Where two ancient continents crashed 230 million years ago. The rocks here are twisted and folded like crumpled paper from the collision that built Southeast Asia.
The continental collision
The mountains of Loei Province are the scar from a tectonic collision that occurred during the Triassic Period. This event, an important part of the larger Indosinian Orogeny, involved two major landmasses: the Shan-Thai block to the west and the Indochina block to the east. For millions of years, they were separated by a branch of the Paleo-Tethys Ocean. As tectonic plates shifted, this ocean began to close.
The final collision was not a single event but a complex process that peaked in the Late Triassic, around 230 million years ago. The immense pressure of the two continental plates grinding together buckled the crust, creating the Loei Fold Belt, a region of intensely deformed rock that runs roughly north-south through the province. This zone is part of a larger geological feature known as the Nan-Uttaradit Suture, which is the line where the two ancient landmasses were welded together. The formation of this suture was a fundamental step in the assembly of mainland Southeast Asia.
Reading the rocks
The evidence for this ancient crash is written in the local geology. The Nan-Uttaradit Suture Zone contains a chaotic mixture of rocks called a mélange. This includes slices of ophiolites—dark, heavy rocks like serpentinite, gabbro, and basalt—which are the mineralized remnants of the Paleo-Tethys oceanic crust. These deep-ocean rocks were scraped off the descending plate and thrust upwards, ending up high in the mountains.
Alongside the ophiolites are deep-sea sedimentary rocks, such as radiolarian cherts, formed from the silica-rich skeletons of microscopic organisms that rained down on the ancient seafloor. Volcanic rocks are also common, including andesites and rhyolites, remnants of a volcanic arc that formed as the oceanic plate subducted. The entire assembly of rock is intensely folded and faulted, showing how solid rock behaved like a plastic substance under the extreme heat and pressure of the continental collision. Geologists can observe features like imbricate structures, where rock slices are stacked on top of each other like fallen dominoes, to understand the westward direction of the tectonic forces. This geological turmoil also created conditions for significant mineral deposits, and the Loei Fold Belt is known for its concentrations of copper and gold.
💡Fun Facts
The collision occurred shortly after the Permian-Triassic extinction, Earth's most severe extinction event which wiped out over 80% of marine species and 70% of terrestrial vertebrates.
The Loei Fold Belt contains volcanic rocks dating back to the Devonian and Carboniferous periods, long before the main collision.
Some rocks in the suture zone, called serpentinite mélange, have a "block-in-matrix" fabric, where harder chunks of rock are mixed within a softer, sheared matrix, illustrating the chaotic forces at play.
The sandstone in the region contains in volcanic fragments, confirming that a volcanic arc was the primary source of sediment for basins that formed during the collision.
Phu Kradueng National Park is open for visitors from 7:00 AM to 2:00 PM for the hike up; the park itself is accessible 24 hours for those camping. The park is closed annually during the rainy season from June 1st to September 30th.
Admission
Admission to Phu Kradueng National Park is 400 baht for foreign adults and 200 baht for foreign children. Thai residents pay 40 baht for adults and 20 baht for children.
Accessibility
The primary points of interest are on a high plateau. Reaching the plateau requires a strenuous 5.5 km hike that ascends over 900 meters. The trails on the plateau are mostly flat but can be uneven. Bicycles are available for rent on the plateau.