induced magnetizations of measured lower crustal rocks and the source requirements of long wavelength magnetic anomalies may be due to a high temperature viscous magnetization in the deep crust

contrasts in sillimanite deformation in felsic tectonites from anhydrous granulite and hydrous amphibolite facies shear zones western canadian shield abstract the deformation behavior of crustal materials in variably hydrated metamorphic environments can significantly influence the rheological and seismic properties of continental crust optical observations and electron backscatter diffraction ebsd analyses are used to characterize sillimanite deformation behavior in felsic tectonites from two deformation settings in the athabasca granulite terrane western canadian shield under estimated conditions of 0 8 1 0 gpa 725 850 c in the cora lake shear zone the data suggest that sillimanite deformed by dislocation creep with slip in the 001 direction accompanied by subgrain rotation recrystallization where sillimanite locally remained undeformed strain was concentrated in surrounding weaker phases under hydrated conditions of 0 4 0 6 gpa 550 650 c in the grease river shear zone textures and cathodoluminescence imaging point to dissolution precipitation creep as the major deformation mechanism for sillimanite resulting in synkinematic growth of foliation parallel euhedral sillimanite in a preferred orientation with 001 parallel to the lineation the results suggest that temperature fluid content and modal mineralogy of the surrounding phases may all have significant influence on sillimanite deformation but that preferential alignment of sillimanite 001 parallel to the lineation persists regardless of contrasts in the conditions or mechanisms of deformation

investigating the lithospheric velocity structures beneath the taiwan region by nonlinear joint inversion of local and teleseismic p wave data slab continuity and deflection the interaction between two flipping subduction systems shapes the complicated lithospheric structures and dynamics around the taiwan region whether and in what form the eurasian plate subducts deforms under taiwan island is critical to the debate of tectonic models although an east dipping high velocity anomaly down to a depth below 200 km has been reported previously its detailed morphology remains uncertain and leads to different interpretations with a two step strategy of nonlinear joint inversion the slab images of the eurasian plate were retrieved in a geometry that is hyperthin in the south becoming massive and steeper in the central and severely deformed in the north the possible depth and dimension of a slab break were also investigated through synthetic tests of whether the slab had torn moreover the slab deflection found at 23 2 n latitude seems to correspond to where the nonvolcanic tremors and recent nw se striking structures have occurred in southern taiwan

transition from oblique subduction to collision in the northern luzon arc taiwan region constraints from bathymetry and seismic observations we systematically study the detailed bathymetry seismicity and source parameters of large and moderate sized earthquakes that occurred in the northern luzon arc taiwan region between 1964 and 1996 the purpose is to characterize the transition from oblique subduction to regional collision in terms of the distribution of morphological features the characteristics of seismogenic structures and the corresponding state of strain to the south of 21 5 n the existence of a subduction zone is clearly shown by both bathymetry and seismicity the subducted eurasia slab reaches a maximum depth of 200 km and the depth decreases sharply as it approaches taiwan focal mechanisms indicate that the subducted slab is in downdip extension for depths 150 km but switches to downdip compression at greater depths to the north of 23 n collision is clearly the predominant process with broad deformation on both sides of the suture in between the transition is accommodated by a distributed thrust deformation zone in the frontal portion of the accretionary prism closure of the forearc basin and the right lateral ne sw striking taitung canyon fault zone tcfz these structures resolve the differential movements between the subduction and collision zones and will repeatedly change their positions as the collision propagates southwestward the present day plate boundary follows the axis of the manila trench in the south broadens significantly in the zone of transition and finally becomes a zone of several tens of kilometers in width across eastern taiwan the deformation front on the other hand is located between the western foothills and coastal plain in taiwan then connects to the manila trench to the south seismic strain tensors indicate that the region to the north of tcfz has a compression rate 15 times larger than that to the south the northern luzon arc shows characteristics of oblique subduction but the effects of collision are not as significant within the subducted eurasian lithosphere as comparing to the subducted philippine sea plate beneath the southernmost ryukyu arc

