Oblique Thrust Research Articles

Source Parameters of the 1 May 2013 mb 5.7 Kishtwar Earthquake: Implications for Seismic Hazards

Abstract We study the source mechanism and depth of the recent moderate earthquake, which occurred on 1 May 2013 in the Kashmir seismic gap. The epicenter of the earthquake lies southeast of the Kashmir valley and close to the meisoseismal zone of the 1555 Kashmir earthquake (magnitude ∼7.6). This event provides an excellent opportunity in which to study the seismotectonics of the Kashmir Himalaya using global digital seismic data. We modeled the source parameters of the earthquake by least‐squares fitting of the teleseismic P ‐ and SH ‐waveform data. The minimum‐misfit solution reveals that the earthquake occurred on an oblique thrust fault with strike, dip, and rake of 346°, 26°, and 121°, respectively, and originated at a depth of 16±3 km. The strike of the fault plane matches that of the mapped Himalayan thrust faults in the region, and its depth puts it within the Himalayan wedge, close to the basal decollement (main Himalayan thrust [MHT]). However, the dip of the causal fault plane is larger than the inferred dip of the MHT and therefore requires the earthquake to have occurred on a ramp or a splay thrust. The most likely candidate is the main boundary thrust the downdip end of which matches with the geometry of the fault plane and coincides with the observed hypocentral distribution of small‐to‐moderate earthquakes in the region. We synthesize our findings with geodetic measurements from the Kashmir Himalaya to show that the hypocentral zone of this event marks a region of strain accumulation beneath the Kashmir seismic gap. This region could possibly be the point of initiation of a future great earthquake. We conclude by highlighting the outstanding questions in assessing seismic hazards in this region and designing a broadband seismological field experiment that is currently being undertaken to address these important issues.

Push-up inversion structures v. fault-bend reactivation anticlines along oblique thrust ramps: examples from the Apennines fold-and-thrust belt (Italy)

The study of inversion structures in fold-and-thrust belts that originated as extensional faults is valuable for the development of geometric and kinematic models that are helpful in unravelling the structural style and for detecting structural hydrocarbon traps. We document push-up inversion structures versus fault-bend reactivation anticlines occurring along oblique thrust ramps, by examining examples of buried Apulian structures in the Central–Southern Apennines and outcropping thrust-related folds in the Central–Northern Apennines of Italy. The different geometric features between push-up inversion structures and fault-bend reactivation anticlines are functionally controlled by the mechanical characteristics of the involved sedimentary multilayer, the original architecture of the former extensional basin, and the amount of thrust displacement. The recognition of such inversion structures is important in understanding the anatomy and structural style of foreland fold-and-thrust belts. In addition, their identification is critical in interpreting the geometry and types of structural traps that may be expected along oblique thrust ramps. This study reveals that push-up inversion structures and fault-bend reactivation anticlines could coexist along oblique thrust ramps within a curved thrust system and may be detected among analogous foreland thrust belts that include inherited extensional basins.

Tectonic evolution of a crustal-scale oblique ramp, Hellenides thrust belt, Greece

This study investigates the tectonic evolution of the Omalos transverse zone, which served as a crustal-scale oblique ramp in the External Hellenides thrust belt on Crete island. The Omalos oblique ramp developed above an inherited Mesozoic fault zone that strikes NE–SW, oblique to the regional SSW-directed tectonic transport. During the Early Miocene–Pleistocene evolution of the thrust belt, the oblique ramp was repeatedly reactivated localizing deformation above the inherited structure. Geological and structural mapping combined with kinematic analysis of ductile and brittle structures suggest that the Omalos oblique ramp generated a local kinematic field, which deviated significantly from the regional kinematic pattern in the thrust belt. The most conspicuous feature in the tectonic evolution of the oblique ramp is a change from a ductile wrench-dominated to a brittle, primarily reverse faulting regime across the brittle–ductile transition, followed by brittle wrench deformation after the final exhumation of high-pressure (HP) rocks. Deflections of transport and compression orientations from the regional pattern are attributed to buttressing against basement-cover offsets produced by the pre-existing fault zone, to oblique ramping, and to transfer faulting, respectively. Our findings are potentially applicable to other examples of crustal-scale oblique thrust ramps in various tectonic settings.

