Deep structure and tectonic controls of the Majiaoba area inferred from gravity and magnetics data by multi-criteria-evaluation-driven adaptive focusing Inversion

Abstract

Abstract The Majiaoba area, on the northwestern margin of the Longmen Mountains within the Sichuan Basin, exposes a complete Silurian–Triassic succession. Silurian–Devonian strata are widespread, Carboniferous–Triassic formations are well developed, and metamorphic or igneous rocks are rarely exposed at the surface. With the exception of the weakly magnetic Lower Triassic Feixianguan Formation, most units are essentially non-magnetic. Despite this, the region exhibits high-amplitude magnetic anomalies (up to ~103 nT) whose genesis remains uncertain, and basin-scale magnetic highs in central-western Sichuan are commonly ascribed to rifting even though their driving mechanisms and lateral continuity are debated. Two high-precision profiles across Majiaoba were acquired, and gravity and magnetic datasets were inverted independently using a minimum-support, adaptive iterative focusing algorithm selected under multi-criteria evaluation, yielding high-resolution images of subsurface structure beneath both lines. Rock-magnetic measurements on Feixianguan samples, combined with the inversion results, reveal a pronounced vertical offset between the principal magnetic source and the surface-exposed reddish mudstone-shale of the Feixianguan Formation. Along profile L2, a closed high-susceptibility body is resolved at ~400–600 m depth. The anomaly pattern is inconsistent with a purely shallow sedimentary origin and is best explained by mafic (diabase) intrusions. At the regional scale, the deep magnetic source is interpreted as a product of sustained activity related to the large-scale magmatic episode along the western margin of the Yangtze Block during the Late Permian, with magmatism in the northern Longmen Mountain segment plausibly persisting into the Early Triassic (Feixianguan time). Considering gravity–magnetic anomaly patterns across northwestern Sichuan, the magmatic system was likely widespread rather than local in extent.