Gediz Graben Research Articles

Complex erosional response to uplift and rock strength contrasts in transient river systems crossing an active normal fault revealed by 10Be and 26Al cosmogenic nuclide analyses

AbstractUnderstanding the influence of bedrock lithology on the catchment‐averaged erosion rates of normal fault‐bounded catchments and the effect that different bedrock erodibilties have on the evolution of transient fluvial geomorphology remain major challenges. To investigate this problem, we collected 18 samples for 10Be and 26Al cosmogenic nuclide analysis to determine catchment‐averaged erosion rates along the well‐constrained Gediz Fault system in western Türkiye, which is experiencing fault‐driven river incision owing to a linkage event ~0.8 Ma and has weak rocks overlying strong rocks in the footwall. Combined with existing cosmogenic data, we show that the background rate of erosion of the pre‐incision landscape can be constrained as <92 mMyr−1, and erosion rates within the transient reach vary from 16 to 1330 mMyr−1. Erosion rates weakly scale with unit stream power, steepness index and slip rate on the bounding fault, although erosion rates are an order of magnitude lower than slip rates. However, there are no clear relationships between erosion rate and relief or catchment slope. Bedrock strength is assessed using Schmidt hammer rebound and Selby Rock Mass Strength Assessments; despite a 30‐fold difference in erodibility, there is no difference in the erosion rate between strong and weak rocks. We argue that, for the Gediz Graben, the strong lithological contrast affects the ability of the river to erode the bed, resulting in a complex erosional response to uplift along the graben boundary fault. Weak covariant trends between erosion rates and various topographic factors potentially result from incomplete sediment mixing or pre‐existing topographic inheritance. These findings indicate that the erosional response to uplift along an active normal fault is a complex response to multiple drivers that vary spatially and temporally.

Some thoughts on the morphotectonic development of the Denizli region Basic

The Denizli region is located to the east of the Buldan horst. Although they follow the same trends as the Büyük Menderes and Gediz grabens, the Denizli region has undergone semi-independent evolution during the Quaternary. The essential morphotectonic elements of the area are the Denizli basin and the surrounding Babadağ and the Güzelpınar horsts. The Babadağ horst is the most distinct morphotectonic element of the region. It is bounded in the North by the Denizli fault, a listric normal fault, possibly a major detachment fault. The Laodicea and Güzelpınar horsts located within the downthrown block of the fault may be regarded as antithetic structures.

Assessment of geothermal power potential in the Gediz Basin, Turkey

By the end of 2020, the installed geothermal power capacity in Turkey reached 1643 MWe. Approximately one-fifth of the capacity of the installed power plants is located at the Gediz graben, which is geologically the second largest structure of the Menderes Massif. The Gediz graben has several geothermal surface manifestations and geothermal resources that have been discovered. When considering the structural/geological similarities between the Buyuk Menderes and the Gediz graben, both regions can be assessed as geothermal basins.The scope of this study is to estimate the geothermal power potential of the Gediz graben. To achieve this, a different stochastic approach is used rather than the conventional geothermal resource assessment. This approach involves using temperature depended volume distributions obtained from drilled wells, which lead to improved and solid results. Three hot anomalies that are identified from temperature dependent volumes are evaluated using the Monte Carlo Simulation technique. In these simulations, distribution data of other parameters are used along with data obtained from the drilled wells in the region.The power potential (MWe) values for three identified anomalies in terms of P10, P50 and P90 values are estimated to be 825, 1157 and 1524 MWe, respectively. Furthermore, additional geothermal potential discovery is predicted in the graben, provided that extensive efforts are exerted.

