Pacific Rim Introd.pdf

Tectonophysics 464 (2009) 1

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Contents lists available at ScienceDirect

Tectonophysics

journal homepage: www.elsevier.com/locate/tecto

Preface

Interpreting the tectonic evolution of Paci

c Rim margins using plate kinematics and

slab window volcanism

The possibility that slab windows might form in the wake of ridge

subductionwas

suggesting asthenospheric

upwelling as a mechanism for both back-arc plateau lavas and

extension within the Scotia Sea plate.

Three papers consider the implications of slab windows in three

dimensions. One of these papers constructs 3D thermal and mechan-

ical models for the overriding plate at a subduction boundary (Groome

and Thorkelson, this issue) that will serve as a foundation for future

3D physics-based models. The 3D thermal model is used to investigate

transient heating patterns and resultant metamorphism due to ridge

subduction and slab-windowmigration through time. Thermal results

are used to constrain the rheology of a 3D mechanical model that

predicts deformation behavior in the overriding plate. Another paper

constructs a re

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rst conceptualized in the late 1970s as earth scientists

explored the implications of plate tectonic theory in three dimensions.

Acceptance of slab-windowoccurrence, however, has been a long time

coming (McCrory andWilson, this issue). With records of both current

and past slab-window transients now

rmly established through

detailed studies of volcanism and tectonism tied to kinematic plate

reconstructions, we are able to exploit these spatially and temporally

restricted events as paleogeographic markers. Slab-window research

encompasses and integrates a broad range of disciplines including

earthquake seismology, plate kinematics, volcanic geochemistry,

lithospheric dynamics, seismic tomography, and structural geology.

In this special issue we use this powerful new tool to constrain and

interpret the Cenozoic evolution of Paci

ned 3D tomographic model of p -wave velocities

c Rim subduction margins

from Alaska, to Patagonia, to Antarctica, to Tonga.

Tectonophysics seeks to foster international research in emerging

North American plate boundary (Biasi, this issue)

that considers evolution of the lithosphere through Cenozoic time.

Low velocity anomalies in the mantle are attributed to relict slab

windows. High velocity anomalies are attributed to one of two

sources: (1) detached lithospheric mantle roots beneath granitic

batholiths or (2) downwelling lithospheric mantle roots delaminated

from N

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c Rim with a series of studies tying

slab-window magma generation to kinematic plate motions, model-

ing slab-detachment processes, modeling margin evolution in three

dimensions, and examining the rheologic implications of elevated

heat-

rst to explore

the implications of slab-window heating on promoting the fault-block

style of deformation that marks the San Andreas transform boundary

in southern California (McCrory et al., this issue) and the effect of

microplate capture on triggering break-up of the continental margin.

This paper also offers two animated models showing how California

may deform 5 million years into the future based on current fault-

block motion. Another paper focused on the Paci

S converging batholiths. The third paper is the

ow events. The collection of papers represents an outgrowth of

a special session at the Fall 2006 American Geophysical Union (AGU)

meeting.

The following thirteen papers convey the breadth and caliber of

slab-window research. Three papers consider aspects of slab detach-

ment as a mechanism for initiating slab windows or slab tears. One of

these papers presents a 2D thermo-mechanical model of slab and

mantle rheology (Andrews and Billen, this issue) that de

North American

plate boundary employs a new stratigraphic facies methodology

(Burnham, this issue) to constrain the older fault history of San

Andreas fault system. This structural geology paper correlates early

Cenozoic conglomerate units that are derived in part from volcanic

rocks which erupted above the paleogeographically-restricted slab

windows associated with subduction of the East Paci

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nes expected

behavior for relatively strong and weak slabs. The paper provides both

conceptual and physical models of the dynamics of slab detachment by

two failure modes

yielding (shear) and necking (thermal). Another

paper uses relocated seismicity and stress orientations inferred from

moment tensors to map the geometry of a deep detached slab

fragment beneath Tonga (Bonnardot et al., this issue). This paper also

identi

—

c Rise system

region since 55 Ma accompanies a paper describing its plate kinematic

history (Eagles et al., this issue) based on matching oceanic free-air

gravity anomaly patterns and fracture zones combined with

attributed to asthenospheric upwelling. The third paper traces the

con

nite

rotations of plate pairs, isochrons, and reconstructed plate boundaries.

