reassessment of the rifting process in the western corinth rift from relocated seismicity the seismic activity in the western part of the corinth rift greece over the period 2000 2007 monitored by a dense network of three component stations is analysed in terms of multiplets and high precision relocation using double difference techniques this detailed analysis provides new insights into the geometry of faults at depth the nature and the structure of the active zone at 6 8 km depth previously interpreted as a possible detachment and more generally into the rifting process the seismicity exhibits a complex structure strongly varying along the rift axis the detailed picture of the seismic zone below the rift indicates that its shallower part at depths of 6 8 km is 1 1 5 km thick with a complex microstructure and that its deeper part at depths of 9 12 km gently dipping to the north 10 20 is 0 1 0 3 km thick with a microstructure consistent with the general slope of the structure although the nature of this seismic zone remains an open question the presence of seismicity beneath the main active area the strong variability of the structure along the rift over short distances and the complex microstructure of the shallower part revealed by the multiplet analysis are arguments against the hypothesis of a mature detachment under the rift this active zone more likely represents a layer of diffuse deformation the geometry of the mapped active faults is not well defined at depth as no seismicity is observed between 0 and 4 km except for the aigion fault rooting in the seismic layer at 6 km depth with a dip of 60 a distinct cloud of seismicity may be associated with the antithetic kalithea fault on which the 1909 fokis earthquake m s 6 3 may have occurred the link between the 1995 rupture m s 6 2 and the faults known at the surface has been better constrained as the relocated seismicity favours a rupture on an offshore blind fault dipping at 30 rather than on the deeper part of the east helike fault consequently the 1995 event is expected to have decreased the coulomb stress on the east helike fault to explain these seismic observations along with the geodetic observations a new mechanical model for the rifting process in this region is proposed involving non elastic mostly aseismic uniform ns opening below the rift axis coupled with the downward and northward growth of a yet immature detachment the reported gps rates would mainly result from this deep silent source and the seismicity would reveal the detachment position not yet connected to the ductile lower crust in such a model the strong fluctuations of microseismicity would result from small strain instabilities undetected by continuous gps and possibly related to pore pressure transients

subsurface fault geometries and crustal extension in the eastern basin and range province western u s abstract we provide the first synthesis of seismic reflection data and active present day crustal deformation for the greater wasatch fault zone we analyzed a number of previously unpublished seismic reflection lines horizontal and vertical crustal velocities from continuous gps and surface geology to investigate the relationships between interseismic strain accumulation subsurface fault geometry and geologic slip rates on seismogenic faults across the eastern third of the northern basin and range province the seismic reflection data show recent activity along high angle normal faults that become listric with depth and appear to sole into preexisting decollements possibly reactivating them we interpret these listric normal faults as reactivated sevier age structures that are connected at depth with a regionally extensive detachment horizon these observations of subsurface structure are consistent with the mapped geology in areas that have experienced significant extension we modeled the crustal deformation data using a buried dislocation source in a homogeneous elastic half space the estimated model results include a low angle dislocation 8 20 at a locking depth of 7 10 km and slipping at 3 2 0 2 mm yr despite the model s relative simplicity we find that the predicted location of the dislocation is consistent with the interpreted seismic reflection data and suggests an active regionally extensive sub horizontal surface in the eastern basin and range this result may imply that this surface represents aseismic creep across a reactivated low angle fault plane or the onset of ductile flow in the lower crust at or beneath the brittle ductile transition zone under the present day basin and range extensional regime this result may also have implications for crustal rheology and suggests that geodesy might under some circumstances serve as an appropriate tool for inferring deeper crustal structure

localization of listric faults at thrust fault ramps beneath the great salt lake basin utah evidence from seismic imaging and finite element modeling reflection seismic data from the great salt lake basin utah show that the major basin bounding normal faults decrease in dip from 60 at the surface to 10 20 at depths as shallow as 4 6 km this rapid decrease in fault dip at depths shallower than the brittle ductile transition zone in the basin and range province suggests an explanation other than a gradual change of rheology and stress orientations with depth using a dense grid of seismic data gravity data borehole data and published geologic information from islands in the lake we constrain the position of the sevier age willard thrust and a footwall imbricate and show their reactivation as normal faults during tertiary extension in the absence of surface geologic information we use available subsurface information from the lake to draw an analogy with the ogden duplex in the wasatch front where cenozoic normal faulting was superimposed on an earlier sevier age thrust regime to give rise to listric normal faults our interpretations are consistent with finite element modeling results which demonstrate that extensional slip on preexisting thrust ramps leads to the formation of energetically favored synthetic normal faults some of which may merge with the thrust ramp and obtain listric geometries further slip on these listric faults gives rise to secondary synthetic and antithetic faults resulting in hanging wall grabens

evidence for seismogenic normal faults at shallow dips in continental rifts several recent observations indicate that normal faulting earthquakes occasionally occur on faults dipping 35 dips often considered shallow most of these occur in the woodlark and aegean rifts the woodlark and aegean rifts are found to generate significantly more earthquakes than others and are the most rapidly extending so display the widest variety of fault behavior even within the woodlark rift system extension rates vary along strike with the shallowest dipping faults confined to the most rapidly rifting segment here several events mw 6 0 6 8 feature nodal planes dipping 23 35 these planes are subparallel to shear zones bounding nearby metamorphic core complexes including one imaged to 8 9 km depth by seismic reflection profiling in the western gulf of corinth at least one large event mw 6 4 occurred on a fault dipping 33 as the woodlark example this rift segment exhibits a high opening rate 10 20 mm yr several other cases elsewhere based on older historic data microseismicity or geologic inference suggest seismic slip at similar or shallower dips still no documented large earthquake exhibits seismic slip on sub horizontal surfaces dip 10 15 stress rotation may explain the 23 35 dips but thus far no realistic mechanism has been found more likely these faults represent surfaces somewhat weaker than surrounding rock through some combination of modest cohesion of the surrounding rock and slightly lower frictional coefficients on the fault such weakening may be a consequence of high slip rates which rapidly generate large offsets and mature fault systems