Surface heat flow in Western Anatolia (Turkey) and implications to the thermal structure of the Gediz Graben

Knowledge of heat flow density on the Earth's surface and subsurface temperature distribution is essential for the interpretation of several processes in the crust such as for the evaluation of the geothermal potential of a region. With this study, we investigate the conductive heat flow distribution in western Anatolia to understand the thermal state and its relationship to regional tectonics in the region. The new heat flow data are collected and combined with previously published data to obtain the new heat flow map of western Anatolia. Analysis of data sets after appropriate corrections yields a better picture of the regional distribution of heat flow within the region. Generally, high values are observed around the grabens of Menderes Massif due to the intense tectonic activity. We also present the 2D steady-state thermal model of Gediz. The modeled temperatures are validated by temperature measurements from two deep wells. Numerical simulation results show that the dominant heat transfer mechanism in Gediz graben can be explained by conduction. Temperature distribution in the deep subsurface of the graben is controlled by both thickness distribution and thermal properties of the different stratigraphic sections. Thermal conductivity contrast between different stratigraphic sections causes anomalously elevated heat flow values at the edges of the graben. The comprehensive results of this study will bring a new perspective to geothermal studies in particular Enhanced Geothermal Systems (EGS) resource estimations in Gediz graben.

SEISMIC EVENTS IN THE UPPER MIOCENE – PLIOCENE SEDIMENTARY SUCCESSION IN THE GULF OF İZMİR (WESTERN ANATOLIA): IMPLICATIONS FOR HYDROCARBON PROSPECTIVITY

An analysis of multi‐channel seismic reflection data integrating reflection tomography, pre‐stack depth migration, AVO analysis, seismic modelling and seismic attribute analysis was used to investigate the Miocene – Quaternary stratigraphy of the Gulf of İzmir, western Anatolia. In this area, the east‐west oriented Gediz graben intersects with the NE‐SW oriented Bakırçay Graben. A velocity‐depth model together with pre‐stack depth migration allowed two seismic stratigraphic units (SSU1 and SSU2) to be distinguished. These units can be correlated with the stratigraphic succession at the offshore Foça‐1 well and correspond to the Upper Miocene to Recent Bozköy, Ularca and Bayramiç Formations with a combined thickness of 1.75 km. The units rest on acoustic basement (SSU3) which has a basin‐and‐ridge morphology, and which corresponds to the Lower‐Middle Miocene Yuntdağ Volcanics. A number of lateral velocity variations were identified. In particular, a ~90 m wide and ~500 m long lenticular‐shaped low‐velocity zone with an interval velocity of 1.68 km/s was identified in the Quaternary Bayramiç Formation. The structure is bounded by negative reflections whose amplitude increases with offset at the top and by strong positive reflections whose amplitude increases with offset at the base, interpreted as possible bright and flat spots respectively. These amplitude events point to the presence of gas‐saturated sediments within the study area. The lenticular structure is bounded by strike‐slip faults on either side, and by a Miocene – Pliocene unconformity surface below and by shales of the Bayramiç Formations above. It is therefore interpreted as a possible structural – stratigraphic trap. The strike‐slip faults may allow the migration of hydrocarbons from source rocks located at greater depths. The presence of a low‐velocity zone above the lenticular structure reaching up to seafloor may indicate the upward leakage of hydrocarbons from the trap. These observations will contribute to future hydrocarbon exploration activities in the study area.

Geothermal Waters of Urganli (Manisa) as a Hydrogeological Model, Western Anatolia, Turkey

The area of Urganlı is located in the western part of Gediz graben and consists of Paleozoic metamorphic rocks with intercalations of various schists and marbles, Mesozoic limestones and ultrabasic rocks, and Pliocene to recent travertine deposits and alluviums. Mica schists as intercalations in Paleozoic metamorphic rocks form impermeable basement rocks hydrogeochemically. The marble as intercalations in metamorphic rocks is of reservoir rock for geothermal waters. Moreover, these marbles are also aquifer for groundwaters. As impermeable rocks, sedimentary rocks with high quantities of clay contents in Pliocene to recent age play an essential role for enhancement of geothermal waters. Geothermal waters of Urganlı with temperatures of up to 75 °C and reservoir temperatures of up to 180 °C are of Na-HCO3 type whereas groundwaters display Ca-Mg-HCO3 type. Geothermal waters of Urganlı were represented as an hydrogeological model within this study. The Kula volcano in the Gediz rift zone with a last eruption age up to 18,000 a is responsible for heating geothermal waters.

