skip to main content

Research

::: Home/Research - Research Fields

Research Fields

Geology and Active Tectonics

Research Fellows/Professors︰Sun-Lin ChungJian-Cheng LeeYa-Ju HsuYu-Chang ChanKuo-Lung WangKwan-Nang PangHsin-Hua HuangYunung Nina LinAlexandre Canitano
Research Scientists︰Horng-Yue ChenHao-Yang Lee

The research fields of geology and active tectonics at IES involve topics of various time and space scales for understanding the processes of crustal evolution and recent crustal deformation and surface processes. Research on orogenic belts and mountain building in IES aims at establishing magmatic records by systematic geochronological and geochemical analyses, and elucidating underground structure and earthquake source characteristics by deploying portable broadband seismic networks. These efforts lead to better understanding of various evolutionary stages of the orogenic belt. Key study areas include orogenic belts of the Tibet-Himalaya and Caucasus-Iran-Anatolia, two of the most important collision orogens in the world, accretionary orogens of Central Asian orogenic belt and South East Asia, and the young and classic Taiwan orogenic belt. Comparative research on these types of orogens is currently conducted by IES scientists.

In the field of active tectonics, by combining continuous GPS networks data and borehole strainmeter measurements, IES investigates and studies the correlation between the spatial-temporal patterns of crustal deformation prior to earthquakes and other surface processes such as landslides. IES also develops airborne LiDAR and uses the derived high-resolution DEMs for understanding surface geology, active faulting, landslide and erosion mechanisms. We employ new generations of SAR techniques for investigating earth deformation and surface processes. In addition, IES has carried out a series of geological, seismological and geodetic studies in the Longitudinal Valley in eastern Taiwan. Current research includes GPS and leveling measurements, Chihshang Fault Observatory, and numerical simulations. The purpose is to determine the surface break locations of active faults and the extent and rate of crustal deformation, which will help us understand the mechanical properties of subsurface faults. The research results from the fields of geology and active tectonics provide solid scientific foundation for prevention and mitigation of natural hazards.

Research Highlights

How did it happen, the 2020 Hpakant Jade Mine Disaster in northern Myanmar?
How did it happen, the 2020 Hpakant Jade Mine Disaster in northern Myanmar?
In July 2020, one of Myanmar’s most devastating landslides occurred in the Wai Khar open-pit jade mine in the Hpakant township in the northern state of Kachin, Myanmar. At least 170 jade miners were dead in this incident. Based on the multi-sensor satellite data, the primary cause of the incident was attributed to the seepage failure which caused continuous instability on the slope, rather than extreme weather. We suggest that governments should improve mining site regulations and enforce proper management. Furthermore, the proposed method in this study is recommended for the long-term monitoring of mining sites.
Lin, Yunung Nina
More
Evaluation of single-frequency receivers for studying crustal deformation at the longitudinal Valley fault, eastern Taiwan
Evaluation of single-frequency receivers for studying crustal deformation at the longitudinal Valley fault, eastern Taiwan
Applications of low-cost single-frequency continuous GPS receivers for monitoring volcano and landslide activities as well as to complement dual-frequency receivers have been demonstrated to produce stable and accurate positioning. In studies of crustal deformation, the relative distance between monitoring stations may vary from several kilometers to tens of kilometers, hence the differential single-frequency observations cannot model the ionospheric delay or other distance dependent errors. The 55 low-cost single-frequency continuous stations have been deployed together with 52 continuous dual-frequency stations in southeastern Taiwan since 2008. All of the single-frequency stations have applied corrections using dual-frequency stations to eliminate the distance dependent errors. Comparing velocity estimates from 8 co-located, the differences in horizontal and vertical components are less than 3 mm/yr and 6 mm/yr, respectively. Our study shows that the combination of single- and dual-frequency GPS data can provide robust results to study the fault slip behavior on the Longitudinal Valley fault.