geodynamic effects of subducted seamount at the manila trench insights from numerical modeling abstract we used numerical modeling to investigate the geodynamic effects of subducted seamounts at the manila trench a series of numerical modeling experiments were conducted with variable parameters including the activation volume vact and cohesion c which influence lithospheric rheology the plate convergence velocity and the age of subducting slab modeling results indicate that varying the vact and c within an appropriate range have limited effects on the geodynamic process of subduction a lower vact allows the slab to sink more easily and results in a steeper dip angle a slab break off is more likely to occur under subduction at depths of 100 300 km while the existence of a seamount further promotes the break off process the convergence rate is a key parameter affecting the break off timing and depth in contrast under subduction where subducted oceanic plate move faster upper plate the model results exhibit non break off steady subduction slab age is another factor controlling break off where break off time extends with slab age a subduction without seamount will cause a 2 myr delay in break off timing we suggest that the low velocity zone under the manila trench at 17o n is the result of a break off event due to subduction of the zhenbei huangyan seamount chain further to the north such as the location at 19o n the absence of seamount and an older oceanic crust would favor a delay in break off timing during subduction

seismic tomography of taiwan improved constraints from a dense network of strong motion stations in this study a large collection of 41 141 s p times from the untapped records of the taiwan strong motion instrumentation program tsmip network is combined with the p and s wave arrival times from the taiwan central weather bureau seismic network cwbsn to image the v p and v p v s structures beneath taiwan the records from the 680 tsmip stations throughout taiwan in the past 15 years enhance the path coverage and the resolution in the tomography inversions tremendously our result for the v p structure largely confirms previous studies but brings better constraint on the v p v s structure the colliding luzon volcanic arc is characterized by a belt of high v p and high v p v s with high seismicity that includes the offshore islands of lutao and lanyu and the coastal ranges in eastern taiwan at the depth between about 13 and 25 km this high v p v s belt can be traced to the subduction zone in the region between hualian and ilan in the deeper portion the shallow portions of the southwestern coastal plain and the pingtung region are also characterized by a belt of high v p v s with lower seismicity most of the events occurred at the base of the high v p v s zones we suggest that material strength in those regions may be too low to accumulate stress which may indicate water saturated young sediments finally the central range region is characterized by a low v p v s belt

international geomagnetic reference field the eleventh generation the eleventh generation of the international geomagnetic reference field igrf was adopted in december 2009 by the international association of geomagnetism and aeronomy working group v mod it updates the previous igrf generation with a definitive main field model for epoch 2005 0 a main field model for epoch 2010 0 and a linear predictive secular variation model for 2010 0 2015 0 in this note the equations defining the igrf model are provided along with the spherical harmonic coefficients for the eleventh generation maps of the magnetic declination inclination and total intensity for epoch 2010 0 and their predicted rates of change for 2010 0 2015 0 are presented the recent evolution of the south atlantic anomaly and magnetic pole positions are also examined

three dimensional p and s wave velocity structures beneath taiwan deep structure beneath an arc continent collision the island of taiwan is a byproduct of one of the few active collisions between a continent represented by the eurasian continental shelf and an island arc represented by the northern extension of the luzon arc on the philippine sea plate to understand better the evolution and current tectonics of this collision we selected 1260 well recorded earthquakes from an initial set of 50 000 located by the taiwan telemetered seismograph network for the determination of one and three dimensional p and s wave velocity structures beneath the island the results for both structures and earthquake relocations reveal that the island is divided tectonically into three distinct zones in the east the philippine oceanic plate is underthrusting the eurasian plate east of a north south boundary that is well defined by both seismic activity and a region of high velocity in the south the eurasian continental plate is underthrusting the philippine sea plate south of an east west boundary at about 23 n that is sharply defined by both subcrustal seismicity and a zone of relatively low velocities the dip of the subducted continent is shallow until it reaches the luzon island arc 50 km east of the main island structure under the main part of the island north of 230n reveals a shallow dipping zone of low velocities in the west above 25 km depth that narrows and steepens below that depth to at least 50 km beneath the central range the dip of this low velocity region is outlined by a narrow zone of seismicity that extends to depths of 100 km this seismic zone lies in a velocity saddle and marks an apparent offset in the central low velocity region at 24 n evidence for the subduction of the eurasian continent beneath taiwan therefore exists everywhere beneath the island and the low velocity regions in the mantle support but do not require the subduction of some 6 16 km of lower continental crust to depths of at least 50 km the distinct change in both the velocity structure and the seismic activity at 23 n and the offset of the low velocity regions at 24 n argue for abrupt changes in the nature of subduction across these latitudes these changes may be caused by an interaction of the luzon island arc with the subducted continental shelf that mimics a similar interaction evident at the surface finally the velocity structure of the subducting philippine sea plate suggests that the earthquakes beneath 70 km depth are not occurring within the highest velocity regions of the plate but are probably located near the upper edge of the plate there is also evidence from earthquake locations and velocity structure that suggests that the subducting plate is segmented and that subduction is presently occurring as far south as 22 n

