Old igneous rocks hold key to evolution of crustal thickness

Voice of editors is a blog from the AGU Publications Department.

One of the main goals of geology is to understand the formation and evolution of the Earth’s continental crust, including changes in thickness and elevation. Scientists can use the chemical composition of igneous rocks and their minerals to understand how the thickness and elevation of the crust has changed over time.

A recent article in Geophysics Opinion explores the use of chemical parameters to determine crustal thickness and elevation. Here, we asked the authors to explain the fundamentals of continental crust, the methods scientists use to determine thickness, and remaining research questions.

Simply put, what is continental crust and why is it important to understand its composition and evolution?

The chemical differentiation of rocky planets results in the formation of an outer layer, the crust. In this respect, the Earth is unique because it develops two contrasting types of crust: a thin and dense, rather homogeneous basaltic (silica-poor) oceanic crust, which is recycled into the mantle by subduction in less than a few hundred million years, and a thick, less dense and heterogeneous continental crust, on average andesitic (relatively rich in silica) which tends to survive for billions of years.

Repeatedly broken down and reassembled by plate tectonics, and reworked by igneous, metamorphic, and sedimentary processes, continental crust has interacted intensively with the atmosphere, hydrosphere, and biosphere over the eons. Unraveling the evolution of its composition, thickness and elevation is not only a fundamental goal of geology, but also has major implications for understanding the evolution and diversification of life, climate, as well as the distribution of various natural resources.

What is the “Mohorovicic discontinuity”?

Named after the Croatian geophysicist Andrija Mohorovičić who discovered it in 1909, the Mohorovicic or “Moho” discontinuity is marked by a noticeable change in the vertical velocity gradient of seismic waves, being considered in continental and oceanic environments as the boundary between the crust and the mantle.

The observed velocity contrast can be attributed to major compositional differences: the shallow mantle is largely dominated by mafic minerals, olivine and pyroxenes, in which seismic waves propagate rapidly, while crustal lithologies contain plus various mafic minerals varying proportions of felsic minerals, quartz and feldspar, which tend to slow the propagation of these waves considerably.

In general, the Moho is sharp and clear beneath oceans and ancient continental interiors and, due to ongoing tectonic and magmatic processes, more gradual beneath orogens developing at convergent plate margins.

Why is it difficult to quantify the thickness of ancient continental crust and its evolution?

Continental crust can be extensively and repeatedly reworked by tectonic and petrogenetic processes and by erosion, and therefore its thickness can change significantly over time. Because the geophysical methods employed to limit depths at Moho rely on directly observable physical properties of earth materials, such as density or seismic wave velocity, they provide valuable information about the current thickness of the crust (c i.e. the depth of Moho relative to the surface topography), but are blind to its past modifications.

The only practical way to quantify crustal thicknesses and their variations in the geological past is to find and exploit Moho-sensitive information recorded in crustal rocks with known formation ages. The biggest challenge in this approach is to identify the parameters that can provide sufficiently accurate and precise Moho depth estimates to track potential changes in crustal thickness over time.

How can igneous rock compositions help limit crustal thickness?

It has long been recognized that the concentrations of a range of chemical compounds in lavas emitted by young arc volcanoes along subduction zones correlate with the geophysically constrained thickness of the arc crust. Similar correlations have recently been identified for igneous rocks formed in orogens developed along continental collision zones. Moreover, detrital zircons from these rocks not only have compositions that correlate with crustal thickness, but can also be directly used to constrain the timing of their igneous crystallization and can retain all of this information long after their host igneous has decomposed by weathering and erosion.

Once established for young igneous rocks, these correlations can be applied to ancient igneous rocks and zircons of known chemical composition and age to quantify Moho depths beneath ancient mountain ranges that have experienced variations in thickness. in time. Importantly, because orogens tend towards isostatic equilibrium, mountain elevations correlate with Moho depths and therefore can also be estimated by the same approach.

Comparison of the Cretaceous paleotopography of the Northern Peninsular Ranges reconstructed using igneous rock chemistry (a) with the current smoothed topography of the same region (b). Although greatly reduced in elevation over the past >80 Ma, the current topography retains key features developed during the Cretaceous (note the different elevation scales for the two images). Credit: Luffi and Ducea [2022]figure 27

What is “chemical mohometry”?

By the term “chemical mohometry” we refer to all techniques in which the chemical parameters of rocks and minerals are used to estimate the depth of the Moho discontinuity. Currently, all of these approaches are based on observed empirical relationships between young igneous rocks and the geophysically constrained Moho depth beneath their location.

Representative surface of the chemical mohometer model: La/Yb ratios in combination with MgO (% by weight) in igneous rocks can be used to predict Moho depths. Credit: Luffi and Ducea [2022]Figure 19c

How have online geochemical databases advanced our understanding of crustal thickness?

Modern online geochemical databases collect the chemical compositions of various rock samples and their minerals studied around the world, which have been extracted from tens of thousands of research papers published over the past decades and can be mined for different purposes. In particular, these databases support the quantitative assessment of past orogenic crustal thicknesses and elevations in two fundamental ways.

First, they represent an unprecedented source for the exploration, selection and statistical treatment of large datasets of young igneous rocks, which, in combination with various geophysical models, allow us to formulate robust scale models. world linking various chemical parameters to the depths and elevations of Moho. the present.

Second, they host compositions of ancient igneous rocks and zircons which, when connected to these models, help us estimate the thickness and elevation of orogens from the geological past in which they formed.

Combining multiple mohometer models improves crustal thickness estimates for ancient orogens. The approach can be illustrated by the example of the well-sampled Peninsular Ranges Batholith (PRB) in northern Baja, Mexico and extreme southern California, which is characterized by significant east-west contrast in l thickness of the crust during the magmatism of the Cretaceous arc. Magmas to the east (E-PRB) settled into continental crust about 50 kilometers thick, while to the west (W-PRB) they penetrated thinner crust about 30 kilometers of island arc terranes. Credit: Luffi and Ducea [2022]Figure 26c

What are the key unresolved questions or knowledge gaps where further research, data or modeling efforts are needed?

Exactly how various mantle and crustal processes contribute to observed correlations between crustal composition and thickness is still poorly constrained and controversial, and thus chemical mohometry lacks a robust quantitative petrogenetic basis. Once quantified, such a base will allow further refinement of existing models and will be particularly useful for predicting the thickness of collisional orogens, to which the models developed for subduction zone magmatism are not always applicable, and for which the availability of data is naturally limited (there are far fewer young collisional orogens than subduction zones) hampers the improvement of specific empirical mohometric models.

Another problem is that, while online chemical databases have grown dramatically over the past two decades, they still lack data on many ancient orogens. Therefore, estimating the thickness of these crustal pieces may involve considerable effort to gather the required chemical data from the literature and/or new analyses.

—Peter Luffi ([email protected]; 0000-0001-9536-5405), Institute of Geodynamics and Geological Institute of Romania, Romania; and Mihai Ducea ([email protected]; 0000-0002-5322-0782), University of Bucharest, Romania, and University of Arizona, USA

Editor’s note: It is the policy of AGU Publications to invite authors of articles published in Reviews of Geophysics to write an abstract for Eos Editors’ Vox.

Quote: Luffi, P. and M. Ducea (2022), Ancient igneous rocks hold key to evolution of crustal thickness, Eos, 103, https://doi.org/10.1029/2022EO225028. Published September 7, 2022.
This article does not represent the opinion of AGU, Eos, or any of its affiliates. This is the author’s opinion only.
Text © 2022. The authors. CC BY-NC-ND 3.0
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