Hydrogeochemical similarities and differences between high temperature geothermal systems with similar geologic settings in the Büyük Menderes and Gediz Grabens of Turkey

The Büyük Menderes Graben (BMG) and the Gediz Graben (GG) are two major graben systems and they have important medium-high temperature geothermal systems, with suitable reservoir temperatures (e.g. 170–276 °C) for electricity production such as Kızıldere (Denizli city), Germencik, Salavatlı, Pamukören (Aydın city) in the BMG and Alaşehir (Manisa city) in the GG.These high temperature geothermal systems are located in a highly fractured zone because of the Aegean Extensional Zone effect. Although the main reservoir rocks retain similar compositions for each graben system, the geothermal fluids chemistry characterization can vary along the BMG and GG as a result of differing variables such as a paleo sea effect, the concentration of non-condensable gases (NCG), as well as mineral effects. The water chemistry results and δ18O-δD isotope values show remarkable differences within the Germencik and Kızıldere systems along the BMG, while the main reservoir rocks are of a similar composition. Chemical, isotope compositions and pH values of thermal waters in Alaşehir differ from those in the BMG geothermal systems.Results for δ13C isotopes indicate that the origin of the large amounts of CO2 is the marble in metamorphic rocks in the reservoirs in both graben systems. δ34S, δ34S- SO4 and δ18O-SO4 values indicate that the origin of SO4 indicates gypsum and also bacteria effect in the eastern part of the BMG, while different mechanisms such as; oxidation of sulfur, pH, temperature and possible buffer mineral effects on H2S dissolution to the west of the BMG and GG display a greater impact.

Evaluation of relative tectonic activity along the Priene-Sazlı Fault (Söke Basin, southwest Anatolia): Insights from geomorphic indices and drainage analysis

The West Anatolia Extensional Zone, which has a width of about 300 km, is located within the Alpine-Himalayan belt and is one of the regions with intense seismic activity in the world. The most important geomorphological structures in this area are three main graben structures resulting from regional N-S extension since the Early Miocene. These structures are the E-W trending Buyuk Menderes, Kucuk Menderes, and Gediz grabens. Soke Basin is located at the SW end of the Buyuk Menderes graben. The lineaments which control the NW of Soke Basin have a length of approximately 40 km and have been defined as the Priene-Sazli Fault (PSF). The PSF is seismically active, and the last large earthquake (the Soke-Balat earthquake; Ms: 6.8) was produced on July 16th of 1955. The ancient city of Priene, which was located in the study area, suffered from destructive earthquakes (in the 4th century and 2nd century BC, in the 2nd century AD, during the Byzantine period and after the 12th century BC). This study aims to reveal the effect of the PSF on the morphotectonic evolution of the region and the relative tectonic activity of the fault. To this end, it was the first time the stream length gradient index (SL: 130–1303), mountain-front sinuosity (Smf: 1.15–1.96), valley floor height and valley width ratio (Vf: 0.27–1.66), drainage basin asymmetry (AF: 0.15–0.76), hypsometric curve (HC) and hypsometric integral (HI: 0.22–0.86) and basin shape index (Bs: 1.04–5.75) along the mountain front that is formed by the PSF. Using a combination of the mountain-front sinuosity (Smf), valley floor height and valley width ratio (Vf), it is found that the uplift ratio in the region is not less than 0.05 mm/yr and the relative tectonic activity of PSF is high. According to the relative tectonic activity index (Iat) obtained from geomorphic indices, the southwest part of the PSF is relatively more active than the northeast part. As a result, I posit that the PSF has the potential to produce earthquakes in the future similarly to those that were produced in the past, and that the most destructive earthquakes will likely occur on the southwest segments of the fault according to geomorphic indices.