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

eclogites of the snowbird tectonic zone petrological and u pb geochronological evidence for paleoproterozoic high pressure metamorphism in the western canadian shield eclogite occurs within the southern domain of the east athabasca mylonite triangle in northern saskatchewan situated at the boundary between the archean rae and hearne provinces of the western canadian shield the east athabasca mylonite triangle is a fundamental exposure of the 3 000 km long snowbird tectonic zone the eclogite occurs in association with a variety of lower crustal high pressure granulites that record a complex metamorphic history from 2 6 to 1 9 ga temperatures of the eclogite facies metamorphism are constrained by garnet clinopyroxene exchange thermometry at 920 1 000 c minimum pressure conditions are recorded by the jadeite quartz albite geobarometer at 1 8 2 0 gpa a near isothermal decompression path to granulite facies conditions is inferred from retrograde reaction textures involving the formation of granulite facies assemblages such as orthopyroxene plagioclase and pargasite plagioclase u pb idtims zircon geochronology of the eclogite yields a weighted mean 207pb 206pb date of 1 904 0 0 3 ma which we interpret as the time of peak eclogite facies metamorphism shrimp in situ analyses of metamorphic zircons included within omphacitic clinopyroxene support this interpretation with a weighted mean 207pb 206pb date of 1 905 19 ma inclusion suites of high pressure phases and the petrographic setting of zircon are a direct link between zircon growth and eclogite facies metamorphism zircon from one eclogite sample has older cores that are 2 54 ga which is a minimum age for the emplacement or earliest metamorphism of the gabbroic protolith u pb rutile data indicate slow cooling at 1 c ma below 500 c from 1 88 to 1 85 ga the formation and exhumation of the eclogites at ca 1 9 ga has important implications for the tectonic significance of the snowbird tectonic zone during the paleoproterozoic the eclogites described here are consistent with transport of continental crust to mantle depths during the paleoproterozoic followed by rapid buoyancy driven exhumation to normal lower crustal depths

crustal scale seismic profiles across taiwan and the western philippine sea abstract we have used combined onshore and offshore wide angle seismic data sets to model the velocity structure of the taiwan arc continent collision along three cross island transects although taiwan is well known as a collisional orogen relatively few data have been collected that reveal the deeper structure resulting from this lithospheric scale process our southern transect crosses the hengchun peninsula of southernmost taiwan and demonstrates characteristics of incipient collision here 11 km thick transitional crust of the eurasian plate eup subducts beneath a large rapidly growing accretionary prism this prism also overrides the n luzon forearc to the east as it grows just west of the arc axis there is an abrupt discontinuity in the forearc velocity structure because this break is accompanied by intense seismicity we interpret that the forearc block is being detached from the n luzon arc and philippine sea plate psp at this point our middle transect illustrates the structure of the developing collision steep and overturned velocity contours indicate probable large scale thrust boundaries across the orogen the leading edge of the coherent psp appears to extend to beneath the east coast of taiwan deformation of the psp is largely limited to the remnant n luzon arc with no evidence of crustal thickening to the east in the huatung basin our northern transect illustrates slab continent collision the continuing collision of the psp and eup as the psp subducts the collisional contact is below 20 km depths along this transect ne of hualien this transect shows elements of the transition from arc continent collision to ryukyu arc subduction both of our models across the central range suggest that the paleozoic to mesozoic basement rocks there may have been emplaced as thick coherent thrust sheets this suggests a process of partial continental subduction followed by intra crustal detachment and buoyancy aided exhumation although our models provide previously unknown structural information about the taiwan orogen our data do not define the deepest orogenic structure nor the structure of western taiwan additional seismic active and passive geologic and geodynamic modeling work must be done to fully define the structure the active deformation zones and the key geodynamic process of the taiwan arc continent collision

can remanent magnetization in the deep crust contribute to long wavelength magnetic anomalies long wavelength aeromagnetic anomalies require sources in the deep crust that are far more magnetic than the measured induced magnetization of lower crustal rocks several scientists have suggested that remanence in the deep crust may provide the missing magnetization this is not likely either through stable and or viscous remanence bodies large enough to contribute to surface anomalies and carrying stable remanence probably cooled slowly enough that they are subdivided into zones of alternating polarity this subdivision sharply reduces the power in the long wavelength portion of the anomaly spectrum on the other hand if such a body carries a viscous remanence the viscous component will be acquired in such a way as to bring the total magnetization of the body into equilibrium with the earth s field this equilibrium total magnetization is only about 25 greater than the induced magnetization not enough to explain the discrepancy

