Chow-Son Chen

Magnetotelluric evidence for thick-skinned tectonics in central Taiwan

Taiwan is the type example of an arc-continent collision. Numerous tectonic models have been proposed for this orogen, and include both thin-skinned and thick-skinned lithospheric deformation. These models predict very different structures at middle and lower crustal depths, but insuffi cient geophysical data exist to unequivocally distinguish between them. Long-period magnetotelluric (MT) data were collected in central Taiwan in 2006‐2007 to constrain the crustal resistivity structure. A two-dimensional inversion of these MT data revealed a prominent electrical conductor that extends across the decollement predicted by the thin-skinned model. This feature is interpreted to be due to 1%‐2% saline fl uids, and is inconsistent with the thin-skinned model. In contrast, the thick-skinned model predicts this feature since fl uids are generated in the crustal root through metamorphism. Quantitative correlation of the resistivity and seismic velocity models supports small-volume, high-salinity fl uids in a thickened crust as the cause of this conductor.

Preliminary result of magnetotelluric soundings in the fold-thrust belt of Taiwan and possible detection of dehydration

Abstract Until the magnetotelluric (MT) soundings were conducted in the last two years, there was no systematic view of the deep electric conductivity in the fold-thrust belt of the Taiwan orogen, being a typical example of arc-continental collision. Presented in this paper are the initial results and preliminary interpretation of the MT survey in Taiwan. The determinant response of the impedance tensor was calculated for inversion and Occams inversion which could generate smooth models was mainly used for interpreting MT data at each site. The existence of a conductive zone with depths of 10–20 km beneath the Island of Taiwan is undoubtedly the most important feature found in this study. The depths are well correlated with the inferred depth of dehydration reactions of Suppe (1981) and the top of the aseismic lower crust observed by Wang et al. (1994). Thus, the authors believe that the cause of this conductive zone is probably the fluids released from dehydration reactions. Additionally, close to the Lishan fault the MT data show strong distortions which may be the result of induction in more complex three-dimensional structures. Further multi-dimensional analysis of the data is required in the future for delineating a more realistic image of the subsurface heterogeneous structure.

Preliminary report on the Sanyi-Puli seismic zone conductivity anomaly and its correlation with velocity structure and seismicity in the Northwestern Taiwan

As part of seismotectonic investigations in Northwestern Taiwan, eight magnetotelluric (MT) soundings were conducted across the Sanyi-Puli seismic zone, a distinct NW-SE trending linear seismic zone in the fold-thrust belt of Northwestern Taiwan. A preliminary one-dimensional resistivity model according to the determinant MT response was computed, and the resistivity model includes a high-conductivity anomaly beneath this seismic zone. This conductivity anomaly raises the possibility of metamorphic dehydration triggering the seismic events in the seismic zone. The correlation between the conductivity anomaly and active seismicity in this area is tentatively discussed in this study.

Taiwan model: combination of MT and seismicity

SUMMARY A strong correlation between the relevant seismic image and the geoelectric model of Taiwan, thus electrical conductivity image is interpreted with the constraints afforded by velocity information. The results show that both the velocity profile and the resistivity section indicate the slab of the subducting Philippine Sea plate. The conductive surface of the slab is thought to be related to fluids associated with the surface of the subducting plate. The slab northward deepens near latitude of 24°N to a depth of 50 km at the northern tip of Taiwan. The angle of subduction is interpreted to be 30o. At the depth of the Moho discontinuity, 40 km, a corresponding sharp boundary can be traced either from the conductivity profile or from the reflectivity section.

The subsurface structure of Taiwan: Preliminary results of a magnetotelluric study

The island of Taiwan is located at the active boundary between the Philippine Sea Plate and the Eurasian Plate. The relative velocity between these plates is about 7 cm/year in the direction N50°W. The Philippine Sea Plate is subducting northeastward from the Ryukyu Trench, northeast of Taiwan, and the Eurasian Plate is subducting beneath the Philippine Sea Plate along the Manila Trench, south of Taiwan. The overall plate configuration in the vicinity of Taiwan is well defined by the seismicity. Thus Taiwan lies in the region where the polarity of subduction changes.

