Plants produce resinous secretion, which serves to heal injuries and obstruct predators (Labandeira, 2014). The resinous secretion is an organic material often highly resistant to weathering and thus is able to deposit in developing sediments. Throughout transportation, deep burial, thermal-pressure effect, and long-term maturation, the resin might transform into hard resin amber, and it is not regarded as a mineral due to the lack of a crystallographic structure. Moreover, before the eventual depositional place, transportation is a long-term process that takes weeks to millennia (Labandeira, 2014). Typically, transportation from the source to the initial depositional area in a fluvial, lake, deltaic, lagoon, or nearshore marine environment, frequently, is from a few to tens of kilometers; much less commonly, a few hundred of kilometers (Martínez-Delclòs et al., 2004; Girard et al., 2008; Girard et al., 2009).

Due to the amber’s botanical structural characteristics and composition (Anderson and Winans, 1991; Anderson et al., 1992) global amber classification system classified amber resinites into four classes. Class I is the most abundant, comprised of labdatriene carboxylic acids such as communic or ozic acids. The most famous origins are from the Baltic Sea, the Dominican Republic, and New Zealand. Class II resinites are formed from the polymer of bicycling sesquiterpenoid hydrocarbons and Triterpenoid. The most probable botanical affinity is Dipterocarpaceae, which is mainly distributed in the tropical forests of Southeast Asia. Class III, whose basic structural character is Polystyrene, was found in New Jersey and Germany, and the most probable botanical affinity is Hammelidaceae. Class IV resinites originate from Bovey-Tracy and Moravia, and the basic structural character is sesquiterpenoid, which is primarily based on the Cedrane skeleton.

In the present tropical rain forests of the Indian Peninsula and the Indo-China Peninsula, the Dipterocarpaceae is the dominant tree family and is one of the confirmed botanical provenances of the resinites (Anderson and Winans. (1991); Anderson et al., 1992). Dipterocarpaceae are also well known in Cenozoic fossil floras. Several fossils tension are recorded around the south, southeast, and east Asia regions, except in Taiwan, so far (Dutta et al., 2011; Rust et al., 2010; Böhme et al., 2013; Langenheim and Beck, 1965; Brackman et al., 1984; Van Aarssen et al., 1990; Kocsis et al., 2020; Hillmer et al., 1992; Feng et al., 2013; Shi et al., 2014; Naglik et al., 2018; Wang et al., 2018).

In this study, we first report and quantitatively analyze the amber resin of the family Dipterocarpaceae in Lichi Mélange at the southern tip of the Coastal Range, Eastern Taiwan. Moreover, it is also the first finding to report on the amber resin in the mélange unit. The chaotically tectonic origin of the Lichi Mélange and the arc-continental collision of Taiwan island implies the complicated journey and indeterminate era of resin production. This discovery fills the gap in the fossil record of the Dipterocarpaceae family in Taiwan and illustrates that the forearc basin gathers some tropical sediment sources far away from today’s Taiwan island.

Taiwan is an example of an ongoing arc-continental collisional orogeny produced by an oblique convergence between the passive continental margin of the Eurasian Plate and the Luzon volcanic arc on the Philippine Sea Plate (Chai, 1972; Suppe, 1981; Hayes and Lewis, 1984; Suppe, 1984; Tsai, 1986; Yu et al., 1997). The Eurasian Plate is adjacent to the South China Sea oceanic lithosphere southeastward that is subducted beneath the Philippine Sea Plate along the Manila trench, whereas the Philippine Sea Plate northward is subducted beneath the Eurasian Plate along the Ryukyu Trench (Figure 1A ). The arc-continent collision is initiated at 5 Ma (e.g., Teng, 1990), with the onset increased in clastic sedimentation in Western and Eastern Taiwan, in the northern part of Taiwan and continuously propagates to the south.

The island of Taiwan consists of five morphotectonic units from west to east: the Coastal Plain, the Western Foothills, the Hsuehshan Range, the Central Range, and the Coastal Range. The Coastal Plain and Western Foothills consist of Pliocene to modern shallow marine siliciclastic deposits. The Hsuehshan Range and the western part of the Central Range are composed of Eocene to Miocene deep marine turbidites. The eastern part of the Central Range is featured by two metamorphic belts, including Tailuko and Yuli belts, which consist of the Paleozoic to the Mesozoic metamorphic basement and Oligocene to Miocene greenschist to blueschist facies, respectively. Lastly, the Miocene to Pliocene-accreted Tuluanshan volcanic arc and the Miocene to Pleistocene-deformed flysch forearc basins are exposed in the Coastal Range, Eastern Taiwan.