shape and microstructure of microgranitoid enclaves indicators of magma mingling and flow abstract at tarandore point 328 km south of sydney new south wales australia two tonalite phases are in contact with a gabbroic diorite all belonging to the moruya batholith lobate to crenulate contacts pillowing of the more mafic phase in the more felsic phase net veining of the more mafic phase by the more felsic local chilling of the more mafic phase against the more felsic and an abundance of rounded to elongate microgranitoid enclaves all suggest mingling of magmas at the site of emplacement therefore all phases were emplaced before the others solidified and an order of intrusion cannot be determined the variety of microgranitoid enclaves suggests that repeated magma mingling has occurred linear geochemical trends indicate either magma mixing or restite unmixing as a prime cause of variation in the members of the moruya suite of granitoids direct evidence of mixing is not present but evidence that the magmas were substantially liquid at the time of emplacement sugests that mixing is possible restite unmixing is unlikely to be the prime process because near minimum melts which would occur in all granitoid phases according to that model should mix easily rather than mingling in fact mingling has occurred between tonalite phases of very similar composition which would not be expected if the liquid parts of enclaves were minimum melts available experimental and theoretical data on mixing in both magma chambers and conduits cannot be applied directly to the occurrence at tarandore point owing to inadequately constrained parameters but mixing generally should be favoured by higher temperatures which is consistent with our inference that relatively high proportions of melt were present in the granitoid phases at the time of emplacement conduit mixing may be reflected in a local zone of strong to extreme elongation of enclaves in which the distortion has been accomplished by magmatic flow without solid state deformation as evidenced by strong dimensional preferred orientations coupled with a lack of microstructural evidence of intragranular plastic strain this suggests that elongate enclaves should not be used as indicators of solid state deformation or as tectonic strain markers without microstructural evidence that suitably strong intracrystalline plastic deformation has taken place

subduction of a buoyant plateau at the manila trench tomographic evidence and geodynamic implications we determined p wave tomographic images by inverting a large number of arrival time data from 2749 local earthquakes and 1462 teleseismic events which are used to depict the three dimensional morphology of the subducted eurasian plate along the northern segment of the manila trench dramatic changes in the dip angle of the subducted eurasian plate are revealed from the north to the south being consistent with the partial subduction of a buoyant plateau beneath the luzon arc slab tears may exist along the edges of the buoyant plateau within the subducted plate induced by the plateau subduction and the subducted lithosphere may be absent at depths greater than 250 km at approximate to 19 degrees n and approximate to 21 degrees n the subducted buoyant plateau is possibly oriented toward nw se and the subducted plate at approximate to 21 degrees n is slightly steeper than that at approximate to 19 degrees n these results may explain why the western and eastern volcanic chains in the luzon arc are separated by approximate to 50 km at approximate to 18 degrees n whereas they converge into a single volcanic chain northward which may be related to the oblique subduction along the manila trench caused by the northwestern movement of the philippine sea plate a low velocity zone is revealed at depths of 20 200 km beneath the manila accretionary prism at approximate to 22 degrees n suggesting that the subduction along the manila trench may stop there and the collision develops northward the taiwan orogeny may originate directly from the subduction of the buoyant plateau because the initial time of the taiwan orogeny is coincident with that of the buoyant plateau subduction

