Balanced Geological Cross-sections Research Articles

Comment on “Fragmentation of the Adriatic Promontory: New Chronological Constraints From Neogene Shortening Rates Across the Southern Alps (NE Italy)” by Moulin & Benedetti

AbstractMoulin and Benedetti (2018),https://doi.org/10.1029/2018TC004958present a new interpretation of the Neogene‐Quaternary tectonic evolution of the Eastern Southern Alps (ESA) in Veneto‐Friuli. After the reinterpretation of literature field data by means of remote sensing analysis (Digital Elevation Model interpretation), they calculated deformation rates of the tectonic structures through age interpretation of geomorphological surfaces of the Veneto‐Friuli piedmont plain. The authors linked the result of surface analysis to the thrust and fold architecture of the ESA basing on the Castellarin et al. (2006),https://doi.org/10.1016/j.tecto.2005.10.013interpretation of TRANSALP project and the Friuli geological map at the scale 1:150,000 (Carulli, 2006). Discussing their new architecture of the ESA, the authors finally yielded rates of Europe‐Adria plates convergence and suggested fragmentation of Adria over the last 1–2 Ma. The present comment is aimed at discussing several critical points concerning: the use of the geomorphological and chronological data; the misinterpretation of the Digital Terrain Model; the reconstruction of the balanced geological cross section. Moreover, the application of a structural model defined in a certain area to another without considering peculiar structural complexities available in the literature results is geologically and methodologically questionable.

Quantifying shortening across the central Appalachian fold-thrust belt, Virginia and West Virginia, USA: Reconciling grain-, outcrop-, and map-scale shortening

AbstractWe present a kinematic model for the evolution of the central Appalachian fold-thrust belt (eastern United States) along a transect through the western flank of the Pennsylvania salient. New map and strain data are used to construct a balanced geologic cross section spanning 274 km from the western Great Valley of Virginia northwest across the Burning Spring anticline to the undeformed foreland of the Appalachian Plateau of West Virginia. Forty (40) oriented samples and measurements of >300 joint orientations were collected from the Appalachian Plateau and Valley and Ridge province for grain-scale bulk finite strain analysis and paleo-stress reconstruction, respectively. The central Appalachian fold-thrust belt is characterized by a passive-roof duplex, and as such, the total shortening accommodated by the sequence above the roof thrust must equal the shortening accommodated within duplexes. Earlier attempts at balancing geologic cross sections through the central Appalachians have relied upon unquantified layer-parallel shortening (LPS) to reconcile the discrepancy in restored line lengths of the imbricated carbonate sequence and mainly folded cover strata. Independent measurement of grain-scale bulk finite strain on 40 oriented samples obtained along the transect yield a transect-wide average of 10% LPS with province-wide mean values of 12% and 9% LPS for the Appalachian Plateau and Valley and Ridge, respectively. These values are used to evaluate a balanced cross section, which shows a total shortening of 56 km (18%). Measured magnitudes of LPS are highly variable, as high as 17% in the Valley and Ridge and 23% on the Appalachian Plateau. In the Valley and Ridge province, the structures that accommodate shortening vary through the stratigraphic package. In the lower Paleozoic carbonate sequences, shortening is accommodated by fault repetition (duplexing) of stratigraphic layers. In the interval between the duplex (which repeats Cambrian through Upper Ordovician strata) and Middle Devonian and younger (Permian) strata that shortened through folding and LPS, there is a zone that is both folded and faulted. Across the Appalachian Plateau, slip is transferred from the Valley and Ridge passive-roof duplex to the Appalachian Plateau along the Wills Mountain thrust. This shortening is accommodated through faulting of Upper Ordovician to Lower Devonian strata and LPS and folding within the overlying Middle Devonian through Permian rocks. The significant difference between LPS strain (10%–12%) and cross section shortening estimates (18% shortening) highlights that shortening from major subsurface faults within the central Appalachians of West Virginia is not easily linked to shortening in surface folds. Depending on length scale over which the variability in LPS can be applied, LPS can accommodate 50% to 90% of the observed shortening; other mechanisms, such as outcrop-scale shortening, are required to balance the proposed model.

