Vol 53, No 2 (2019)

There are three stages in tectonic evolution of the Earth: (1) nucleation, from the origin of protocratons to their assembly into the Kenorland supercontinent (2.7–2.5 Ga); (2) cratonization, from the breakup of Kenorland (2.45 Ga) to the assembly of Columbia (1.85 Ga) and its reorganization into Rodinia (1.0–0.72 Ga); and (3) modern plate tectonics, from the breakup of Rodinia 720 Ma until the present. Analysis of the time-space reorganizations of Archean granulite–gneiss terranes, which correspond to continental lithospheric keels, reveals five groups of protocratons (Nena, Ur, Congo–Sahara, NAsia and Atlantica) that remained almost intact during long time intervals. After the breakup of Kenorland, the continental crust rotated counterclockwise. NAsia and Atlantica rotated the least and drifted relative to Nena; however, the latter rotated by 180^○. Congo‒Sahara, Ur, and Kalahari rotated the most. The assembly and breakup of the supercontinents clearly correlates with secular changes in dominant types of base, precious, and ferrous metal deposits, as well as the formation and emplacement of diamonds.

The results of detailed geological mapping, coupled with the isotope-geochemical study of a metamorphosed mafic-ultramafic complex known as the Central Belomorian Belt located in the Belomorian province of the Fennoscandian Shield, are reported. The protholith of the complex is ~2.9‒3.1 Ga old. It has been subjected to two 2.87 and 1.87 Ga structural-metamorphic reworking. This complex is one of the oldest in the Belomorian Province. We present several lines of evidence showing that these lithologies constitute a tectonic remnant of the Mesoarchean oceanic lithosphere, rather than any other mafic-ulramafic complex from modern geodynamic settings. The Central-Belomorian high grade mafic-ultramafics reveal a clear geochemical coherency, which implies their genetic relationships. Their mafic protholiths stem from the partial melting of the mantle peridotite protholith. The petrologic modelling has shown that primary melts were formed in the garnet lherzolite field at pressure of 3.5–3.8 GPa at ambient mantle potential temperatures of 1520–1550°С which led to an emergence of ~25‒30 thick oceanic crust. The available geochemical data suggest that the complex was formed at the initial stage of subduction. It marks the start of early continental crust-forming processes in the Belomorian Province.

The Earth’s lithosphere is commonly investigated by both direct and indirect methods, corresponding to rock sampling and geophysical surveys, respectively. The interpretation of geophysical data is generally based on the combination of values measured in the lithosphere with those obtained in laboratories from rock samples. However, petrophysical properties of numerous lithotypes overlap, yielding the misleading interpretation of geophysical surveys in many areas of the world. A heated debate particularly concerns non-volcanic rifted margins, fuelled by the possible presence of giant oil and gas fields: thinned continental crust or serpentinized oceanic basement. One of the possible causes of ambiguity is related to the intimate similarity of oceanic serpentinites and various crustal rocks (e.g. basalts, gabbros, limestones, sandstones, shales, etc.), in terms of petrophysical properties. Can variably serpentinized peridotites mimic typical continental crustal rocks, such as granites and granodiorites? To answer this question, we compared literature data of worldwide samples of such lithologies. The results show the complete overlap of the considered petrophysical properties (density, magnetic susceptibility, V_P, V_S, V_P/V_S, and Poisson’s ratio) of these lithotypes (P = 10–1000 MPa, depth = 0.33–33.33 km), further confirming the difficulty in discriminating variably serpentinized mantle rocks from crustal lithologies. Therefore, the recognition of buried serpentinite geobodies, being potential sites of exploitable gas and oil reservoirs, like those probably ensconced in non-volcanic rifted margins, necessitates a robust lithological model inferred from direct methods, namely the study of core drillings, deep-seated xenoliths and tectonic exposures of deep-crustal sections to substantiate the interpretation of geophysical data.

Crustal-scale extensions occurred in the Tibetan Plateau during the post-collision stage, and leucogranites, N–S and E–W faults and other tectonic-thermal events were developed in Tethys Himalaya, which formed a series of Pb–Zn–Sb–Au polymetallic deposits. The ore deposit may be distributed around the dome (with core of leucogranites), or along the N–S and E–W faults. Due to the lack of deep geophysical data, many different genesises of mineral deposit have been proposed by predecessors. This paper establishes the spatial relationship of deep tectonic-thermal events in the Tethys Himalaya Pb–Zn–Sb–Au belt by the N–S magnetotelluric (MT) profiles covering Cuonadong dome, the Southern Tibet Detachment System (STDS) and other tectonic-thermal events (length: 72 km, the basic point distance: 1 km): (a) a partial melting body was observed about 15 km below the Tethys Himalayan, which intruded in the form of leucogranites and formed domes; (b) the STDS and its secondary faults extended to deep the partial melting body. In combination of time relationship of tectonic-thermal events, a view has been presents that the Tethys Himalaya Pb–Zn–Sb–Au belt was formed in one tectonic‒thermal coupling metallogenic system in the post-collision stage. Two types of metallogenic models were formed based on whether the partial melting intruded or not: (a) the tectonic‒thermal coupling metallogenic model of leucogranites and the surrounding detachment faults of the dome (partial melting intruded in the form of leucogranites which driven ore-forming fluid to migrate in the surrounding detachment faults); (b) the tectonic‒thermal coupling metallogenic model of non-intruded partial melting and fault systems (under the extension stress, the N–S extension of Tibetan Plateau had formed the STDS and its secondary fault faults that extended to the partial melting body). This results in instantaneous low pressure, which decoupled the partial melting body and magmatic fluid and drove magmatic fluid and deep formation fluid to flow into fault system. Finally, the two fluids were mixed with atmospheric water to form ore. Also a hydrodinamical model for the long distance migration of ore-forming fluid along the fault systems within the Tethys Himalaya Pb–Zn–Sb–Au belt has been established. This study will provide a reference for subsequent geophysical prospecting in the belt.

Geometric and seismic parameters of the Qoshadagh Fault (QDF) were investigated to evaluate seismic hazard along this fault, which consists of three segments. The central E–W striking, dextral-reverse segment is the longest and terminates at both ends into NW–SE striking splay arrays. Both eastern and western splay arrays form locally transtensional bends. Paleoseismic data obtained from three excavated trenches across the fault combined with dated offset geomorphic features revealed that the central segment experienced at least 5 surface rupturing earthquakes during the past 2.5 ka, with maximum moment magnitude of M_w = 6.8 ± 0.2. The mean recurrence interval for the identified paleoearthquakes is 452 ± 143 years (±2σ) and the calculated amount of slip per event is ca. ≈0.85 m. These results imply that the QDF slips at an average rate of 1.9 ± 0.1 mm yr^–1 for over the past 2.5 ka. The obtained values define the seismic behavior of the fault and are essential to remediate ensuing seismic risks.