The Gediz Supradetachment System (SW Turkey): Magmatism, Tectonics, and Sedimentation During Crustal Extension

AbstractUnraveling the evolution of supradetachment basins developed in the hanging wall of low‐angle detachment faults may be an invaluable tool in reconstructing the tectonic evolution of highly extended terrains. These basins may record major regional tectonic events related to the exhumation of metamorphic core complexes, and the reconstruction of their evolution helps to quantify the amount of extension accommodated by such processes. Here we present stratigraphic and structural field evidence and micropaleontological constraints to the Neogene‐to‐Quaternary evolution of the supradetachment Gediz Graben that developed on top of the exhuming Central Menderes Massif (SW Turkey). This basin displays three different structural styles during its evolution: (i) it initiated as a ramp basin following the activation of the Gediz Detachment in the Middle Miocene, (ii) evolved as a half graben during the late Miocene following the activation of high‐angle brittle faults at its southern margin, and (iii) reached its final symmetric graben configuration in Late Pliocene (?)‐Quaternary times following the activation of its northern margin. New micropaleontological data document a short‐lived upper Tortonian marine episode in the basin, and major along‐strike variations in exhumation are documented on its southern margin. Our reconstruction shows how sedimentary basins originally formed in the hanging wall of detachment faults may eventually end up in tectonic contact with the mylonitic footwall. Finally, we highlight the importance of magmatism in localizing the deformation in highly extended terrains and in controlling the evolution of supradetachment systems.

Determination of the block movements in the eastern section of the Gediz Graben (Turkey) from GNSS measurements

The source parameters of an earthquake should be determined in detail to calculate deformations caused by that earthquake and to identify faults that are likely to break. Today, the GNSS method is frequently used to determine these source parameters. However, to date, no detailed study has been conducted for this purpose in the eastern section of the Gediz Graben. Therefore, in this study, the GNSS technique was used to determine the tectonic movements in the area between Alaşehir and Sarıgöl in the Gediz Graben. For the implementation of the GNSS technique, 11 benchmark sites, which had been previously established for different purposes in the area, were used, and the measurements were performed in 2013, 2014, and 2015. The GNSS measurements obtained at these sites were evaluated by GAMIT/GLOBK software, and the annual velocity values of the sites were obtained (with respect to the Eurasian plate). The resulting annual horizontal velocity values obtained at the GNSS sites ranged from 12 mm/yr to 27 mm/yr. It was observed that the sites with the highest vertical movements were within the graben. The annual vertical velocity value at the TRAZ site located in the east of the graben was obtained to be approximately −90 mm/yr. This site was followed by BGCL (-33 mm/yr) and AKCL (−13 mm/yr) toward the west of the graben. Furthermore, significant strain accumulations were derived in the study area, and their compliance with the tectonic structure of the region was revealed. The maximum and minimum principal strain parameters and the direction of maximum principal strain that provided the average drift and compression directions for the region were obtained to be 0.604 μs/yr, -0.375 μs/yr, and 11.27 °, respectively. Block modeling in the study area revealed that the annual strike slip ranged from 4 ± 1.2 mm/yr to 19.3 ± 0.6 mm/yr, and the dip slip varied between -23.3 ± 0.5 mm/yr and -16.0 ± 0.5 mm/yr.

Three-Dimensional Seismic Velocity Structure of the Aegean Region of Turkey from Local Earthquake Tomography