Chen, Horng-Yue
More
Tracking Slow Landslide Movements with Optimized MTInSAR Workflow (2023)
Tracking Slow Landslide Movements with Optimized MTInSAR Workflow (2023)
This study focuses on detecting and monitoring slow-moving landslides in Taiwan using a sophisticated radar technique. Identifying over 2500 pre-existing landslides is crucial for assessing their activity, especially before typhoon seasons. The proposed method, "multi-snap2stamps," effectively analyzes nine slow-moving landslides, revealing seasonal patterns in two sites and accelerated movement in one. The study showcases the potential of the method for large-scale landslide detection and monitoring.
Chan, Yu-Chang
More
Tatun Volcanic System: Thermal Patterns and Earthquake Correlation (2023)
Tatun Volcanic System: Thermal Patterns and Earthquake Correlation (2023)
Satellite-detected land surface temperature (LST) and seismic records are used to study the volcanic activity of Tatun Volcanic Group. Analyzing four decades of LST and three decades of earthquake magnitude data reveals a correlation between LST trends and energy released from volcanic earthquakes, validating the connection between surface temperatures and volcanic activity.
Chan, Yu-Chang
More
Seismogenic structure beneath the northern Longitudinal Valley revealed by the 2018-2021 Hualien earthquake sequences
Seismogenic structure beneath the northern Longitudinal Valley revealed by the 2018-2021 Hualien earthquake sequences
This study uses the 3D crustal velocity model and the relocated earthquake hypocenters, including the 2018–2021 earthquake sequences, to re-assess the seismogenic structures at the northern Longitudinal Valley. Earthquake focal mechanisms and relocated hypocenters from earthquake clusters suggest a gentle west-dipping fault existing under the Longitudinal Valley and the Coastal Range. Earthquake clusters associated with this west-dipping fault indicate it develops along the base of high-velocity Central Range metamorphic rocks and is likely branched out from the previously recognized Central Range Fault (CRF). Both the 3D velocity model and the geometry of earthquake clusters suggest this structure truncates the Longitudinal Valley Fault north of 23.7° N, separating the northernmost LVF into the shallow and the deep segments. The shallow segment then plausibly evolves to be a transpressional fault system that mainly accommodates the left-lateral motions. This interpretation coincides with the geomorphological and geodetic observations showing that the northern LVF is dominated by the left-lateral faulting, instead of showing a significant reverse component as in the southern Longitudinal Valley. The limited fault width and geometry of the shallow LVF segment also imply its seismic potential is relatively limited, while the underling west-dipping fault and the deeper segment of the LVF are the major seismogenic structure. Such development of the major CRF-related west-dipping structure could accommodate the northwestward subduction of the Philippine Sea Plate and also likely reactivate part of the Offshore Eastern Taiwan Thrust Belt.
Huang, Hsin-Hua
More
Coulomb stress changes triggering surface pop-up during the 2016 Mw 6.4 Meinong earthquake with implications for earthquake-induced mud diapiring in SW Taiwan
Coulomb stress changes triggering surface pop-up during the 2016 Mw 6.4 Meinong earthquake with implications for earthquake-induced mud diapiring in SW Taiwan
We investigated the 2016 Meinong earthquake (Mw 6.4) in southwestern Taiwan, which caused surface pop-up in an area of 10x15 km2 with maximum uplift of 12 cm, where lies an array of mud volcanoes and possible underlying mud diapir. We calculated 3D strain tensor in a 3D mesh with 5x5x2 km grids in the epicentral area induced by the Coulomb stress change due to coseismic fault slip. We obtained substantial contraction strain (10−5–10−6) that occurred in a lobe showing “squeezing” at the depth of 5–14 km below the surface pop-up area. Dilatation strain (10−5–10−6) occurred at shallow level (0–3 km) with a radial pattern around the surface pop-up area. Combining with local geology, which is composed of Mio-Pliocene ~5-km-thick mudstone in a fold-thrust belt, we interpret that the 2016 Meinong coseismic surface pop-up was closely related to mud diapirs/volcanoes, which were likely reactivated by sudden increase of fluid pore-pressure in the basal reservoir (at 5–6 km depth) and dilatation in the shallow level. We also explored the potential effects of the Coulomb stress transfer on nearby receiver faults – including three arrays of mud diapir, the regional decollement, a suspected backthrust and one thrust close to the pop-up area. Our results show that the Coulomb stress transfer a) favors NNE-trending mud diapirs in the coseismic pop-up area, with a combination of clamping stress changes at 5–6 km depth and unclamping stress changes at 0–4 km depth, and b) it does not favor triggered thrust slip on the regional thrusts.
