Jianen Jing

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.

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.

Two types of marine controlled source electromagnetic transmitters

Marine controlled source electromagnetic methods are used to derive the electrical properties of a wide range of sub-seafloor targets, including gas hydrate reservoirs. In most marine controlled source electromagnetic surveys, the deep-tow transmitter is used with a long horizontal electric dipole being towed above the seafloor, which is capable of transmitting dipole moments in the order of up to several thousand amperemetres. The newly developed deployed transmitter uses two horizontal orthogonal electrical dipoles and can land on the seafloor. It can transmit higher frequency electromagnetic signals, can provide accurate transmission orientation, and can obtain higher signal stacking, which compensates for the shorter source dipole length. In this paper, we present the study, key technologies, and implementation details of two new marine controlled source electromagnetic transmitters (the deep-tow transmitter and the deployed transmitter). We also present the results of a marine controlled source electromagnetic experiment conducted from April to May 2014 in the South China Sea using both the deep-tow transmitter and the deployed transmitter, which show that the two types of marine transmitters can be used as effective source for gas hydrate exploration.