Fault Junction Research Articles

Hierarchic features of stress field in the Baikal region: Case study of the Buguldeika Fault Junction

The hierarchy of stress patterns at different evolution stages of the Central Baikal region has been studied by structural-genetic analysis of faults of different size ranks combined with other tectonophysical methods. Structural measurements were performed in the zones of the Primorsky, Olkhon, Prikhrebtovy, Buguldeika, and Kurtun faults that meet at the Buguldeika junction in the northwestern shore of Lake Baikal. Tectonic stress fields were reconstructed at the local, subregional, and regional levels in three largest fault zones (Primorsky, Olkhon, and Prikhrebtovy) and at the local and subregional levels in the smaller faults of Buguldeika and Kurtun. The stress settings in the area of the Buguldeika fault junction changed in the course of its tectonic history from Paleozoic-Mesozoic compression to Early Cenozoic strike-slip and finally to Late Cenozoic extension. The principal horizontal stress σ1 during the compression stage was oriented mainly in the NW and N–S directions. The Mesozoic N–S compression maintained northward propagation of imbricate thrusts from the Angara River headwaters as far as the Buguldeika area. The stage of strike-slip stress with N–S σ1 and W–E σ3 principal directions was reconstructed from structural measurements combined with slickenside data in the Primorsky fault zone and from slickenside data only in the Olkhon and Prikhrebtovy faults. The results show prolonged left-lateral strike-slip motions with large amounts of displacement in the Primorsky fault which delineates the Siberian craton margin. The Late Cenozoic extension stress setting has been modeled in terms of fan-like opening of large faults and clockwise rotation of upper crust blocks in the Central Baikal region.

Multifault Models of the 2019 Ridgecrest Sequence Highlight Complementary Slip and Fault Junction Instability

AbstractThe 2019 Ridgecrest Mw 6.4 and Mw 7.1 earthquakes ruptured a complex fault system, posing challenges in understanding their physical processes. Modeling of the ruptures relies on fault geometries at depth, which are usually assumed based on surface traces and aftershocks. Here we use seismic and geodetic data to jointly constrain the fault geometries and slip distributions. We first represent the first‐order rupture processes with a series of subevents, then conduct slip inversions with subevent‐guided fault geometries. We find that the foreshock sequentially ruptured the NW and SW striking faults starting from their junction. The mainshock initiated at a complex three‐fault junction along the extension of the foreshock NW rupture, with major slip first occurring bilaterally near the hypocenter and then minor unilateral slip later to the southeast end. The slip distributions of the foreshock and mainshock are complementary to each other on the overlapping fault section.

Inchworm-like source evolution through a geometrically complex fault fueled persistent supershear rupture during the 2018 Palu Indonesia earthquake

How does fault slip follow an earthquake rupture front propagating faster than the local shear-wave velocity (i.e., at supershear speed)? How does a supershear rupture front pass through a geometrically complex fault system? Resolving the evolution of such complex earthquake ruptures is fundamental to our understanding of earthquake-source physics, but these events have not been well captured by conventional waveform inversions of observational data. We applied a new framework of finite-fault inversion to globally observed teleseismic waveforms and resolved both the spatiotemporal evolution of slip and the fault geometry of the 2018 Palu earthquake (moment magnitude 7.6) in Sulawesi, Indonesia. We show that supershear rupture propagation for this event was sustained by transient slip stagnation and advancement as the rupture front passed through the geometrically complex fault system. This peculiar inchworm-like slip evolution was caused by the rupture front encountering fault bends with favorable and unfavorable orientations for rupture propagation. Our analysis also identified the possible existence of a fault junction beneath Palu Bay connecting an unmapped primary fault in northern Sulawesi with the Palu-Koro fault in the south.