Multidetachment analogue models of fold reactivation in transpression: The NW Persian Gulf

Two deformation events have been documented in the NW Persian Gulf during the Late Cretaceous and the Late Cenozoic. The most distinctive feature in this part of the Persian Gulf is the reactivation of the Late Cretaceous NNE-SSW Arabian trending folds by NE-SW shortening during the Late Cenozoic Zagros orogeny. In general, if a set of folds with horizontal axes is shortened roughly parallel to its fold axis, a dome-basin fold interference pattern is produced. In the NW Persian Gulf, reactivation of some old folds occurred instead of a fold interference pattern. Reactivation may be influenced by the following factors: i) the presence of incompetent layers (i.e. evaporites), ii) a variable overburden, iii) basement faults, and iv) obliquity between the younger deformation shortening axis and fold axis. It is this last factor that we investigated by means of analogue modelling. The experimental apparatus is similar to that commonly used for experiments with brittle-ductile systems at the Laboratory of Experimental Tectonics of Geosciences Rennes. The model consisted of an alternation of ductile and brittle horizontal layers with a stratigraphy similar to the one found in the NW Persian Gulf. The model was deformed by two deformation events with an angle α between the two directions of shortening. After deformation, the resulting structure resembled a fold facing the static wall with internal thrust faults and detachment faults arranged into a geometry similar to a fish tail. In the second shortening event, the fold was reactivated without formation of an interference pattern. Moreover, the displacement on both the reactivated and newly formed faults varied between almost pure thrust faults for low α and oblique thrust faults with a strike-slip component for high α. The models suggest that the presence of incompetent layers plays an important role in fold reactivation and confirm that basement faults are not necessary.

Rupture process of the Wenchuan earthquake (Mw 7.9) from surface ruptures and fault striations characteristics

On 12 May 2008, the Wenchuan earthquake (Mw 7.9) produced complicated thrust-type co-seismic surface rupture zones, which encompass the dextral-slip thrust of the Yingxiu–Beichuan fault, the approximately pure thrust of the Guanxian–Anxian fault, and the sinistral-slip thrust of the Xiaoyudong rupture zone located between the former two. In order to understand the faulting mechanism, we discuss the rupture process by examining the segmentation and kinematics of the surface rupture zones, together with the co-seismic fault striations at various sites. Based on the two along-strike main displacement peaks (6–6.5 m and 11–12 m) and on the different geometric and kinematic patterns for the southern and northern segments of the surface rupture zones, we find that the Wenchuan earthquake might have consisted of two rupture stages, which is in agreement with seismic wave inversion results. By comparing the kinematics of fault striations occurring in the Bajiaomiao and Beichuan areas, it suggests that during the first stage, thrusting along both the Yingxiu–Beichuan fault and Guanxian–Anxian fault produced the ~ 80–100 km-long Yingxiu–Qingping surface rupture segment and the ~ 80 km-long Guanxian–Anxian surface rupture zone, respectively. Then, faulting was triggered along the Yingxiu–Beichuan fault by the first rupture process, yielding the second rupture stage, which was characterized by dextral strike-slip (or dextral oblique thrusting). Due to the overlap between the two rupture stages, the southern segment (Yingxiu–Qingping) of the Yingxiu–Beichuan rupture zone comprises two different processes while the northern segment (Gaochuan–Beichuan–Shikan) only suggests one rupture phase.

The October 28, 2012 Mw 7.8 Haida Gwaii underthrusting earthquake and tsunami: Slip partitioning along the Queen Charlotte Fault transpressional plate boundary