This study brings new insights to elucidate the 3-D seismic velocity structure of the Aegean region of Turkey by using Local Earthquake Tomography (LET). The study area has remarkable potential for geothermal resources. To provide the subsurface geological structure of seismogenic layers and geothermal areas, we develop new high-resolution depth-cross sections through Buyuk Menderes, Kucuk Menderes and Gediz grabens. Travel times of almost 265.000 readings (14.963 P-phase and 11.969 S-phase picks) from 2.085 well-located events recorded between 2007 and 2016 by a permanent seismic network of 75 broad-band seismometers were used to precisely interpret the 3-D Vp (lithological) and Vp/Vs (petrological) models of the study area. We detected four main layers down to 35-40 km depths with different Vp velocities ranging from 3.5 to 8.5 km/s. Mid-crustal interface (Conrad discontinuity) is discovered at 15 km depth in nearly all depth-cross-sections. Our results suggest an average of 25 km Moho depth in the Aegean region of Turkey. The depths range from around 18 km beneath SE of Aydin to 29 km beneath Aliaga, and approximate values of 19, 25 and 31 km beneath the Doganbey, Kutahya and Selendi-Demirci regions, respectively. The geothermal areas of the studied region are imaged by Vp and Vp/Vs anomalies. We propose the Aliaga, Denizli, Doganbey and Kosk areas as low Vp, low Vp/Vs anomalies which are an indicator of steam, CO2 or a mixture of both. Low Vp, high Vp/Vs models, suggesting geothermal fluids, are clearly visible near the Buharkent, Gumuskol, Guzelhisar, Kosk, Kuyucak, Saraykoy and Suzbeyli regions. We also report that the Bademler, Candarli, Kalekoy, Karahalli, Merdivenli, Ortakoy, Saruhanli, Yelki and Yuntdagikoseler regions might be good candidates for new potential geothermal resources.

The conductivity structure of the Gediz Graben geothermal area extracted from 2D and 3D magnetotelluric inversion: Synthetic and field data applications

In this study, we have created a virtual three-dimensional conductivity model of the Gediz Graben geothermal area using real topographical variations and geological settings as prelude to the interpretation of acquired MT field data. The thicknesses of the geological units were obtained from previously studied seismic sections and boreholes drilled inside the study area. A large magnetotelluric grid was created using 234 sites with 1km spacing between each site. The three-dimensional magnetotelluric response of the model was calculated at each site and used as an input for two- and three-dimensional inversion to assess resolving capability for subsurface features. The inversion results were compared to the virtual graben model, and the efficiency of the inversion methods was examined. Comparisons show that the three-dimensional inversion of magnetotelluric data has many advantages with respect to the two-dimensional inversion results for the interpretation of the three-dimensional geological structure of the graben fill. Although the two-dimensional inversion is a rapid tool for exploration of the conductive anomalies, it is inadequate for interpreting the resistive basement rock structure under the sedimentary cover. We also applied three-dimensional inversion to the field data previously recorded inside the study area. The target borehole locations were defined according to the interpretation of the three-dimensional inversion model. A resistivity cross-section extracted from the three-dimensional model of the field data is presented with the borehole observations. The lithological boundaries of the actual borehole data and the resistivity transactions of the three-dimensional model show close correlation. The synthetic data examples show that the virtual graben model is a practical tool for understanding the advantages and disadvantages of the magnetotelluric inversion methods on extensional tectonic basins. Additionally, the field data experiments show that the three-dimensional inversion of magnetotelluric data is an efficient tool for interpreting the geological structure of extensional graben systems.

The effects of extensional structures on the heat transport mechanism: An example from the Ortakçı geothermal field (Büyük Menderes Graben, SW Turkey)

The extensional stress regime is very effective in western Turkey since the Latest Oligocene-Early Miocene. Geothermal fields are positioned on the high-angle normal faults along the northern and southern margins of the Büyük Menderes and Gediz grabens, respectively. The Ortakçı geothermal field is located on the tectonically active northern margin of Büyük Menderes graben. The discharge rate and temperature of the Ortakçı hot spring is 2.4l/s and 50.5°C, respectively. Hot water discharges in the relay ramp representing the overlapping regions of two E–W normal faults related to N–S-oriented extension. The high-angle normal faults provide pathways for the fluid flow by increasing dilations. The field observations prove that all hot water outflows are related to the high-angle normal faults on the hanging wall of the detachment faults. Detailed structural analyses were performed on the Ortakçı geothermal field, particularly on one of the active hot springs at the intersection of the E–W and N–S-oriented faults, to evaluate the tectonic activity in the area and the flow pathways for geothermal fluids. The heat source is the shallow mantle of a thinned lithosphere due to large extension rates, resulting in increased heat flow. The tectonic effects are on the heat transport mechanism, and the heat transport is controlled by faults in a convection dominated systems. The result is a high geothermal gradient because water can circulate deep and transports the heat upwards to the surface.