Rupture Characteristics and Bedrock Structural Control of the 2016 Mw 6.0 Intraplate Earthquake in the Petermann Ranges, Australia

ABSTRACT The 20 May 2016 surface-rupturing intraplate earthquake in the Petermann Ranges is the largest onshore earthquake to occur in the Australian continent in 19 yr. We use in situ and Interferometric Synthetic Aperture Radar surface observations, aftershock distribution, and the fitting of P-wave source spectra to determine source properties of the Petermann earthquake. Surface observations reveal a 21-km-long surface rupture trace (strike=294°±29°) with heterogeneous vertical displacements (<0.1–0.96 m). Aftershock arrays suggest a triangular-shaped rupture plane (dip ≈ 30°) that intersects the subsurface projection of the major geophysical structure (Woodroffe thrust [WT]) proximal to the preferred location of the mainshock hypocenter, suggesting the mainshock nucleated at a fault junction. Footwall seismicity includes apparent southwest-dipping Riedel-type alignments, including possible activation of the deep segment of the WT. We estimate a moment magnitude (Mw) of 6.0 and a corner frequency (fc) of 0.2 Hz, respectively, from spectral fitting of source spectra in the 0.02–2 Hz frequency band. These translate into a fault area of 124 km2 and an average slip of 0.36 m. The estimated stress drop of 2.2 MPa is low for an intraplate earthquake; we attribute this to low-frictional slip (effective coefficient of friction >0.015) along rupture-parallel phyllosilicate-rich surfaces within the host rock fabric with possible additional contributions from elevated pore-fluid pressures.

КРИВОРОЖСКОЕ-III ЗЕМЛЕТРЯСЕНИЕ 23 июня 2013 г. сКР=11.6, Mwрег=4.5, I0=6 (Украина, Днепропетровская область)

On June 23, 2013, on the platform territory of Ukraine near Krivoy Rog (Dnipropetrovsk region), there was a noticeable earthquake with intensity in the epicenter zone I0=6. The quake originated in the upper part of the earth's crust of the large Dnieper block of the Ukrainian shield under the action of horizontal forces of compression and has been confined to the complex area of the junction of Krivoy Rig-Kremenchug and Lede-kinskiy deep faults. The type of movement in the source is a strike-slip with upthrust components. The most likely plane of rupture is of the North–North-East direction oriented along the Krivoy Rog-Kremenchug fault. The major axis of the first isoseist is elongated in the same direction. In terms of energy characteristics (КР=11.6, Mwreg=4.5) and macroseismic effect (I0=6), this seismic event is one of the strongest for the period of instrumental observations. By the spectra of records of volumetric seismic waves at the four stations of the Crimea, the dynamic parameters of the source have been restored using the Brune dislocation model. Detailed instrumental and macroseismic data are given. The comparison of the obtained parameters with similar parameters for other Krivoy Rog earthquakes has been given.

Spatial variations of rock damage production by earthquakes in southern California

We perform a comparative spatial analysis of inter-seismic earthquake production of rupture area and volume in southern California using observed seismicity and basic scaling relations from earthquake phenomenology and fracture mechanics. The analysis employs background events from a declustered catalog in the magnitude range 2≤M<4 to get temporally stable results representing activity during a typical inter-seismic period on all faults. Regions of high relative inter-seismic damage production include the San Jacinto fault, South Central Transverse Ranges especially near major fault junctions (Cajon Pass and San Gorgonio Pass), Eastern CA Shear Zone (ECSZ) and the Imperial Valley – Brawley seismic zone area. These regions are correlated with low velocity zones in detailed tomography studies. A quasi-linear zone with ongoing damage production extends between the Imperial fault and ECSZ and may indicate a possible future location of the main plate boundary in the area. The regions around the 1992 M6.1 Joshua Tree, M7.3 Landers and M6.3 Big Bear earthquakes have background seismic activity before 1990. This may represent a regional weakening process by damage production in future rupture zones. The depth of background seismicity and damage production decreases steadily from SW of the coastline to NE of the San Andreas fault, and also to the SE near the US–Mexico border. The seismicity and rock damage become more pronounced and continuous along-strike of main faults with increasing depth.