Large scale quantification of aquifer storage and volumes from the Peninsula and Skurweberg Formations in the southwestern Cape

The Western Cape Province of South Africa is a relatively water-scarce area as a result of the Mediterranean climate experienced. Due to the increased usage of groundwater, and the requirement to know how much water is available for use, it is imperative as a 1 st step to establish an initial estimate of groundwater in storage. The storage capacity, namely, the total available storage of the different aquifers, and the storage yield of the fractured quartzitic Peninsula and Skurweberg Formation aquifers of the Table Mountain Group (TMG), are calculated with a spreadsheet and Geographic Information System (GIS) model. This model is based on the aquifer geometry and estimated values (based on measured data) for porosity and specific storage (calculated using the classic Jacob relation). The aquifer geometry is calculated from 1:50 000 and 1:250 000 geological contacts, faults and major fractures, with dips and aquifer formation thickness calculated through structural geology 1 st principles using a Digital Elevation Model (DEM). Balanced geological cross-sections constructed through the model areas provide an important check for the aquifer top and bottom surface depth values produced by the GIS model. The storage modelling undertaken here forms part of the City of Cape Town TMG Aquifer Feasibility Study and Pilot Project, with modelling focusing on the 3 main groundwater target areas at Theewaterskloof (Nuweberg), Wemmershoek and Kogelberg-Steenbras. In the storage models, the Peninsula and Skurweberg Formation aquifers have confined pore volumes ranging from approximately 29 bn. to 173 bn. m 3 and 4 bn. to 26 bn. m 3, respectively (based on using different porosity values ranging from 2.5% to 15%). Using an average head decline of 1 m across the confined aquifer areas across all 3 groundwater exploration areas, and confined pore volumes based on a porosity of 5%, 6.9 Mm 3 and 1.1 Mm 3 of groundwater, from the Peninsula and Skurweberg Formation aquifers, respectively, is available. The aquifer storage model intentionally makes use of low, geologically reasonable values for porosity and aquifer compressibility, so as to provide minimum large-scale 1 st estimates of potential yields; however, when new data become available these initial porosity and compressibility assumptions will probably be revised upward. The storage yield approach is also very conservative, as it does not take into account the annual replenishment of the aquifer, and constitutes the yield potential during drought conditions (zero recharge) from the confined portion of the aquifer only. The yield model therefore provides a quantitative perspective on the common public and decision-maker perception that groundwater abstraction from the deep confined Peninsula Formation aquifer will significantly dewater the system, with (often unspecified) adverse ecological conse quences. Even where the regionally-averaged decline in hydraulic head approaches 20 m, the volume released by aquifer compression generally remains in the order of 0.24% of the total volume in slow circulation within the deep groundwater flow system. A vastly greater volume of groundwater is essentially non-extractable by any practical and/or economical means.

A constant slip rate for the western Qilian Shan frontal thrust during the last 200 ka consistent with GPS-derived and geological shortening rates

Active thrust faulting at the front of the Qilian Shan accommodates the northeastward growth of the Tibetan Plateau, however, the lifespan of individual faults and their slip history on different timescales remain largely unknown. Here, we show that the main range-bounding thrust fault of the western Qilian Shan has accrued tectonic slip at an almost constant rate during the last ∼200 ka, and possibly since fault initiation in the mid-Miocene. Our finding is based on 10Be ages from a flight of five deformed fluvial terraces along the Hongshuiba river, which constrain the vertical slip rate of the Qilian Shan frontal thrust to be 1.2±0.1 m/ka during the last 200 ka. With a fault dip of 30±5° constrained by seismic reflection data, we obtain a horizontal shortening rate of 2.0±0.3 m/ka. This value is consistent with both the short-term shortening rate of 1.7±0.3 mm/a derived from GPS data and the long-term shortening rate of 2.1±0.4 m/ka, which is based on a balanced geological cross-section. The latter provides a total shortening estimate of 25±3 km since the thrust fault initiated 12±2 Ma ago. The agreement between the shortening rates on the range of timescales between 100 and 107 years suggests that the western Qilian Shan frontal thrust has slipped at a steady rate since its initiation and implies that this fault is the main structure responsible for the growth of the western Qilian Shan.