Magnetic susceptibility and VLF-R investigations for determining geothermal blowout contaminated area: a case study from Alaşehir (Manisa/Turkey)

Western Turkey is one of the most spectacular regions of widespread, active continental extension in the world. The most prominent structures of this region are E–W trending Gediz and Buyuk Menderes Grabens. Geothermal activity around city of Manisa in Gediz Graben has been investigated by many researchers and many geothermal boreholes were drilled in order to produce electricity. In the middle of May 2012, a geothermal blowout occurred 150 m away from the deep drilling (depth 1,100 m), in Alkan village of Alasehir resort in city of Manisa. After that, four big blowouts took place at the same area and thermal water had been diverted to Alkan stream for almost 4 months. The boron (B) and Fe levels of thermal water are 87 and 11.2 ppm where the background B and Fe values of topsoil vary between 0.2–1.5 ppm and 3.08–5.63 ppm, respectively. The objective of this study was to utilize joint magnetic susceptibility and VLF-R investigations to determine possibly contaminated area with B, related to high Fe level on the topsoil and thermal water in the subterranean layers. It is observed that magnetic susceptibility values vary between 500 and 790 cgs (×10−6) in the vicinity of blowouts and 150–340 cgs (×10−6) at a distance of 50 m around, where the background average value is 20 cgs (×10−6), by using the data collected at 346 stations. Besides, the VLF-R studies were carried out along 18 profiles at three different frequencies. Laterally constrained two-layer inversion was applied to each station; in addition to the inversion of all profiles for each frequency, all lines were stacked, 2-D resistivity maps were obtained, and the contaminated area was determined approximately as 35,000 m2. The results show not only satisfactory agreement between magnetic susceptibility and VLF-R data, but also provide fast and valued interpretation of geochemical and geophysical properties of the geothermal contaminated area.

Environmental dose rate assessment from an active fault zone in Western Anatolia, Turkey: towards retrospective epidemiology

In this study we present data regarding natural radioactivity from a populated active tectonic area based on site measurements using a field gamma spectrometer. The results will provide data to the scientists for dating studies as well as epidemiological studies towards ancient human life, also termed as retrospective epidemiology. The study area covers an active tectonic region in western Anatolia surveyed using on-site gamma spectrometer. More than 500 sites were measured across the region on a 150- km grid. Radiation background shows relatively non-homogenous level of radiation in the area. The gamma dose rate measured in Gediz graben was found in the range between 0.8120- 1.7540 mGya–1 (94-203 nGyh–1) with an average value of 1.2292 mGy/a (142 nGyh–1), whereas it was in the range between 0.5530-1.1491 mGy/a (64-133 nGyh–1) with an average value of 0.8397 mGy/a (97 nGyh–1) across Buyuk Menderes graben. The relative contribution of 238U decay chain to gamma dose rate is between 0.2002- 0.4630 mGya–1, of 232Th decay series is 0.3089- 0.8136 mGya–1, and of 40K is 0.1270- 0.3468 mGya–1 in the whole studied area. The average annual effective dose equivalents from the calculated outdoor terrestrial gamma radiation was between 90-215 μSva–1, above world average of 70 μSv per year, and the measured outdoor dose rate (terrestrial plus cosmic) on the ground was between 133-257 μSva–1. The significance of all these numbers is discussed in the framework of epidemiological studies, in order to correlate these enhanced dose rate values directly to specific diseases such as cancer.