Lee, Jian-Cheng
More
Unveiling Tatun volcanic plumbing structure induced by post-collisional extension of Taiwan mountain belt
Unveiling Tatun volcanic plumbing structure induced by post-collisional extension of Taiwan mountain belt
The Tatun Volcanic Group (TVG) is proximal to the metropolis of Taipei City (population of ca. 7 million) and has long been a major concern due to the potential risks from volcanic activity to the population and critical infrastructure. While the TVG has been previously considered a dormant or extinct volcano, recent evidence suggests a much younger age of the last eruption event (~ 6000 years) and possible existence of a magma reservoir beneath the TVG. However, the location, dimension, and detailed geometry of the magma reservoir and plumbing system remains largely unknown. To examine the TVG volcanic plumbing structure in detail, the local P-wave travel time data and the teleseismic waveform data from a new island-wide Formosa Array Project are combined for a 3D tomographic joint inversion. The new model reveals a magma reservoir with a notable P-wave velocity reduction of 19% (ca. ~ 19% melt fraction) at 8–20 km beneath eastern TVG and with possible northward extension to a shallower depth near where active submarine volcanoes that have been detected. Enhanced tomographic images also reveal sporadic magmatic intrusion/underplating in the lower crust of Husehshan Range and northern Taiwan. These findings suggest an active volcanic plumbing system induced by post-collisional extension associated with the collapse of the orogen.
Huang, Hsin-Hua
More
The 2020 Hpakant Jade Mine Disaster, Myanmar: A multi-sensor investigation for slope failure
The 2020 Hpakant Jade Mine Disaster, Myanmar: A multi-sensor investigation for slope failure
A quarry failure along the slopes of the Wai Khar open-pit jade mine in Hpakant, Myanmar has led to the deaths of at least 172 jade miners on 2 July 2020. This paper conducts a systematic investigation of the incident by integrating data from multiple sensors, including high-resolution optical imagery, Sentinel-1 synthetic aperture radar (SAR) images, unmanned aerial system (UAS) footage, SRTM and ALOS digital elevation models (DEMs), soil moisture product from multi-spectral Landsat-8 satellite and precipitation records from the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS). Optical imagery, UAS footage and DEMs allow us to build a comprehensive mapping of tailing areas and quarry scarps from 2010 and reconstruct the 2D pit geometry prior to failure. Deformation signals from multi-temporal SAR interferometry (MTInSAR), soil moisture variations and precipitation trends further allow us to identify possible failure causes. To evaluate the quality of deformation obtained from different distributed-scatterer phase estimators, we develop an empirical mapping function based on areal fraction values to facilitate the comparison of temporal coherence values that are differently formulated in each phase estimator. The comparison shows that phase linking algorithm outperforms the small baseline subset method in terms of signal recovery and phase reliability. Our investigation points out that the mining site is under aggressive mining cycles that are exacerbated by frequent, uncontrolled landslides. Seepage failure, which involves the expulsion of water from rapidly compacting tailings, may be a critical factor in the 2020 incident. Instead of extreme weather, the failure had occurred under normal to drier conditions. This means that the sliding planes were already in a critical state, which is evident from the accelerated deformation around the collapse area since the beginning of 2020. Based on these findings, we provide recommendations to improve mining site regulations and management practices for safer open-pit mining in Myanmar and probably in similar contexts outside Myanmar.