magnetism of the lower crust observations from the chipman domain athabasca granulite terrain northern canada abstract magnetic properties of lower crustal rocks produce anomalies seen in satellite aeromagnetic and ground studies and are assumed to be responsible for observed long wave length anomalies lwa of 20 nt the soon to be launched swarm satellites will provide extensive data on the magnetization of the lower to middle crust in anticipation of this event we are investigating magnetic properties in a superbly exposed section of lower crust in northern saskatchewan the athabasca granulite terrain agt is a complex region of felsic and mafic lower crustal rocks part of the snowbird tectonic zone stretching ne sw across the canadian shield the agt is composed of a sequence of rocks identified as lower crustal in origin by their high pressure 1 0 gpa and high temperature 800 c metamorphism dated at 2 6 ga and 1 9 ga with uplift and exhumation at 1 85 1 80 ga the agt is characterized by low negative aeromagnetic anomalies with distinct large positive anomalies in the southern and central regions the chipman domain on the east side consists of tonalites mafic granulites and granite intruded by the chipman dike swarm at 1 9 ga where anomalies cut across mapped lithologic boundaries susceptibility measurements from both field and lab readings range over several orders of magnitude from 1 10 5 to 3 10 1 si with higher values related to both mafic granulite and some tonalite samples remanence values also show considerable variability from 0 1 ma m to 90 a m with the weakest magnetization found in the chipman dikes and the fehr granite forty samples out of 89 have koenigsberger ratios greater than 1 but low initial remanence limits its influence on anomalies hysteresis and low temperature measurements identify magnetite as the predominant iron oxide this section of lower crustal rocks has paramagnetic granites and dikes with ferromagnetic mafic granulites and bimodal tonalites defined by geographic location

a genetic algorithm for filter design to enhance features in seismic images abstract we present a novel method to enhance seismic data for manual and automatic interpretation we use a genetic algorithm to optimize a kernel that when convolved with the seismic image appears to enhance the internal characteristics of salt bodies and the sub salt stratigraphy the performance of the genetic algorithm was validated by the use of test images prior to its application on the seismic data we present the evolution of the resulting kernel and its convolved image this image was analysed by a seismic interpreter highlighting possible advantages over the original one the effects of the kernel were also subject to an automatic interpretation technique based on principal component analysis statistical comparison of these results with those from the original image by means of the mann whitney u test proved the convolved image to be more appropriate for automatic interpretation

magnetic mineralogy in an archean crustal cross section implications for crustal magnetization a traverse across precambrian rocks from the kapuskasing uplift in ontario provided 31 samples from a crustal cross section that is now exposed at the surface about half the samples are dominantly paramagnetic magnetite accompanied in a few samples by minor amounts of pyrrhotite is the only significant magnetic mineral in the other half magnetic susceptibilities of these samples are constant between room temperature and about 540 c above which temperature they drop rapidly to zero magnetic susceptibility does not depend on rock type apparent original depth in the crust or degree of metamorphism the average induced magnetization of the most magnetic units in this section is less than 1 a m which is much lower than the source requirements for the deep crust deduced from aeromagnetic and satellite magnetometer surveys

tomographic evidence for the eurasian lithosphere subducting beneath south taiwan 1 a tomographic image of the crust and upper mantle beneath the taiwan island as derived from local regional and teleseismic data simultaneously revealed a high velocity zone from the surface down to a depth of 300 km locating beneath south taiwan to the south of latitude 24 n we interpret this high velocity zone to be the subducted eurasian lithosphere our result is different from a global tomography study that shows the eurasian plate subducting beneath most part of the taiwan island down to 670 km depth this down going model of the eurasian lithosphere implies that the tectonic framework of taiwan has been changing from subduction in the south to collision in the north which provides geophysical evidence supporting the previous proposed models the subducted eurasian lithosphere colliding with the subducted philippine sea plate would have contributed to the mountain building active seismicity and crustal deformation in the study region due to the lower part of the eurasian continental plate subducted beneath south taiwan

a tomographic image of indian lithosphere break off beneath the pamir hindukush region summary the p and s seismic anomalies in the upper mantle beneath the pamir hindukush collision zone and surrounding areas are investigated using more than 4 10 5 traveltimes from 5775 events located in the study region recorded by the stations of the worldwide seismic network and reported in the isc catalogue all available epicentral distance ranges of seismic rays are considered all sources are relocated using an algorithm which includes the double difference method in addition source parameters are corrected in the main inversion step simultaneously with the velocity models the high reliability of the resulting p velocity images is confirmed by various tests the s model provides much smoother patterns which are however generally consistent with p anomalies our model shows a complex image of lithospheric plate under the study region under sw hindukush the northwards subducting indian plate is imaged throughout the upper mantle and transition zone further to the ne within a 300 km wide zone the upper part of the plate overturns and then the slab breaks off after it narrows at 250 km depth the tomographic model also provides evidence for the southward subduction of the lithosphere beneath the southern tien shan down to at least 250 km depth to the north of tarim we observe a northward dipping of the tarim plate down to 500 km depth at the crustal depth our model shows alternating belts of high and low velocities parallel to the main thrusting faults which indicate complex crustal structure in this region