The Pacific/North American plate boundary is undergoing predominantly right-lateral strike–slip motion along the Queen Charlotte and Fairweather transform faults. The Queen Charlotte Fault (QCF) hosted the largest historical earthquake in Canada, the 1949 MS 8.1 strike–slip earthquake, which ruptured from offshore northern Haida Gwaii several hundred kilometers northwestward. On January 5, 2013 an Mw 7.5 strike–slip faulting event occurred near the northern end of the 1949 rupture zone. Along central and southern Haida Gwaii the relative plate motion has ∼20% oblique convergence across the left-stepping plate boundary. There had been uncertainty in how the compressional component of plate motion is accommodated. The October 28, 2012 Mw 7.8 Haida Gwaii earthquake involved slightly (∼20°) oblique thrust faulting on a shallow (∼18.5°) northeast-dipping fault plane with strike (∼320°) parallel to the QCF, consistent with prior inferences of Pacific Plate underthrusting beneath Haida Gwaii. The rupture extended to shallow depth offshore of Moresby Island beneath a 25–30km wide terrace of sediments that has accumulated in a wedge seaward of the QCF. The shallow thrusting caused seafloor uplift that generated substantial localized tsunami run-up and a modest far-field tsunami that spread across the northern Pacific, prompting a tsunami warning, beach closure, and coastal evacuation in Hawaii, although ultimately tide gauges showed less than 0.8m of water level increase. The mainshock rupture appears to have spread with a ∼2.3km/s rupture velocity over a length of ∼150km, with slip averaging 3.3m concentrated beneath the sedimentary wedge. The event was followed by a substantial aftershock sequence, in which almost all of the larger events involve distributed intraplate normal faulting extending ∼50km oceanward from the QCF. The highly oblique slip partitioning in southern Haida Gwaii is distinctive in that the local plate boundary-parallel motion on the QCF may be accommodated either by infrequent large strike–slip ruptures or by aseismic creep, as seems to be the case for deeper oblique relative plate motion beneath Haida Gwaii, while the sedimentary terrace accumulates plate boundary-perpendicular compressional strain that releases in almost pure thrust faulting earthquakes, seaward of the QCF.

Seismogenesis and earthquake triggering during the Van (Turkey) 2011 seismic sequence

article i nfo Article history: A unique and very interesting earthquake of magnitude Mw 7.2 occurred in the Van region of Turkey on October 23, 2011 that caused a heavy loss of human lives and properties. The earthquake occurred on a blind oblique thrust fault oriented towards the NE-SW direction and dipping towards NW as evidenced by focal mechanism solution and aftershock distribution. In this study, we analyzed the seismogenesis and earthquake triggering during this sequence with the help of estimated seismological parameters (b-value of frequency-magnitude relation, p-value of aftershocks temporal decay and D-value of fractal dimension), 2D mapping of b- and p-values, 3D mapping of b-value and coseismic Coulomb stress modeling. The estimat- ed seismic b-value equal to 0.89 reveals that the mainshock occurred in a highly stressed region and sequence comprised larger magnitude aftershocks due to the presence of large size asperities within the rupture zone. The normal estimate of p-value (0.98) suggests a tectonic genesis of the aftershocks sequence. The estimated D-value equal to 1.80 reveals that rupture propagated in a two-dimensional plane filled up by fractures. The spatial 2D and 3D mapping of seismic b-value suggests that the Van earthquake originated in a highly hetero- geneous fractured rock matrix with fluid intrusions into it at deeper depth beneath the mainshock hypocen- ter region. The estimated coseismic Coulomb stress using the variable slip model for depth range 0-30 km exhibits a 'butterfly' pattern and most of the aftershocks fall (90%) in the region of enhanced Coulomb stress. This suggests that most of the aftershock activities have been triggered by transfer of positive Coulomb stress due to coseismic slip of the mainshock. The results estimated in the present study have potential useful im-

蒙古国西部早–中侏罗世含煤盆地的演化和后期改造—以希奈金斯盆地为例<br> Evolution and Later Reformation of Early-Middle Jurassic Coal-Bearing Basins in Western Mongolia—A Case Study from the Shinejinst Basin