Lin, Yunung Nina
More
Inherited State of Stress as a Key Factor Controlling Slip and Slip Mode: Inference From the Study of a Slow Slip Event in the Longitudinal Valley, Taiwan
Inherited State of Stress as a Key Factor Controlling Slip and Slip Mode: Inference From the Study of a Slow Slip Event in the Longitudinal Valley, Taiwan
Tectonic faults display a large range of slip behaviors, ranging from fast slip (earthquakes) to episodic or continuous aseismic slow slip. Episodic aseismic slip events (typically called 'slow slip events') are now widely observed in active regions and play an important role in stress redistribution in the Earth's crust. The Longitudinal Valley Fault is the most active fault in Taiwan, hosting large to destructive earthquakes. However, very little is known about the presence and the role of slow slip events on the fault deformation. We document a 13‐day long slow slip event with a magnitude of 5.45, which represe'nts the largest event detected on the fault to date. We demonstrate that the slow event was likely encouraged by stress accumulation due to the 2003 Chengkung earthquake. Monitoring and characterizing the sources of aseismic slip is fundamental to identify areas with high seismic hazard on the fault and to shed some light on the interactions between seismic and aseismic processes.
Canitano, Alexandre
More
How does the performance of SAR-based landslide detection changes at different radar wavelengths?
How does the performance of SAR-based landslide detection changes at different radar wavelengths?
The study evaluates the performance of different SAR data properties on landslide detection, including radar wavelengths, spatial resolutions, polarizations, and viewing geometry. We discuss the detection performance in two landslide cases, the earthquake-triggered landslides due to the 2018 Hokkaido Eastern Iburi Earthquake in Japan, and the rainfall-triggered landslide caused by the 2017 heavy rain in the catchment of Putanpunas River, southern Taiwan.
Lin, Yunung Nina
More
Surface traces and related deformation structures of the southern Sanyi Fault, Taiwan, as deduced from field mapping, electrical-resistivity tomography, and shallow drilling
Surface traces and related deformation structures of the southern Sanyi Fault, Taiwan, as deduced from field mapping, electrical-resistivity tomography, and shallow drilling
The southern Sanyi Fault extends across a highly urbanized area in central-west Taiwan. It is within 1–2 km from and generally parallels the northern Chelungpu Fault, which last ruptured in 1999 (Mw = 7.6 Chi-Chi Earthquake) and caused about 2400 fatalities. Several questions thus arise: is the Sanyi Fault likewise active? what is its geometry and extent? and should it be zoned or otherwise designated to mitigate risk through restrictions on development or engineering design? We address these questions by mapping river-exposed outcrops, by interpreting nine, near-surface geophysical surveys (electrical resistivity tomography [ERT]), and by compiling and assessing existing shallow borehole data. Outcrops in the Dajia River show that the Sanyi Fault is mainly a low-angle thrust that juxtaposes poorly lithified sandstone over alluvial gravels. Fault branches are common both in the hanging wall and footwall. To the south, the fault is not marked by a break in topography and its trace has not been clearly mapped within the alluvial plain, in part motivating the current effort. Here the ERT surveys identify the hanging-wall sandstone by a relatively low resistivity of <100 Ωm, whereas the footwall conglomerates and alluvial gravels generally exceed ~300 Ωm and ~ 100 Ωm, respectively. Borehole data likewise constrain the extent and character of the Sanyi Fault, which we identify as extending across the eastern part of Fengyuan, a city with a population now exceeding 160,000. The fault system is clearly imaged in two of five surveyed sites, and we integrate these data with drilling data to map the Sanyi fault surface trace as a ~ 400 m wide zone in the city of Fengyuan. Compiling previously documented paleoseismic data at one site, the Sanyi Fault system likely slipped in the thousand-year range throughout the Holocene. The Sanyi Fault is therefore ‘active’ and warrants zoning or other forms of mitigation to minimize potential, seismically induced loss of life and property.