evidence for paleoseismic slip on a continental low angle normal fault tectonic pseudotachylyte from the west salton detachment fault ca usa abstract the potential of continental low angle normal faults lanf to nucleate large 6 0 m w earthquakes at low angles remains unclear despite much focused research we document evidence for ancient seismicity along a continental lanf detachment fault that formed and slipped at low angles and produced tectonic pseudotachylyte these thick and laterally persistent pseudotachylyte accumulations along the west salton detachment fault wsdf salton trough usa preserve convincing evidence for a frictional melt origin including spherulitic microlites ductile flow structures preservation of high temperature phases as clasts and injection veins cumulative thickness of pseudotachylyte along the fault ranges from 0 1 to 1 5 m and pseudotachylyte cataclasite in the fault core and damage zone are exposed along 2 6 km length of the fault reworked fragments of pseudotachylyte in cataclasites and multiple generations of cataclasites provide evidence for the preservation of multiple earthquake cycles the limited exposure

a review of the western part of the altaids a key to understanding the architecture of accretionary orogens abstract the altaids is one of the largest accretionary collages in the world and the tectonic styles of the accretionary processes have been interpreted in several ways including as an amalgamation of multiple terranes as a result of oroclinal bending of a long single arc or as a caledonian continental collision based on recent tectonostratigraphic analyses together with paleomagnetic data the tectonic styles of the neoproterozoic to paleozoic accretionary processes of the altaids are discussed the western altaids is the main focus of the study which was mainly composed of several independent linear components such as arcs and microcontinents with proterozoic basement and cover rocks various kinds of arcs existed in the paleo asian ocean including a complicated type of arc alaskan type which is a combination of the japan and mariana type intra oceanic arcs and the cordillera type continental arcs these linear components rotated and collided with each other with multiple subduction polarities which could have been an important result of multiple linear element amalgamation and which has contributed greatly to the architecture of the eurasian continent the basic tectonic styles of the altaids can be summarized as arc arc collision oroclinal bending and large scale rotation and multiple subductions with a complicated archipelago paleogeography these basic features of accretionary orogens in general can be attributed to the amalgamation of complicated multiple linear elements some mesozoic to cenozoic accretionary orogens in the world are also characterized by processes of multiple linear element amalgamation more attention should be paid to the multiple linear element amalgamation of ancient accretionary orogens which will shed light on lateral and vertical continental growth

6 kyr record of flood frequency and intensity in the western mediterranean alps interplay of solar and temperature forcing abstract the high resolution sedimentological and geochemical analysis of a sediment sequence from lake savine western mediterranean alps france led to the identification of 220 event layers for the last 6000 years 200 were triggered by flood events and 20 by underwater mass movements possibly related to earthquakes that occurred in 5 clusters of increase seismicity because human activity could influence the flood chronicle the presence of pastures was reconstructed through ancient dna which suggested that the flood chronicle was mainly driven by hydroclimate variability weather reanalysis of historical floods allow to identify that mesoscale precipitation events called east return events were the main triggers of floods recorded in lake savine the first part of this palaeoflood record 6 4 kyr bp was characterized by increases in flood frequency and intensity in phase with northern alpine palaeoflood records by contrast the second part of the record i e since 4 kyr bp was phased with southern alpine palaeoflood records these results suggest a palaeohydrological transition at approximately 4 kyr bp as has been previously described for the mediterranean region this may have resulted in a change of flood prone hydro meteorological processes i e in the balance between occurrence and intensity of local convective climatic phenomena and their influence on mediterranean mesoscale precipitation events in this part of the alps at a centennial timescale increases in flood frequency and intensity corresponded to periods of solar minima affecting climate through atmospheric changes in the euro atlantic sector

geophysical confirmation of low angle normal slip on the historically active dixie valley fault nevada the december 16 1954 dixie valley earthquake ms 6 8 followed the nearby fairview peak earthquake ms 7 2 by 4 min 20 s waveforms from the fairview peak event contaminate those from the dixie valley event making accurate fault plane solutions impossible a recent geologic study of surface rupture characteristics in southern dixie valley suggests that the dixie valley fault is low angle 30 along a significant portion of the 1954 rupture to extend these observations into the subsurface we conducted a seismic reflection and gravity experiment our results show that a portion of the dixie valley ruptures occurred along a fault dipping 25 to 30 as such the dixie valley event may represent the first large low angle normal earthquake on land recorded historically our high resolution seismic reflection profile images the rupture plane from 5 to 50 m depth medium resolution reflections as well as refraction velocities show a smoothly dipping fault plane from 50 to 500 m depth stratigraphic truncations and rollovers in the hanging wall show a slightly listric fault to 2 km depth gravity profiles conservatively constrain maximum basin depth and define overall geometry extension along the low angle section may have occurred in two phases during the cenozoic current fault motion postdates a 13 to 15 ma basalt imaged in the hanging wall and inherits from a fault formed during an earlier extensional pulse concentrated at 24 2 to 24 4 ma the earlier extension suggests extraordinary slip rates as high as 18 mm yr resulting in the formation of the low angle fault break sections of the dixie valley fault where there is no evidence for current low angle slip correlate well with areas where no pre 15 ma slip has been documented

