In a recent study published in the journal Tectonophysics by my colleague Tony Yates and I, we investigate what we believe to be the largest known impact structure in the known world buried deep in the Southern New Half, based on years of experience studying asteroid impacts. day. Wales.

The Deniliquin structure, which has not yet been tested by drilling, has a diameter of 520 kilometers. This exceeds the size of the nearly 300 km wide Vredefort impact structure in South Africa, which is now considered the largest in the world.

Hidden traces of Earth’s early history

The history of the bombardment of Earth by asteroids is largely hidden. There are several reasons for this. The first is erosion: the process by which gravity, wind, and water slowly erode the earth’s materials over time. When an asteroid collides, it creates a crater with a raised core. When you throw a pebble into a pond, it’s like a drop of water splashing out of a short-lived crater.

This central raised dome is a key feature of large impact structures. However, it can degrade over thousands or millions of years, making the structure difficult to identify. Structures can also be buried by precipitation over time. Or they could be destroyed as a result of subduction, when tectonic plates collide and slide under each other into the Earth’s mantle.

Still, new geophysical discoveries herald a paradigm shift in our understanding of how Earth has evolved over the ages, revealing signs of impact structures formed by asteroids that could be tens of kilometers across. These include pioneering discoveries of impact “ejectors”, that is, materials ejected from the crater during the impact.

Researchers believe the oldest layers of these emissions, found in sedimentary deposits of the first landmasses on Earth, could mark the end of Earth’s late heavy bombardment. Recent evidence suggests that Earth and other planets in the Solar System were heavily bombarded by occasional asteroids until about 3.2 billion years ago and thereafter.

Some important outcomes are associated with mass extinction events. For example, the Alvarez hypothesis, named after father-and-son scientists Luis and Walter Alvarez, explains how non-flying dinosaurs were wiped out by a massive asteroid impact about 66 million years ago.

Deniliquin structure disclosure

The continent of Australia and its predecessor Gondwana have been the target of numerous asteroid collisions. This resulted in at least 38 confirmed and 43 potential impact structures ranging from relatively small craters to large and fully buried structures.

As you recall from the pool and gravel analogy, when a large asteroid hits Earth, the underlying crust responds with a temporary elastic recoil that creates a central dome.

Such domes, which may gradually collapse and/or sink over time, may be all that remains of the original impact structure. They represent a deep impact “root zone”. Famous examples can be found in Mexico’s Vredefort impact structure and the 170 km wide Chicxulub crater. Second, it symbolizes the effect that caused the extinction of the dinosaurs.

Between 1995 and 2000, Tony Yates suggested that magnetic patterns beneath the Murray Basin in New South Wales likely represent a large, buried impact structure. Analysis of the region’s current geophysical data from 2015 to 2020 confirmed the existence of a 520 km diameter structure with a seismic dome at its centre.

The Deniliquin structure has all the features you would expect from a large scale impact structure. For example, magnetic readings of the field reveal a symmetrical fluctuation pattern in the shell around the core of the structure. This probably occurred during the impact, as the extremely high temperatures created intense magnetic forces.

The central region of the low magnetic field corresponds to the 30 km deep deformation of the mantle dome determined by the seismic method. The top of this dome is about 10 km shallower than the top of the regional mantle.

Magnetic measurements also show evidence of “radial faults”: faults radiating from the center of a large impact structure. Additionally, they are accompanied by minor magnetic anomalies that may represent magmatic “dykes”, which are layers of magma injected into cracks in a pre-existing rock mass.

Radial faults and the eruptive rock layers formed within them are typical of large impact structures and can be found at the Vredefort and Sudbury structures in Canada. Currently, most of the evidence for the Deniliquin effect is based on geophysical data obtained from the surface. To confirm the impact, we will only need to collect physical evidence of impact from the deep drilling of the structure.

When did the Deniliquin effect appear?

The Deniliquin Structure was probably located in the eastern part of the Gondwana continent before splitting into several continents much later (including the Australian continent). The effect that caused this may have occurred during the so-called Late Ordovician mass extinction. In particular, I believe it may have triggered the Hirnantian glaciation stage, which lasted from 445.2 to 443.8 million years ago and is also described as an Ordovician-Silurian extinction event.

This massive glaciation and mass extinction wiped out about 85% of the species on the planet. This was more than double the effect of Chicxulub, who destroyed the dinosaurs. It is also possible that the Deniliquin Structure predates the Hirnant Event and is of Early Cambrian origin (about 514 million years ago). The next step will be to collect samples to determine the exact age of the structure. This would require drilling a deep hole in its magnetic center and dating the extracted material.

It is hoped that further studies of the Deniliquin impact structure will shed new light on the nature of the early Paleozoic Earth.