Lee, Jian-Cheng
More
Mid-Miocene volcanic migration in the westernmost Sunda arc induced by India-Eurasia collision
Mid-Miocene volcanic migration in the westernmost Sunda arc induced by India-Eurasia collision
The migration of arc magmatism that is a fundamental aspect of plate tectonics may reflect
the complex interaction between subduction zone processes and regional tectonics. Here we
report new observations on volcanic migration from northwestern Sumatra, in the westernmost
Sunda arc, characterized by an oblique convergent boundary between the Indo-Australian
and Eurasian plates. Our study indicates that in northwestern Sumatra, volcanism ceased at
15–10 Ma on the southern coast and reignited to form a suite of active volcanoes that erupt
exclusively to the north of the trench-parallel Sumatran fault. Younger volcanic rocks from the
north are markedly more enriched in K2O and other highly incompatible elements, delineating
a geochemical variation over space and time similar to that in Java and reflecting an increase
in the Benioff zone depth. We relate this mid-Miocene volcanic migration in northwestern
Sumatra to the far-field effect of propagating extrusion tectonics driven by the India-Eurasia
collision. The extrusion caused regional deformation southward through Myanmar to northwestern Sumatra and thus transformed the oblique subduction into a dextral motion–governed
plate boundary. This tectonic transformation, associated with opening of the Andaman Sea, is
suggested to be responsible for the volcanic migration in northwestern Sumatra.
Chung, Sun-Lin
More
Variable vertical movements and their deformation behaviors at convergent plate suture: 14-year-long (2004-2018) repeated measurements of precise leveling around middle Longitudinal Valley in eastern Taiwan
Variable vertical movements and their deformation behaviors at convergent plate suture: 14-year-long (2004-2018) repeated measurements of precise leveling around middle Longitudinal Valley in eastern Taiwan
To better characterize the vertical movements and the deformation behaviors across the plate suture of an arc-continent collision, we conducted annual repeated measurements on two precise leveling routes in a length of 34 and 37 km, respectively, across the middle part of the Longitudinal Valley in eastern Taiwan in 2004–2018. The 14-year-long results showed that the Longitudinal Valley fault (LVF) dominates the surface deformation: a) the middle LVF (Juisui fault) exhibited partially locked in the upper few kilometers, with a cumulative uplift rate of 9–10 mm/yr in a distance of 4 km; b) the southern LVF (Chihshang fault) showed a creeping behavior with a vertical rate of 24–27 mm/yr. In addition, we are able to characterize other features, including 1) tilting upward to the west in the eastern Central Range, suggesting activity on the west-dipping Central Range fault; 2) the hanging wall of the LVF showed tilting downward behavior to the east; 3) the Chimei fault, a suspected active fault, revealed active slip on the sub-vertical fault plane, that caused a vertical rate of 8–9 mm/yr. Putting the results under global ITRF system, the whole Juisui route was moving downward, supporting the notion that NNW subduction of the Philippine Sea plate starts around the latitude of the middle of the Longitudinal Valley. Finally, the co-seismic vertical deformation of the 2013 ML6.4 Juisui earthquake was characterized by tilting upward to the west, consistent with stick–slip on the deeper part of a west-dipping interface of the forearc basement.