Brines in deep horizons of the Udachnaya kimberlite pipe

The study was focused on groundwaters sampled from boreholes drilled to deep horizons of the Udachnaya kimberlite pipe and the host sedimentary strata. Brines in the rocks significantly complicate underground mining. Analysis of the hydrogeological setting is required to ensure safety during mining to the design levels. The features of chemical composition and the geochemical evolution of brines in the crust can be clarified in a more detail on the basis of new reliable data on strong chloride saline solutions that formed in complex geological and tectonic conditions. Kimberlite and water samples were taken from the ore bodies and host sedimentary strata at the depth of 680–980 m. Conventional methods of quantitative and instrumental analysis were applied to study the chemical composition of brines. Mineral composition of kimberlite was determined by powder diffractography and X-ray fluorescence methods. Geological, structural and tectonophysical methods were used to reveal and describe the tectonic structure of the kimberlite pipe area. Groundwaters with salinity of 280–406 g/L are strong and very strong calcium chloride brines. The chlorine-bromine ratio has a small range of 48–57; the sodium-chlorine ratio varies from 0.11 to 0.18. According to their geochemical features, the studied groundwaters are metamorphosed brines that have analogues across the Siberian platform. Physical and chemical processes were simulated to investigate the degrees of saturation of strong brines relative to the minerals of water-bearing rocks. The simulation results show that the brines in the Western ore body of the Udachnaya pipe are strongly undersaturated in the deep horizons in comparison to carbonate, sulfate and chloride minerals. This suggests possible dilution of brines during their geochemical evolution. A detailed study of the tectonic structure identified structural elements that control the distribution and migration of groundwaters in the rocks. In the mining sites, brines occur mainly in the fault zones, fault junctions and intersections with the contacts of kimberlite bodies. Integration of hydrogeological and geostructural data can provide a basis for prediction and assessment of the sites with increased water inflow in the deep horizons involved in mining.

Cobar Deposits - Structural control

The Cobar Superbasin is located in the Central Subprovince of the Lachlan Orogen in the central part of New South Wales, about 700km northwest of Sydney. The Cobar Superbasin is the most mineralised Palaeozoic sedimentary basin in Lachlan Orogen. It has an estimated pre‐mining resource of 202 t of Au, 4.597 t Ag, 2.5 Mt Cu, 4.8 Mt Zn and 2.8 Mt Pb metals. The term ‘Cobar Superbasin’ is introduced to refer to a series of deep-water troughs/basins, inferred to have formed as half graben and shallow water shelfs. Its northern portion is dominated with siliciclastic sedimentary sequences whilst the southern portion comprises sediments, volcaniclastics, volcanic rocks, granites and minor limestone. The basin formed in the Early Devonian by NE-SW transtension and was closed by NW transpression in Late and Middle Carboniferous. The overall inversion structural style is NW-SE folding, overprinted by NE-SW trending and NNW-trending eastwards oblique left-lateral reverse faulting, in a combined thick- and thin-skinned tectonic environment. The Cobar Style mineralisation is a common name for mineral deposits hosted in the Cobar Superbasin and includes massive sulphides (VMS), clastic hosted Pb-Zn mineralisation and epithermal gold. In the Cobar Superbasin, the primary location of mineral deposits is controlled by basement architecture and then overprinted and modified with secondary controlling factors of inversion tectonics. Primary control is related to the location of mineral deposits within basin architecture and directly depends om major basement structures such as: - basin marginal growth fault; - intersection of growth and transform/transfer faults; and - intersection transform/transfer faults. Secondary control is related to deposit geometry and is direct consequence of invasion tectonic such as: - intersection termination and deflection of strike-slip faults; - overlap of en-echelon strike-slip; and - junction of major faults. Cobar Style mineralisation occurs in form of sheeted veins characterised by narrow width (5m - 10m), short strike (50m – 10m) and a significant depth extension (> 2000m). The deposits occur as a group of lenses in an en-echelon array with a steep plunge