The kinematic reconstruction tracks the location and timing of ridge

subduction ( i.e. , slab-window formation) and the occurrence of alkali

basalt volcanism. The apparently long lag time between initial slab-

window formation and the oldest known volcanic rocks along the

Antarctic Peninsula argues for continued

guration of late Cenozoic slab windows beneath Patagonia and

the Antarctic Peninsula (Breitsprecher and Thorkelson, this issue), and

ties the overlying volcanic rocks to reconstructed slab-window

positions via their geochemistry. These results support previous

research suggesting a ca . 12 Ma tear in the Nazca slab north of the

‘

slab window. This paper also draws a correlation between

adakitic volcanoes within the Austral Volcanic Zone (AVZ) and the

’

eld work in this challenging

region to more fully constrain the initiation time of volcanic pulses.

see front matter. Published by Elsevier B.V.

doi: 10.1016/j.tecto.2008.10.017

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eastern edge of the Antarctic plate

beneath the Paci

elds through dissemination of new results in special issues such as

this one. This special issue features important progress in tectonic

reconstructions around the Paci

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before being offset great distances by fault motion.

An animated plate tectonic reconstruction of the South Paci

es an adjacent deep slab tear along with high heat

Patagonia

0040-1951/$

Preface

Five papers consider the relationship between magmatism in

southern Alaska, southwestern Canada or western US, and the location

of slab-window transients during the early Cenozoic. One of these

papers examines west-to-east geochemical and isotopic trends within

plutons of the Sanak

fth paper examines trace-element

and isotopic variations within unusual intrusive rocks of the Eocene

‘

belt (Duke, this issue) to infer a mixture of magma sources

from (1) subducted Kula slab and associated metasedimentary rocks,

(2) preCambrian continental mantle, and (3) asthenospheric or lower

mantle. The paper correlates changing magma composition through

time with the inferred migration of a Kula

’

Baranof belt in southern Alaska during

progressive subduction of a segmented ridge system (Farris and

Paterson, this issue) which indicate an increase in degree and duration

of heating attributed to changes in slab-window development.

Signi

–

Farallon slab window

in particular, passage of the Kula slab edge

through the mantle transition zone followed by mantle upwelling to

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cant gaps between plutons in the chain are attributed to

intervals when fracture zones were being subducted rather than

ridge segments. This paper presents a novel approach to mapping the

shape of a slab window using the distribution of plutonic centers as

well as offering an indirect estimate of the convergence rate. Another

paper provides a detailed examination of geochemical and isotopic

variations within the Paleocene Kodiak batholith of the Sanak

ll the slab gap. The paper proposes that the interaction of the slab

edge with transition zone mantle is the primary mechanism for

producing kimberlite and carbonatite melts, and further proposes

concurrent continental extension as the primary mechanism allowing

these melts to traverse the thick continental lithosphere to the surface.

The thirteen papers described brie

Baranof

belt (Ayuso et al., this issue) to test whether magma sources are

compatiblewith subduction of a ridge system. The authors do attribute

the geochemical and isotopic trends to a migrating slab window that

yields a complex mix of crustal and asthenospheric magma sources.

The third paper traces the origin and mixing of forearc magmas in

southern Alaska and western California (Cole and Stewart, this issue)

by carefully analyzing geochemical and isotopic data to con

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y above provide guidelines for

recognizing slab tears and slab detachments in the geologic record, for

recognizing the geochemical

ngerprint of slab-window magmatism,

and for constraining new global reconstruction models. The anima-

tions that accompany two of the papers offer an intuitive means to

examine slab-window formation through Cenozoic time, to forecast

tectonic deformation into the future, and to identify gaps where

further investigation would prove fruitful. Finally, this special issue

lays the framework for the continued development of 3D thermal and

mechanical models of slab-window transients and testable hypoth-

eses regarding surface manifestation of their passage.

rm their

genesis in slab-window settings. In these cases, the strong depleted

mantle signature of the basalts implies little interaction with crustal

rocks as the magma ascended, supporting the necessity of crustal

extension to provide conduits that minimize crustal assimilation. This

paper also documents lag times of several million years between the

development of a slab window and the initiation of overlying

volcanism at several locations around the Paci

c Rim, suggesting

such lag periods may result frommagma that is trapped at the base of

the continental crust until it undergoes an extensional episode. In

southwestern British Columbia, isotopic dating and geochemical

analysis of volcanic rocks in the Princeton Group (Ickert et al., this

issue) are used to identify magma sources and pathways through the

continental crust. The authors' novel interpretation of the origin of

primitive adakites from melting of Mesozoic basaltic dikes in the

lithospheric mantle

Patricia A. McCrory

U.S. Geological Survey, Menlo Park, CA, USA

Corresponding author. Tel.: +1 650 329 5677; fax: +1 650 329 5163.

E-mail address: pmccrory@usgs.gov .

Douglas S. Wilson

Department of Earth Science, University of California,

Santa Barbara, CA, USA

E-mail address: dwilson@geol.ucsb.edu .

rather than from the more commonly proposed

melting of meta-basaltic slab

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21 October 2008

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required careful comparisonwith index

rocks from those two settings. The

Black Hills

beneath the region

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