seismic reflection evidence for seismogenic low angle faulting in southeastern arizona focal mechanisms for large m 6 earthquakes in extensional terranes suggest that seismogenic normal faults have dips that range from approximately 30 degree to approximately 70 degree geologic relations suggest that low angle faults have accommodated large scale upper crustal extension these disparate observations are often reconciled by arguments that low angle faults move aseismically or rotate to low angles from initially high angles seismic reflection data from the tucson basin in southeast arizona image a low angle normal fault the santa rita fault that crops out along the trend of late quaternary fault scarps caused by large magnitude m approximately 6 7 7 6 earthquakes velocity independent dip analysis from shot records of the santa rita fault indicates that it has a true dip of approximately 20 degree to a depth of at least 6km this observation suggests that low angle extensional faults indeed may be seismogenic and that actual mechanisms for accommodation of upper crustal extension depend on local conditions of stress and preexisting geologic structure

postmylonitic deformation in the raft river metamorphic core complex northwestern utah evidence of a rolling hinge much of the recent debate over low angle normal faults exposed in metamorphic core complexes has centered on the rolling hinge model the model predicts tilting of seismogenic high angle normal faults to lower dips by footwall deformation in response to isostatic forces caused by footwall exhumation this shallow brittle deformation should visibly overprint the mylonitic fabric in the footwall of a metamorphic core complex the predicted style and magnitude of rolling hinge strain depends upon the macroscopic mechanism by which the footwall deforms two end members have been proposed subvertical simple shear and flexural failure each mechanism should generate a distinctive pattern of structures that strike perpendicular to the regional extension direction subvertical simple shear svss should generate subvertical faults and kink bands with a shear sense antithetic to the detachment for an svss hinge the hinge related strain magnitude should depend only on initial fault dip rolling hinge structures should shorten the mylonitic foliation by 13 for an initial fault dip of 30 in flexural failure the footwall behaves as a flexed elastic beam that partially fails in response to bending stresses resulting structures include conjugate faults and kink bands that both extend and contract the mylonitic foliation extensional sets could predominate as a result of superposition of far field and flexural stresses strain magnitudes do not depend on fault dip but depend on the thickness and radius of curvature of the flexed footwall beam and vary with location within that beam postmylonitic structures were examined in the footwall of the raft river metamorphic core complex in northwestern utah to test these predictions observed structures strike perpendicular to the regional extension direction and include joints normal faults tension gash arrays and both extensional and contractional kink bands aside from the subvertical joints the extensional structures dip moderately to steeply and are mainly either synthetic to the detachment or form conjugate sets range wide the extensional structures accomplish about 4 elongation of the mylonitic foliation contractional structures dip steeply mainly record shear antithetic to the detachment and accomplish 1 contraction of the foliation these observations are consistent with the presence of a rolling hinge in the raft river mountains but a rolling hinge that reoriented a high angle normal fault by svss is excluded the pattern and magnitudes of strain favor hinge related deformation mainly by flexural failure with a subordinate component of svss

a note comparing parameters controlling low angle normal and thrust movement abstract a simple model for the initiation of normal and thrust motion along pre existing low angle faults shows that the parameters necessary for fault movement are nearly the same in both cases the parameters controlling fault movement which are tectonic tension and tectonic compression as well as fault friction or fault strength are considered in terms of available observations of crustal stresses and possible crustal stresses predicted from laboratory rock friction experiments because tectonic compression can exceed tectonic tension it is concluded that thrust movement along low angle zones of weakness is somewhat more likely than normal movement the relative difficulty of the different motions depends upon the ratio of tectonic compression to tectonic tension if available observational data of maximum and minimum horizontal stresses in the crust represent reasonable upper and lower limits to tectonic compression and tectonic tension then low angle movement is greatly facilitated by low coefficients of fault friction similar to values obtained in a number of recent studies of weak faults

paleozoic crustal growth and metallogeny of central asia evidence from magmatic hydrothermal ore systems of central kazakhstan abstract the ore deposit spectrum of central kazakhstan reflects a range of lithospheric magmatism controlled by the polycyclic geodynamic evolution of an active continental margin which develops from back arc oceanic settings volcanic hosted massive sulphide cu au ore deposits to subduction controlled calcalkaline magmatism copper porphyries with subsequent stages of crust differentiation of low degree partial melting and extended intramagmatic fractionation molybdenum porphyries on to continental rifting peralkaline ree zr nb rich systems mesothermal gold deposits mark transcrustal shear zone controlled fluid conduits formed during arc slicing and thermal relaxation in a post orogenic setting the ore deposits characterize successive and spatially overlapping stages of crustal growth and consolidation the general subduction setting of this range of ore deposits is reflected by calc alkaline trends in the associated subalkaline high k felsic rocks with a wide range in silica contents the predominant metal spectrum of au cu and mo indicates relatively oxidized magmatic hydrothermal systems in accordance with the metaluminous i type nature of their igneous host rocks i e oxidized igneous precursor material despite of presumably highly variable ratios of mantle crust components in individual magmatic systems gabbros to leucogranites most rocks have very similar positive initial e nd values of 0 to 5 5 and depleted mantle model ages in the range of 500 800 ma the ubiquitous relatively young mantle extraction age is likely to characterize the lower crust of central kazakhstan this basement is interpreted as back arc oceanic crust that formed behind oceanwards drifting continental margin slivers during the initial stages of the active continental margin evolution of the late precambrian angara baltica supercontinent the early back arc oceanic crust was transformed into the present lower crust during the paleozoic orogenic evolution in which the back arc crust was buried by magmatic arc and sedimentary material and recurrently affected by high grade metamorphism basaltic melt injection underplating and granitic melt extraction anorogenic rift related permian peralkaline riebeckite granites with ree zr nb mineralization have very high positive e nd of 5 to 8 which is probably inherited from young subcrustal lithosphere at the mantle crust boundary that was affected by partial remelting due to asthenospheric upwelling during rifting

