Ancient rocks reveal a dramatic journey from deep inside Earth

Surface rocks can reveal records of regions they occupied millions of years ago, even if those areas were kilometers below ground. On Türkiye's Marmara Sea's southern shores and islands, granitic rocks that are now exposed originated as molten material deep in Earth's crust.

A new study by researchers at Istanbul Technical University (ITU) has outlined how these rocks cooled and moved toward the surface, identifying two key stages in northwestern Anatolia's tectonic history. Instead of using only one dating method, the team combined several mineral-based geological clocks that lock at different temperatures. This approach traced the rocks' cooling from about 800°C to 60°C, illustrating their journey from deep magma formation to later cooling during exhumation.

The findings indicate the process was uneven: rapid cooling during the Eocene, a slowdown, followed by another phase of rapid cooling and uplift in the Oligocene. These two phases may point to different underlying processes. The study was published in the Journal of Asian Earth Sciences.

Reading the history recorded in minerals

Northwestern Anatolia is part of the eastern Mediterranean with complex geology, shaped by the collision and interaction of continental fragments and a long history of subduction between the African and Eurasian plates. Researchers have long debated the drivers of widespread Eocene magmatism in the region, with explanations ranging from continental collision-related compression to various forms of extension tied to changes deep within the lithosphere.

Past studies have dated Eocene granitoids here, examining their geochemistry and tectonic origins. For example, a 2023 study dated granitoid emplacement at around 57–41 million years ago, linking post-collisional magmatism to slab break-off. Earlier work used apatite fission-track thermochronology to study low-temperature histories near Marmara Island and the Kapıdağ Peninsula.

The new study highlights a gap: an integrated reconstruction connecting emplacement depth, crystallization, cooling, and exhumation across a wide temperature range for the Eocene Southern Marmara granitoids. Previous research mainly focused on high- and intermediate-temperature data, leaving low-temperature history less explored.

To address this, researchers studied three granitic bodies—the Northern Kapıdağ, Southern Kapıdağ, and Avşa plutons—and used a combination of zircon U-Pb dating, biotite 40Ar/39Ar dating, zircon and apatite (U-Th)/He dating, geothermobarometry, and inverse thermal-history modeling. Each method captures a different stage of the rocks' temperature history, and together, they act like checkpoints tracing the rocks' cooling journey.

From deep underground to today's landscape

The results suggest that the three plutons crystallized during distinct episodes between roughly 48 and 37 million years ago. The researchers estimate they were originally emplaced at mid-crustal depths, averaging about 8.8 to 13.6 kilometers. Despite their different ages and locations, these plutons exhibit remarkably similar thermal histories, which the researchers divided into three stages.

Initially, there was rapid Eocene cooling, with the rocks cooling from approximately 800°C to about 350°C at estimated rates of 150–200°C per million years. This cooling then slowed significantly between roughly 350°C and 180°C, occurring at rates of 15–40°C per million years.

Finally, during the early Oligocene (around 32–28 million years ago), the rocks experienced another phase of rapid cooling below 180°C, with rates exceeding 100°C per million years, indicating rapid exhumation. The similarity in thermal histories across the three plutons suggests regional processes influenced their cooling patterns more than local conditions around each body.

Two stages in a changing landscape

The reconstructed timeline enables researchers to differentiate between various explanations for how northwestern Anatolia evolved over millions of years. Initially, the team deliberated whether the rocks cooled naturally after hot magma intrusion or if circulating hot fluids facilitated heat removal. Since the rocks originated deep underground and lack signs of extensive fluid alteration, cooling alone seems insufficient. Instead, they suggest that during the Eocene, deeper processes caused parts of the dense material beneath the crust to break away or be removed, allowing hotter material to ascend. This likely heated and weakened the overlying rocks, causing uplift and bringing granites closer to the surface for faster cooling.

Geological evidence indicates this process started deep below, not through large-scale crust stretching. Later, a separate rapid cooling episode is linked to regional changes, as similar ages are found elsewhere around the Marmara region, showing these rocks didn't rise independently but as part of a broader regional tectonic shift.

Across the Aegean, evidence shows rocks surfaced progressively later from north to south, related to the southward retreat of the African plate. The rapid uplift of the Southern Marmara rocks around 32–28 million years ago aligns with this pattern. Collectively, the findings reveal that the Marmara region's exposed rocks represent two major phases: an initial deep-seated transformation and, much later, a regional tectonic process bringing buried rocks to the surface.

"By combining geological clocks that record different temperatures, we can follow these rocks through much more of their history, from crystallization deep in the crust to their later journey toward the surface. That gives us a way to distinguish tectonic events that would otherwise be difficult to separate," explained Prof. Şafak Altunkaynak of the Department of Geological Engineering at Istanbul Technical University.

Rocks as archives of a changing Earth

Today, the granite around Kapıdağ and Avşa may seem like ordinary landscape features near the Marmara Sea. However, its minerals hold evidence of an extraordinary journey that started over 10 kilometers beneath Earth's surface and tens of millions of years ago.

By analyzing various mineral "clocks," researchers identified two main stages in this journey: an initial rapid cooling during the Eocene, driven by deep-earth processes that uplifted the rocks, followed by a later phase where tectonic activity related to the evolving Aegean region brought the rocks closer to the surface.

This distinction helps scientists establish a clearer timeline of how deep-seated geological forces gradually shape the surface landscape. Northwestern Türkiye lies in a highly active geological zone, where shifting tectonic plates and deep Earth processes have repeatedly transformed the terrain over millions of years.

"These rocks are like geological archives. Their minerals record different stages of cooling, and when we put those records together, we can reconstruct changes that occurred far deeper than the rocks we see at the surface today," said Prof. Şafak Altunkaynak.

These rocks are remarkable not just because of their age. The granite seen at the surface today originally formed kilometers underground, where it was subjected to temperatures exceeding hundreds of degrees Celsius. Tiny minerals embedded within it have preserved evidence from that extensive journey, enabling scientists to determine when the rocks formed, cooled, and started ascending toward the surface.

This means that the landscape visible today holds the record of a world once concealed deep beneath our feet. By learning to interpret this record, researchers can look beyond the surface and understand the powerful forces that have continuously shaped Earth over tens of millions of years.