Early Mesozoic tectonic settings of the northern North China craton

Abstract

This paper aims to reconstruct early Mesozoic tectonic settings of the northern North China craton based on investigations of Triassic–Middle Jurassic successions and age determinations of some lithostratigraphic units and dikes cutting tilted early Mesozoic strata. It used to be regarded that the northern North China craton experienced fold–thrust deformations in the Triassic to Early Jurassic based on the occurrences of unconformity beneath Triassic/Lower Jurassic units, synorogenic conglomerate, and Late Triassic thrusting. We revisited early Mesozoic stratigraphy of the Xiabancheng basin in the middle Yanshan belt to restore tectonic environments during that period of time. Detrital zircon data reveal that the Liujiagou and Ermaying Formations are actually of Middle and Late Triassic ages, respectively. Field observations show that the Ermaying and Xingshikou Formations are conformable, making up a sequence from fluvial to alluvial-fan facies. It is also noticeable that Lower–Middle Jurassic Xingshikou through Xiahuayuan succession is typified by fining- and deepening-upward depositional trend and contains abundant volcaniclastic rocks, indicative of continued subsidence of the Xiabancheng basin in a volcanic setting. All the results cast doubts on the long-held view that the first phase of shortening in the northern North China craton happened in the Late Triassic or Early Jurassic. The angular unconformity beneath Upper Jurassic Tiaojishan volcanics registered shortening in the northern North China craton, which plausibly took place around 170 Ma on account of zircon U–Pb ages of two dikes penetrating deformed Middle Triassic–Lower Jurassic strata and ages of Tiaojishan volcanics above the unconformity. Given the widespread occurrence of magmatism, rift basins and metamorphic core complexes during the Triassic and Early Jurassic, it follows that the northern North China craton was more likely under an extensional setting during the early Mesozoic. • Triassic stratigraphy of the middle Yanshan belt is revised. • No unconformities exist in Triassic–Middle Jurassic successions. • Extensional tectonics was prevailing in early Mesozoic. • First-phase shortening possibly occurred around 170 Ma.