因基底构造、同造山应力场和区域演化的不同，蒙古东、西两部早–中侏罗世含煤盆地有不同的几何学和运动学特征。控制中带磨拉石盆地西段发育的是两条近北西向的压扭性断裂带，与蒙古–鄂霍次克缝合线西段的走向一致。近北西向断裂由北西西–近东西向的斜冲段和北西向的斜冲–走滑段组成，前者的前锋断层控制了下盘的含煤盆地发育。这些控盆断裂向北(东)斜冲，成盆动力来自中国西部特提斯洋早侏罗世末闭合和嗣后的碰撞产生的向北推挤。希奈金斯盆地演化可分为三个阶段，后期经历了四期改造。西部寻找下–中侏罗统大煤田的方向是希奈金斯盆地南次盆类型的构造盆地；其主要特征，一是成煤期盆地发育受北西西–近东西向斜冲断层及近北东向左行剪切断层控制，二是早白垩世的拉张事件中处于正断层的上盘，三是新生代隆升时位于山前或山麓地区。 Owing to variation of the basement structures, syn-orogenic stress fields and regional tectonic evolution, the Early-Middle Jurassic coal-bearing basins in western Mongoliashow different geometric and kinematic features from the simultaneous molasse basins in easternMongolia. The west segment of the Mongolia-Okhotsk suture is a NW- striking transfer zone, which constraints two transpressional fracture belts developing in western Mongolia. The tran- spressional fracture consists of the oblique thrust with NWW- to nearly E-W-striking, and the NW-orientated oblique thrust-strike slipping fault. The front fault of the oblique thrust controls a coal-forming basin in its lower plate. The ba- sin-controlling fault thrusts north(east)wards, with the basin dynamics resulting from a northward push-compression caused by closure of Tethyan ocean in the end of Early Jurassic in western China and followed continent-continent col- lision. The evolution of the Shinejinst Basin can be divided into three stages, which experiences four episodes of refor- mation after its reversion. The coal-search direction for the Lower-Middle Jurassic large coal fields in western Mongo- lia should be a kind of the South Sub-Basin of Shinejinst Basin, which could be indicated by three characters: 1) the basin-controlling faults being NWW- to nearly E-W-striking oblique thrust as well as NE-striking sinistral shearing fault, 2) to be situated at upper side of normal fault in Early Cretaceous extension, and 3) to be located along the piedmont belt of thrust or foothill of range in the Cenozoic uplifting.

Paleoseismology and kinematic characteristics of the Xiaoyudong rupture, a short but significant strange segment characterized by the May 12, 2008, Mw 7.9 earthquake in Sichuan, China

Three faults were involved in the rupture process of the May 12, 2008 Mw7.9 Wenchuan earthquake, representing a rarely documented case of multiple faults rupturing simultaneously. Distinct from the other two major NE-trending surface ruptures, the ~7km long Xiaoyudong surface rupture is oriented NW340° and shows thrust- and left‐slip faulting. In order to understand what role Xiaoyudong rupture played in the tectonic evolution of Longmenshan thrust system, we conducted a detailed investigation including geomorphic feature observation, coseismic displacement measurement, paleo-rupture event trenching, and fault kinematic estimation of the Xiaoyudong area. Our results not only document an in situ paleoearthquake occurring at 3.2–2.3ka BP, but also present the Xiaoyudong rupturing event as an oblique thrust with minor components of left lateral slip. We also found vertical and sinistral slip rates estimated as 1.1–1.5mm/yr and 0.5–0.7mm/yr, respectively. Finally, we suggest that the Xiaoyudong rupture is not a passive tear fault but an active participator of slip partitioning on multiple faults within the Longmenshan thrust system, and moreover, it gave strong response to both the 2008 and the penultimate earthquake while it kept silence with the antepenultimate event.

The 23 July 2010 mN 4.1 Laurier-Station, Quebec, Earthquake: A Midcrustal Tectonic Earthquake Occurrence Unrelated to Nearby Underground Natural Gas Storage

Abstract On 23 July 2010, at 17:24 coordinated universal time (13:24 local time), a Nuttli magnitude ( m N ) 4.1 ( M w 3.5) earthquake occurred near Laurier‐Station, Quebec, approximately 40 km southwest of Quebec City, Quebec. The earthquake was widely felt by most people within 50 km of the epicenter and by some up to 170 km. The event had an oblique thrust mechanism at a depth of approximately 23.5 km. This focal depth indicates that the earthquake occurred in the Precambrian basement, much deeper than the 6‐km‐thick sedimentary layers at the surface. The earthquake could represent a reactivation of one of the regional normal faults imaged in the magnetic field and on reflection profiles. The immediate epicentral region is not very seismically active, and for this reason, the question arose as to whether local deep natural gas storage or shale gas hydraulic fracturing close to the area could have triggered the earthquake. The only aftershock‐recorded m N 2.1 was within 2 hours of the mainshock and took place before a field instrument was in place. Based mainly on the mid‐crustal focal depth of the earthquake, our study shows that the earthquake is not linked to the near‐surface natural gas storage or to the shale gas hydraulic fracturing in the area.