geometry and kinematics of mykonos detachment cyclades greece evidence for slip at shallow dip 1 the eastern part of mykonos island cyclades greece shows the detailed internal structure of a two branch shallow dipping extensional detachment system of miocene age this paper shows that the last stage of slip along the detachment during the deposition of syn rift sediments occurred with a very low dip and proposes that this geometry prevailed since the initiation of extension extensional deformation during and after a 13 ma old granite intrusion is taken up by two main shallow dipping shear zones 1 the lower livada detachment which extends laterally across tinos island is a ductile structure located at the interface between the granite and the upper cycladic nappe metabasites a pervasive top to the ne shearing deformation is observed throughout the granite in this island with a strong gradient toward the intrusive contact later brittle faults shallowly and steeply dipping rework the ductile deformation with the same overall shear sense 2 the upper mykonos detachment is brittle and separates the metabasites from late miocene sandstones and conglomerates the detachment fault dips 12 15 toward the nne and its sense of shear is consistent with that of the livada detachment soft sediment deformation during the time of detachment faulting and the presence of steep normal faults that root into the detachment gouge indicate a shallow depth of deformation rotations about vertical and horizontal axes can be reconstructed within and below the detachment zone indicating that the overall direction of extension is nne ssw with a sense of shear toward the nne and the dip of the fault is throughout very low the two detachments have accommodated several tens of kilometers of horizontal extension during the formation of the aegean sea which emphasizes the importance of low angle extensional faults and shear zones in extensional tectonics

methane rich fluid evolution of the baogutu porphyry cu mo au deposit xinjiang nw china abstract baogutu is the first porphyry cu mo au deposit discovered in western junggar xinjiang nw china the ore bearing intrusion is a dioritic intrusive complex that includes stage 1 diorites and minor stage 2 diorite porphyries the stage 1 diorites have produced concentric potassic and propylitic alteration zone and overprinted phyllic alteration and host much of the cu mo au mineralization at baogutu the baogutu porphyry cu mo au deposit consists of unusually disseminated and minor vein style mineralization the fluid evolution occurred in stage 1 diorite from late magmatic stage to hydrothermal stage stages 1b and 1c is constructed based on alteration and fluid inclusion analysis by microthermometry raman spectroscopy and quadrupole mass spectrometer five fluid types are distinguished and they are l1 liquid rich 2 phase l2 liquid vapor 2 phase v1 vapor rich 2 phase v2 mono phase vapor and h multi phase solid the dominant inclusions rich in reductive ch4 plus h2o while minor inclusions rich in ch4 and co2 the inclusions in quartz from the late magmatic stage contain the most brine inclusions and less other inclusions with ch4 rich and h2o rich compositions the inclusions in quartz from stage 1b contain all fluid types and have ch4 rich and ch4 h2o compositions with rare co2 the inclusions in quartz from stage 1c are characterized by more vapor inclusions without brine inclusions and a clear co2 concentration in a few assemblages with ch4 co2 composition using microthermometry we estimate fluid trapping conditions at t 400 c and p 1500 to 3100 bar in late magmatic stage t 200 to 400 c and p 50 to 320 bar in disseminated quartz in stage 1b t 180 to 400 c and p 20 to 260 bar in vein quartz in stage 1b and t 170 to 400 c and p 20 to 230 bar in stage 1c such late magmatic conditions are compatible with the fluid evolution as a result of ch4 rich inclusions derived from mantle magma such hydrothermal conditions in stages 1b and 1c with small p t fluctuations are compatible with the dominant disseminated mineralization at baogutu that indicates a weak fracturing the overall fluid path at baogutu is toward lower pressure and small changed temperature with composition transit of fluid from a high halite ch4 rich system to low halite ch4 co2 system it may lead to the formation of this cu mo au deposit

architecture and seismotectonics of a regional low angle normal fault zone in central italy information from surface geology subsurface geology boreholes seismic reflection and refraction profiles and seismicity are used to depict the geometry and the possible seismogenic role of the altotiberina fault af a low angle normal fault in central italy the af extends along the inner umbria region for a length of 70 km with an average dip of 30 and an horizontal displacement up to 5 km it emerges west of the inner border of the tiber basin and deepens beneath the umbria marche carbonate fold and thrust belt to a depth of 12 14 km close to the af surface trace low angle synthetic east dipping normal faults extensively outcrop whereas high angle antithetic west dipping normal faults prevail farther east integrating geological and seismologic information it can be stated that the af behaves as an active extensional fault zone and represents the basal detachment of the west dipping seismogenic normal faults of the umbria marche region the af belongs to a regional ne dipping low angle normal fault system etrurian fault system efs which extends for 350 km from northwestern tuscany to southern umbria early preliminary considerations suggest that the efs may play an important role in controlling active extension and related seismicity in northern central italy

