controls on the geometry of transfer zones in the suez rift and northwest red sea implications for the structural geometry of rift systems two main types of transfer zones are mapped in the suez rift and the northwestern red sea these are transfer zones between indi vidual faults fault to fault transfer zones f f tz and transfer zones between half grabens with different fault domains half graben to half graben transfer zones h h tz relay ramps and linking transfer faults are two main recognized types of f f tz different varieties of these f f tz have the form of en echelon fault belts with single or mixed polarity tilted fault blocks at rift offsets and zigzag fault arrays at major fault or rift jumps relay ramps transfer throw between overlapping normal faults of the same polarity linking transfer faults join normal faults that have the same polarity with no overlapping segments leading to a characteristic zigzag pattern if the linked faults are listric local anticlines and synclines develop at the corners of the zigzag fault array three h h tz in the suez rift and northwest red sea show the effect of prerift structures both faults and major folds on the location orientation and style of deformation of the transfer zones of continental rifts the widest one of these three h h tz 40 60 km has a broad antiformal structure lying between the overlapping ends of the listric faults bounding the two linked half grabens and is deformed by dip slip normal faults of opposite polarity the other two h h tz reactivate prerift faults by strike slip faulting due to the angular relationship between these faults and the direction of extension in the rift

changing stress field in the middle segment of the tan lu fault zone eastern china abstract the tan lu active fault zone is an important tectonic boundary in east china minor structures associated with the fault show that the nature of displacement on the middle segment of the fault zone has changed in consecutive episodes since the cretaceous normal faulting during the cretaceous sinistral strike slip and reverse faulting at the end of the cretaceous or in the early tertiary and dextral strike slip and reverse faulting from the late tertiary to the present the regional orientation of the maximum principal compressive stress based upon the statistical study of minor shear fracture orientation is 6 s52 w in the late tertiary to quaternary this stress field may be related to the collision between india and tibet and the microspreading of the japan sea

motion of the pacific plate relative to eurasia and its potential relation to cenozoic extension along the eastern margin of eurasia extension along the eastern margin of eurasia has been regarded commonly as a far field effect of the india eurasia collision however some aspects of the timing and location of this extension make a link between the collision and extension difficult we suggest that some extensional features commonly interpreted as effects of the collision may be related more simply to changes in plate convergence rates along the eastern plate boundary of eurasia an analysis of the motion of the pacific plate relative to eurasia suggests that the rate of pacific eurasia convergence varied significantly during the tertiary period from a late cretaceous convergence rate of 120 140 mm yr the rate declined substantially during early tertiary time and reached a minimum in eocene time of 30 40 mm yr in oligocene to earliest miocene time the average convergence rate increased moderately to 70 95 mm yr then decreased again to 65 70 mm yr during early to middle miocene time from late miocene to the present the rate of convergence increased to an average of 100 110 mm yr the paleocene through middle miocene episode of relatively slow convergence correlates with a period of widespread extension along the eastern margin of eurasia decreased convergence may have been related to a net reduction in horizontal compressional stress transmitted between the pacific and eurasian plates which resulted in widespread extension adjacent to the margin of eurasia