Relationships between thrusts and normal faults in curved belts: New insight in the inversion tectonics of the Central-Northern Apennines (Italy)

The relationships between thrusts and normal faults represent primary constraints in the reconstruction of the modes and timing of pre-, syn- and post-orogenic deformation events in fold-and-thrust belts. Such relationships are well exposed in curved orogenic belts where the thrusts are oblique to the trend of normal faults.We study the NNE–SSW-trending Olevano-Antrodoco-Sibillini oblique thrust and its crosscutting relationships with NW–SE-trending normal faults in order to constrain the Neogene–Quaternary deformation history of the Central-Northern Apennine (Italy). The analysis of structural and geological data allowed us to reconstruct the geometric and kinematic constraints of two inversion events: 1 – During the Pliocene, positive inversion reactivated the NNE–SSW-trending pre-existing Ancona-Anzio normal fault as the Olevano-Antrodoco-Sibillini oblique thrust ramp and caused the shortcut of NW–SE-oriented normal faults; 2 – During the Quaternary, negative inversion reactivated NW–SE-trending pre-thrusting normal faults.The growth of the NW–SE Quaternary normal faults causes seismicity and is responsible of the development of wide Quaternary intramontane basins. Their distribution and the related seismicity have been controlled and compartmentalized by NNE–SSW-trending oblique thrusts. Thus, the crosscutting relationships between thrusts and normal faults are crucial in seismic hazard assessment.

Strike-slip faulting in the central part of the Sanandaj-Sirjan Zone, Zagros Orogen, Iran

The Sanandaj-Sirjan Zone (SSZ) is one of the main basement tectonic blocks located close to the northeastern margin of the Zagros Orogenic Belt. New observations in the central part of the zone indicate that the fault pattern is dominated by NW-trending longitudinal faults. The components of movement on the fault planes are interpreted as dextral oblique thrusting and dextral strike-slip. The identified structures along the faults were associated with the strike-slip faults (e.g., Hasan-Robat and Najafabad dextral restraining stepovers) and rotated tectonic blocks (arranged in a ‘domino’ configuration) near to the Foladshahr and Kolah-Ghazi mountains. Along the longitudinal faults, the dextral offsets of the rock units, streams, alluvial deposits and rivers were measured. The dextral offset values range from 2.4 to 2.7 km, while the estimated offsets based on the geomorphological features are between 50 and 61 m for streams, up to approximately ∼50 m for alluvial deposits and up to 2.2 km for rivers. The dextral strike-slip component on the fault planes occurs along the northern and southern margins of the SSZ, as well as directly within the zone, west and east of the study area. The evidence for strike-slip faulting in the internal part of the zone suggests that the central part of the SSZ probably was horizontally sheared in a manner consistent with a simple shear ‘card-deck model’.

Lithospheric flexure in the Sichuan Basin and Longmen Shan at the eastern edge of Tibet

The mountain range at the steep eastern edge of the Tibetan Plateau, the Longmen Shan, was deformed by a Mw7.9 earthquake with oblique thrust and strike‐slip motion in 2008. The tectonic processes and structure of the lithosphere beneath this range have been controversial. Gravity measurements reflect the distribution of mass within the Earth, including the large load of rock above the geoid in mountain ranges. We investigate the response of the lithosphere to the load of the Longmen Shan and estimate the flexural rigidity or effective elastic thicknessTeusing new gravity data acquired by recent satellites and the combined GOCO2S and EIGEN‐6c datasets. The free‐air gravity anomalies over the Longmen Shan show that its mass is supported by flexure of the adjacent Sichuan Basin lithosphere, similar to the flexural support of the Himalayas. Analysis of a stacked profile of the free‐air anomalies shows that the effective elastic thickness of the basin lithosphere is greater than 10 km, but has a broad minimum misfit function with no upper limit on the thickness. Two‐dimensional admittance analysis shows theTe of easternmost Tibet is very low, approximately 7 km.

