Crustal Failure on Icy Moons and Satellites from a Strong Tidal Encounter

Crustal Failure on Icy Moons and Satellites from a Strong Tidal Encounter

Authors:

Quillen et al

Abstract:

Close tidal encounters among large planetesimals and satellites should have been more common than grazing or normal impacts. Using a mass spring model within an N-body simulation, we simulate the deformation of the surface of an elastic spherical body caused by a close parabolic tidal encounter with a body that has similar mass as that of the primary body. Such an encounter can induce sufficient stress on the surface to cause brittle failure of an icy crust and simulated fractures can extend a large fraction of the radius of body. Strong tidal encounters may be responsible for the formation of long graben complexes and chasmata in ancient terrain of icy moons and satellites such as Dione, Tethys, Ariel and Charon.

Did the Late Heavy Bombardment Destroy the Hadean Crust?

Excavation and Melting of the Hadean Continental Crust by Late Heavy Bombardment

Authors:

Shibaike et al

Abstract:

No Hadean rocks have ever been found on Earth's surface except for zircons---evidence of continental crust, suggesting that Hadean continental crust existed but later disappeared. One hypothesis for the disappearance of the continental crust is excavation/melting by the Late Heavy Bombardment (LHB), a concentration of impacts in the last phase of the Hadean eon. In this paper, we calculate the effects of LHB on Hadean continental crust in order to investigate this hypothesis. Approximating the size-frequency distribution of the impacts by a power-law scaling with an exponent {\alpha} as a parameter, we have derived semi-analytical expressions for the effects of LHB impacts. We calculated the total excavation/melting volume and area affected by the LHB from two constraints of LHB on the moon, the size of the largest basin during LHB, and the density of craters larger than 20 km. We also investigated the effects of the value of {\alpha}. Our results show that LHB does not excavate/melt all of Hadean continental crust directly, but over 70% of the Earth's surface area can be covered by subsequent melts in a broad range of {\alpha}. If there have been no overturns of the continental crust until today, LHB could be responsible for the absence of Hadean rocks because most of Hadean continental crust is not be exposed on the Earth's surface in this case.

Australia's Lake Eyre Caused by Fossil Subduction Zone, to Disappear in 30 Million Years

Geoscientists have, for the first time, discovered the origins of Australia's two largest basins: Lake Eyre and the Murray-Darling Basin. The research also implies that in 30 million years' time both basins will cease to exist.

Monash University geoscientist Associate Professor Wouter Schellart, and his colleague Professor Wim Spakman from Utrecht University, have discovered how the floor of an entire ocean basin that was destroyed 70 to 50 million years ago off the North coast of New Guinea is currently located at 800-1200km depth below Central and South-eastern Australia.

Using supercomputers, the researchers found that this dense piece of ocean floor material (called a lithospheric slab) is slowly sinking into the Earth's mantle and is responsible for the formation of the Lake Eyre Basin, one of the Earth's largest internally drained basins and home to the lowest point in Australia at 15m below sea level, as well as the Murray-Darling Basin, home to the largest river system in Australia. With a combined surface area exceeding 2 million square kilometres, both basins are located directly above the deep mantle slab.

The research also predicts that in 30 million years from now, when Australia has moved about 1500km northwards, the fossil slab will be located below the Southern Ocean and, as a consequence, the Lake Eyre Basin and Murray Darling Basin will cease to exist.

Using geological and geophysical data from the New Guinea region, Schellart was able to reconstruct the geological evolution of the region over the last 70 million years, including the motion of the tectonic plates and plate boundaries. He discovered that the occurrence of deep ocean floor rocks, volcanic rocks and deformed rocks, which are currently found in the mountain ranges of New Guinea, point to the existence of a 4000km wide subduction zone. At subduction zones such as these, an oceanic tectonic plate sinks (subducts) into the Earth's interior, the mantle.

With these plate tectonic reconstructions Schellart was able to predict where the fossil subduction zone was during its lifetime some 50-70 million years ago, and therefore where the lithospheric slab disappeared into the mantle. With a global seismic tomography model that makes use of seismic waves to map the internal structure of the Earth's mantle, Schellart and Spakman were able to identify the fossil slab structure below central and south-eastern Australia at a location and depth predicted by the reconstructions.

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Is Continental Crust Lost During Supercontinents Assembly?

Increased loss of continental crust during supercontinent amalgamation

Authors:

Roberts

Abstract:

The volume of Earth's continental crust depends on the rate of addition of continental crust from the mantle compared to the rate of continental loss back to the mantle, which at present is roughly balanced. Models for the growth rate of continental crust vary, with isotope data suggesting various episodes of increased growth rate throughout Earth's history; these episodes have been correlated with the supercontinent cycle, but may be a consequence of preferential preservation of continental crust during these cycles. The global balance between addition and loss of continental crust is controlled by: 1) the extent of internal orogens versus exterior orogens, with the latter favouring continental addition, and 2) the balance between exterior orogens in retreating mode versus those in advancing mode, with the latter favouring continental loss. A greater balance of continental addition versus loss should exist during supercontinent break-up, due to a high magmatic flux in retreating accretionary orogens, whereas the amalgamation of supercontinents should involve increased continental loss due to increased sediment subduction and tectonic erosion. Zircon U–Pb and Hf isotopes provide insight to models of crustal growth rate since they sample the continental crust at their time of formation. Using the distribution of data within εHf(t)-time space of a global zircon database, it is demonstrated that the data are in accord with the concept of increased continental loss during supercontinent amalgamation. Periods featuring increased continental addition relative to continental loss, and hence increased continental crust growth rate, occur at ~ 1.7–1.2 Ga, ~ 0.85–0.75 Ga, and ~ 0.45–0.35 Ga, and follow the formation of the Columbia (Nuna), Rodinia and Gondwana supercontinents respectively. Distinct increases in continental loss compared to continental addition, i.e. decreased continental growth rate, occur at ~ 1.0–0.9 Ga, and ~ 0.6–0.55 Ga, correlating with the periods of Rodinia and Gondwana amalgamation respectively. Formation of Pangea by introversion rather than extroversion, means that continental addition in exterior orogens was concurrent with continental loss in interior orogens; a similar process may have been responsible for formation of the Columbia supercontinent. Peaks in the compilation of U–Pb zircon ages correlate with the timing of supercontinent amalgamation, and are likely to be a consequence of preferential preservation of continental crust during this part of the supercontinent cycle.