Lee, Jian-Cheng
More
Tracking the magmatic response to subduction initiation in the forearc mantle wedge: Insights from peridotite geochemistry of the Guleman and Kızıldağ ophiolites, Southeastern Turkey
Tracking the magmatic response to subduction initiation in the forearc mantle wedge: Insights from peridotite geochemistry of the Guleman and Kızıldağ ophiolites, Southeastern Turkey
The initiation of subduction is associated with sequential magmatic responses that lead to the formation of the forearc lithosphere, yet the detailed characteristics of these magmatic activities are not well constrained. Here we use mineral chemistries and bulk-rock trace-element contents of highly-depleted harzburgites from the Guleman and Kızıldağ ophiolites in Southeast Turkey to examine mantle wedge melting dynamics during subduction initiation. We focus on how different components from the subducting slabs potentially contribute to various stages of magmatism throughout the process. Mineral and bulk-rock compositions of these harzburgites are significantly different from those of abyssal peridotites, suggesting that our harzburgites cannot be explained as residues of anhydrous adiabatic melting and melt-rock interaction at mid-ocean ridges alone. This implies that the petrogenesis of SE Turkey harzburgites involves additional processes and components. Harzburgites with the most depleted heavy-rare earth element (HREE) contents are the ones with the highest abundance of strongly incompatible elements, which can be explained by open-system processes where the peridotites in the mantle wedge experienced melting and infiltration of enriched components simultaneously. Open-system dynamic melting models with continuous flux of sediment-derived melts can account for the observed correlation, but are numerically too low compared to the measured values. Based on the observed fractionation between Zr, Hf, and elements with similar incompatibility (middle REEs), we hypothesized the involvement of amphibolite-derived melt and modeled its numerical trace-element contents. Binary mixing between this hypothetical melt and residues of the former open-system model can coherently account for the majority of the obtained trace-element data. This indicates that magmatic events during subduction initiation likely involve multiple components and occur in multiple stages, and that melt-mantle interaction plays a significant role in oceanic forearc lithosphere formation. Based on our model, we suggest the high (Zr/MREE)N signatures in some boninites and depleted harzburgites found in modern forearcs and ophiolites could be inherited from amphibolite-derived melts. Moreover, the existence of slab melts agrees with current constraints on the reconstructed geothermal gradients during subduction initiation based on the petrology and geochemistry of metamorphic soles.
Wang, Kuo-Lung
More
Tracing Argoland in eastern Tethys and implications for India-Asia convergence
Tracing Argoland in eastern Tethys and implications for India-Asia convergence
Incremental accretion of continental fragments from East Gondwana to Eurasia resulted in the growth of Asia and rise of the Tibetan Plateau, yet its detailed evolution remains uncertain. Argoland, a continental fragment that rifted from NW Australia during the Late Jurassic, played a key role in the initial opening of the Indian Ocean and the evolution of eastern Tethys. However, its present identity remains elusive, with East Java-West Sulawesi currently assumed to be the most likely option. To constrain the missing Argoland and its role in India-Asia convergence, we report new detrital zircon data from Sulawesi, Indonesia, and West Burma, Myanmar, and synthesize literature results from relevant regions in Southeast Asia, which (>15,000) reveal age profiles of West Sulawesi, the central Sulawesi metamorphic belt, and southeast Borneo comparable to that of Bird's Head, New Guinea, whereas age patterns of West Burma and East/West Java are similar to those of NW Australia. Notably, the most dominant age populations in NW Australia are rarely detected in Sulawesi and Borneo. These observations, combined with previous geological records and recent paleomagnetic data, suggest that West Burma is the mysterious Argoland, opposing the currently favored East Java-West Sulawesi model, with East Java and West Sulawesi probably having originated from NW Australia and Bird's Head, respectively. We estimate an average northward motion of ∼6–8 cm/yr for West Burma, which split from NW Australia to approach the equator during ca. 155–95 Ma, shedding new light on the reconstruction and breakup of northern East Gondwana, progressive building of Southeast Asia, and India-Asia convergence.
Chung, Sun-Lin
More
Diachronous initiation of post-collisional magmatism in the Arabia-Eurasia collision zone
Diachronous initiation of post-collisional magmatism in the Arabia-Eurasia collision zone
The continental collision between Arabia and Eurasia which gave rise to the Caucasus-Iran-Anatolia (CIA) volcanic province provides a unique opportunity for understanding collisional zone magmatism. This study reports a comprehensive dataset of ages and geochemical compositions of volcanic rocks formed during the initial phase of post-collisional magmatism in the CIA province. The age data indicate a diachronous onset of volcanism that began ~17 Ma in SE Anatolia, and propagated northward from ~11 to 9 Ma toward NE Anatolia and NW Iran. The rocks are characteristically bimodal, with dominantly basic (SiO2 = 48–52 wt%) and silicic (SiO2 = 58–71 wt%) components that feature significant isotopic variations (εNd = +6 to −5), suggesting two principal magma sources: (1) a juvenile mantle-derived component, and (2) an older continental crust component. We therefore attribute the volcanic initiation to a migrating post-collisional extension regime caused by the successive breakoff of subducted Neo-Tethyan slabs. Subsequent volcanism that began from ~6.5 Ma resulted in a wide spectrum of calc-alkaline and alkaline rocks, with mafic to felsic lithologies in the entire province. From ~2 Ma, volcanism ceased in the western CIA province, and started propagating eastward and southeastward to SE Iran, following the Urumieh-Dokhtar magmatic belt, consistent with the notion of an oblique continental collision between Arabia and Eurasia.