Testing the effects of topography, geometry, and kinematics on modeled thermochronometer cooling ages in the eastern Bhutan Himalaya

Abstract. In this study, reconstructions of a balanced geologic cross section in the Himalayan fold–thrust belt of eastern Bhutan are used in flexural–kinematic and thermokinematic models to understand the sensitivity of predicted cooling ages to changes in fault kinematics, geometry, topography, and radiogenic heat production. The kinematics for each scenario are created by sequentially deforming the cross section with ∼ 10 km deformation steps while applying flexural loading and erosional unloading at each step to develop a high-resolution evolution of deformation, erosion, and burial over time. By assigning ages to each increment of displacement, we create a suite of modeled scenarios that are input into a 2-D thermokinematic model to predict cooling ages. Comparison of model-predicted cooling ages to published thermochronometer data reveals that cooling ages are most sensitive to (1) the location and size of fault ramps, (2) the variable shortening rates between 68 and 6.4 mm yr−1, and (3) the timing and magnitude of out-of-sequence faulting. The predicted ages are less sensitive to (4) radiogenic heat production and (5) estimates of topographic evolution. We used the observed misfit of predicted to measured cooling ages to revise the cross section geometry and separate one large ramp previously proposed for the modern décollement into two smaller ramps. The revised geometry results in an improved fit to observed ages, particularly young AFT ages (2–6 Ma) located north of the Main Central Thrust. This study presents a successful approach for using thermochronometer data to test the viability of a proposed cross section geometry and kinematics and describes a viable approach to estimating the first-order topographic evolution of a compressional orogen.

3-D geologic architecture of Xiong’an New Area: Constraints from seismic reflection data

Studying the geologic architecture of the Xiong’an New Area will provide important basis for the evaluation of crustal stability, urban planning and infrastructural projects in this region, and it is also of great significance in exploring the occurrence of oil and gas, geothermal, hot dry rock and other resources. The stratigraphic system of the study area is established by using latest high quality seismic reflection and deep borehole data. Characteristics of the major faults developed in the study area are finely depicted with the method of structural analysis. Tectonic evolution of Xiong’an and adjacent areas is reconstructed by using balanced geological cross-section technique. The tectonic activity of the study area is discussed on the basis of the development of secondary faults and the distribution of active earthquakes across the region. This study demonstrates that Xiong’an New Area is located at the transfer zone of the central and northern Jizhong Depression. There are three regional unconformities developed in this area, by which four structural layers are sub-divided. Controlled by the Taihang Mountain piedmont fault, the Daxing fault, the Rongcheng fault and the Niudong fault, the structural framework of the study area is characterized by intervening highs with sags. This structural pattern has an important controlling over the reservoir characteristics, hydrocarbon accumulation and the distribution of geothermal resources and hot dry rock within this region. Rifting in this area began in the early Paleogene, exhibiting typical episodic character and became inactive in Neogene. The development feature of secondary faults along with the distribution of active earthquakes indicate that Xiong’an New Area has been in a relatively stable tectonic setting since the Neogene, while the Baxian Sag and other structural units to the east of it have obviously been in a rather active environment.