Mantle origin of methane in the Black Sea

A new distribution map of gas seeps and mud volcanoes has been compiled of the Black Sea. It has been derived from the published coordinates for ca. 5000 gas seeps and 80 mud volcanoes. An analysis of the stable isotopic composition of methane has been performed for authigenic carbonates and sediments. The δ13C values of carbonates and sediments form 2 distinct tight groups depending of the geological environments. The diagrams of values of δ13C vs. δD and δ13C vs. C1/(C2 + C3) have been applied to assess earlier classifications of methane. The origin of methane from most of samples has turned out to be uncertainly determined. Based on seismic reflection data, the feeder channels of gas releases and mud volcanoes penetrate to the Pre-Mesozoic basement beneath the Polshkov High, Andrusov and Tetyaev Ridges and Sorokin Trough at a depth of up to 12 km. In the Central Black Sea the feeder channel of mud volcano reaches the mantle surface where a depth is 19 km. The highest concentration of gas seeps distribution is observed at the triple junction of the mantle faults in the NW Black Sea. Assessments of different mechanisms have been made for their ability to produce an unprecedented methane concentration in the Black Sea water column. There occurs thermogenic methane in the sediments from gas seepage and mud volcanoes areas. The thermogenic methane results from post genetic alternation of biogenic methane of the Paleogene—Neogene sediments. Biological methane produced from organic matter plays negligibly small role in accumulating the world’s largest quantities of anaerobic methane in the Black Sea. Nonorganic methane is most likely to form the tremendous reservoir of dissolved gas in the Black Sea water column below 150—200 m. The seeming scarcity of abi-otic methane is accounted for its recycling by microbial activity, misclassificating origin and producing by chemical syntheses, with its biological methane δ13C values.

Faults in the Baikal region: morphostructural and structure-genetic features (case study of the Buguldeika fault junction)

Lineament analysis is applied to map the pattern of the Obruchev fault system in the Buguldeika Village area, where several fault zones (Olkhon, Primorsky, Prikhrebtovyi, Buguldeika, and Kurtun) make up a junction. As inferred from the predominant directions of genetically related lineaments, the Olkhon, Primorsky, and Prikhrebtovyi faults originated under NW-SE extension and compression. The extension and compression settings within these zones are reconstructed by analysis of lineaments that have prominent and poor geomorphic expression, respectively. However, the pattern of lineaments well expressed in the surface topography within a weakly deformed block corresponds to reverse slip, while that of poorly pronounced lineaments corresponds to left-lateral strike slip. The study confirms the idea that the latest extension (rifting) stage in the Baikal region reactivated fault zones but did not deform blocks. The blocks store record of residual deformation produced by previous settings of regional compression and shear. The obtained results agree with earlier tectonophysical analysis of faults and fractures in the area and prove the applicability of the suggested approach to map the fault patterns and reconstruct their respective stress settings in areas that underwent multiple deformation events of different ages.

Shallow/upper crustal shear wave structure of the Tehran region (Central Alborz, Iran) from the inversion of Rayleigh wave dispersion measurements

Surface wave dispersion curves from microearthquakes are used to obtain group velocity dispersion maps. The calculation of the local dispersion curves for each grid point from these maps then produces the input data to retrieve the 3D shear wave velocity model of the Tehran region. The group velocity maps indicate that the tomographic results agree well with the three main tectonic features and the geological units in the study area. The tomographic maps generally possess high-velocity structures across most of the mountain belts (Central Alborz and east-southeast mountains), whereas the Tehran Basin correlates to a low-velocity structure. Increasing the period in the study area highlights four independent low-velocity zones that reflect faults and fault junction systems. The shear wave velocity profiles indicate that the depth to bedrock exhibits southward variation ranging from ~ 300 m to ~ 1500 m. We also focus our analysis on the existence of faults within the shear wave profiles and discuss the low shear wave velocity anomalies deeper than 2 km result from the main fault structures (e.g., North Tehran, North-South Rey and Parchin). Furthermore, we argue that the dip angle of the North Tehran fault varies along fault strike, whereas the North-South Rey fault possesses a constant dip angle. Moreover, initial model uncertainties and checkerboard resolution tests are used to identify reliable and robust anomaly features in the 3D shear wave velocity model and 2D tomographic maps, respectively. Microearthquake analysis provides an effective approach for studying the upper crustal structure heterogeneity, especially the fault structure, of the Tehran region.