syntectonic generation and segregation of tonalitic melts from amphibolite dikes in the lower crust striding athabasca mylonite zone northern saskatchewan vapor absent melting in a swarm of amphibolite dikes in the lower crust has produced segregations of tonalitic and trondhjemitic composition in a variety of textures and structures that dramatically illustrate the mechanisms of melt segregation during deformation the 3 2 ga chipman dikes intrude the mylonitized chipman tonalite within the striding athabasca mylonite zone of northern saskatchewan dike emplacement spans a major sinistral transpressive ductile deformation and granulite facies metamorphic event older dikes are intensely folded and sheared younger dikes are undeformed major and trace element analyses indicate that the dikes are tholeiitic basalts similar in composition to midocean ridge basalt thermobarometry finds conditions of 750 850 c 1 0 gpa the youngest most pristine dikes contain hornblende and plagioclase with minor clinopyroxene and garnet older migmatitic dikes have tonalitic to trondhjemitic segregations spatially associated with garnet crystals where small the segregations occupy tails or strain shadows on every garnet crystal where garnets and segregations are large leucosomes form an interconnected network that extends into the host tonalite tonalitic pools probably of dike origin collect in boudin necks and fold hinges the chipman dikes are interpreted to have been emplaced solidified and partially melted during ductile shearing in the lowermost crust perhaps near the base of an archean island arc they appear to offer an exceptional view of magma genesis where underplated mantle derived mafic magmas provide not only heat but also a component of material to new felsic magmas deformation is essential to the process allowing access of mafic magma into the lower crust and facilitating the segregation of the new felsic melts from their source rocks

thermal modeling of continental subduction and exhumation constrained by heat flow and seismicity in taiwan abstract the thermal evolution of crustal exhumation subsequent to the subduction of the continental margin is modeled through numerical solutions to the two dimensional heat conduction equation the boundary conditions used in the modeling are basically constrained by available geophysical and geological observations in the taiwan area temperature distributions are calculated at one million year intervals over a 5 ma duration although this study presents a tentative estimation based on some reasonable assumptions the preliminary results not only explain the sharp change in seismicity within the crust but also agree with the general heat flow pattern on the surface of taiwan more specifically both the high heat flow and the aseismic belt beneath the eastern central range which are probably attributed to higher temperatures within the crust are modeled by crustal exhumation after the subduction of the continental margin however the occurrence of deeper earthquakes east of the central range is a result of the lower thermal gradient which is due to the subduction of the cold continental crust into the uppermost mantle in summary preliminary thermal modeling supports the possibility of a tectonic model of active continental subduction and crustal exhumation for the taiwan orogen

thermal modeling of the sw ryukyu forearc taiwan implications for the seismogenic zone and the age of the subducting philippine sea plate huatung basin subduction mega thrust earthquakes in the sw ryukyu trench pose a seismic and tsunami hazard one of the objectives of this study is to estimate the downdip width of the seismogenic zone using numerical modeling to determine the temperature distribution along the plate interface however this approach depends strongly on the thermal parameters of the subducting slab while the philippine sea plate psp subducting beneath the central and eastern ryukyu arc is of eocene age 35 50 ma its age west of the gagua ridge is uncertain with proposed ages ranging from lower cretaceous 140 ma to upper eocene 35 ma since the sparse available heat flow data are insufficient to resolve this debate both end member hypotheses are tested as input parameters we examined two transects at 122 5 e and 123 5 e on either side of the n s trending 4 km high gagua ridge the shallow forearc geometry is obtained from wide angle seismic data the deep slab geometry was obtained from hypocenter distribution and tomography for an eocene slab age we obtain a 100 km and 110 km wide seismogenic zone between the 150 c and 350 c isotherms west and east of gagua ridge respectively this is in good agreement with the observed distribution of hypocenters using a cretaceous slab west of gagua ridge predicts a deep seismogenic zone 25 km 60 km depth inconsistent with observed thrust earthquakes tomographic images at 122 5 e and 123 5 e show a similar slab thickness of 70 80 km suggesting that the oceanic lithosphere has a young eocene thermal age the westernmost psp huatung basin may have been thermally rejuvenated by mantle convection near the slab corner the tectonic history since 6 ma transition from subduction to collision beneath taiwan may have also perturbed the thermal structure