Eastern Taiwan is featured by the arc-continental collision along the Longitudinal Valley. Nowadays, with a length of approximately 150 km and a width of approximately 10 km, the Coastal Range is composed of four geological units: 1) the Miocene accreted volcanic arc of the Tuluanshan Formation, 2) the overlapped Plio-Pleistocene deformed sequence of unmetamorphosed flysch deposits, and the conglomerate of the Fanshuliao and Paliwen Formations, 3) the Miocene to Pleistocene chaotic disrupted blocks-in-matrix Lichi Mélange, and 4) the Pleistocene Peinanshan conglomerate (Hsu, 1956; Chang, 1967; Chang, 1969; Teng, 1987; Chen, 1988).

“Mélange” is a term for mappable units of chaotic mixed rocks, including blocks of different lithology, origins, and ages (blocks-in-matrix) (Greenly, 1919; Festa et al., 2010). It is common to find the mélange unit at many collisional- and accretionary-type orogenic belts worldwide. In Taiwan, the Lichi Mélange, widely exposed along the southwestern flank of the Coastal Range, consists mainly of poorly stratified mudstone in which some large or small rock fragments or blocks of hard greyish sandstone, gabbro, serpentinite, and a minor slate are present (Hsu, 1956). Other exotic block lithologies include andesite, volcaniclastic rocks, limestone, ophiolite-bearing sedimentary rocks, amphibolite, low-grade meta-sandstones, and flysch blocks similar to the Fanshuliao and Paliwen formations scattered in the Lichi Mélange (Liou, 1977; Page and Suppe, 1981; Sung, 1991). The characteristic mesoscopic structure of the Lichi Mélange is that the curviplanar surfaces of the penetrative scaly foliation are generally polished and have aligned minerals and slickensides (Hsu, 1976; Teng, 1981; Chen, 1991; Chen, 1997; Chang et al., 2000; Chang et al., 2001). The origin of the Lichi Mélange is still debated. Prior studies have shown evidence of sedimentary (Liou, 1977; Page and Suppe, 1981) and tectonic processes (Chen, 1997; Chang et al., 2000; Chang et al., 2001) of the Lichi Mélange.

The result of the Raman spectra of the resin is shown in Figure 3. The most characteristic bands in the Raman spectrum of resin are the medium-strong and robust features at 1,646 and 1,450 cm⁻¹, respectively, which may be assigned to ν(C=C) stretching modes and δ(CH₂) deformation modes of characteristic peaks (Edward and Farwell, 1996; Brody et al., 2001). In the literature (Moreno et al., 2000; Brody et al., 2001; Winkler et al., 2001; Badea et al., 2015), the intensity ratio of the ν(C=C)/δ(CH₂) and I1646/I1450 is reported (Badea et al., 2015) to be indicative of the maturation process. The intensity ratio of the Lichi Amber is < 1 (0.82), owing to the degradation of the ν(C=C) unsaturation in the fossilized resin, probably through aerial or microbial oxidation (Edward and Farwell, 1996).

The FTIR-ATR result of the Lichi Amber is shown in Figure 4. The wavenumbers 3000 to 2,800 cm⁻¹ are saturated hydrocarbons for CH₃ and CH₂ stretching vibrations, and the wavenumbers 1385–1,370 cm⁻¹ are CH₂ bending vibrations. With different origins and the amber’s maturation process, the proportion of functional groups (C-H, C=C, and C=O bonds) will have different absorption ratios (Chen, 2019; Chen and Chang, 2020). In addition, the Lichi Amber sample has higher exocyclic methylene at 887 cm⁻¹, so it can be judged that the sample is less affected by heat or encounters short-buried history (Guiliano et al., 2007). Compared with different origins of amber (Chen, 2019; Chen and Chang, 2020; Chang and Chang, 2020; Chang, 2022; Chang et al., 2022), the pattern of the Lichi Amber has a high similarity with that of the Sumatra Amber. However, it shows a different proportion of functional groups from the pattern of the Zhangpu Amber, which is the closest geographic location. This result implies that the Lichi Amber might come from tropical Southeast Asia rather than the southeast of mainland China. The ratio of the empirical formula, Factor(H) versus Factor(O) (Figure 5), also shows the same result that the Lichi amber is correlated to the Southeast Asian amber (Chen, 2019; Chen and Chang, 2020; Chang and Chang, 2020; Chang, 2022; Chang et al., 2022).

