Many geologists share the hypothesis that about 155 million years ago a piece of continent 5,000 kilometers long separated from Western Australia and moved away in a northerly direction. There are geological signs that indicate this, among which are a hidden basin in the depths of the ocean known as the Argo abyssal plain.
This underwater relief also gives name to the continent that would have been formed by this Australian cleavage: Argoland or Argolandia.
The structure of the seabed shows that this continent must have drifted towards the northwest and ended up where the islands of Southeast Asia are today. But surprisingly, there is no large continent hidden beneath those islands, just remnants of small continental fragments that are also surrounded by much older ocean basins.
So what happened to Argoland? Geologists from the University of Utrecht (Netherlands) have managed to reconstruct part of the history of the lost continent and now present their results in an article published (October 2023) in the specialized journal Gondwana Research.
It turns out that Argoland has survived in the form of fragments, a microcontinent in the form of an archipelago.
Geologists differentiate the Earth's crust into heavier oceanic crust and lighter continental crust. These lighter continents may be partially hidden below sea level, as was also the case with another "lost" continent, Greater Adria. Like Argoland, it also consisted of different fragments separated by narrow ocean basins, which over time became part of a single tectonic plate. At some point in the past, Greater Adria sank into the Earth's mantle, but the upper layer was left behind and folded to form the mountains of southern Europe. Argoland, however, left no trace in the form of folded rock strata.
“If continents can sink into the mantle and disappear completely, leaving no geological trace on the Earth's surface, then we wouldn't have much idea what the Earth might have been like in the geological past. It would be almost impossible to create reliable reconstructions of ancient supercontinents and the geography of the Earth in past times,” explains Utrecht University geologist Douwe van Hinsbergen. “These reconstructions are vital for our understanding of processes such as the evolution of biodiversity and climate, or for finding raw materials. And at a more fundamental level: to understand how mountains form or to discover the driving forces behind plate tectonics; two phenomena that are closely related.”
Van Hinsbergen and his colleague Eldert Advokaat were curious about what the geology of Southeast Asia might say about the fate of Argoland. “But we were literally dealing with islands of information, which is why our investigation took so long. We spent seven years putting the puzzle together,” says Advokaat.
“The situation in Southeast Asia is very different from places like Africa and South America, where a continent was clearly broken into two pieces. Argoland was divided into many different fragments. “That obstructed our view of the continent’s journey.” But that was until he realized that the fragments arrived at their current locations at around the same time, clarifying how they were once connected to each other. The fragments formed a collage: Argoland is hidden under the green jungles of much of Indonesia and Myanmar.
The history of this land mass goes back in time and is typical of a fragmentation as a microcontinent. There was never a single clearly delineated and coherent continent in Argoland, but rather an "Argopelago" of microcontinental fragments separated by older ocean basins, the authors of the new study explain. In that it is similar to Greater Adria, which has now been almost completely submerged in the Earth's mantle, or Zeelandia, the largely submerged continent east of Australia. “The fragmentation of Argoland began about 300 million years ago,” adds Van Hinsbergen.
The puzzle that Advokaat and Van Hinsbergen have solved fits perfectly between the neighboring geological systems of the Himalayas and the Philippines. Their detective work also explains why Argoland is so fragmented: the breakup accelerated about 215 million years ago, when the continent shattered into thin fragments. Geologists conducted field work on several islands, including Sumatra, the Andaman Islands, Borneo, Sulawesi and Timor, to test their models and determine the age of key rock strata.