structure and dynamics of the upper mantle beneath the alpine himalayan orogenic belt from teleseismic tomography abstract the upper mantle structure beneath the alpine himalayan orogenic belt ahb at depths from 100 to 500 km has been studied by its tomographic inversion of p wave travel times from earthquakes that occurred in the region and were recorded by the worldwide seismological network at teleseismic distances from the isc catalogues the resulting maps were obtained as a sum of independent inversions in about 60 overlapping blocks 600 800 km in diameter this approach is similar to high frequency spatial filtration and allows investigation of much subtler structures than in global inversion high velocity linear patterns along the ahb have been interpreted as traces of continental or oceanic lithosphere sinking to the upper mantle under the effect of active regional compression the tomographic maps show well pronounced traces of subduction in the region of cretan arc hindukush and burma also confirmed by independent data as well as other less certain evidence of subduction positive anomalies in the western part of the ahb are attributed to subduction in the area of cyprus and along the caucasus kopet dagh lut plate in the tomographic maps one can see traces of the subducted indian slab under western part of himalayas beneath eastern himalayas the upper mantle is rather hot zones of sinking lithosphere are traceable around the tarim block a strong low velocity anomaly in mongolia is apparently related to the hangai plume whose existence was inferred from geological and geophysical evidence

petrological and geochronological constraints on high pressure high temperature metamorphism in the snowbird tectonic zone canada the upper deck of the east athabasca mylonite triangle eamt northern saskatchewan canada contains mafic granulites that have undergone high p t metamorphism at conditions ranging from 1 3 to 1 9 gpa 890 960 c coronitic textures in these mafic granulites indicate a near isothermal decompression path to 0 9 gpa 800 c the godfrey granite occurs to the north adjacent to the upper deck high p t domain well preserved corona textures in the godfrey granite constrain igneous crystallization and early metamorphism in the intermediate pressure granulite field opx pl at 1 0 gpa 775 c followed by metamorphism in the high pressure granulite field grt cpx pl at 1 2 gpa 860 c u pb geochronology of zircon in upper deck mafic granulite yields evidence for events at both c 2 5 ga and c 1 9 ga the oldest zircon dates are interpreted to constrain a minimum age for crystallization or early metamorphism of the protolith a population of 1 9 ga zircon in one mafic granulite is interpreted to constrain the timing of high p t metamorphism titanite from the mafic granulites yields dates ranging from 1900 to 1894 ma and is interpreted to have grown along the decompression path but still above its closure temperature indicating cooling following the high p t metamorphism from c 960 650 c in 4 10 myr zircon dates from the godfrey granite indicate a minimum crystallization age of 2 61ga without any evidence for 1 9 ga overgrowths the data indicate that an early granulite facies event occurred at c 2 55 2 52 ga in the lower crust c 1 0 gpa but at 1 9 ga the upper deck underwent high p t metamorphism then decompressed to 0 9 1 0 gpa juxtaposition of the upper deck and godfrey granite would have occurred after or been related to this decompression in this model the high p t rocks are exhumed quickly following the high pressure metamorphism this type of metamorphism is typically associated with collisional orogenesis which has important implications for the snowbird tectonic zone as a fundamental boundary in the canadian shield

p wave velocity and gradient images beneath the okinawa trough abstract we determined the high resolution 3 d p wave velocity structure under the okinawa trough in the region between south taiwan and north kyushu japan by inverting 102 511 absolute travel time data from 5012 local earthquakes and 29 153 relative travel time residuals calculated from 1873 teleseismic events for the first time a combined data set recorded simultaneously by three seismographic networks located in kyushu okinawa trough taiwan and fujian province of china has been used which enables us to determine the high resolution p wave velocity structure in the crust and upper mantle under the entire arc system the resulting tomographic and gradient images provide a new insight into the structural heterogeneities related to the opening of the okinawa trough and dehydration of the philippine sea slab phs and confirm some of the features proposed by previous studies a high velocity zone is imaged clearly at depths of 50 300 km along the ryukyu arc system from south kyushu to north taiwan showing good agreement with the spatial distribution of seismicity under the subduction zone the relative residuals from different quadrants also exhibit early relative arrivals at the stations corresponding to the high vp regions and we therefore interpret the high vp zone as the subducting phs slab along the volcanic front from south kyushu to north taiwan low vp areas are visible which are mainly caused by the fluids liberated by the extensive dehydration process of the subducting phs slab this fluid related low velocity zone is located at depths of 20 120 km along the arc with a total length of 1200 km from north taiwan to south kyushu using the inverted p wave velocity model we also calculated the gradient images which are independent of the selection of reference model or color scale to provide further information and help us to understand and explain the tomoagraphic structures three sub blocks of the ryukyu arc system are revealed from the inverted velocity images and the calculated gradient maps these features of the velocity anomalies together with other parameters determined by the present study suggest that the okinawa trough extension arc magmatism and seismicity are mainly attributed to the dehydration process associated with the phs slab subduction and the oblique subduction direction of the plate