FAULTS IN THE BAIKAL REGION: MORPHOSTRUCTURAL AND STRUCTURE-GENETIC FEATURES (case study of the Buguldeika fault junction)

FAULTS IN THE BAIKAL REGION: MORPHOSTRUCTURAL AND STRUCTURE-GENETIC FEATURES (case study of the Buguldeika fault junction)

Pathways of volatile migration in the crust beneath Harrat Lunayyir (Saudi Arabia) during the unrest in 2009 revealed by attenuation tomography

Harrat Lunayyir is a relatively young basaltic field in Saudi Arabia located at the western margin of the Arabian Peninsula. In April–June 2009, strong seismic activity and ground deformations at this site marked the activation of the magma system beneath Harrat Lunayyir. In this study, we present new three-dimensional models of the attenuation of P and S waves during the unrest in 2009 based on the analysis of t*. We measured 1658 and 3170 values of t* for P and S waves, respectively, for the same earthquakes that were previously used for travel time tomography. The resulting anomalies of the P and S wave attenuation look very similar. In the center of the study area, we observe a prominent high-attenuation pattern, which coincides with the most active seismicity at shallow depths and maximum ground deformations. This high-attenuation zone may represent a zone of accumulation and ascending of gases, which originated at depths of 5–7km due to the decompression of ascending liquid volatiles. Based on these findings and previous tomography studies, we propose that the unrest at Harrat Lunayyir in 2009 was triggered by a sudden injection of unstable liquid volatiles from deeper magma sources. At some depths, they were transformed to gases, which caused the volume to increase, and this led to seismic activation in the areas of phase transformations. The overpressurized gases ultimately found the weakest point in the rigid basaltic cover at the junction of several tectonic faults and escaped to the surface.

STRUCTURAL-COMPOSITIONAL MODEL OF THE NYURBINSKAYA KIMBERLITE PIPE FORMATION (SREDNE-MARKHA AREA OF THE YAKUTIAN DIAMONDIFEROUS PROVINCE)

The paper presents the results of comprehensive study of the primary diamond deposit of the Nyurbinskaya pipe in the Yakutian diamondiferous province. It is established, that the pipe is confined to the fault junction of four directions and is composed of the kimberlite of four phases. Analysis of different faults and tectonic fracturing allowed to reconstruct the tectonic stress fields acting at the stage of the kimberlite body formation and to determine their occurrence sequence in time. The data obtained about regularities of the Nyurbinskaya pipe compositional structure and results of geologo-structural studies are combined in a single structural-compositional model of the deposit formation. Peculiarities of the fault network operation during the deposit formation stage are confirmed by experimental results using polarization-optical method. The model allowed to formulate the basic structural characteristics of the prospecting works object within which the formation of kimberlite body type of the Nyurbinskaya pipe is possible and to determine the elements of the fault network which are promising for the kimberlite pipes discovery.

Central and eastern Anatolian crustal deformation rate and velocity fields derived from GPS and earthquake data

