Abstract Available major, trace element and Sr–Nd isotope data for the late Mesozoic mafic rocks in the eastern North China Block (NCB) show chemical and isotopic differences between rocks from different tectonic units. Such differences are interpreted as signatures inherited from the melted mantle sources, which had experienced distinctive enrichment processes during lithospheric evolution. The subcontinental lithospheric mantle beneath the NCB interior is characterized by long-term light REE (LREE) enrichment and EM1-like Sr–Nd isotopic signatures. Such a lithospheric mantle is mainly composed of chemically refractory peridotites that are common in cratonic regions. In contrast to that of the NCB interior, beneath the northern part of the NCB a relatively chemically fertile mantle was enriched in large ion lithophile elements and LREE and depleted in Nb–Ta and Th–U. It has higher 87 Sr/ 86 Sr(i) and ɛ Nd ( t ) than that of the interior of the block, and is interpreted to have been modified by recycled lower continental crust components related to the palaeo-Asian Ocean subduction. The lithospheric mantle beneath the southern NCB has the highest 87 Sr/ 86 Sr(i) and the lowest ɛ Nd ( t ), and is chemically transitional between the interior and northern part of the block. Formation of such an enriched lithospheric mantle was closely associated with modification from the subducted Yangtze lower–middle crust during Triassic collision between the North China and Yangtze Blocks. A lithospheric extension–thinning model is proposed to explain the petrogenesis of these late Mesozoic mafic rocks in the eastern North China Block. This process was amplified by effects from surrounding plate interactions, including the rapid northward movement of the palaeo-Pacific Ocean, compressional forces from the Siberian plate, the Tethyan tectonic belt and possibly the Indo-China Block. The resultant forces triggered lithospheric extension, asthenospheric upwelling, and decompressional melting of the enriched mantle sources.

Abstract The palaeotemperature recorded by vitrinite reflectance ( R o ) in the pre-Cenozoic uplifted stratigraphic strata, and in Palaeozoic–Mesozoic remnant basins outside the Cenozoic depocentres, has not been overprinted by later thermal events in the eastern North China Craton (NCC). Based on downhole R o data from the Palaeozoic and the Mesozoic subsections, we reconstruct the temperature gradients when the subsections reached their maximum palaeotemperatures in the Middle Triassic and the Cretaceous, and calculate the corresponding heat flow histories since the early Mesozoic. The temperature gradient and heat flow were much higher in the Cretaceous (35–43 °C km −1 and 73–83 mW m −2 , respectively) than in the Middle Triassic and at the present. The high palaeo-heat flow during the Late Mesozoic implies that the thickness of the ‘thermal’ lithosphere at that time was c . 65 km, about half the thickness of c . 135 km estimated for the Early Mesozoic. The change from a stable thermal regime to an active thermal regime took place during the Late Jurassic–Early Cretaceous ( c . 110 Ma). This tectonothermal event was accompanied by extensive surface erosion, and is also evidenced in the areas adjacent to the NCC, such as the South Yellow Sea and East China Sea basins. Our study provides not only geothermal evidence for the Late Mesozoic lithospheric thinning, but also additional constraints on the thinning mechanism, which is currently being debated.

The Carpentaria Conductivity Anomaly of western Queensland is a major element in the electrical conductivity structure of the Australian continent. Investigation of it is significant both for its own sake, and as a case history in the general understanding of continental conductivity structure. Following Its earlier discovery by reconnaissance magnetometers arrays, detailed magnetotelluric observations were carried out in 1997 along a transect crossing the anomaly between Cloncurry and Julia Creek. The magnetotelluric results define a good conductor within the crust beneath the sediments of the Eromanga Basin. The conductor extends over a depth range of tens of kilometres. This structure, evidently shown also by aeromagnetic and gravity data, is interpreted as the eastern boundary of the Mt Isa Block at a plate suture, which was later covered by the sediments of the Eromanga Basin. Seismic tomographic results show a major gradient in seismic-wave speed in the region. It appears the potential-field, electromagnetic and seismic methods have detected different characteristicsof the same geologic structure, with complementary results. The electromagnetic results, new to this paper, define horizontal position well, and give evidence of highly conducting material from the crust to a depth of tens of kilometres. The seismic results extend the depth of the boundary into the upper mantle. The case history supports the hypothesis that the major conductivity anomalies of the geomagnetic deep-sounding method mark continental sutures of fundamental significance in recording the creation of continents.