Contexte géographique et géologique du site de Kocabaş, Bassin de Denizli, Anatolie, Turquie

The site of Kocabaş is located in the Denizli Basin, in western Anatolia, 400km southwest of Ankara, 360km south of Istanbul, 200km east-southeast of Izmir and 150km northwest of Antalya. The Denizli Basin depression, at the junction of the Buyuk Menderes and Gediz grabens, which began to form at the beginning of the Miocene, was filled in around the edges, along fault lines, by significant travertine formations, some of which are still active today. The age of the Kocabaş travertines, which yielded the Kocabaş Homo erectus skullcap, was evaluated by thermoluminescence at 828,000years, by electron spin resonance (ESR) at 1,110,000 years and by the 26Al/10Be cosmogenic nuclide method to more than 1.22 Ma and less than 1.5 Ma, a date which has been confirmed by magnetostratigraphy and biochronology.

Geodetic aspects of the 19 May 2011 Simav earthquake in Turkey

On 19 May 2011, an earthquake of magnitude Mw:5.9 occurred just 9.1 km below the surface of the earth on the Simav fault in Turkey. The region is located north-west of the Gediz graben, one of the major tectonic structures in the extensional regime of western Anatolia. In this study, from 2009 to 2011, the data from nine Continuously Operating Reference Stations (CORS-TR) in the region and one permanent Global Navigation Satellite System station working locally were assessed and analysed using GAMIT/GLOBK software. The behaviour of these stations before and after the earthquake was analysed using calculated time series. At the same time, the strain and rotation rates of current deformation in the Simav region were computed using the 2008–2011 data from the CORS-TR network. The directions obtained from the focal mechanism solutions and strain analysis of the Simav earthquake were observed as being in harmony. Nonetheless, Simav earthquake caused a displacement of 1–2 mm (approximately) in nearby locations. The Simav earthquake shows that the extensional regime in western Anatolia has recently been active in the region and is a good example of the evolution of the graben system.

Fault kinematics in supradetachment basin formation, Menderes core complex of western Turkey

The Neogene Alasehir supradetachment basin in western Turkey occurs in the footwall of the modern Alaşehir–Gediz graben and is a major component of the Aegean extensional province in Anatolia that developed during the late Cenozoic exhumation of the Menderes core complex. The size, geometry, accommodation space and internal structure–stratigraphy of this basin were strongly controlled by the kinematics and interplay of four different fault generations throughout its early Miocene-Pleistocene evolution. The ~ E − W-striking, NNE-dipping (10°–30°) Alaşehir detachment fault separates the high-grade crystalline rocks of the core complex below from the Neogene deposits of the basin in the hanging wall above. The basin-parallel, synthetic and antithetic high-angle normal faults represent a block-faulting stage of the continued horizontal extension that terminated the northerly slip along the detachment surface and produced horst-graben structures in and across the basin. These faults were also responsible for the back-tilting of the basinal strata into steeper dips and the rotation of the previously formed faults into gently dipping planes. The mutually crosscutting relationships between the intra-basinal low-angle and basement-involved high-angle and normal faults indicate their contemporaneous development and operation. The NNE-striking, basin-perpendicular oblique–slip hinge faults are reminiscent of transfer (cross) faults in both continental and oceanic extension, and divide the supradetachment basin into a series of fault blocks with their own internal structure and stratigraphy. Differential uplift and exhumation of the core complex rocks along these hinge faults and the varied displacement and slip along and across them within the basinal strata caused differential extension and block-uplift, leading to the formation of local unconformities in the supradetachment basin. Significant changes in the dip directions and strike orientations of the sedimentary beds and the observed deviations from the inferred direction of maximum extension in the adjacent fault blocks resulted from rotational deformation due to hinge faulting. Fault segmentation of the Alaşehir basin as a result of extension-parallel hinge-faulting is a characteristic feature of its supradetachment basin evolution.

