A Gravity Study of the Longmenshan Fault Zone: New Insights Into the Nature and Evolution of the Fault Zone and Extrusion‐Style Growth of the Tibetan Plateau Since 40 Ma

Abstract

The Longmenshan Fault Zone (LFZ) is a tectonic boundary between the Tibetan Plateau and the Sichuan Basin of the South China Block. Knowledge of the nature and history of the LFZ is important for understanding the growth of continental plateaus and mechanisms for major earthquakes along their margins, as exemplified by the magnitude 7.9 M Wenchuan earthquake of 12 May 2008. Flexural modeling of new and existing gravity survey data along three transects, combined with published seismic reflection profiles and earthquake focal mechanism data, indicates that the central-northern LFZ is a lithospheric-scale fault zone that has low elastic strength but with strong episodic dextral transpressional motions. In contrast, the southern LFZ is a crustal-scale thrust zone dominated by shallow-angle thrust motion of the Tibetan Plateau over a moderately stiff South China Block. In conjunction with the record of Cenozoic basin erosion in the western Sichuan Basin and a regional kinematic analysis, we suggest that the lithospheric-scale LFZ started 40 Myr ago and became an external boundary of a northeasterly directed extrusion-style plateau growth, predominantly through crustal thrusting and thickening.Plain Language Summary The occurrence of magnitude 7.9 M Wenchuan earthquake on 12May 2008 at the eastern foothill of the Tibetan Plateau surprised many in the earthquake community. More intriguingly, the large number of aftershocks related to that earthquake almost exclusively occurred northeast of the main shock along the Longmenshan Fault Zone (LFZ), a phenomena current models have trouble explaining. Here we combine gravity modeling with a synthesis of an array of geological and geophysical observations to establish a regional hypothesis featuring (1) contrasting behaviors between the southern and central-northern LFZ, with the southern LFZ being a crustal-scale thrust zone, whereas the central-northern LFZ being a lithospheric-scale fault zone with strong episodic dextral transpressional motions;(2) such contrasting behaviors are driven by differential motions of two crustal blocks from the western highland (the Tibetan Plateau), pushed by India's northward motion; (3) as a consequence, the LFZ features a twisted fault plane with the Wenchuan earthquake being located at the junction of the LFZ and the bounding fault between the two crustal blocks to the west, and (4) the LFZ has been behaving like this since around 40 Myr ago, and it became an external boundary for a northeasterly directed extrusion-style growth of the Tibetan Plateau.

The Interaction of the Eastward Extrusion of the Songpan‐Ganzi Terrane and the Crustal Rotational Movement of the Sichuan Basin Since the Late Paleogene: Evidence From Cretaceous and Paleogene Paleomagnetic Data Sets of the Sichuan Basin

“…The ductile lower crust of the SGT was extruded eastwards and was resisted by the relatively rigid and stationary lower crust of the Sichuan Basin, which induced the pure‐shear thickening of the ductile lower crust of the SGT. Profiles of A–B and C–D are modified from Sun et al (), Jiang et al (), and Tan et al (). QCF = Qingchuan fault; HYF = Hanyuan fault; MJF = Minjiang fault; WMF = Wenchuan‐Maoxian fault; BYF = Beichuan‐Yingxiu fault; JGF = Jiangyou‐Guanxian fault; KLF = Kunlun fault; LRBF = Longriba fault; SDF = Shuangshi‐Dachuan fault; LQF = Longquan fault; XSHF = Xianshuihe strike‐slip fault.…”

“…This indicates that the early topography and the onset of Cenozoic mountain building of the eastern margin of Tibetan Plateau were formed during the thickening of the upper crust. The regional geological and thermochronological studies of Jiang et al (2019) indicate that the central and northeastern segments of the LMSFB probably commenced lithospheric-scale right-lateral transpressional faulting at~40 Ma, which is much earlier than the previously estimated Miocene time (Wang et al, 2014). Comprehensive analysis of seismic reflection profiles and low temperature thermochronological data from the southwestern segment of the LMSFB suggests that this area experienced crustal shortening during 40-25 and 15-10 Ma, driven by the eastward growth of the Tibetan Plateau (Li et al, 2019).…”

“…GPS observations show that the crustal extrusion of the SGT was gradually divided into two branches in the eastern edge of the Tibetan Plateau (Figure a). Adjacent to the northeastern segment of the LMSFB, the crustal extrusion of the northern SGT changed from eastward to northeastward, which probably induced right‐lateral strike‐slipping of the northeastern segment of the LMSFB (Jiang et al, ; Li & Li, ; Wang et al, ; Zhang, , ). Although the southern SGT still maintained eastward extrusion, the crustal strain velocity decreased abruptly from ~20 mm/year in the SGT to less than ~3–4 mm/year in the central and southwestern segments of the LMSFB, relative to the South China reference frame.…”

Crustal density structure and flexure mechanism of the Tarim Basin, China, constrained by latest in situ gravity observations

“…Combining with the chronology results makes it possible to analyse the dynamic process with static gravity data (Jiang et al, 2019; Wang et al, 2018). Our results suggest that the evolution process of the Tarim Basin has undergone at least two main stages (Figure 4).…”

Quantifying the contribution of upper‐middle crustal shortening and lower crustal thickening to surface uplift in the south‐eastern Tibetan Plateau

“…According to the lower crustal flow model, the margin of the Tibetan Plateau is usually divided into two typical end-members: (Clark & Royden, 2000;Jiang et al, 2013Jiang et al, , 2019. As the plateau material began to flow into the south-eastern margin region at 9-13 Ma and continued to propagate southward into the SCDB in the Pliocene (Li et al, 2015;Schoenbohm et al, 2006;Zhang et al, 2004), with over 1000 km distance escape of plateau material, the gravitational energy of the lower crustal flow became very weak when it reached the SCDB.…”