3 d seismic interpretation of tectonic wrenching and faulting in la cira infantas a current problem in oil recovery is knowledge of the reservoir s external geometry and the effect of geologic heterogeneities on hydrocarbon flow geologic heterogeneities compartmentalize reservoirs into flow units preventing efficient drainage and sweep of reservoirs understanding 3 d geometry spatial organization genesis and evolution of folds and faults is essential to providing a realistic conceptual model for flow studies wireline log based structural models of mature giant oil fields such as la cira infantas colombia do not provide clear delineation of production boundaries and compartments in la cira infantas the characteristics and evolution of its structural style are not well understood published interpretations suggest that la cira infantas is a complex combination of coeval folds thrust faults and frequent normal faults the field is the westernmost expression of a thrust belt that extends out into the middle magdalena valley basin from the eastern cordillera mountains yet structural elements are not concordant with characteristics of a typical compressive tectonic regime as previously claimed in the literature to solve these uncertainties this paper presents a rigorous model of faulting for la cira infantas a very thorough and detailed description and documentation of the structural features based on the interpretation of a 3 d seismic data set support the model la cira infantas field is in the middle magdalena valley basin mmvb near the center of colombia about 250 km north of bogota figure 1 this basin is an elongated depression that covers approximately 30 000 km2 it is 500 km in length n s and 60 km in width figure 1 geologic map and producing oil fields of the middle magdalena valley basin after geotec 1988 ingeominas 1976 and ecopetrol 2000 mmvb s geologic evolution involved distinct stages of tectonic development 1 aulocogen basin in the triassic to jurassic 2 passive margin basin in the cretaceous to paleocene and 3 foreland basin

uplift and extension at the gulf of suez indications of induced mantle convection reconstruction of tectonic movements since the initiation of the gulf of suez rift indicates that there has been 25 27 km of extension the uplift bordering rift indicates that lithosphere heating greatly exceeds that producible by uniform lithosphere extension i propose that small scale convection induced by the rifting augments the heating introduced by extensional rifting and produces the broad uplifts flanking the rift

tectonic evolution of the tancheng lujiang tan lu fault via middle triassic to early cenozoic paleomagnetic data the north striking tancheng lujiang tan lu fault is a conspicuous and controversial feature of the eastern asian landscape near the southeast extremity of the fault in anhui province we collected paleomagnetic samples at 17 middle triassic t2 and 10 upper cretaceous k2 to lower cenozoic e1 sites t2 remanent magnetizations are interpreted as primary in two of three areas the three areas are rotated 37 to 137 counterclockwise with respect to the south china block scb reference direction k2 e1 remanent magnetization directions pass regional fold and reversals tests and are not rotated with respect to surrounding areas counterclockwise rotation of t2 strata therefore ended before k2 and is attributed to left lateral shear acting along tan lu during the north china block ncb scb collision in shandong province 700 km north of the anhui sites four areas containing 33 upper jurassic j3 and cretaceous sites have negligible declination differences except for one which has dispersed directions the fold test is inconclusive for this latter area and positive for the other three regional concordance of the j3 e1 paleomagnetic data including paleolatitudes together with observed deformation patterns suggest that an extensional regime prevailed in the late cretaceous and cenozoic euler pole positions that constrain the north south china collision and account for tan lu motion suggest at least 500 km of sinistral shear took place along the fault and either 1 subduction and related ultrahigh pressure uhp metamorphism occurred near the present location of the qinling dabieshan and sulu uhp belts while tan lu acted as a transform fault that connected the two subduction zones or 2 tan lu and sulu were parts of the same transform fault system and no uhp rocks formed in situ at sulu in either case uhp rocks originally exhumed near dabieshan could have been transported by plate capture toward sulu along tan lu after north and south china impacted near dabieshan the tan lu fault grew within the scb as the dabieshan corner indented the scb causing folds in scb cover rocks to conform to the ncb margin late cretaceous to cenozoic reactivation of tan lu with both right lateral strike slip and normal fault motion occurred as the scb extruded east relative to the ncb under the influence of the india asia collision

