期刊
GEOPHYSICAL JOURNAL INTERNATIONAL
卷 203, 期 1, 页码 692-706出版社
OXFORD UNIV PRESS
DOI: 10.1093/gji/ggv321
关键词
Tomography; Surface waves and free oscillations; Continental margins: convergent; Crustal structure
资金
- State Key Laboratory of Isotope Geochemistry [SKBIG-RC-14-03]
- National Science Foundation of China [41021063]
- NSF [EAR-1246925]
- Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation [EAR-1261681]
- National Science Foundation [CNS-0821794]
- Division Of Earth Sciences
- Directorate For Geosciences [1246925] Funding Source: National Science Foundation
Recently constructed models of crustal structure across Tibet based on surface wave data display a prominent mid-crustal low velocity zone (LVZ) but are vertically smooth in the crust. Using six months of broad-band seismic data recorded at 22 stations arrayed approximately linearly over a 440 km observation profile across northeastern Tibet (from the Songpan-Ganzi block, through the Qaidam block, into the Qilian block), we perform a Bayesian Monte Carlo joint inversion of receiver function data with surface wave dispersion to address whether crustal layering is needed to fit both data sets simultaneously. On some intervals a vertically smooth crust is consistent with both data sets, but across most of the observation profile two types of layering are required: a discrete LVZ or high velocity zone (HVZ) formed by two discontinuities in the middle crust and a doublet Moho formed by two discontinuities from 45-50 km to 60-65 km depth connected by a linear velocity gradient in the lowermost crust. The final model possesses (1) a mid-crustal LVZ that extends from the Songpan-Ganzi block through the Kunlun suture into the Qaidam block consistent with partial melt and ductile flow and (2) a mid-crustal HVZ bracketing the south Qilian suture coincident with ultrahigh pressure metamorphic rocks at the surface. (3) Additionally, the model possesses a doublet Moho extending from the Qaidam to the Qilian blocks which probably reflects increased mafic content with depth in the lowermost crust perhaps caused by a vertical gradient of ecologitization. (4) Crustal thickness is consistent with a step-Moho that jumps discontinuously by 6 km from 63.8 km (+/- 1.8 km) south of 35A degrees to 57.8 km (+/- 1.4 km) north of this point coincident with the northern terminus of the mid-crustal LVZ. These results are presented as a guide to future joint inversions across a much larger region of Tibet.
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