Reply to Comments on “the Cenozoic Fold‐and‐Thrust Belt of Eastern Sardinia: Evidences from the Integration of Field Data With Numerically Balanced Geological Cross Section” by Arragoni et al. (2016)

AbstractThe comment by Berra et al. (2017) on the evidence of Alpine tectonics in Eastern Sardinia proposed by Arragoni et al. (2016) is based on the sedimentological interpretations of few local outcrops in a marginal portion of the study area. The Cenozoic Alpine fold‐and‐thrust setting, which characterizes this region, presents flat‐over‐flat shear planes acting along originally stratigraphic contacts, where stratigraphic continuity is obviously maintained. The ramp sectors present steeply dipping bedding attitudes, and there is no need to invoke and to force prograding clinoforms with unrealistic angles to justify them. The balanced geological cross section proposed by Arragoni et al. (2016) is fully supported by robust newly collected structural data and is compatible with the overall tectonic setting, while the interpretation proposed by Berra et al. (2017) lacks a detailed structural investigation. We believe that the partial application of the techniques available to modern geology may lead to incorrect interpretations, thus representing an obstacle for the progress of knowledge in the Earth sciences.

Balanced geological cross-section of the outer ukrainian carpathians along the pancake profile

The Outer Ukrainian Carpathians (OUC) are part of the Eastern Carpathians, which is a segment of the European Alpine belt. The OUC is crossed by recently acquired WARR (wideangle reflection and refraction) PANCAKE profile (Starostenko et al., 2013). To study the basin architecture of the OUC along the profile a balanced geological cross-section was onstructed. Reconstruction of the balanced geological cross-section has shown that the present day fold-and-thrust belt of the Outer (Flysch) Carpathians was formed in the Miocene from the deposits of the sedimentary basin∼460km in width. Evolution of the sedimentary basin to the present fold-and-thrust belt 70km in width occurred in two stages, characterized by accelerating the shortening rate of the basin from 11.5km/Ma between∼32 and 22 Ma to 22.5km/Ma between∼22 and 12 Ma. The detachment of the flysch complexes of the OUC occurred on the top of the basement located at a depth of 10−12km. The main phase of Miocene thrusting was followed by significant exhumation and erosion during the period of 12−2 Ma related to isostatic uplift in the OUC, which reached a maximum (up to 7km) in the central nappes (Krosno and Dukla-Chornohora). The collision regime, obviously, followed the subduction one; this could happen approximately at 22 Ma which is evident after changing the shortening rate of the basin.

Active Tectonic and Mechanic Interaction Between Cusiana and Yopal Faults Interpreting Seismic and Terraces Geometry

Along the Colombian Llanos Foothills, different rock formations lie affected by tectonic movements, expressing geomorphological changes at the surface associated with subsurface geologic structures (anticlines and synclines). Some of them were created during the most recent tectonic movements throughout the last period of geological time. This evidence is plotted in the geological map of the area extended from the Colombian Eastern Cordillera to the Colombian Llanos Orientales basin (figure 1). There, folded and faulted Quaternary deposits can be observed. This is the case of the alluvial terraces in the study area (figure 1), which have been deformed due to the recent tectonic activity. Therefore, to test this theory, this study generates a three dimensional kinematic model based on serial balanced geological cross sections built by seismic, geological and well data. Hence, we can analyze the geometry of faults in the area (Cusiana and Yopal), with the geometry of the Alluvial Terraces deposited during t...

Heterogeneous strength and fault zone complexity of carbonate-bearing thrusts with possible implications for seismicity