We present a new strain-rate and associated kinematic model for the eastern and central parts of Turkey. In the east, a quasi N-S compressional tectonic regime dominates the deformation field and is partitioned through the two major structural elements of the region, which are the conjugate dextral strike-slip North Anatolian Fault Zone (NAFZ) and the sinistral strike slip East Anatolian Fault Zone (EAFZ). The observed surface deformation is similar to that inferred by anisotropy studies which sampled the region of the mantle closer to the crust (i.e. the lithospheric mantle and the Moho), and is dependent on the presence or absence of a lithospheric mantle, and of the level of coupling between it and the overlaying crust. The areas of the central and eastern parts of Turkey which are deforming at elevated rates are situated above areas with strong gradients in crustal thickness. This seems to indicate that these transition zones, situated between thinner and thicker crusts, promote more deformation at the surface. The regions that reveal elevated strain-rate values are 1) the Elaziğ–Bingol segment of the EAFZ, 2) the region around the Karlıova triple-junction including the Yedisu segment and the Varto fault, 3) the section of the NAFZ that extends from the Erzincan province up to the NAFZ-Ezinepazarı fault junction, and 4) sections of the Tuz Gölü Fault Zone. Other regions like the Adana basin, a significant part of the Central Anatolian Fault Zone (CAFZ), the Aksaray and the Ankara provinces, are deforming at smaller but still considerable rates and therefore should be considered as areas well capable of producing damaging earthquakes (between M6 and 7). This study also reveals that the central part of Turkey is moving at a faster rate towards the west than the eastern part Turkey, and that the wedge region between the NAFZ and the EAFZ accounts for the majority of the counter clockwise rotation between the eastern and the central parts of Turkey. This change in movement rate and direction could be the cause of the extensional deformation and respective crustal thinning, with the resulting upwelling of warmer upper mantle observed in tomographic studies for the region between the Iskenderun bay and the CAFZ. The partitioning of deformation into an extensional regime could be the cause of the relatively low levels of strain-rate in the south-west part of the EAFZ and the northern part of the Dead Sea Fault Zone. Finally, using this new compilation of GPS data for the central-eastern part of Turkey, we obtained a new Anatolia–Eurasia rotation pole situated at 2.01°W and 31.94°N with a rotation rate of 1.053±0.015°/Ma.

Subsurface geometry of the San Andreas-Calaveras fault junction: Influence of serpentinite and the Coast Range Ophiolite

While an enormous amount of research has been focused on trying to understand the geologic history and neotectonics of the San Andreas-Calaveras fault (SAF-CF) junction, fundamental questions concerning fault geometry and mechanisms for slip transfer through the junction remain. We use potential-field, geologic, geodetic, and seismicity data to investigate the 3-D geologic framework of the SAF-CF junction and identify potential slip-transferring structures within the junction. Geophysical evidence suggests that the San Andreas and Calaveras fault zones dip away from each other within the northern portion of the junction, bounding a triangular-shaped wedge of crust in cross section. This wedge changes shape to the south as fault geometries change and fault activity shifts between fault strands, particularly along the Calaveras fault zone (CFZ). Potential-field modeling and relocated seismicity suggest that the Paicines and San Benito strands of the CFZ dip 65° to 70° NE and form the southwest boundary of a folded 1 to 3 km thick tabular body of Coast Range Ophiolite (CRO) within the Vallecitos syncline. We identify and characterize two steeply dipping, seismically active cross structures within the junction that are associated with serpentinite in the subsurface. The architecture of the SAF-CF junction presented in this study may help explain fault-normal motions currently observed in geodetic data and help constrain the seismic hazard. The abundance of serpentinite and related CRO in the subsurface is a significant discovery that not only helps constrain the geometry of structures but may also help explain fault behavior and the tectonic evolution of the SAF-CF junction.

Strain heating in process zones; implications for metamorphism and partial melting in the lithosphere

