Joint inversion of Rayleigh wave ellipticity and phase velocity for crustal structure in Taiwan

Highlights

• High-resolution shear wave velocity transects across the Taiwan Island from joint inversion.

• Deflection of Rayleigh wave packet from the great-circle path.

• Coupling of the mountain building with the subsurface slab interactions in Taiwan.

Abstract

The convergence between the Eurasian Plate and the Philippine Sea Plate results in rapid uplifting of the Taiwan Orogen. To study the mountain building in Taiwan, we present three transects of shear wave velocity across the Taiwan mountain belt with joint inversion of teleseismic Rayleigh wave H/V ratios and phase velocities derived from ambient noise tomography. The Particle Swarm Optimization algorithm is introduced into the joint inversion and is proved to be an efficient tool in solving the geophysical inverse problems. In determining the H/V ratios, cross-correlation techniques are used to reduce the non-great circle effects of the Rayleigh wave packets and to correct sensor misorientations. The cross-correlation techniques improve the accuracy of the H/V measurements, especially at the short periods. The observed up-to-middle crustal thickening in the central segment of the Central Range suggests lithospheric collision tectonism, whereas the lower crustal thickening in southern segment is proposed to be related to subduction of the Eurasian continental crust. In the north, however, no crust thickening is observed. The variations of the crust architecture along the axis of the Taiwan Orogen favor a close coupling relationship between mountain building and slab interactions in the mantle. Beneath the Central Range, the twist of the fate of the Eurasian continental crust from consumption in the south to collision in the central segment is proposed to be resulted from the slab breakoff beneath central Taiwan.

Introduction

Due to the convergence between the Eurasian Plate (EUP) and the Philippine Sea Plate (PSP), the Taiwan Orogen is experiencing violent deformation as manifested by the significant seismicity in and around its surroundings. East to the Taiwan Orogen, the PSP is subducting northwestward beneath the EUP along the Ryukyu trench, whereas to the south, the PSP is overriding the EUP along the Manila trench. Overall, the PSP is advancing towards the EUP at ~90 mm/yr (Sella et al., 2002). Acting as one of the youngest orogens of the world, the Taiwan Orogen has attracted much attention from geoscientists, especially since the deployments of the broadband seismological array and GPS network in this region (e.g., Roecker et al., 1987 Wu et al., 2007 Kuo-Chen et al., 2012 Huang et al., 2014a, Huang et al., 2014b Huang, 2017).

With regard to the slab interactions near Taiwan, a series of geophysical observations advocate southwestward propagating slab tearing along the Chinese continental margin, i.e. following the geometry of the continental-ocean boundary (e.g., Lallemand et al., 2001 Teng et al., 2000 Ustaszewski et al., 2012). Specifically, Teng et al. (2000) inferred that the Eurasian slab has broken off all the way from under the southern Ryukyu to north Taiwan. Correspondingly, the north Taiwan has undergone a post-collisional lithospheric extension resulted from the Eurasian slab breakoff (Teng, 1996 & Teng et al., 2000; Ustaszewski et al., 2012 Wang et al., 1999), as manifested by the horizontal GPS measurements (Bos et al., 2003). On the other hand, the geodetic leveling indicates that the Taiwan mountain belt uplifted at 0.2–18 mm/yr in the most recent 20 years, and the maximum uplift rates, 22.9 mm/yr and 25.8 mm/yr, are observed at the Central Range and Coastal Range respectively (Ching et al., 2011b). It is therefore expected that the central Taiwan is still in the process of mountain building, in contrast to the northeastern Taiwan suffering the post-collisional lithospheric extension. In this tectonic scenario, the mountain building process in Taiwan is a typical example that begins with the formation of the collisional orogen and ends with subsequent collisional collapse (Teng, 1996).

To understand the mountain building in Taiwan, several conceptual models have been proposed based on the present-day deformation pattern and the observed seismicity distribution. For example, the thin-skinned model suggests that the EUP subducts beneath the PSP with an accretionary wedge overlying a planar decollement fault (Suppe, 1981 Dahlen, 1990). Similarly, the lithospheric collision hypothesis advocates strong crust thickening and shortening occur on both sides of the Longitudinal Valley which is the suture zone that separates the PSP and EUP (Wu et al., 1997). Essentially, these models give only two-dimensional views of the Taiwan orogeny. Recently, new results on slab interactions beneath Taiwan are reported (e.g., Huang et al., 2014a, Huang et al., 2014b Huang et al., 2015 Ai et al., 2019b). Especially, we have provided a crustal and uppermost mantle model beneath the Taiwan Island, Taiwan Strait and southwestern Ryukyu Arc from ambient noise tomography (Ai et al., 2019b). This model, however, has a poor resolution in the crustal part limited to the frequency band of the surface wave dispersions. To decipher how the mountain building in Taiwan is linked to the subsurface slab interactions, a high-resolution crust model is desired. In the present study, we present three 2-D shear wave velocity transects that are located in the northern, central and southern Taiwan respectively (Fig. 1) by joint inversion of teleseismic Rayleigh wave H/V ratios and phase velocities. By introducing Rayleigh wave ellipticity as an additional type of observable into shear wave velocity inversion, we can overcome the low depth-resolution of the shallow structure that obtained from inversion with phase velocity alone (Ai et al., 2019b), as the H/V ratio is more sensitive to the shallow structure than the phase velocity at the same period (Boore and Nafi Toksöz, 1969; Lin et al., 2012 Lin and Schmandt, 2014 & Lin et al., 2014).

Similar seismological studies aimed at resolving the arc-continent collision structure were also conducted along the three transects in which we apply the joint inversion (Van Avendonk et al., 2014 & Van Avendonk et al., 2015; Kuo et al., 2016). Compared to the P wave tomography studies, the present joint inversions constrain the crust in western Taiwan well, which is dominated by relatively lower seismicity, though the lateral resolution in our results may be limited by the inter-station distance. Overall, the shear wave velocity images obtained here are complementary to the P wave velocity profiles and the 3-D lithosphere-scale models and provide more details on the evolution of the Taiwan Orogen associated with the slab interactions.