low angle normal fault earthquakes and triggering geologic evidence for low angle normal faulting is overwhelming and recent seismic geodetic and neotectonic studies have identified probable active examples thus apparent low levels of historical seismicity on such faults is enigmatic however one large mainshock was a low angle normal event and three others may have been one earthquake sequence had significant moment release from a triggered low angle normal fault and three earthquakes may have had normal subevents on low angle planes such sub events are hard to recognize may be underrepresented in catalogs helping to explain the apparent seismic enigma and pose increased seismic hazard due to delayed strong motions triggered sub events may be along strike or down dip from their triggering shock in cases where the triggered slip is downdip of the steep fault and under its hanging wall elevated pore fluid pressure or fault weakness may be required

surface faulting of the 1954 fairview peak ms 7 2 and dixie valley ms 6 8 earthquakes central nevada abstract the 1954 fairview peak earthquake was followed 4 min and 20 sec later by the dixie valley earthquake surface ruptures were distributed among six different faults that define a complex north trending zone 100 km long and over 15 km wide the rupture sequence progressed from south to north the initial event produced a 32 km rupture of right oblique slip along the east dipping fairview fault where the components of right lateral offset and vertical separation reached 2 9 and 3 8 m respectively the northernmost ruptures occurred during the subsequent event along a 46 km portion of the east dipping dixie valley fault where the sense of slip was normal and reached a maximum vertical separation of 2 8 m average surface offset for both events is about 1 2 m the west dipping west gate louderback mountains and gold king faults also ruptured during the earthquake sequence these faults form a complex structural linkage within a 15 km left step that separates the surface traces of the fairview and dixie valley faults the west dipping faults likely played an important role in the redistribution of static stress from the fairview peak earthquake and subsequent triggering of the dixie valley earthquake right lateral offsets of more than a meter occurred along both the west gate and louder back mountains faults slip was limited to normal motion of 1 m or less along the gold king fault the change from right oblique motion along the fairview fault to pure normal motion on the dixie valley fault may reflect the transition from principally northwest extension in the south to more westerly extension in the north alternatively slip at the latitude of the dixie valley fault may be partitioned between pure normal slip along the dixie valley fault and strike slip motion on faults immediately to the west observations of fault dip provide insight to the complex three dimensional geometry of the rupture sequence of particular note a significant section of the dixie valley fault appears to be characterized by dip angles as low as 25 at the surface slip variation or earthquake endpoints do not generally appear to have been influenced by changes in bedrock type we do not observe systematic relations between slip variation and aspects of fault trace geometry such as salients and reentrants yet the relative competence of footwall bedrock appears to have influenced the long term development of large salients and reentrants along both the dixie valley and the fairview faults as well as the style of rupture along the southern portion of the dixie valley fault

creep and locking of a low angle normal fault insights from the altotiberina fault in the northern apennines italy while low angle normal faults have been recognized worldwide from geological studies whether these structures are active or capable of generating big earthquakes is still debated we provide new constraints on the role and modes of the altotiberina fault atf in accommodating extension in the northern apennines we model gps velocities to study block kinematics faults slip rates and interseismic coupling of the atf which is active and accounts with its antithetic fault for a large part of the observed chain normal 3 mm yr tectonic extension a wide portion of the atf creeps at the long term slip rate 1 7 0 3 mm yr but the shallow locked portions are compatible with m 6 5 earthquakes we suggest that positive stress accumulation due to atf creep is most likely released by more favorable oriented splay faults whose rupture may propagate downdip along low angle normal fault surface and reduce the probability of occurrence of a seismic rupture of the shallower locked portion

coupled basin evolution and late stage metamorphic core complex exhumation in the southern basin and range province southeastern arizona abstract records of lithospheric extension and mountain range uplift are most continuously contained within syntectonic sedimentary rocks in basins adjacent to large structural culminations in southeastern arizona metamorphic core complexes form mountain ranges with the highest elevations in the region and supposedly much less extended terranes lie at lower elevations adjacent to the santa catalina rincon metamorphic core complex within the tucson basin stratigraphic sequence geometries evident in a large suite of 2 d seismic reflection data suggest a two phase basin evolution model controlled by the emplacement and subsequent uplift of the core complex in its earliest stage phase i of basin formation was characterized by extensive faults forming relatively small scale proto basins which coalesced with the larger basin bounding detachment fault system synextensional sedimentation within the enlarging basin is evidenced by sediment growth packages derived from adjacent footwall material fanning into brittle hanging wall faults during this phase volcanism was widespread and growth packages contain interbedded sediments and volcanic products but paradoxically no mylonitic clasts from the adjacent metamorphic core complex phase ii of basin evolution begins after a significant tectonic hiatus and consists of a symmetric deepening of the central basin with the introduction of mylonitic clasts in the basin fill this is coupled with the activation of a series of high angle normal faults ringing the core complex these observations suggest a two phase model for metamorphic core complex evolution with an initial stage of isostatic core complex emplacement during detachment faulting that resulted in little topographic expression this was followed after a significant tectonic hiatus by late stage exhumation and flexural uplift of the santa catalina rincon metamorphic core complex through younger high angle faulting moreover the geometry of upper basin fill units suggests an extremely low effective elastic thickness in the region and that flexural uplift of the core complex induced asymmetric transfer of ductile mid crustal rocks from beneath the subsiding tucson basin to the uplifting mountain range