Since the late 1970s, most earth scientists have discounted the plausibility of melting by shear–strain heating because temperature-dependent creep rheology leads to negative feedback and self-regulation. This paper presents a new model of distributed shear-strain heating that can account for the genesis of large volumes of magmas in both the crust and the mantle of the lithosphere. The kinematic (geometry and rates) frustration associated with incompatible fault junctions (e.g. triple-junction) prevents localisation of all strain on the major faults. Instead, deformation distributes off the main faults forming a large process zone that deforms still at high rates under both brittle and ductile conditions. The increased size of the shear-heated region minimises conductive heat loss, compared with that commonly associated with narrow shear zones, thus promoting strong heating and melting under reasonable rheological assumptions. Given the large volume of the heated zone, large volumes of melt can be generated even at small melt fractions.There are clear examples of volcanism correlated with fault complexities that remain enigmatic and that could be related to that mechanism of “process zone heating”. We propose here a simple dislocation model to define the process zones, determine off-fault strain rates and quantify how much plastic work can be dissipated as heat. To provide an example, we examine the case of the junction between the East and North Anatolian shear zones in eastern Turkey. This is chosen because the composition and age of emplacement of the quite extensive volcanics are well known, as are the rates of motion on the associated strike-slip faults. The geometry of the system also allows the dislocation method to be easily adopted. We conclude that melting of the crust and the lithospheric mantle could start only a few Myrs after the onset of deformation. Conservative assumptions for rheological parameters and melting points can explain both the timing and the bimodal nature of the volcanism. Predictions of melt volume are within the rheological uncertainties. While the current paper is focused on strike-slip faults, the approach can be applied to kinematic complexities associated with thrust faults as well, and, maybe more generally, to regions in the lithosphere where oblique boundary conditions force deformation to occur partly in a distributed manner, while still at high rates.

Kinematic and Dynamic Analysis of North-Tehran Tectonic Wedge Formed in South Central Alborz, Iran

Abstract To answer the question “Whether the North-Tehran tectonic wedge is a dynamic tectonic wedge or not?” we applied paleostress techniques to investigate fault slip data. The mean reduced stress tensor is defined for all stabilized stress regions. Unscaled Mohr’s circles drawn for fault slip data were used to obtain the relative slip tendency of clusters on diagrams. It showed that the slip tendency in the vicinity of fault junction is much lower than expected. The mean σ1 defined for a combination of fault slip data trends N14°E nearly parallel with the overall pressure in Iranian crust at the latitude of Central Alborz. This trend suggests the least effect of boundary faults and the wedge between them on stress orientations inside the wedge. Finally, the stress trajectory map was prepared showing the configuration and relative intensity of σ1. The map did not illustrate any convergence in σ1 trajectories and the consequent concentration of stress and seismic potential in fault junction. That is a direct evidence for disagreeing the dynamicity of this tectonic wedge.