wrench faulting in the northern pakistan foreland the acquisition processing and interpretation of reflection seismic gravity and magnetic data in conjunction with detailed geological mapping throughout the kohat plateau have resulted in a reinterpretation of the structural style of this portion of the northern pakistan foreland our interpretation of the recently acquired geologic and geophysical data as well as a critical reevaluation of previously acquired data and regional geology indicates that the kohat plateau has undergone major episodes of strike slip faulting basement involved reverse faulting and minor thrust faulting consistent with transpressional tectonics our interpretation contrasts with previous models that invoked only southward directed thrust faulting the principal key to the reinterpretation of seismic data was depth migration before stack mbs seismic processing which was performed on five seismic lines to resolve the structural geology mbs processed seismic data show complex deformation and steep dips more accurately than other methods and allowed us to identify the high amplitude structures in the kohat plateau surface geological mapping in the kohat plateau and adjacent areas indicates tight anticlines consistent steep dips horizontal motion fabrics and a conspicuous lack of low angle thrust faults aeromagnetic and gravity data indicate that precambrian basement is involved in the deformation modern seismicity indicates that faults mapped at the surface and subsurface are dominated by strike slip displacement the kohat plateau is located near the intersection of the transcurrent chaman fault system a 1000 km long system of north south striking sinistral strike slip faults and east west striking contractual structures that form the grain of the himalayan mountain chain in this area of northwest pakistan surface and subsurface structures fit the expected regional patterns that result from transpressional deformation in this complex zone of intersection our reinterpretation has applicability to other deformed belts worldwide that exhibit analogous characteristics and criteria of transpressional tectonics but are interpreted as being dominated by low angle thrust tectonics

collision between the north and south china blocks a crustal detachment model for suturing in the region east of the tanlu fault a crustal detachment model based on the interpretation of linear aeromagnetic anomalies surface geological observations and deep seismic profiles is proposed for the continent continent collision between the north and south china blocks east of the tanlu fault the model suggests that during the mid mesozoic collision between the two continental blocks the upper crust of the south china block in the subei yellow sea region was detached from the lower crust and thrust over the north china block for 400 km whereas the lower part of the lithosphere was subducted under the north china block along a subsurface suture running east from nanjing the sinistral offset of the qinling suture by the tanlu fault is only 110 120 km in the deep crust much less than previously suggested

coesite bearing eclogite from the dabie mountains in central china coesite and coesite pseudomorphs are recognized in eclogite from the dabie mountains a collision zone between the sino korean and yangtze cratons in central china this third known occurrence of coesite in deep crustal rocks is within an archean gneiss terrane and is the first locality where coesite has been identified as an inclusion in both omphacite and garnet crystals at the same location coesite bearing eclogite is estimated to form at pressures of 28 kbar and temperatures from 600 to 710 c the dabie mountains eclogite underwent retrograde metamorphism to amphibolite facies after its formation the preservation of coesite in eclogite is found to be strongly controlled by the pressure temperature time path that coesite bearing eclogite followed during its uplift and retrograde metamorphism

the cenozoic rift system of the north china plain and the deep internal process abstract the cenozoic north china plain rift system generated on the site of a mesozoic rift system is the largest rifted basin in east china it trends nne with a total length of about 1000 km maximum width up to 400 km in its middle part and a width of 50 100 km at its south and north the largest thickness of the cenozoic deposits reaches 12 000 m and the accumulated thickness of the cenozoic basalts about 2000 m this rift system is a region of strong seismicity it has quite considerable differential crust movement at the present day the geophysical data indicate that the thickness of the crust and lithosphere beneath the rift system is approximately 4 8 km and 40 km thinner than that of the surrounding mountains this rift system has rather high heat flow values and residual gravity anomalies so far no low velocity anomaly of pn has been found in this region we infer that the formation of this rift system is caused by upwelling of the asthenosphere which is probably related to the convective roll induced by subduction of the pacific plate underneath the asiatic continent and the disturbance of the upper mantle in this region during the mesozoic