The understanding of fault-slip behaviour in carbonates has an important societal impact due to the widespread occurrence and propagation of earthquakes in these rocks. Fault rock variations in carbonates are systematically controlled by the lithology of the faulted protolith: cataclasis and hydraulic fracturing with evidence of past seismic slip commonly affect fault rocks in competent limestone formations whereas widespread pressure-solution and sliding along clay foliation are observed in marly rocks. We performed a series of friction experiments on carbonatic fault rocks sampled from mature thrusts (>2 km displacement) in the Apennines of Italy. We sheared both intact wafers and powdered fault materials at low (10 MPa) and in situ (53 MPa) normal stress under room-humidity and water-saturated conditions. We used velocity steps (1 to 300 μm/s) and slide–hold–slide (3–1000 s holds) to assess the frictional stability and healing behaviour of these rocks. We observe that cataclastic fault rocks derived from competent limestones are characterized by high friction coefficients coupled with significant post-slip restrengthening and velocity-weakening behaviour. Conversely, intact foliated marly tectonites, sheared under the same conditions, show low friction, null post-slip healing and stable velocity-strengthening behaviour suggesting that these rocks deform aseismically. To extrapolate these opposite mechanical behaviours to the entire fault surface we developed a fault model integrating our mechanical data, field observations and balanced geological cross-sections. The mechanical heterogeneities highlighted in the model provide constraints for the distribution of fault patches with higher seismogenic potential.

Summit low‐angle faults in the Central Apennines of Italy: Younger‐on‐older thrusts or rotated normal faults? Constraints for defining the tectonic style of thrust belts

AbstractLow‐angle faults that juxtapose younger rocks over older ones are widely documented in fold‐thrust belts, and reconstruction of tectonic style is strictly dependent on their interpretation. Various modes exist for generating hinterland‐dipping low‐angle faults with younger‐on‐older relationships. Indeed, in the Central Apennines of Italy, the hinterland‐dipping younger‐on‐older low‐angle faults, which rest on the summits of the major anticlines (i.e., summit low‐angle faults), have been interpreted variously as younger‐on‐older thrusts within out‐of‐sequence thrust systems, postorogenic normal faults, gravity‐driven slides, or as rotated prethrusting normal faults. In this study, we provide a new and robust structural‐geological data set, corroborated with stratigraphic timing constraints and balanced geological cross sections, bringing an essential contribution to the interpretation of the hinterland‐dipping younger‐on‐older low‐angle faults as preexisting normal faults rotated within the shortcut anticlines during the Neogene thrust‐related fold emplacement. A detailed geological and structural characterization carried out on four remarkable examples from the Central Apennines allowed reconstructing an inversion tectonics model. The lack or reverse reactivation of the rotated prethrusting normal faults here analyzed is consistent with fault bend and fault propagation folding models associated with break forward in‐sequence thrust propagation implying more conservative estimates of shortening for the Central Apennines thrust system, compared to the previous out‐of‐sequence models. Taking into account the various possible causes for the development of hinterland‐dipping younger‐on‐older low‐angle faults, the structural‐geological characterization presented in this study and the results achieved could be critically applied when examining similar structures in other thrust belts.

Geometry and kinematics of fault-propagation folds with variable interlimb angle

Several conceptual approaches have been proposed to account for the development of fault-propagation folds whose geometry and kinematics depend on the amount of displacement along a basal decollement level, the ramp angle and the slip to propagation ratio. Among these, the variable interlimb angle model of Mitra (1990) is able to explain open and close natural folds but its application is limited because the fold geometry and bed thickness evolution rely on imposed parameters that cannot be measured directly. Here, we use the ramp and the interlimb angles as input data to develop a forward fold model that accounts for thickness variations in the forelimb. The relationship between the fold amplitude and fold wavelength is subsequently applied to construct balanced geological cross-sections from surface parameters only and to propose a kinematic restoration of the folding through time. The model can catter for a wide variety of folds, reconstruct the deep architecture of anticlines and deduce the kinematic evolution of the folding with time. We consider three natural examples to validate the variable interlimb angle model. Along-strike thickness variation in the forelimb of the Turner Valley anticline in the Alberta foothills of Canada precisely corresponds to the theoretical values proposed by our model. Reconstruction at depth of the Alima anticline in the southern Tunisian Atlas implies that the decollement level is localised in the Triassic-Liassic series, as highlighted by seismic imaging. The kinematic reconstruction of the Ucero anticline in the Spanish Castilian mountains is also in agreement with the fold geometry derived from two cross-sections. The variable interlimb angle model predicts that the fault-propagation fold can be symmetric, normal asymmetric (with a greater dip value in the forelimb than in the backlimb), or reverse asymmetric (with greater dip in the backlimb) depending on the shortening amount.