the northwestern margin of the basin and range province part 1 reflection profiling of the moderate angle 30 surprise valley fault abstract seismic reflection profiling demonstrates that the active significant offset surprise valley fault that marks the western boundary of the basin and range province in northernmost california dips at a moderate angle only 30 to 2 km depth a nearby seismic refraction wide angle reflection profile albeit of lower resolution shows the fault plane remains approximately planar to at least 7 km depth variably tilted volcanic strata of known ages within the surprise valley basin demonstrate that 7 km of normal slip occurred along the surprise valley fault within the past 8 4 myr at a time averaged slip rate of 1 2 mm yr and that the fault rotated from an initial dip of as much as 60 to its present dip of 30 the rotation of this isolated fault was not accomplished by domino style rotation as seen elsewhere in the basin and range province but rather by flexural rift shoulder uplift aided by mid crustal flow

lithosphere dynamics and continental deformation the unifying theme in this section is the remarkable weakness of major faults i will consider the diverse new evidence for weakness and the evidence for high pore pressure localized in faults as a fundamental cause with this background one can better understand why faults remain active even after large rotations with respect to stress i will look at large neogene 23 7 million year old rotations about horizontal axes in the basin and range province and about vertical axes along the pacific margin recent developments will be summarized from studies of neogene tectonics neotectonics in california alaska and the mississippi embayment in the context of a weak north american stress field that results mainly from topographic forces to close i will present new geophysical studies relevant to the continuing controversy over whether the basic structure of the north american mantle lithosphere was altered by an early tertiary episode of flat subduction

tectonic evolution of a low angle extensional fault system from restored cross sections in the northern apennines italy 1 broad geological and geophysical documentation is available on regional extensional systems driven by low angle normal faults however little information exists about the three dimensional geometry and the offset distribution of such extensional structures we present a new set of balanced geological sections across the extensional fault system driven by the altotiberina low angle normal fault in the northern apennines of italy we document this extensional system throughout a large set of surface field surveys and geological maps and subsurface data seismic reflection profiles and boreholes the subsurface data allowed us to define the fault deep geometry and to obtain its structural contours the fault geometry is characterized by both along dip and along strike irregularities in cross section the fault displays a staircase trajectory with the shallowest part being dome shaped and flattened to horizontal this bending could be due to the footwall uplift triggered by a footwall uploading greater than about 115 mpa the sequential restoration of five geological cross sections yields a maximum extension of about 10 km accumulated over approximately 3 ma the resulting long term slip rate is about 3 mm yr which is of the same order as the present day extensional rate measured by gps 2 5 3 0 mm yr suggesting an almost steady state extension over the last 3 ma the distribution of the extension values along the fault strike is bell shaped as expected for a continuous surface

identification of multiple generations of crosscutting domino style faults insights from seismic modeling seismic modeling and geometric analysis provide clues for identification of multiple generations of cross cutting planar rotational faults in seismic reflection profiles many elements characteristic of this structural geometry are apparent although contorted at shallow levels in unmigrated and migrated synthetic seismograms structural features potentially signaling an earlier generation of rotational faulting within a tilt block terrain include first generation fault segments intrablock terminations of horizons steep dips of prefault strata anomalously high fault strata intersection angles composite basin geometry truncations of early synextension sedimentary wedges and anomalous crustal thinning evaluation of published profiles from the bay of biscay and galicia bank indicates that the crosscutting domino fault model probably is not applicable in these areas as previously suggested while the model may be appropriate for the basement structure of spring valley nevada seismic evidence is equivocal

present activity and seismogenic potential of a low angle normal fault system citta di castello italy constraints from surface geology seismic reflection data and seismicity abstract we present new constraints on an active low angle normal fault system in the citta di castello sansepolcro basin csb of the northern apennines of italy new field data from the geological survey of the carta geologica d italia carg project define the surface geometry of the normal fault system and lead to an interpretation of the crop 03 deep crust seismic reflection profile castiglion fiorentino urbania segment with particular attention paid to the geometry of the plio quaternary extensional structures surface and sub surface geological data are integrated with instrumental and historical seismicity in order to define the seismotectonics of the area low angle east dipping reflectors are the seismic expression of the well known altotiberina fault af a regional extensional detachment on which both east and west dipping high angle faults bounding the csb sole out the af breakaway zone is located 10 km west of the csb within the extensional allochthon synthetic east dipping planes prevail displacement along the af is 4 5 km which agrees with the cumulative offset due to its synthetic splays the evolution of the csb has mainly been controlled by the east dipping fault system at least since early pleistocene time this system is still active and responsible for the seismicity of the area a low level of seismic activity was recorded instrumentally within the csb but several damaging earthquakes have occurred in historical times the instrumental seismicity and the intensity data points of the largest historical earthquakes 5 events with maximum mcs intensity of ix to ix x allow us to propose two main seismogenic structures the monte santa maria tiberina mmax 5 9 and citta di castello mmax up to 6 5 normal faults both are synthetic splays of the af detachment dipping to the ne at moderate 45 50 to low 25 30 angles and cutting the upper crust up to the surface this study suggests that low angle normal faults at least with dips of 25 30 may be seismogenic