한반도 동남부의 주단층대에 의해 구분된 지질블록별 백악기-고제3기 화강암의 차별적 냉각-지열 이력: 피션트랙 열연대학적 증거

한반도 동남부의 주단층으로 구분된 지질블록별 백악기-고제3기 화강암체의 피션트랙(FT) 열연대학적 기록은 차별적인 냉각사와 후기 지열사를 입증한다. 대부분 화강암체에는 관입후 단조로운 냉각패턴(J 자-형)이 그대로 보존되어 있으며, 일부 백악기 암체는 <TEX>$300^{\circ}C$</TEX> 등온선의 리셋 후 재냉각된 복잡한 패턴(N 자-형)을 기록한다. 각 암체의 온도구간별 차별적 냉각속도는 상대적 암체규모, 모암종류에 따른 초기 열손실, 후기화성활동 존재와 근접성 등에 따라 설명된다. 큰 규모의 저반상 암체의 경우에도 단일 암체 내에서 위치에 따라 각 등온선별 냉각연대는 대체로 부합된다. 양산단층을 가로질러 양쪽에 인접한 두 암체의 유사한 냉각연대는 그들이 거대한 수평변위 이전의 동시대 관입체임을 입증한다. 양산/동래단층 등 주단층대를 따라 국지적으로 기록된 후기 지열상승 범위는 인회석 부분안정영역(<TEX>$70-125^{\circ}C$</TEX>)에 달했으며 <TEX>$200^{\circ}C$</TEX>에는 미치지 못하였다. 양산-울산단층 교차점의 파쇄대 내 강한 변질증거는 FT 스핀연대(31Ma)가 전기 올리고세에 단층교차점을 포함한 포항-감포블록의 구조적 침강에 따른 <TEX>$290^{\circ}C$</TEX> 이상의 지온상승에 의한 리셋 후 재냉각연대임을 지시하며, 한편 일치된 FT 저콘 및 인회석연대(24Ma)는 올리고세 말기에 동일 지역의 급작스런 구조적 융기에 관련된 <TEX>$200-105^{\circ}C$</TEX> 등온선의 급작스런 냉각을 암시한다. <TEX>$300-200-100^{\circ}C$</TEX> 등온선별 냉각연대가 울산단층 동쪽(포항-감포블록)에서 서쪽보다 현저하게 낮은 경향(FT 스핀 및 K-Ar 흑운모=19%; FT 저콘=20%; FT 인회석=27%)은 훨씬 뒤에 관입한 고제3기 암체의 늦은 냉각시점 때문이며, 후기 지열영향으로 부분감소된 인회석연대가 반영되었기 때문이다. Fission-track (FT) thermochronological records from SE Korean Cretaceous-Paleogene granitic plutons in different fault-bounded blocks reveal contrasting cooling and later thermal histories. Overall cooling patterns are represented by a monotonous (J-shaped) curve in most plutons except some Cretaceous granites retaining a complicated (N-shaped) path due to post-reset re-cooling. Discriminative cooling rates over different temperature ranges can be explained for individual plutons with respect to relative pluton sizes, differences in initial heat loss depending on country rocks, and the presence and proximity of later igneous activity. Even within a single batholith, cooling times for different isotherms were roughly contemporaneous with respect to positions. Insignificant deviations in cooling ages from two different plutons in succession across the Yangsan fault may suggest their contemporaneity before major horizontal fault movement. The extent of later thermal rise recorded locally along the Yangsan and Dongnae fault zones were reached the Apatite Partial Stability Zone (<TEX>$70-125^{\circ}C$</TEX>), but did not exceed <TEX>$200^{\circ}C$</TEX>. Thermal alteration from fractured zones in the Yangsan-Ulsan fault junction may suggest a thermal reset above <TEX>$290^{\circ}C$</TEX> resulting a complete reset in FT sphene age (31 Ma), caused by a tectonic subsidence in Early Oligocene. A consistency in FT zircon/apatite ages (24 Ma) may imply a sudden rapid cooling over <TEX>$200-105^{\circ}C$</TEX>, plausibly related to the abrupt tectonic uplift of the Pohang-Gampo Block including the fault junction in Late Oligocene. A remarkable trend of lower cooling ages for <TEX>$300-200-100^{\circ}C$</TEX> isotherms (i.e., 19% for FT sphene and K-Ar biotite; 20% for FT zircon; 27% for FT apatite) from the east of the Ulsan fault (Pohang-Gampo Block) comparing to the west of the fault may be attributed to retarded cooling times from the Paleogene granites and also reflected by their partially-reduced apatite ages due to later thermal effects.

Applying Wedge Theory to Dynamic Rupture Modeling of Fault Junctions

Wedges, such as accretionary prisms and thin‐skinned fold‐and‐thrust belts, occur frequently in nature and can be the site of devastating earthquakes. Critical wedge theory can be applied to these settings, but this steady‐state description of wedge deformation is at odds with the periodic occurrence of earthquakes. We discuss how critical wedge theory applies to the seismic cycle, and we use elastic wedge theory to constrain realistic stress states. Our goal is to determine the rupture behavior of an earthquake in a wedge. If rupture initiates on the basal sliding surface, will it stay confined to the basal surface, or will it propagate onto a fault branch interior to the wedge? This information can significantly alter the seismic hazard in areas where fault intersections occur. We answer this question using numerical models of dynamic rupture propagation through branched geometries for which the stress state is a pivotal input parameter. We apply wedge theory to constrain the stress state, but inherent to this theory is the assumption of a weak basal fault. We investigate the role of this assumption and determine that rupture is unlikely to propagate from a weak basal fault onto a strong branch fault without the aid of a physical process such as pore fluid migration along the branch. This framework is applied to the rupture of the 2008 Wenchuan earthquake. We find that we are able to reproduce the behavior at some fault intersections, but our 2D model is not able to reproduce all the behaviors, possibly due to the oblique nature of this event.