Unraveling the central Appalachian fold-thrust belt, Pennsylvania: The power of sequentially restored balanced cross sections for a blind fold-thrust belt

We present a kinematic model for the sequential development of the Appalachian fold-thrust belt (eastern U.S.) across a classic transect through the Pennsylvania salient. New map and strain data are used to create a balanced geologic cross section from the southern edge of the Valley and Ridge Province to the northern Appalachian Plateau. This region of the central Appalachian fold-thrust belt is an ideal location to illustrate the incorporation of strain data in balanced cross sections, because it cannot be balanced without quantifying grain-scale strain. We use a sequentially restored, balanced cross section to show how layer-parallel shortening (LPS) is distributed above and ahead of thrust and fold shortening and constrain the geometric and kinematic evolution of a passive roof duplex. By combining line length and area balancing of a kinematically viable cross section with LPS estimates in both the Valley and Ridge Province (20%) and Appalachian Plateau (13%), we document the total magnitude of shortening in both the folded cover sequence and the duplexed lower layer of the fold-thrust belt. Restoration of the cross section indicates a total of 77 km (22%) of shortening between the southern margin of the Valley and Ridge Province in central Pennsylvania and a pin line immediately north of the northern limit of documented LPS in the foreland. The 24 km (13%) of LPS on the Appalachian Plateau is interpreted as being above the Salina (salt) decollement. This magnitude of shortening is 14 km greater than the amount of displacement on the Nittany Anticlinorium, the northernmost structure of the fold-thrust belt that cuts upsection from the Cambrian Waynesboro Formation to the Silurian Salina decollement. Because the fault that cores the Nittany Anticlinorium can only facilitate 10 km of shortening on the plateau, an early history of Appalachian Plateau LPS in Silurian and younger rocks is required to balance the section. We propose that the additional 14 km of LPS on the plateau occurred early in the deformation history and was kinematically linked to two fault-bend folds that have a lower decollement in the Cambrian Waynesboro Formation and an upper, subhorizontal detachment in the Silurian Wills Creek Formation (in the Valley and Ridge) and the Salina Group on the Appalachian Plateau. This upper detachment feeds displacement from these early horses in the duplex system onto the Appalachian Plateau and is expressed there as LPS shortening. This early shortening is followed by the development of in-sequence horses that repeat the mainly thrust-faulted Cambrian–Ordovician sequence using both the main decollement in the Cambrian Waynesboro and the Ordovician Reedsville Formations as an upper detachment horizon. In the south, shortening in the Late Ordovician through Devonian layers is accommodated by both LPS and forced folding of the overlying folded cover sequence. We propose that the Reedsville Formation becomes weaker to the north, facilitating shorter wavelength detachment folds. The development of gentle open folds on the Appalachian Plateau, as well as the last 10 km of LPS on the plateau, is linked to the most forelandward horse in the duplex. This horse forms the broad Nittany Anticlinorium, the northern boundary of the Valley and Ridge.

Advances in Paleostructures Research

The formation and evolution of paleostructures have an important impact on the migration, accumulation and distribution of ancient and present petroleum. This paper makes a particularly description of several common paleostructural restoration methods, including thickness method, tectonic reconstruction method, backstripping analysis method, paleo-horizon restoration method, balanced geological cross-section method and line-drawing method. Among them, line-drawing method is a new way to do research in paleostructures using seismic sections. Besides, this paper also concludes the factors which affect restoration of paleostructures.