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

right lateral shear along the northwest pacific margin and the india eurasia collision right lateral shear along the eastern margin of asia from the eocene to the present has led to the opening of pull apart basins intracontinental such as the bohai basin or oceanic such as the japan sea we suggest in this paper that this right lateral shear is a consequence of indentation of asia by india as in small scale analog experiments we conclude that antithetic wrench faults accommodate the counterclockwise rotation of large domino blocks between two major left lateral shear zones tien shan baikal stanovoy for the northern one and qin ling for the southern one we discuss the compatibility of this mechanism which involves a rather small amount of extrusion with the fast eastward expulsion described for southeast asia we re emphasize the role played in the opening of marginal basins by the pacific subduction as a free boundary to the east

neogene volcanism and holocene earthquakes in the tanlu fault zone eastern china abstract in this paper we study the relationship between neogene volcanism and holocene earthquakes in the tanlu fault eastern china we find that fault segments through which neogene and quaternary magma have extruded do not show holocene slip because they are covered by unfaulted basalts in contrast fault branches that are away from the quaternary volcanic centers display quaternary faulting and are responsible for earthquakes as recorded both historically and geomorphologically therefore magma intrusion appears to modify fault activity in two different ways 1 within the faults the cooling magma serves as a cohesion or barrier welding the faults so that they become stronger in resisting slip 2 beneath the faults the upwelling magma promotes slip of faults above the magma body and hence generation of earthquakes physically the first case results from contraction of the cooling magma which causes a relative increase in fault normal stress so that the fault failure resistance is enhanced the second case results from the upward dynamic force and the heat brought in by the magma body both of which cause the effective fault normal stress to decrease so that the fault failure resistance is reduced that could explain why earthquakes occur on faults bypassing the volcanic centers as typified in the yishu fault zone and in the regions where heat flows are relatively high as shown in the bohai bay area

formation of the steer s head geometry of sedimentary basins by differential stretching of the crust and mantle if the lithospheric mantle is stretched over a wider region than the crust postrift stratigraphic onlap will occur at basin margins giving rise to a steer s head geometry space problems are avoided by making the total amount of extension in the lithospheric mantle equal to that in the crust the predicted pattern of onlap agrees well with that observed if the lithospheric mantle is stretched over a fractionally wider region than is the crust thus extensive stratigraphic onlap can be produced easily without recourse to sea level rise or flexural rigidity

seismogenic strike slip faulting and the development of the north china basin the actively subsiding north china sedimentary basin is associated with an unusually high level of seismic activity this oil and gas producing basin has been the site of nine large m 7 destructive earthquakes since 1600 a d an analysis of faulting during the tangshan earthquake sequence which includes some of the largest shocks that have occurred in this basin during the past 400 years showed that the dominant pattern of deformation during this sequence was associated with displacement on right stepping right lateral strike slip faults a large amount of subsidence 1 0 1 5 m occurred in pull apart regions between steps of the north northeast nne trending strike slip faults the directivity of the p wave radiation pattern and the distribution of aftershocks of the bohai gulf earthquake of july 18 1969 also indicate strike slip faulting on planes of similar nne trend together with the four largest shocks of the hsingtai sequence in 1966 earthquakes with right lateral slip on nne trending faults account for about two thirds of the seismic moment released in this region for the past 100 years the average regional strain due to seismic slip has a dominant component of dextral simple shear on nne striking planes that is approximately twice as large as north south extension or east west shortening based on a combination of published field and borehole data the dominance of horizontal dextral simple shear over subsidence seems to have begun no later than the mid pliocene and the total amount of extension over the entire basin is only about 20 30 this pattern of deformation the spatially variable heat flow values the scattered locations of rapid deposition of quaternary sediments and the fluctuating rate of sedimentation in space and time during the neogene in this basin are difficult to explain by pure extension and thermal subsidence we propose that these observations and the shape of the whole north china basin that of a lazy z are a consequence of pulling apart associated with right lateral movement on the nne trending strike slip fault systems and that the basin as a whole has formed and evolved as a composite pull apart basin due to right lateral slip on numerous right stepping faults since the eocene