Estimation of model parameter of Sumatra earthquake using empirical Green’s function technique and generation of hypothetical earthquake scenario for Andaman Island, India

Empirical Green’s function (EGF) technique is considered to be most effective technique for simulation of ground motions due to a finite earthquake source. In the present paper, this technique has been used to simulate ground motion due to a great earthquake. The coastal region of Sumatra Island has been visited by a great earthquake on December 26, 2004. This earthquake has been recorded at several broadband stations including a nearest broadband station PSI in Indonesia. The shear wave contributions in both horizontal components have been simulated at PSI station using EGF technique. The comparison of simulated and observed waveform has been made for various possibilities of rupture parameters in terms of root mean square error. The final rupture model supports rupture velocity of 3.0 km/s with nucleation point supporting northward propagating rupture that coincide with high-slip asperity defined by Sorensen et al. (Bull Seism Soc Am 97:S139–S151, 2007). The final modeling parameters have been used to simulate record at MDRS station in coastal state of Tamilnadu, India. In an attempt to model a scenario of great earthquake in the Andaman Island, a hypothetical rupture plane is modeled in this region. The event occurred on August 10, 2008 of magnitude 6.2 (M w ) recorded on strong motion array at Port Blair has been used as EGF to simulate records due to the hypothetical great earthquake. Possibilities of earthquake due to the oblique strike-slip and thrust mechanism have been modeled in the present paper. Several possibilities of nucleation point for both cases has been considered, and it is seen that variation of peak ground acceleration at Port Blair station for strike-slip and thrust mechanism is 126–738 gals and 647–2,571 gals, respectively, which indicate high seismic hazard potential of Andaman Island.

Numerical Study on the Role of Basin Geometry and Kinematic Seismic Source in 3D Ground Motion Simulation of the 22 February 2011 Mw 6.2 Christchurch Earthquake

Almost six months after the MW 7.1 Darfield (Canterbury) earthquake, on 22 February 2011 at 12:51 p.m. (local time), an MW 6.2 earthquake struck the city and suburbs of Christchurch—the largest city on the South Island of New Zealand, with about 400,000 inhabitants. The Christchurch earthquake can be considered one of the greatest natural disasters recorded in New Zealand. The death toll was more than 180, with around 2,000 people injured, and structures already weakened by the Darfield event and its aftershocks were badly affected (Cubrinovski and Green 2010; Tonkin and Taylor Ltd. 2010; Kam et al. 2011). The earthquake was generated by an oblique thrust fault located between the Australian and Pacific plates, within about 6 km of the city center. It is worth recalling that prior to the Darfield event there was no surface evidence of the fault that generated the Christchurch earthquake on February 2011, nor of the Greendale fault, recognized as responsible for the September 2010 earthquake (Quigley et al. 2010). During the last decade a set of seismic surveys across the Canterbury Plains had been carried out (Green et al. 2010), but they did not reveal any convincing evidence of the Greendale fault and there was no clear indication that a major earthquake was imminent in this particular region. Beyond the effects and the consequences of the seismic event, the attention of the scientific community was drawn to two aspects that had a primary role in the Christchurch earthquake: 1) the extremely severe, strong ground shaking observed, especially on the vertical component; and 2) the widespread liquefaction phenomena across the city (Cubrinovski and Green 2010; Green et al. 2011, page 927 of this issue). Between September 2010 and June 2011 the Canterbury area experienced three major earthquakes with M …

Structural Controls and Evolution of Gold-, Silver-, and REE-Bearing Copper-Cobalt Ore Deposits, Blackbird District, East-Central Idaho: Epigenetic Origins

