Did Oligocene crustal thickening precede basin development in northern Thailand? A geochronological reassessment of Doi Inthanon and Doi Suthep

Abstract

The Doi Inthanon and Doi Suthep metamorphic core complexes in northern Thailand are comprised of amphibolite-grade migmatitic gneisses mantled by lower-grade mylonites and metasedimentary sequences, thought to represent Cordilleran-style core complexes exhumed through the mobilization of a low-angle detachment fault. Previous studies have interpreted two metamorphic events (Late Triassic and Late Cretaceous), followed by ductile extension between the late Eocene and late Oligocene, a model which infers movement on the detachment at ca. 40 Ma, and which culminates in a rapid unroofing of the complexes in the early Miocene. The Chiang Mai Basin, the largest such Cenozoic Basin in the region, lies immediately to the east. Its development is related to the extension observed at Doi Inthanon and Doi Suthep, however it is not definitively dated, and models for its development have difficulty reconciling Miocene cooling ages with Eocene detachment movement. Here we present new in-situ LA-ICP-MS and SIMS U-Pb age data of zircon and monazite grains from gneiss and leucogranite samples taken from Doi Inthanon and Doi Suthep. Our new zircon data exhibit an older age range of 221-210 Ma, with younger ages of ca. 72 Ma, and 32-26 Ma. Our monazite data imply an older age cluster at 83-67 Ma, and a younger age cluster of 34-24 Ma. While our data support the view of Indosinian basement being reworked in the Cretaceous, they also indicate a late Eocene-Oligocene tectonothermal event, resulting in prograde metamorphism and anatexis. We suggest that this later event is related to localized transpressional thickening associated with sinistral movement on the Mae Ping Fault, coupled with thickening at the restraining bend of the Mae Yuan Fault to the immediate west of Doi Inthanon. Further, this upper Oligocene age limit from our zircon and monazite data would imply a younger Miocene constraint on movement of the detachment, which, when combined with the previously recorded Miocene cooling ages, has implications for a model for the onset of extension and subsequent development of the Chiang Mai Basin in the early mid-Miocene.

The crustal architecture of Myanmar imaged through zircon U-Pb, Lu-Hf and O isotopes: Tectonic and metallogenic implications

“…Late Cretaceous (~120 Ma) ages are reported from granites with I-type affinity outcropping into the metasedimentary sequences in the vicinity of Mandalay (Barley et al, 2003, and this study), and Eocene and Miocene magmatic ages are also recorded (Barley et al, 2003, and this study;Gardiner et al, 2016a;Mitchell et al, 2012). Within the country rocks of the Mogok Metamorphic Belt, at least two phases of post-collisional Barrovian-type regional metamorphism up to sillimanite grade have been identified through zircon and monazite U-Pb geochronology: one pre-to Mid Palaeogene, and one Eocene-Oligocene (Barley et al, 2003;Searle et al, 2017;Searle et al, 2007), this latter age similar to that of Doi Inthanon in northern Thailand (Dunning et al, 1995;Gardiner et al, 2016b). Localized partial melting of the country rock also produced leucogranitic melts, with a tourmaline and garnet Himalayan-type…”

“…Inthanon Zone, reflecting minor magmatism likely related to the era of Neo-Tethys ( Figure 4) (Dunning et al, 1995;Gardiner et al, 2016b).…”

Exhumation of west Sundaland: A record of the path of India?

“…2 and 6) is located in NW Thailand and experienced a complex polyphase deformation. Two major episodes of metamorphism postdate the Indosinian Orogeny viz., 1) a thermal event associated with magmatic emplacement from ~84 Ma to 72 Ma (Late Cretaceous), followed by, 2) prograde metamorphism during Eocene to Oligocene crustal thickening (Dunning et al, 1995;Gardiner et al, 2016). The final exhumation of the dome occurred in the Miocene via a low angle extensional detachment (Gardiner et al, 2016).…”

“…The ensuing crustal thickening was accompanied or followed by the formation of a number of metamorphic domes, such as Stong, Khanom, Khlong Marui, Ranong, Klaeng, Khabila, Doi Inthanon, and Mogok/shan scarp, which are often attributed to extensional detachments (or large normal faults) and major strike-slip displacements (Figs. 1b and 2) (e.g., Bertrand et al, 1999;Gardiner et al, 2016;Macdonald et al, 2010;Watkinson et al, 2011;Zhang et al, 2012a).…”

Structural interpretation of Mae Suai Basin, Chiang Rai Province, based on gravity data analysis and modelling

“…Some small basins are associated with significant recent earthquakes.

Fig. 1Regional map of Central and Northern Thailand, showing the locations of major Cenozoic faults and sedimentary basins with moment tensor analyses of recent earthquakes (modified from Gardiner et al., 2016; Morley et al., 2011).

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