geologic structural and thermochronologic constraints on the tectonic evolution of the sierra mazatan core complex sonora mexico new insights into metamorphic core complex formation 1 the sierra mazatan in northwestern mexico is the southernmost metamorphic core complex in the north american cordillera large magnitude tertiary extension at sierra mazatan involved both ductile and brittle slip along a major normal fault that presently dips 10 15 west extension was polyphase and involved an early period of extension from 25 to 23 ma followed by major slip from 21 to 16 ma total slip was 20 km and occurred at rates of 3 4 mm a this extension predated the plate boundary change to transtension at 12 ma and was largely decoupled from relative pacific north american plate motion numerous lines of evidence suggest that the presently low angle normal fault initiated at a steep dip 50 60 and was rotated to lower angles during slip when corrected for this tilting fault corrugations at sierra mazatan had a similar geometry to the segmentation of many active normal faults which is compatible with their origin as primary fault features many aspects of the sierra mazatan are comparable to large active normal faults indicating that this core complex formed owing to prolonged extension on an otherwise typical high angle normal fault therefore core complexes need not represent a fundamentally unique mode of crustal extension

low angle normal faults and seismicity a review although large low angle normal faults in the continental crust are widely recognized doubts persist that they either initiate or slip at shallow dips 30 because 1 global compilations of normal fault focal mechanisms show only a small fraction of events with either nodal plane dipping less than 30 and 2 andersonian fault mechanics predict that normal faults dipping less than 30 cannot slip geological reconstructions thermochronology paleomagnetic studies and seismic reflection profiles mainly published in the last 5 years reinforce the view that active low angle normal faulting in the brittle crust is widespread underscoring the paradox of the seismicity data for dip slip faults large enough to break the entire brittle layer during earthquakes m w 6 5 consideration of their surface area and efficiency in accommodating extension as a function of dip suggests average recurrence intervals of earthquakes r tan assuming stress drop rigidity modulus and thickness of the seismogenic layer do not vary systematically with dip if the global distribution of fault dip normalized to total fault length is uniform the global recurrence of earthquakes as a function of dip is shown to be r tan sin this relationship predicts that the frequency of earthquakes with nodal planes dipping between 30 and 60 will exceed those with planes shallower than 30 by a factor of 10 in good agreement with continental seismicity assuming major normal faults dipping more than 60 are relatively uncommon revision of andersonian fault mechanics to include rotation of the stress axes with depth perhaps as a result of deep crustal shear against the brittle layer would explain both the common occurrence of low angle faults and the lack of large faults dipping more than 60 if correct this resolution of the paradox may indicate significant seismic hazard from large low angle normal faults

cenozoic crustal extension in southeastern arizona and implications for models of core complex development abstract in conventional models of cordilleran style metamorphic core complex development initial extension occurs along a breakaway fault which subsequently is deformed into a synform and abandoned in response to isostatic rebound and new faults breaking forward in the dominant transport direction the catalina core complex and associated geology in southeastern arizona have been pointed to as a type example of this model from southwest to northeast the region is characterized by the nw se trending tucson basin the catalina core complex the san pedro trough and the galiuro mountains the catalina core complex is bounded by the top to the southwest catalina detachment fault along its southwestern flank and the low angle northeast dipping san pedro fault along its northeastern flank the galiuro mountains expose non mylonitic rocks and are separated from the san pedro trough to the southwest by a system of low to moderate angle southwest dipping normal faults this galiuro fault system is widely interpreted to be the breakaway zone for the catalina core complex it is inferred to be folded into a synform beneath the san pedro trough to resurface to the southwest as the san pedro fault and to have been abandoned during slip along the younger catalina detachment this study aimed to test this model through analysis of field relations and geochronological age constraints and reprocessing and interpretation of 2 d seismic reflection data from the catalina core complex and san pedro trough in contrast to predictions of the conventional breakaway zone model we raise the possibility of a moderate angle southwest dipping detachment fault beneath the san pedro trough that could extend to mid crustal depths beneath the eastern flank of the catalina mountains we present an alternative kinematic model in which extension was accommodated by a pair of top to the southwest normal fault systems the catalina and galiuro detachment faults with the only major difference between the two being the magnitude of displacement which was greater for the catalina detachment

origin of methane in quartz crystals from the tertiary accretionary wedge and fore arc basin of the western carpathians abstract quartz crystals from mineralised joints of the carpathian flysch contain fluid inclusions covering the compositional range from gaseous ch4 co2 c2 mixture through ch4 saturated oil to ch4 saturated aqueous solution the c isotope composition of ch4 extracted from the fluid inclusions 31 and 36 v pdb is indicative of a thermogenic dry gas released from overmature rocks although the maturation level of the surrounding flysch corresponds to a wet gas generating zone trapping pt conditions for the ch4 bearing fluids range between 130 and 205 c and 0 5 and 3 7 kbar the fluid inclusion derived crystallisation temperatures are inconsistent with the vitrinite reflectance data strongly fluctuating fluid pressures during growth of the quartz crystals indicate the existence of a crack seal mechanism with recurrent hydrostatic and lithostatic regimes fluid pressures exceeding the lithostatic load have been recorded in the dukla nappe the overpressures have been generated by fluid volume increase due to thermal decomposition of oil and kerogen to ch4 and pyrobitumen occurrence of oil and condensate in the ch4 bearing inclusion fluids trapped at temperatures considerably above oil window suggest a short termed transient post orogenic heating rather than a prolonged heating related to burial the thermal event giving rise to the quartz crystallisation has been coincidental with regional collapse and uplift of the western carpathians