Figure 6 shows the Py-GC-MS analysis pattern of the Lichi Amber. The amber contains sesquiterpenes, mainly Calamenene isomers, accompanied by β-amyrin, α-amyrin, and Hop-22 (29)-en-3β-ol triterpenoids. According to the composition of these terpenoids, the Lichi Amber in Lichi Mélange, Eastern Taiwan, belongs to the plants in the family of Dipterocarpaceae (Shi et al., 2014; Bonaduce et al., 2016; Chang and Chang, 2020; Chang et al., 2022; Simoneit et al., 2020).

The petrography of the 46 sandstone blocks’ samples in the southern tip of the Lichi Mélange shows two groups of different sandstone compositions (Figure 7A), which are the quartzose greywacke sandstone and the volcaniclastic sandstone. The quartzose greywacke mainly contains 65%–99% quartz, 0%–4% feldspar (mostly plagioclase), and 0%–34% lithic fragments. Under the thin section (Figure 7B), the quartz’s angular and less polycrystalline quartz, which might come from the metamorphic rocks, can be found Moreover, the pyroclastic sandstone contains 2%–15% quartz, 78%–82% feldspar, and 5%–15% lithic fragment. In Figure 7C, the pyroclastic thin section shows that the quartz and plagioclase particles are angular. The petrographic result is similar to previous studies (Chen and Wang, 1988; Sung, 1991; Yang et al., 2012) and can be compared with the Type-I volcaniclastic sandstone and Type-II Quartzose-greywacke sandstone reported by Sung. (1991). The petrography of the amber’s surrounding matrix also shows a Quartz-rich composition (quartz ∼74.78%). However, limited by the insufficient number of samples collected, a single sample cannot summarize the most favorable preservation environment. This article is based on a single sample for observation and description. The petrographic studies show that most of the sandstone blocks in the Lichi Mélange are quartz-rich sandstones. Among the quartz granules, the primary source is from the low metamorphic facies region (Sung, 1991). Furthermore, most sandstone blocks belong to the turbidite facies; the original depositional environment also belongs to the deep-sea submarine fan of continental slopes or uplifts. Thus, the most numerous sandstone blocks in Lichi Mélange are from accretionary prisms from the Eurasian margin (Sung, 1991; Lai et al., 2021).

Through the Py-GC-MS and the biomarker, the polymer of bicycling sesquiterpenoid hydrocarbons and Triterpenoid indicated that the resin came from the Dipterocarpaceae species. The FTIR absorption spectrum shows that the pattern is more highly correlated to Sumatra’s pattern than Zhangpu’s pattern, implying that the source may come from tropical southeastern Asia. The petrographic and sedimentary facies analyses indicate that amber’s wall rock source comes from deep-sea fans on the continental slope or continental uplift (Lai et al., 2021) at the margin of the Southeast Eurasia Plate.

The arc-continental collision between the Eurasian plate and the Luzon arc formed the history of the Taiwan orogenic belt and accumulated a variety origin of the sediment deposit in the forearc basin and was mixed in the Lichi Mélange from 15 to 5 Ma (Suppe, 1981; Teng, 1990; Chang et al., 2000, 2001). The Lichi amber, as one of the sources, comes from the southeastern Asia tropical forest, implying a fluke clue of a geomarker despite its lack of geochronological evidence. With several analyses in this study and the tectonic evolution of South China Sea rifting and the arc-continental collision of the Eurasian Plate and the Philippine Sea Plate, the hypothesis model is that (1) with the spreading of the Dipterocarps from India since the Cretaceous (Khan et al., 2020), 2) the Dipterocarps migrated to the Southeast Asian rainforest in Neogene 3). The nannofossils study of the exotic blocks in Lichi Mélange suggested that the Lichi amber might form and transport from the Sumatra and associated regions to the basin and be preserved in sandstone on the continental slope or the continental uplift at the margin of the Eurasian Plate in 18–5.6 Ma (NN3-NN11, Chi, 1982) 4). From 5 Ma to the present, the sandstone and the amber slumped into the forearc basin, and during the arc-continental collision, the slumping or the shortening of the forearc basin with intensive shearing formed the Lichi Mélange and the amber was exposed to the surface (Suppe, 1981; Teng, 1990; Chang et al., 2000; Chang et al., 2001; Lai et al., 2021).