Wenbo Wei

Partially molten middle crust beneath southern Tibet : Synthesis of project INDEPTH results

INDEPTH geophysical and geological observations imply that a partially molten midcrustal layer exists beneath southern Tibet. This partially molten layer has been produced by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation. It is confined on the south by the structurally imbricated Indian crust underlying the Tethyan and High Himalaya and is underlain, apparently, by a stiff Indian mantle lid. The results suggest that during Neogene time the underthrusting Indian crust has acted as a plunger, displacing the molten middle crust to the north while at the same time contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene evolution of the Himalaya is essentially a record of the southward extrusion of the partially molten middle crust underlying southern Tibet.

Electrically Conductive Crust in Southern Tibet from INDEPTH Magnetotelluric Surveying

The crust north of the Himalaya is generally electrically conductive below depths of 10 to 20 km. This conductive zone approaches the surface beneath the Kangmar dome (dipping north) and extends beneath the Zangbo suture. A profile crossing the northern Yadong-Gulu rift shows that the high conductivity region extends outside the rift, and its top within the rift coincides with a bright spot horizon imaged on the INDEPTH CMP (common midpoint) profiles. The high conductivity of the middle crust is atypical of stable continental regions and suggests that there is a regionally interconnected fluid phase in the crust of the region.

Geoelectric Structure of Lithosphere Beneath Eastern North China: Features of Thinned Lithosphere from Magnetotelluric Soundings

Abstract It is widely accepted that disassembly of the North China ancient craton and the thinning process of lithosphere at depth are essential for the dynamic evolution model of the China mainland since Mesozoic and Cenozoic times. To address these issues, the deep structure of the lithosphere beneath North China should be well delineated, which relies on a new generation of geophysical technology including ultra-broadband high-precision magnetotelluric (MT) sounding. In 2001 and 2005, MT survey was carried out along the Yingxian–Shanghe (HB-MT01) and Wenshui–Rizhao (HB-MT02) profiles in eastern North China, respectively. The results show that the crust and upper mantle along the profiles can be divided into four electric sections, and high-conductivity zones and high-conductivity layers exist in the lower crust and upper mantle, respectively. It is inferred that decoupling between crust and mantle is present beneath eastern North China which is associated with large-scale tectonic movements. The thickness of the lithosphere in eastern North China is estimated to be 50–80 km, which has apparently thinned with respect to usual continental cratonic regions. It involves the northern Taihangshan uplift, northern North China rifted basin and Luxi (western Shandong) fault-bounded uplift, which is different from the previous claim that the lithospheric thinning is confined to east of the Taihangshan gravity gradient zone. This study also demonstrates the importance of the modern high-precision MT sounding and combination of geophysics, geology, and geochemistry in the research of continental dynamics.

Three‐dimensional electrical structure of the crust and upper mantle in Ordos Block and adjacent area: Evidence of regional lithospheric modification

Long-period magnetotelluric (MT) data from project SINOPROBE were acquired and modeled, using three-dimensional (3D) MT inversion, to study the electrical structure of Ordos Block, a component of the North China Craton. For the first time, a high-resolution 3D resistivity model of the lithosphere is defined for the region. Contrary to what would be expected for a stable cratonic block, a prominent lithospheric conductive complex is revealed extending from the upper mantle to the mid-to-lower crust beneath the northern part of Ordos. Correlating well with results of seismic studies, the evidence from our independent magnetotelluric data supports regional modification of the lithosphere under the north Ordos and lithosphere thinning beneath Hetao Graben. The abnormally conductive structure may result from upwelling of mantle material in mid-to-late Mesozoic beneath the northern margin of the Ordos block.

Integrated geophysical-petrological modeling of lithosphere-asthenosphere boundary in central Tibet using electromagnetic and seismic data

We undertake a petrologically driven approach to jointly model magnetotelluric (MT) and seismic surface wave dispersion (SW) data from central Tibet, constrained by topographic height. The approach derives realistic temperature and pressure distributions within the upper mantle and characterizes mineral assemblages of given bulk chemical compositions as well as water content. This allows us to define a bulk geophysical model of the upper mantle based on laboratory and xenolith data for the most relevant mantle mineral assemblages and to derive corresponding predicted geophysical observables. One-dimensional deep resistivity models were derived for two groups of MT stations. One group, located in the Lhasa Terrane, shows the existence of an electrically conductive upper mantle layer and shallower conductive upper mantle layer for the other group, located in the Qiangtang Terrane. The subsequent one-dimensional integrated petrological-geophysical modeling suggests a lithosphere-asthenosphere boundary (LAB) at a depth of 80–120 km with a dry lithosphere for the Qiangtang Terrane. In contrast, for the Lhasa Terrane the LAB is located at about 180 km but the presence of a small amount of water in the lithospheric mantle (<0.02 wt%) is required to fit the longest period MT responses. Our results suggest two different lithospheric configurations beneath the southern and central Tibetan Plateau. The model for the Lhasa Terrane implies underthrusting of a moderately wet Indian plate. The model for the Qiangtang Terrane shows relatively thick and conductive crust and implies thin and dry Tibetan lithosphere.