Quantitative structural analysis using remote sensing data: Kurdistan, northeast Iraq

The use of remote sensing data increases the efficiency of field mapping, especially in areas with difficult access or where geologic fieldwork is expensive or hazardous. This study presents a newly developed software tool for interactively mapping and measuring the spatial orientation (i.e., dip angle and dip direction) of finite planar geologic structure from digital elevation models (DEMs). The orientations of planar data (e.g., sedimentary bedding or fault planes) are derived by approximating a virtual plane to the intersection of the planar feature with the DEM topography. To increase the informative value of the DEM, satellite images can be draped onto the topographic data set. The software tool was tested in the Zagros fold and thrust belt, northeast of Erbil (Kurdistan, northeast Iraq), where the stratigraphy has been deformed into subcylindrical fold trains. Computed orientations have been compared with actual dip angles and directions measured in the field. Under favorable conditions (moderately dipping planes, strong competence contrast between stratigraphic boundaries, intersection with a rugged topography, low vegetation), statistical comparison of computed data with the field measurements demonstrates that the spatial data set can be reproduced from the DEM within an average error of approximately 10. The strength of the method is demonstrated by integrating field data with computed values from inaccessible areas, resulting in a reasonably well-constrained balanced geologic cross section.

Deep seismic reflection profiling across the Northern Fossa Magna: The ERI 1997 and the JNOC 1996 seismic lines, active faults and geological structures

The Northern Fossa Magna is a Miocene failed rift and due to subsequent shortening, its basin-fill forms a fold-belt associated with active faults. Seismic reflection data across the middle part of the northern Fossa Magna acquired in late 1990s were reprocessed to reveal the deep geometry of active faults. The reprocessed seismic sections portray the folded and faulted structure of the Neogene basin-fill. The deeper extension of the Western Nagano Basin active Fault (WNBF), which has been revealed for the first time, can be traced down to 4 km, as a reverse fault dipping 40° westward. In the western part, the Itoigawa-Shizuoka Tectonic Line (ISTL) active fault is presented as an emergent thrust dipping 30–35° eastward. Based on the seismic profiles, surface geology and well data, the balanced geologic cross section was constructed. Using simple-shear model of the basin formation, the total amount of Miocene extension is calculated to be ca. 27 km and the total amount of late Neogene to Quaternary shortening is ca. 11 km. The basin formation and shortening deformation are well explained by the tectonic inversion model and fault reactivation.

Seismological and geological characterization of the crust in the southern part of northern Fossa Magna, central Japan

The northern Fossa Magna (NMF) is a Miocene rift basin formed in the final stages of the opening of the Sea of Japan. The northern part of Itoigawa-Shizuoka Tectonic Line (ISTL) bounds the western part of the NMF and forms an active fault system that displays one of the largest slip rates in the Japanese islands. Reflection and refraction/wide-angle reflection profiling and earthquake observations by a dense array were undertaken across the northern part of ISTL in order to delineate structures in the crust, and deep geometry of the active fault systems. The ISTL active fault system at depth (ca. 2 km) shows east-dipping low-angle in Omachi and Matsumoto and is extended beneath the Central Uplift Zone and Komoro basin keeping the same dip-angle down to ca. 15 km. The upper part of the crust beneath the Central Uplift Zone is marked by the high Vp and high resistivity zone. Beneath the folded zone of the NMF, the middle to lower crust shows low Vp, low resistivity and more reflective features. The balanced geologic cross-section based on the reflection profiles suggests that the shortening deformation since the late Neogene was produced by the basin inversion of the Miocene low-angle normal fault.