genesis of post collisional granitoids and basement nature of the junggar terrane nw china nd sr isotope and trace element evidence the post collisional magmatism of the junggar terrane is characterized by intrusion of large amounts of granitoids and minor basic ultrabasic rocks the granitoids comprise two magmatic suites calc alkaline and alkaline which were emplaced contemporaneously at about 294 ma the calc alkaline rocks are typically sodium rich na2o k2o 1 1 3 5 and metaluminous a nk 1 0 a cnk 0 78 1 04 they show mildly fractionated ree patterns la yb n 15 and spidergrams with strong depletion of nb ti and to a lesser extent sr the alkaline granites have high contents of sio2 75 78 alkalis nb hree y sn f and high feo mgo ratios and huge sr and eu depletion in the spidergrams moreover they display characteristic tetrad ree patterns and non charac trace element behaviour the two rock suites have similar initial nd and sr isotopic compositions with end t in the range 5 2 to 7 1 and isr mostly in the range 0 7031 0 7041 this points to a predominance of juvenile components in their sources the calc alkaline rocks are most probably derived by dehydration melting of a basic lower crust leaving behind a granulite residue the process was probably triggered by underplating of mantle derived basic magmas in an extensional regime the alkaline granites are considered to have formed by differentiation of the calc alkaline granitoids our study argues for a juvenile continental crust for the basement of the junggar terrane which is likely dominated by early paleozoic oceanic crust and arc complex that were deeply buried during the late paleozoic subduction and accretion

along strike upper plate deformation in response to metamorphic core complex emplacement se arizona abstract metamorphic core complexes represent concentrated zones of crustal extension universally the hanging walls of these systems are extensively fractured attesting to the significant horizontal extension above the evolving detachment zone recent analysis of a large grid of 2 d seismic reflection lines within the tucson basin of southeastern arizona has facilitated a nearly three dimensional interpretation of subsurface features related to cenozoic crustal extension within the northern tucson basin the catalina detachment fault dips 23 35 to the southwest from the western flank of the catalina rincon metamorphic core complex in the southern portion of the basin the ne trending santa rita normal fault dips 15 20 to the northwest from the western flank of the santa rita mountains and is cut by the catalina detachment beneath the central tucson basin the orientation of the santa rita fault is problematical in that its orientation is nearly perpendicular to extension directions in the region while geologic and seismic reflection evidence indicates that the catalina detachment and santa rita fault were active synchronously one possible explanation is that the santa rita fault is accommodating along strike upper plate deformation in response to core complex emplacement to test this we employed a finite element modeling approach a two dimensional model consisting of a homogenous elastic material undergoing uniform extension is used to study changes in stresses and displacements in proximity to a zone of weakness representing a detachment fault in the upper crust away from the detachment fault primary principal stresses are oriented parallel to the regional extension direction as expected near the detachment extension in the system produces a rotation of the primary principal stresses of nearly 60 with respect to the regional stress field around the end of the fault additionally the model predicts mechanical failure of the upper plate of the detachment system to the south of the catalina core complex this model supports our interpretation that the orientation and early displacement noted on the santa rita fault is the result of a perturbation in the regional stress field caused by the catalina detachment and the associated brittle failure of the upper plate from the extreme crustal extension associated with core complex emplacement

present day motion and deformation of the colorado plateau 1 we analyze data from continuous gps stations in the southwestern united states and present the first observations of a systematic motion of the colorado plateau region relative to a north america fixed reference frame westward velocities increase rapidly across the wasatch fault system which comprises the plateau s northwestern margin but increase more gradually between the great plains and the mojave desert province we detect no significant extension across most of the rio grande rift but extension could be as high as 0 5 mm yr 1 along its southernmost portion we suggest that the motion between the plateau and stable continent may partly be accommodated instead across the jemez lineament only when we consider gps velocities within the plateau s center do the data support a rigid body rotation of 0 103 ma 1 around a pole in the northern rocky mountains we conclude that active extension has encroached into the plateau from both the east and west

reconstruction of mid cenozoic extension in the rincon mountains area southeastern arizona usa and geodynamic implications nsf ear 1649254 u s geological survey national cooperative geologic mapping program under statemap grant 03hqag0114 statemap grant 06hqag0051 g09ac00199

gently dipping normal faults identified with space shuttle radar topography data in central sulawesi indonesia and some implications for fault mechanics abstract space shuttle radar topography data from central sulawesi indonesia reveal two corrugated domal landforms covering hundreds to thousands of square kilometers that are bounded to the north by an abrupt transition to typical hilly to mountainous topography these domal landforms are readily interpreted as metamorphic core complexes an interpretation consistent with a single previous field study and the abrupt northward transition in topographic style is interpreted as marking the trace of two extensional detachment faults that are active or were recently active fault dip as determined by the slope of exhumed fault footwalls ranges from 4 to 18 application of critical taper theory to fault dip and hanging wall surface slope and to similar data from several other active or recently active core complexes suggests a theoretical limit of three degrees for detachment fault dip this result appears to conflict with the dearth of seismological evidence for slip on faults dipping less than 30 the convex upward form of the gently dipping fault footwalls however allows for greater fault dip at depths of earthquake initiation and dominant energy release thus there may be no conflict between seismological and mapping studies for this class of faults