geological characteristics and distribution patterns of hydrocarbon deposits in the bohai bay basin east china abstract the bohai bay basin in east china is a meso cenozoic continental fault bounded basin with abundant petroleum resources based on analyses of sedimentation history and tectonic and thermal evolution the vertical and horizontal distribution patterns of oil and gas are discussed in this paper the basin has characteristics of multiple cycles of depression and uplift and each depression has its own sedimentary system and pattern of oil generation the main source rocks are of lower tertiary sediments characterized by multi cyclic sedimentation multiple source reservoir caprock associations were developed abrupt changes of sedimentary facies and the development of numerous faults caused the formation of various types of oil and gas reservoir that constituted multiple oil and gas accumulation zones based on individual structure zones these accumulation zones are mainly distributed around the centre of source rock deposits and are restricted in the areas where source rocks are distributed

evidence for intracontinental thrust related exhumation of the ultra high pressure rocks in china coesite and diamond bearing ultra high pressure metamorphic rocks produced during the triassic continental collision between the sino korean and yangtze plates are offset by about 530 km along the sinistral tanlu fault in eastern china the tanlu fault terminates abruptly south of the ultra high pressure metamorphic terrain in dabie shan and does not extend into the foreland fold belt along the yangtze river this suggests that in this region the mesozoic major sinistral displacement along the tanlu fault was transferred to southward directed thrusting which led to the exhumation of the ultra high pressure metamorphic rocks formed earlier postcollisional himalayan type intracontinental thrusting and concomitant erosion appear to be the main agents in the exhumation of the very high pressure metamorphic rocks in china

fracture toughness of dry snow slab avalanches from field measurements 1 dry snow slab avalanches release by propagating fractures the first fracture occurs in shear within a thin weak layer underneath a planar slab which eventually fails in tension perpendicular to the weak layer at some distance up slope thus fracture properties of both the weak layer in shear and the slab in tension are important for determining the character of the slab avalanche including its volume and hence destructive potential in this paper the fundamental fracture properties relevant during slab avalanche release are evaluated for both weak layer shear and slab tensile fracture from field measurements data from nearly 300 slab avalanches are used to estimate tensile and shear fracture toughness two important practical results come from the analysis the first is that on average slab tensile toughness is about 5 7 times the weak layer shear toughness with a range of about 2 15 this is of immense practical importance since it allows estimates of slab dimensions length and width from measured estimates of the slab depth d the second important result is that a length scale is provided for a highly stressed tensile stress boundary layer of about 20 cm from the weak layer up through the body of the slab over which the tensile crack should first form this estimate gives an important scale for field evaluation of snow slab instability for measurements from snow profiles for example it gives a guideline for estimating important hardness changes between the weak layer and the slab which have been shown from field data to be associated with human triggering of snow slabs we expect that our results will provide a framework for analysis of rock avalanches and flake type landslides which fail on weak interfaces under slabs

seismic reflection imaging of a major strike slip fault zone in a rift system paleogene structure and evolution of the tan lu fault system liaodong bay bohai offshore china the tan lu fault system in the liaodong bay bohai offshore china affords an exceptional opportunity to document the structural features of a major strike slip fault using two and three dimensional seismic reflection data as well as evolution of a strike slip fault developed coeval with a rift system the fault zone displays a relatively straight throughgoing trace longitudinally bisecting the rift valley it consists of positive and negative flower structures and en echelon folds in the south bay and three parallel flower structure systems northward the middle fault bifurcates northward into two semiparallel vertical fault strands to the north the west strand bends clockwise and merges with the east strand the stepping pattern and orientation of en echelon structures indicate right lateral sense and about n10 35e azimuth of slip the fault apparently accrued about 30 40 km 20 25 mi of post early eocene slip based on the current distribution of deformation zones and depocenters tan lu fault segments with clockwise and counterclockwise orientation relative to the regional slip direction are characterized by divergent and convergent structures i e restraining and releasing bends respectively waning of rifting eliminated the cause of a major restraining bend putting an end to development of associated convergent structures in the south bay area near the central north bay deformation occurred along major normal faults related to the basin rifting although a prominent feature reflecting regional strain partitioning the tan lu fault apparently was not a major factor in the paleogene opening of the liaodong bay basin and the larger north china rift basin