Structural complexities in a foreland thrust belt inherited from the shelf-slope transition: Insights from the Alishan area of Taiwan

Abstract

The Alishan area of Taiwan spans the transition from the platform with full thickness of the Eurasian continental margin in the north to the thinning crust of its slope in the south. This part of the foreland thrust and fold belt includes important along-strike changes in structure, stratigraphy, and seismic velocities. In this paper we present the results of new geological mapping from which we build geological cross sections both across and along the regional structural trend. Fault contour, stratigraphic cutoff, and branch line maps provide 3-D consistency between the cross sections. Minimum shortening is estimated to be~15 km with displacement overall to the northwest. A P wave velocity model helps constrain the structure at depth by providing insight into the possible rock units that are present there. P wave velocities of ≥ 5.2 km/s point toward the presence of basement rocks in the shallow subsurface throughout much of the southeastern part of the area, forming a basement culmination. The changes in strike of thrusts and fold axial traces, the changing elevation of thrusts and stratigraphic contacts, and the growing importance of Middle Miocene sediments that take place from north to south are interpreted to be associated with a roughly northeast striking lateral structure coincident with the northern flank of this basement culmination. These transverse structures appear to be associated with the inversion of Eocene-and Miocene-age extensional faults along what was the shelf-slope transition in the Early Oligocene, uplifting the margin sediments and their higher P wave velocity basement during Pliocene-Pleistocene thrusting.

The deep structure of south-central Taiwan illuminated by seismic tomography and earthquake hypocenter data

“…Eastward, Brown et al (2012), Camanni et al (2014a, b), and Chuang et al (2013) use either surface geology, seismicity, or GPS data to interpret the basal thrust to ramp down into the middle crust and to involve basement in the deformation. These latter observations are further corroborated by the presence of high P-wave velocities (up to 5.5 km/s) close to the surface, suggesting that basement rocks are being uplifted (e.g., Alvarez-Marron et al, 2014;Camanni et al, 2014b;Huang et al, 2014;Kim et al, 2005Kim et al, , 2010Kuo-Chen et al, 2012;Rau and Wu, 1995;Wu et al, 2007). Furthermore, in much of Taiwan Wu et al (1997Wu et al ( , 2004Wu et al ( , 2014 and Gourley et al (2007) have used earthquake hypocenter data to suggest that there are a number of steeply dipping faults that penetrate into the middle and perhaps even the lower crust.…”

“…From west to east these zones are: the Coastal Plain, the Western Foothills, the Hsuehshan Range, and the Central Range. The Coastal Plain is made up of weakly deformed Pliocene to Holocene syn-orogenic sediments of the foreland basin, while the Western Foothills comprise a west-verging thrust system that imbricates the Miocene pre-orogenic and the younger syn-orogenic sediments (Alvarez-Marron et al, 2014;Brown et al, 2012;Hickman et al, 2002;Hung et al, 1999;Lacombe et al, 1999;Rodriguez-Roa and Wiltschko, 2010;Yue et al, 2005). In much of central and southern Taiwan, the boundary between the Coastal Plain and the Western Foothills is generally interpreted to coincide with the tip line of the Changhua Thrust (Fig.…”

“…2;Lin and Watts, 2002;Shyu et al, 2005;Yang et al, 2007). The Western Foothills thrust system appears to be linked to an east-dipping basal thrust that, in the northernmost part of the map area, is located at the base of the syn-orogenic sediments (Brown et al, 2012;Carena et al, 2002;Mouthereau et al, 2002;Mouthereau and Lacombe, 2006;Suppe, 1981;Yue et al, 2005), while in the central and southern parts it cuts down section into Miocene sediments (Alvarez-Marron et al, 2014;Hickman et al, 2002;Lacombe et al, 1999;Mouthereau et al, 2001Mouthereau et al, , 2002Mouthereau and Lacombe, 2006;Mouthereau and Petit, 2003;Suppe, 1976;Suppe and Namson, 1979) and, locally, may lie within undifferentiated pre-Miocene rocks (Hickman et al, 2002;Hung et al, 1999).…”

“…There are, however, a number of pieces of evidence in the surface geology (AlvarezMarron et al, 2014;Brown et al, 2012;Camanni et al, 2014a), as well as in magnetotelluric (Bertrand et al, 2009(Bertrand et al, , 2012, GPS (Chuang et al, 2013), earthquake hypocenter (Gourley et al, 2007; Lacombe and Mouthereau, 2002;Lacombe et al, 2001;Mouthereau and Petit, 2003;Wu et al, 1997Wu et al, , 2004Wu et al, , 2014Wu et al, 2008;Yue et al, 2005) and seismic tomography data (Alvarez-Marron et al, 2014;Camanni et al, A C C E P T E D M A N U S C R I P T…”

“…Alvarez-Marron et al, 2014).The deep level (≥10 km) of the hypocenter cluster along the basement-cover interface make it difficult to link the geometry of the thrust systems developed at the surface(Alvarez-Marron et al, 2014; Hickman et al, 2002; Lacombe et al, 1999; …”