analysis of the field dependence of remanent magnetization curves 1 a new method to calculate and analyze coercivity distributions of measured acquisition demagnetization curves of remanent magnetization is presented the acquisition demagnetization curves are linearized by rescaling both the field and the magnetization axes an appropriate filtering of the linearized curves efficiently removes measurement errors prior to evaluating the coercivity distributions the filtered coercivity distributions are modeled using a set of generalized probability density functions in order to estimate the contributions of different magnetic components an error estimation is calculated for these functions with analytical and numerical methods in order to evaluate whether the model is significantly different from the measured data three sediment samples from baldeggersee switzerland and three samples of urban atmospheric particulate matter pm have been analyzed using this method it is found that the coercivity distributions of some of the magnetic components show significant and consistent deviations from a logarithmic gaussian function large deviations are found also in the coercivity distributions of theoretical af demagnetization curves of single domain and multidomain particles constraints in the shape of model functions affect the identification and quantification of magnetic components from remanent magnetization curves and should be avoided as far as possible the generalized probability density function presented in this paper is suitable for appropriate modeling of gaussian and a large number of non gaussian coercivity distributions

tomographic imaging of lithospheric structures under taiwan abstract tomographic images of the crustal and mantle velocity structures under taiwan are obtained by simultaneous inversion of local earthquake p wave arrival times for hypocenters and p wave velocity structures in northern taiwan a high velocity zone coinciding with the wadati benioff zone can readily be identified as the subducted philippine sea plate the imaged zone dips toward the north at an angle of 40 from a depth range of 20 55 to 100 130 km an upper mantle low velocity wedge ranging from 40 to 80 km in depth exists above the subducted slab above this wedge is the ilan plain of northern taiwan which lies at the west end of the okinawa trough a well recognized back arc basin the crustal velocities under the plain are also relatively low the well defined high velocity zone and the low velocity wedge provide some constraints on the thermal structures of the subduction system under northern taiwan in tomographic images across the central section of taiwan thickening of the crust and up arching of the lower crustal materials under the central range are commonly observed the crust under the western foothills region is clearly thinner and the near surface low velocity layers are well developed the structures under the central range show that although the taiwan orogeny is quite young a root deeper in the north and shallowing to the south has formed the results of our tomography show that a significant portion of the lithosphere is involved in the taiwan orogeny

p and s velocity anomalies in the upper mantle beneath europe from tomographic inversion of isc data we present a new tomographic model for p and s velocity anomalies beneath europe 30 n 55 n 5 w 40 e extending in depth up to 700 km and constrained by inversion of data from the international seismological center isc catalogue the algorithm uses the traveltimes from events located in the study area recorded by all available worldwide stations as well as times from teleseismic events recorded by european stations the events from the isc catalogue have been relocated and combined into composite events all the traveltimes were corrected for crustal structure using the reference model eucrust 07 the resulting velocity anomalies show similar large scale patterns as observed in previous studies but have a higher resolution which allows detection of some features in more detail for example it is now possible to assess the depth extension of the small slow velocity body beneath the eifel region and eger graben the p and the s model show a good consistency in the uppermost 200 km below most of the european area and in some parts even in the deeper layers e g beneath the apennines and the hellenic arc the new model provides clear images of some principal features which were previously detected in a limited number of studies while the comparison between p and s velocity anomalies provides novel constraints to address on their nature e g the gap in the adriatic plate subducted below the central southern apennines in this paper we pay special attention to testing the reliability of the results the random noise effect is evaluated using a test with independent inversion of two data subsets with odd even events the spatial resolution is estimated using different checkerboard tests furthermore we present a synthetic model with realistic patterns which reproduces after performing forward and inverse modelling the same shape and amplitudes of the anomalies as in the case of the real data inversion in this case the parameters of the model can be used to assess the amplitudes of p and s anomalies that is critical for evaluation of other petrophysical parameters temperature density composition etc in the upper mantle 2009 the authors journal compilation 2009 ras