The Cu-Co ± Au (± Ag ± Ni ± REE) ore deposits of the Blackbird district, east-central Idaho, have previously been classified as Besshi-type VMS, sedex, and IOCG deposits within an intact stratigraphic section. New studies indicate that, across the district, mineralization was introduced into the country rocks as a series of structurally controlled vein and alteration systems. Quartz-rich and biotite-rich veins (and alteration zones) and minor albite and siderite veinlets maintain consistent order and sulfide mineral associations across the district. Both early and late quartz veins contain chalcopyrite and pyrite, whereas intermediate-stage tourmaline-biotite veins host the cobaltite. Barren early and late albite and late carbonate (generally siderite) form veins or are included in the quartz veins. REE minerals, principally monazite, allanite, and xenotime, are associated with both tourmaline-biotite and late quartz veins. The veins are in mineralized intervals along axial planar cleavage, intrafolial foliation, and shears. Mineralized intervals are hosted by a variety of metasedimentary rocks, including three phyllitic units of Mesoproterozoic age and two schistose units. All of these units are S-tectonites in the footwall of a regional thrust fault. Specifically, the district lies within an oblique thrust ramp containing a series of structural horses (three domains) in a duplex system. The deposits span the three domains and are hosted by metamorphic rocks that range from lower amphibolite facies in the structurally upper domain to lower-middle greenschist facies in the lower domain (an inverted metamorphic sequence). Early quartz and biotite veins were introduced during progressive folding and prolonged peak metamorphic conditions and they underwent late-tectonic retrograde recrystallization and metamorphic mineral growth, to the same extent as the country rocks in each domain. Where little subsequent deformation occurred, early veins are discordant to bedding but, where folding was polyphase and fabrics are penetrative, mineralized zones are concordant with metamorphic compositional layering. Late quartz veins in the zones are associated with retrograde minerals and textures and are only locally deformed. 40 Ar/ 39 Ar dating of unoriented muscovite from the selvage of a late quartz vein yields a Late Cretaceous age of about 83 Ma, the time of retrograde metamorphism associated with introduction of late quartz veins. Textural data at all scales indicate that the host sites for veins and the tectonic evolution of both host rocks and mineral deposits were kinematically linked to Late Cretaceous regional thrust faulting. Heat, fluids, and conduits for generation and circulation of fluids were part of the regional crustal thickening. The faulting also juxtaposed metaevaporite layers in the Mesoproterozoic Yellowjacket Formation over Blackbird district host rocks. We conclude that this facilitated chemical exchange between juxtaposed units resulting in leaching of critical elements (Cl, K, B, Na) from metaevaporites to produce brines, scavenging of metals (Co, Cu, etc) from rocks in the region, and, finally, concentrating metals in the lower-plate ramp structures. Although the ultimate source of the metals remains undetermined, the present Cu-Co ± Au (± Ag ± Ni ± REE) Blackbird ore deposits formed during Late Cretaceous compressional deformation.

Late Devonian transpressional tectonics in Spitsbergen, Svalbard, and implications for basement uplift of the Sørkapp–Hornsund High

Abstract: The Late Devonian Svalbardian event relates to the final activity in the Caledonian Orogen, and affected Devonian strata in Spitsbergen by major folding, oblique thrusting and basement uplift. In southern Spitsbergen, the Devonian deformation and the complementary, presumed Mid-Carboniferous Adriabukta event deformation caused uplift of the Sørkapp–Hornsund basement high or horst. This high is fault-bounded by Devonian sandstones and a questionably aged Early or Mid-Carboniferous mudstone unit (Adriabukta Formation). The Adriabukta Formation at Hornsund occurs in the core of a major syncline, with underlying Devonian strata in the west limb, all truncated in the footwall by a steep, east-side-up oblique-reverse fault. Mid- to Late Carboniferous rifting reversed the motion and produced rift-fill deposits, and these strata overlie the deformed Devonian rocks and the Adriabukta Formation with an angular unconformity. A similar basin architecture and major syncline bounded by a reverse fault with lateral movement characterize the Svalbardian deformation in the Mimerdalen–Pyramiden area at Billefjorden farther north. Similarity also exists between a Late Devonian unit (Plantekløfta Formation) at Mimerdalen and the Adriabukta Formation at Hornsund, and we question the previous interpretation of the Adriabukta Formation as Carboniferous. Rather, we suggest that the Adriabukta and Svalbardian deformation events may have been part of the same event.