Chung, Sun-Lin
More
Synchronized and asynchronous modulation of seismicity by hydrological loading: A case study in Taiwan
Synchronized and asynchronous modulation of seismicity by hydrological loading: A case study in Taiwan
Delineation of physical factors that contribute to earthquake triggering is a challenging issue in seismology. We analyze hydrological modulation of seismicity in Taiwan using groundwater level data and GNSS time series. In western Taiwan, the seismicity rate reaches peak levels in February to April and drops to its lowest values in July to September, exhibiting a direct correlation with annual water unloading. The elastic hydrological load cycle may be the primary driving mechanism for the observed synchronized modulation of earthquakes, as also evidenced by deep earthquakes in eastern Taiwan. However, shallow earthquakes in eastern Taiwan (<18 km) are anticorrelated with water unloading, which is not well explained by either hydrological loading, fluid transport, or pore pressure changes and suggests other time-dependent processes. The moderate correlation between stacked monthly trends of large historic earthquakes and present-day seismicity implies a modestly higher seismic hazard during the time of low annual hydrological loading.
Hsu, Ya-Ju
More
Magnesium isotopic systematics of the Makran arc magmas, Iran: Implications for crust-mantle Mg isotopic balance
Magnesium isotopic systematics of the Makran arc magmas, Iran: Implications for crust-mantle Mg isotopic balance
Continental crust loses Mg by lower crustal foundering and chemical weathering to become its intermediate-silicic composition. Both processes should preferentially retain heavy Mg isotopes in the crust, yet the estimated Mg isotopic ratio for the bulk continental crust is indistinguishable from that for unmodified mantle. This can be elucidated by two notions that are not mutually exclusive: (i) the basaltic parent to the crust has Mg isotopes lighter than the mantle and has since become mantle-like as a result of Mg loss by igneous and weathering fractionation, and (ii) heavy Mg isotopes in the continental crust are constantly balanced by a hidden input of light Mg isotopes. Arc magmas are arguably building blocks of the continental crust and their Mg isotopic signature could be used to place limits on the above views. Here, we conducted a Mg isotopic study of the Makran arc, a rare continental arc in Iran within the Alpine-Himalayan orogenic belt. The measured Mg isotopic ratios for most mafic and intermediate samples are mantle-like with δ26Mg values ranging from −0.32‰ to −0.20‰. After excluding dubious samples affected by alteration, the silicic samples span a broad range of isotopic ratios with δ26Mg values ranging from −0.32‰ to +0.32‰, beyond that of unmodified mantle (δ26Mg = −0.25 ± 0.04‰). The isotopic data form a trend from mantle-like δ26Mg at relatively high MgO towards elevated δ26Mg at low MgO, requiring the isotopic variability be related to differentiation in an open system involving assimilation of high-δ26Mg crustal rocks. Our results are generally consistent with Mg isotopic data compiled for global arcs where magmas have Mg isotopic ratios ranging from mantle-like to ∼0.7 per mil above the mantle range, and compositions substantially below that range are rare. Thus, heavy Mg isotopes in the continents predicted by igneous and weathering fractionation might have been compensated by a hidden input of light Mg isotopes, which is presumably in the form of carbonates, leading to a bulk crustal Mg isotopic composition that is mantle-like.
Pang, Kwan-Nang
More