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Exposed in the Coastal Range of eastern Taiwan is a portion of the Luzon volcanic arc that has collided with the South China continent since 6 Ma, as well as syn-orogenic sedimentary rocks deposited on the arc and subsequently intensely folded and thrusted. This study sampled syn-orogenic mudstones for the measurement of anisotropy of magnetic susceptibility (AMS) at four sites on the sides of the range. The AMS fabrics reveal a contribution from burial compaction and, less significantly, tectonic shortening which varies slightly across the range. This lateral variation is considered as, for simplicity, a result of two-phase deformation because these rocks were involved in an eastward propagating back thrust system on the colliding arc. A new strain analysis method is developed to estimate the overall tectonic strain by unstraining the pseudo-strains with respect to the AMS measurements at each site with the knowledge about the distribution of the susceptibility ellipsoids in the unstrained state. Strain results reveal that shortening phases occurred before and after the folding, with a shortening ratio of 0.177–0.195 and 0.181–0.248, respectively. The post-folding shortening on the western side is sub-parallel to the range, and the pre-folding shortening on both sides is layer-parallel and nearly perpendicular to the range. Therefore, the mudstones underwent layer-parallel shortening, fold-related tilting and eventually orogen-parallel shortening after deposition. This sheds light on the complexity in deformation of the colliding Luzon arc. • Syn-orogenic mudstones in eastern Taiwan have composite magnetic fabrics. • A new method is developed to estimate strain from AMS data at a site. • Luzon arc mudstones underwent shortening before and after folding. [ABSTRACT FROM AUTHOR]

To better understand the establishment of subduction zones, considerable effort is required regarding various factors, such as the time of subduction initiation and the age of the slab's lithosphere. In this study, adakitic dikes (ca. 17.3 Ma: ε Nd of 7.4–7.9 and 206Pb/204Pb of 18.27–18.42) were recognized to crystallize along with the normal and enriched mid-ocean ridge basalt (N- and E-MORB)-like oceanic crust (ca. 17.8–14.1 Ma: ε Nd of 8.8–13.3 and 206Pb/204Pb of 17.71–18.22) in the East Taiwan Ophiolite (ETO). Given that the outcropping in ultramafic sequence, the distinct Nd–Pb isotopes, and the slight precedence than corresponding arc volcanism (≤ ca. 14.1 Ma), the origins of the ETO adakitic dikes are related to amphibolite-facies melting of the subducted slab beneath the Luzon forearc spreading center during subduction initiation. In this respect, we conclude that the sparsely exposed but isotopically heterogeneous E-MORB-type rocks (ε Nd of 8.8–10.2 and 206Pb/204Pb of 18.06–18.22) are Nb-enriched basalts and relate to the Nb-enriched slab melts derived from further melting of the hot eclogitic residuum of the young South China Sea subducted slab. In addition, the isotopic diversity of N-MORB-like samples can be attributed to the diking of adakitic and Nb-enriched slab melts beneath the Luzon forearc spreading center. Combined with the previously reported early Miocene uplift of the Zambales Ophiolite in the western Luzon, we infer an early Miocene induced-subduction initiation of the South China Sea and the delayed near-trench spreading of the Luzon forearc by <5 Myr. Compared to the absence of Nb-enriched slab melts and >20 Myr subducted slab in the genesis of the in-situ Izu-Bonin-Mariana (IBM) forearc and the voluminous Nb-enriched rocks and ca. 0 Myr subducted slab of the Central Palawan–Amnay ophiolites, the sparse Nb-enriched rocks and <15 Myr subducted slab of the ETO sequence invoke the governing factor of slab's lithosphere age during subduction initiation magmatism. We propose that the lack of Nb-enriched slab melts in the forearc chemostratigraphy of the Oman Ophiolite may indicate the subduction initiation of an old and cold oceanic plate. • Coeval adakitic & Nb-enriched dikes expose in the forearc-type East Taiwan Ophiolite. • Mid-Miocene East Taiwan Ophiolite is representative fragments of the Luzon forearc. • Luzon forearc was due to South China Sea Early Miocene induced-subduction initiation. • Slab's crystallization age is governing factor of subduction initiation magmatism. [ABSTRACT FROM AUTHOR]