Formation and shortening deformation of a back-arc rift basin revealed by deep seismic profiling, central Japan

The northern Fossa Magna (NFM) basin is a Miocene rift system produced in the final stages of the opening of the Sea of Japan. It divides the major structure of Japan into two regions, with north-trending geological structures to the NE of the basin and EW trending structures to the west of the basin. The Itoigawa-Shizuoka Tectonic Line (ISTL) bounds the western part of the northern Fossa Magna and forms an active fault system that displays one of the largest slip rates (4–9 mm/year) in the Japanese islands. Deep seismic reflection and refraction/wide-angle reflection profiling were undertaken in 2002 across the northern part of ISTL in order to delineate structures in the crust, and the deep geometry of the active fault systems. The seismic images are interpreted based on the pattern of reflectors, the surface geology and velocities derived from refraction analysis. The 68-km-long seismic section suggests that the Miocene NFM basin was formed by an east dipping normal fault with a shallow flat segment to 6 km depth and a deeper ramp penetrating to 15 km depth. This low-angle normal fault originated as a comparatively shallow brittle/ductile detachment in a high thermal regime present in the Miocene. The NFM basin was filled by a thick (>6 km) accumulation of sediments. Shortening since the late Neogene is accommodated along NS to NE–SE trending thrust faults that previously accommodated extension and produce fault-related folds on their hanging wall. Based on our balanced geologic cross-section, the total amount of Miocene extension is ca. 42 km and the total amount of late Neogene to Quaternary shortening is ca. 23 km.

後期新生代における東北日本中部背弧域の地殻構造発達−最近の地殻構造探査を中心として−

Northern Honshu, Japan, is a classical example of a trench-arc-back arc basin system, and provides exceptional opportunities for better understanding plate subduction. In this paper, data from deep seismic profiles, petrology and petroleum geology are used to construct a geologic framework for the late Cenozoic development of the back arc area of central northern Honshu. At this time, two rift systems can be recognized in northern Honshu: the Yamato basin rift system along the eastern part of the sea of Japan and the northern Honshu rift system in the western part of northern Honshu island. The eastern part of the Yamato basin rift system is marked by half grabens bound by west-dipping Miocene normal faults, with the thickness of the crust decreasing toward the west due to early Miocene crustal stretching. The northern Honshu rift system is younger, forming in the middle Miocene, and produced the Akita-Yamagata sedimentary basin by a simple shear type of rifting. Basalts were erupted in conjunction with rifting. This produced a high thermal regime, and a shallow brittle/ductile detachment formed along the rift axis with associated thinning of the upper crust. Middle Miocene basin development was mainly controlled by normal faulting along the detachment. Based on our balanced geologic cross-section, the total amount of Miocene extension of the northern Honshu rift system is ca. 40km. Due to subsequent shortening since Pliocene, the low-angle detachment was reactivated as a thrust.

A two-dimensional dislocation model for interseismic deformation of the Taiwan mountain belt

We use a Global Positioning System (GPS)-derived surface velocity field of Taiwan for the time period between 1993 and 1999 to infer interseismic slip rates on subsurface faults. We adopt a composite elastic half-space dislocation model constrained by the observed horizontal velocities projected into the direction of plate motion (306°). The GPS data are divided into northern and southern regions and the velocities in each region are projected into single profiles. The model fault geometry includes a shallowly dipping décollement, based on the balanced geological cross-sections in the Coastal Plain and Western Foothills, and a two-segment fault representing the Longitudinal Valley Fault (LVF) in eastern Taiwan. The décollement is composed of two fault segments, one extending west under the Central Range (CR) and one extending east of the LVF, with estimated slip rates of about 35 and 80 mm/yr, respectively. The optimal geometry of décollement is subhorizontal (2°∼11°) at a depth of 8∼9 km. The inferred surface location of the western end point of dislocation in the northern profile is located 15 km east of the Chelungpu Fault, while in the southern section, it is located beneath the Chukou Fault. The elastic dislocation model successfully matches the horizontal velocity data, and predicts elastic strain accumulation in the Western Foothills that will presumably be released in future earthquakes. However, considered over multiple earthquake cycles, our model cannot explain the topography of the CR and thus fails to predict the active mountain building process in Taiwan. This failure indicates that both horizontal and vertical velocity fields require a more complex rheological model that incorporates inelastic behavior.