The preliminary crustal conductivity anomaly from MT data in Taiwan

Taiwan is located in a complex tectonically active region where the Philippine Sea plate is subducting northwestward along the Ryukyu Tenth while the Eurasia plate is subducting eastward along the Manila Trench. The magnetotelluric method (MT) was used as a part of an integrated project to map this intricate tectonic problem. Nineteen MT soundings, arranged uniformly covering the entire Taiwan, were conducted and analyzed to map the 3D resistivity structures beneath Taiwan. Results show the existence of a distinct conductive zone located beneath the Central Range in the uppercrust of central Taiwan. This zone is not only consistent with the surface distributions of the hot springs in Taiwan, but also matched with the low velocity zone in Taiwan. We infer that the main cause of this crustal conductivity anomaly might have resulted from the heat accutnulated fro in crustal metamorphsm, and from the heat intrusion from the oceanic upper mantle of the Philippine Sea plate, which has been colliding with the Eurasian plate.

High Conductive Zones beneath Mountain Range of Taiwan Imaged by MT Exploration and its Tectonics Interpretation

The Taiwan orogen has formed as a result of the arc-continent collision between the Eurasian continental margin and the Luzon island arc over the last 5 million years and is the type example of an arc-continent collision. The tectonic processes at work beneath Taiwan are still debated, and the available data have been interpreted with both thin-skinned and thick-skinned models. In 2004, the Taiwan Integrated Geodynamical Research (TAIGER) project began a systematic investigation of the crustal and upper mantle structure beneath Taiwan. TAIGER magnetotelluric (MT) data from Central Taiwan favour a thick-skinned model for that region. The Taiwan orogen becomes younger to the south, so the earlier stages of collision were investigated with a 100-km-long MT profile in southern Taiwan. Data were recorded at 15 MT sites and tensor decomposition and two-dimensional inversion were applied to the MT data. The shallow electrical structure is in good agreement with surface geology. The deeper structure shows a major conductor in the mid-crust that can be explained by fluid content of 0.4 -1.4%. A similar feature was observed in Central Taiwan, but with a higher fluid content. The conductor in Southern Taiwan extends to lower crustal depths and is likely caused by fluids generated by metamorphic reactions in a thickened crust. Together the Central and Southern Taiwan MT profiles favor a model with thick skinned deformation.

A magnetotelluric image at subduction zone: Taiwan

As part of an integrated project to map the 3-D structures beneath Taiwan, which is a typical example of arccontinental collision, broadband magnetotelluric (MT) data have been collected and analyzed at 20 sites uniformly covering the entire Taiwan. The determinant response of impedance tensor was calculated for inversion and the Occam’s inversion which could generate smooth model was used for interpreting MT data at each site. A north dipping conductor, at a depth of approximately 10-20 km with resistivities lower than 100 ohm-m beneath the Island of Taiwan is undoubtedly the most significant feature obtained in this study. The depth are correlated well with the inferred depth of dehydration reactions from Suppe (1981) and the top of the aseismic lower crust observed by Wang (1994). We infer that this dipping conductor is associated with the upper surface of the subducting Philippine Sea plate. This finding also provides one more example to support the argument that conductivity anomalies are commonly associated with plate subduction.

Refining TEM data from complicated areas by using trend surface analysis

This case study describes a transient electromagnetic (TEM) survey aimed at delineating sediments on the coastal plain of southwestern Taiwan.

Coastline changes in southwestern Taiwan based on TEM mapping

Results are presented of an investigation of the coastline changes in southwestern Taiwan by using the Transient Electromagnetic (TEM) method. A configuration of coincident loop with SIROTEM instrument was used. The transient voltages were converted in to apparent resistivi ties for qualitative interpretation and then inverted by using onedimensional ridge regression inversions for quantitative interpretation. From displaying and upon comparison with the known geology, we deduced the preliminary shoreline changes. Our conclusion was verified by some drillings. In comparison to traditional geophysical methods, the TEM method can be used effectively to geological mappings along coast in a short time at low coast.