Transposition of structures in the Neoproterozoic Kaoko Belt (NW Namibia) and their absolute timing

Three structural profiles across the Coastal Terrane, the Boundary Igneous Complex and the Orogen Core have been studied in the Kaoko Belt of northwestern Namibia. The oldest known Si fabric is inherited from an older tectono-metamorphic event. It occurs in the Coastal Terrane only and the extent of its reworking increases from south to north. The S1 foliation reactivates or folds Si fabric in the Coastal Terrane and appears as an early planar fabric in granitoids of the Boundary Igneous Complex and migmatites of the Orogen Core domain. Superimposed subvertical S2 fabric corresponds to axial plane cleavage of upright close to isoclinal folds and the extent of its development also increases from south to north. Active migration of partial melt during S2 development in the Orogen Core dates the onset of this deformation at ~550 Ma. Distribution of F2 fold axes and L2 stretching lineations suggests pure shear–dominated deformation associated with development of N–S trending S2 cleavage preserved in the central profile, followed by sinistral simple shear–dominated deformation on newly developed NW–SE trending pervasive cleavage in the northern part of the area. Such spatial variation in the deformation record is attributed to the irregular shape of the Congo Craton indenter that is reflected by heterogeneous development of the S2 cleavage front in the Coastal Terrane and the Boundary Igneous Complex. Common orientation of L1 and L2 stretching lineations and solid-state reworking on both S1 and S2 planes suggest single event of sinistral transpression since 550 Ma with strain partitioning into domains of oblique thrusting (reactivated S1) and transcurrent sinistral shearing (S2 and S3). Such succession of deformation structures suggests that major subvertical shear zones in the Kaoko Belt do not correspond to early crustal discontinuities, but rather reflect late strain localization during cooling.

Regional provenance study of Eocene clastic sedimentary rocks within the South America–Caribbean plate boundary zone using detrital zircon geochronology

Previous on- and offshore studies have postulated that the Caribbean plate has translated hundreds of kilometers eastward during the Cenozoic along strike-slip and oblique thrust faults bounding the northern margin of the continental South America plate. Two previously proposed tectonic-sedimentary models to explain the complex linkages between plate motions and sedimentation within the broad plate boundary zone include: 1) eastward bulldozing by the Caribbean plate of a single, large point source, thick Eocene proto-Maracaibo deltaic system of northwestern South America, over 1000 km to the east and incorporation of these continentally derived sediments into the ∼ 12-km-thick Barbados accretionary prism along the leading edge of the Caribbean plate; and 2) eastward bulldozing by less than 300 km of smaller point and line sources of Eocene and younger clastic sediments derived from erosion of the Guyana shield located in north-central and northeastern South America. We test both models by sampling eight Eocene localities that span a 1200-km-length of the plate boundary zone from the proto-Maracaibo delta in western Venezuela to Barbados Island in the subaerial part of the large accretionary prism bounding the eastern margin of the Caribbean plate. Ages of 972 single grains from samples at these eight localities support the multiple-source model, in which the Barbados prism was partly constructed from the bulldozing and incorporation of smaller point and line sources derived from older-than 1500 Ma crustal provinces of the Precambrian Guyana shield in central and northeastern South America. Eocene clastic sediments of the proto-Maracaibo delta derived from Paleozoic and Precambrian crustal provinces in northwestern South America are distinct in their ranges of detrital zircon ages from the ranges of the Guyana shield sources to the east.

Slip partitioning in the northeast Pamir–Tian Shan convergence zone

Based on a detailed analysis of satellite imagery combined with field geologic and geomorphic observations, we have mapped late Cenozoic folds and faults in the northeastern Pamir–Tian Shan convergence zone. It is a unique example to understand intracontinental ongoing mountain building within India–Eurasia collision system. In the front of northeastern Pamir, our investigations reveal that the NW-WNW-trending folds display a right-stepping en echelon pattern and NW-WNW-striking faults are mainly characterized by south-dipping thrusts with an extensive dextral strike-slip component. Drainage systems across the active faults show a systematic right-lateral offset. In contrast, structural style of the ENE trending fold-and-thrust belts are predominated by south–north directed shortening southwest of the Tian Shan. Our results also infer that oblique thrusting accommodates as long-term dextral slip rate of ca. 4.0 mm/yr during the late Cenozoic time north of the Pamir topographic front. Tectono-stratigraphic evidence suggests that the tectonic deformation was initiated at ca. 3–5 Ma in the study area. We suggest that intracontinental mountain building in the Pamir–Tian Shan convergence zone should be attributed to the crustal shortening caused by folding and thrusting as well as block rotation related to strike-slip faulting within the India–Eurasia collision system.