Constraints on the evolution of crustal flow beneath northern Tibet

Crustal flow is an important tectonic process active in continent-continent collisions and which may be significant in the development of convergent plate boundaries. In this study, the results from multidimensional electrical conductivity modeling have been combined with laboratory studies of the rheology of partially molten rocks to characterize the rheological behavior of the middle-to-lower crust of both the Songpan-Ganzi and Kunlun terranes in the northern Tibetan Plateau. Two different methods are adopted to develop constraints on melt fraction, temperature, and crustal flow velocity in the study area. The estimates of these parameters are then used to evaluate whether crustal flow can occur on the northern margin of the Tibetan plateau. In the Songpan-Ganzi crust, all conditions are satisfied for topography-driven channel flow to be dominant, with partial melt not being required for flow at temperature above 1000°C. Further north, the Kunlun fault defines the southern boundary of a transition zone between the Tibetan plateau and the Qaidam basin. Constrained by the estimated melt fractions, it is shown that channel injection across the fault requires temperatures close to 900°C. The composition of igneous rocks found at the surface confirm those conditions are met for the southern Kunlun ranges. To the north, the Qaidam basin is characterized by colder crust that may reflect an earlier stage in the channel injection process. In the study area, at least 10% of the eastward directed Tibetan crustal flow could be deflected northward across the Kunlun Fault and injected into the transition zone defining the northern margin of the Tibetan plateau.

Time synchronization for marine controlled source electromagnetic recorder

As an effective method to improve the successful rate of marine oil and gas drilling, marine controlled-source electromagnetic method requires acquisition of many marine controlled-source electromagnetic receivers which record electromagnetic data on the same time axis. However, receiver works on the seabed where GPS signal is shielded and relatively constant temperatures, this will bring about a consequence that the land synchronization method could not be implemented. To get the marine receivers which are time synchronization, we combine the GPS and OCXO (Oven Controlled Crystal Oscillator), and then calculate time drift. This method makes time drift down minimum by time drift compensation between different receivers.

Application of Wireless Communication Technology in Geophysical Instrument

Geophysical instruments with high precision and resolution have become an important way during the development of geophysics. Some experts use modern electronic technology and computer technology into geophysical instrument. However, there are few papers on the application of wireless communication in geophysical instrument. This paper shows a kind of geophysical instrument, which is based on wireless communication and consists of data acquisition unit, data coordinator unit and computer. All the data acquisition units acquire the parameters and pass them to the data coordinator unit via wireless communication and the data coordinator unit collects the data, pass them to the computer finally. The system accomplishes the wireless communication with CC1100, a chip of Chipcon. This paper introduces the structure of the system and makes detailed explanation of the design.

Synthesis of natural electric and magnetic Time‐series using Inter‐station transfer functions and time‐series from a Neighboring site (STIN): Applications for processing MT data

Synthesis of natural electric and magnetic Time-series using Inter-station transfer functions and time-series from a Neighboring site (STIN) is a new approach for recovering natural electric and magnetic fields and reduce the influence of anthropogenic noise. The proposed approach modifies the windows of the local electric and magnetic time-series that are affected by noise with time series resulted from modifying the spectra of the magnetic time-series from a neighboring site with the inter-station transfer functions between the local and neighboring sites. The STIN method was tested with artificially contaminated electric and magnetic time-series. Comparison between STIN-corrected time-series and original non-contaminated time-series shows high similarity, both in the time and frequency domains. Differences were quantified using the normalized root-mean-square error, the correlation coefficient, and the signal to noise ratio. The STIN method was also applied to two sites affected by unconstrained anthropogenic noise, thus demonstrating the ability and accuracy of STIN in synthesizing natural electric and magnetic fields and reducing the influence of anthropogenic noise. The synthesized time-series provided by STIN show the method to be valuable for MT geophysical applications, by increasing the reliability when constrains the MT impedance tensor, and by reducing the scatter of data points when the time-series are affected by noise, particularly for longer periods. As STIN is based on inter-station transfer functions, the electric and magnetic time-series can be treated independently, enabling computation of the MT impedance tensor even when the electric and magnetic time-series of the local site were recorded at different times.

A seafloor electromagnetic receiver for marine magnetotellurics and marine controlled-source electromagnetic sounding

In planning and executing marine controlled-source electromagnetic methods, seafloor electromagnetic receivers must overcome the problems of noise, clock drift, and power consumption. To design a receiver that performs well and overcomes the abovementioned problems, we performed forward modeling of the E-field abnormal response and established the receiver’s characteristics. We describe the design optimization and the properties of each component, that is, low-noise induction coil sensor, low-noise Ag/AgCl electrode, low-noise chopper amplifier, digital temperature-compensated crystal oscillator module, acoustic telemetry modem, and burn wire system. Finally, we discuss the results of onshore and offshore field tests to show the effectiveness of the developed seafloor electromagnetic receiver and its performance: typical E-field noise of 0.12 nV/m/rt(Hz) at 0.5 Hz, dynamic range higher than 120 dB, clock drift lower than 1 ms/day, and continuous operation of at least 21 days.