April 2015 Issue - Volume 11, Number 2

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Arc Magmatic Tempos

Scott R. Paterson and Mihai N. Ducea – Guest Editors

Overview

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2015 Topics

Overview

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Next Issue

2015 Topics

Thematic Articles

Arc Magmatic Tempos: Gathering the Evidence

In this issue of Elements we explore the characteristics, potential causes, and implications of episodic magmatism in arcs. A comparison of U–Pb bedrock and detrital zircon ages in arcs with independent calculations of volumetric magma addition rates (MARs) indicates that the former nicely track the episodic temporal histories of arc magmatism but not MARs. MAR estimates indicate that 100–1000 times more magmatism is added to continental arcs during fl are-ups than during lulls and result in plutonic/volcanic ratios of >30/1. Episodic arc magmatism may result from external forcing on arc systems caused by events outside the arc and/or from internal cyclic processes driven by feedback between linked tectonic and magmatic processes within the arc. Along and across arc strike, changes and asymmetries in magmatic, tectonic, and geochemical histories provide important constraints for evaluating these poorly understood driving mechanisms.

High-Volume Magmatic Events in Subduction Systems

By Mihai N. Ducea, Scott R. Paterson, Peter G. DeCelles

The world’s biggest Phanerozoic magmatic arcs formed above subduction zones and comprise the products of continuous magma emplacement into the crust over periods of up to 500 My. However, the intensity of magmatic activity can vary significantly. Punctuated magmatic events lasting from 5 to 20 My can dwarf the volume of magmas generated through the remainder of an arc’s history: these high-volume events are called “fl areups” and can completely rebuild an arc’s crust. In arcs formed on continental lithosphere, fl are-ups typically correlate with regional structural events that shorten and/or thicken the crust. Geochemical and isotopic signatures show that these high magmatic addition rate events involve ~50% recycled upperplate crust and mantle lithosphere; the remaining ~50% comes from the mantle wedge.

Magma Production Rates for Intraoceanic Arcs

By Brian R. Jicha, Oliver Jagoutz

Intraoceanic volcanic arcs have long been recognized as sites where continental crust is created. Yet, despite their importance to understanding magmatic systems and the evolution of our planet, very little is known about their long-term rates of magma production and crust formation. Constraining both crustal construction and destruction processes at intraoceanic arcs allows for improved estimates of magma production. Our revised magma production rates for active intraoceanic arcs are consistent with those calculated for mid-ocean ridge segments that have slow to moderate spreading rates. This is surprising because magma production at intraoceanic arcs has traditionally been assumed to be significantly less than that at mid-ocean ridges.

Quickening the Pulse: Fractal Tempos in Continental Arc Magmatism

By Shanaka L. de Silva, Nancy R. Riggs, Andrew P. Barth

The magmatic history of a continental arc can be characterized as punctuated equilibrium, whereby long periods of low-level activity are interrupted periodically by short bursts of high-volume magmatism (“fl are-ups”). Geochronological records, most notably from zircon, reveal episodicity in volcanism, pluton formation, and detrital sedimentation in, and associated with, arc segments and volcano-plutonic suites. Distinct tempos can be recognized at all resolvable spatial and temporal scales and are broadly fractal, with each scale reflecting the timescale of processes occurring at different levels in the arc crust. The tempos of continental arc magmatism thus reflect modulation of the mantle-power input as it is progressively filtered through the continental crust.

Collisional Processes and Links to Episodic Changes in Subduction Zones

By Jeroen van Hunen, Meghan S. Miller

Continental collision is commonly accompanied by a sequence of several plate–mantle interactions, including accretion of buoyant features, pulses of slab rollback, slab break-off, formation of slab windows, and lithosphere delamination. Using the combined insight from seismic and dynamical modelling studies, we illustrate how these processes and their characteristic rates and timescales played an important role in shaping the Mediterranean and how they dominated the closure of the Tethyan oceans. Older collisions, such as the one that formed the Norwegian Caledonites, probably experienced similarly complex plate–mantle interaction, even though direct evidence of the associated mantle dynamics is absent.

Global Continental Arc Flare-ups and Their Relation to Long-Term Greenhouse Conditions

By Cin-Ty A. Lee, Jade Star Lackey

Continents are long-term storage sites for sedimentary carbonates. Global fl are-ups in continental arc volcanism, when arc magmas intersect and interact with stored carbonates, thus have the potential for elevating the global baseline of deep Earth carbon inputs into the atmosphere, leading to long-lived greenhouse conditions. Decarbonation residues, known as skarns, are ubiquitously associated with the eroded remnants of ancient batholiths, attesting to the potential link between continental arc magmatism and enhanced global CO2 inputs to the atmosphere.