In-situ stress field and mechanics of fault reactivation in the Gediz Graben, Western Turkey

The present-day stress field in the Gediz Graben is characterized by vertical maximum and horizontal 105°N-trending intermediate and 015°N-trending minimum principal stress axes. Stress gradients are 23.4, 18.6 and 15.3MPa/km for maximum, intermediate and minimum stresses, respectively. This stress tensor aligns well with the observed fault pattern of the graben and confirms the ∼N-S-oriented extension. The strike\\dip of optimum planes with highest slip tendency is 105°\\60°S and 285°\\60°N and highest dilation tendency is on 105°N-trending vertical fractures. Faults in the graben fill have near normal frictional strength with friction coefficient of (μ) 0.48. Pore pressure is generally hydrostatic but local overpressure elevated to pore-fluid factor (λ) of 0.71 was observed in close proximity to the master bounding fault of the graben. Data and geological evidence suggest that this fault is a major conduit for mantle degassing and plays a significant role in fluid circulation. The hot CO2-rich fluids ascending through the fault system could be locally trapped to form overpressured CO2 pockets as observed in the shale-rich Alaşehir Formation which is suitable to form clay smears and clay-matrix fault gouges with high seal potential. High temperature and CO2 content is also favorable for healing and sealing of the fractures by carbonate precipitation to re-establish fault cohesion. Temporary fault seal breaching may occur due to shear reactivation of the faults and could relieve excess pore pressure. This may also lead to surface discharge of CO2-rich fluids with hydrocarbons generated in the Alaşehir Formation. Projection of the deduced stress field to 6km depth suggests that brittle reactivation of the low-angle segment of the master bounding fault possibly requires combined operation of overpressuring and fault zone weakening.

Supradetachment basin evolution during continental extension: The Aegean province of western Anatolia, Turkey

We present a detailed structural and stratigraphic record of a Neogene–Quaternary supradetachment sedimentary succession in the Aegean extensional province of western Anatolia, and we compare its tectonic features and evolution to those of other well-documented supradetachment basins around the world. The sedimentary fill of the Alasehir basin records the uplift and exhumation of a core complex in the footwall of a detachment fault within the Central Menderes Massif. Accumulation of footwall-derived clastic sediments in this basin started ca. 20 Ma, shortly after the initiation of the approximately E-W–trending Alasehir detachment and its shear zone, and continued until ca. 2 Ma. Major sedimentary facies types include fluvial and alluvial-fan deposits, debris-flow and mass-flow deposits, and locally developed lacustrine rocks. These sedimentary units were accumulated largely in distal depocenters within the extending basin, as the low-angle (15°–28°) detachment faulting created little accommodation space near the basin margins while producing high back-shed topography in the uplifted Menderes core complex. The drainage system was dominated mainly by extension-parallel transverse streams during the main phases of basin evolution. Extension-parallel, scissor (hinge) faulting produced differential uplift and subsidence in the adjacent fault blocks, changed the direction of sediment transport and drainage patterns over short distances, and resulted in the local uplift of the older basin strata. These processes led to the development of subbasins with lateral variations in basement topography, strata thickness, and sedimentary facies distribution, and generated a segmented basin architecture. High-angle synthetic and antithetic faults that formed extensively after 3 Ma caused back-tilting of the sedimentary strata, formation of half grabens with their own axial drainage systems, and development of angular unconformities. With the onset of this crustal-scale block faulting, the detachment fault ceased to operate, and the Quaternary Gediz graben started to develop at the northern end of the Alasehir supradetachment basin. Our comparative evaluation of select basins shows a maximum sediment thickness of 3 km, average extension rates of 6 to 8–9 mm yr −1 , and accumulation rates of 0.1–0.2 mm yr −1 (uncorrected for compaction) in supradetachment basins in general. The rates and amounts of extension, the geometry of detachment faulting, the rates of footwall uplift, and the kinematics and interplay of different fault systems are the most important factors controlling three-dimensional structural architecture and evolution of supradetachment basins.