Book review

Exhumation Processes: Normal Faulting, Ductile Flowand ErosionEdited by Ring, U., Brandon, M.T., Lister, G.S.,and Willett, S.D. Geological Society SpecialPublication no 154. ISBN 1-86239-032-0. List price:89.00 (UK), $148.00 (US). GSL Price: 39.00 (UK),$65.00 (US).

This book provides an excellent summary of the cur-rent state of knowledge regarding extensional and ero-sional processes that exhume rocks. The editorsprovide an introductory chapter that summarizesmany of the types of processes that have been coveredin the literature and they present some of the unan-swered problems relating to exhumation. The followingpapers are individual studies of exhumed terrains fromall over the world. These papers are organized intothree sections that cover: (1) subduction-related set-tings; (2) collision-related settings; and (3) divergent orrift-related settings. It is convenient to have such acomprehensive compilation of exhumation research ina single volume. This book will prove to be invaluablefor all researchers of exhumed rocks and would makean excellent source book for a graduate seminar onthe topic.

In the introductory chapter Ring et al. review andsummarize the role of tectonics and the role of erosionin exhuming rocks. Terminology used in exhumationstudies is de®ned before the authors move on to dis-cuss exhumed rocks in settings of oceanic and conti-nental rifting, subduction zones and continentalcollision zones. The authors conclude that collisionalprocesses are more e�ective at exhuming rocks thanrifting processes. This paper also looks at the individ-ual role of processes such as normal faulting, erosion,and ductile thinning. Existing data suggests that ero-sion generally exhumes rocks at faster rates than otherprocesses, but that erosion is fastest in areas of activetectonism especially when the climate is wet. The e�-ciency of normal faulting for exhuming rocks dependson such things as fault angle, rate of extension etc.Ductile thinning is more di�cult to quantify. Themethods used in exhumation studies, such as thermo-chronology and thermobarometry are also summar-ized. A special section is dedicated to the discussion of

ultrahigh-pressure rocks, their settings, and mechan-

isms for their formation and exhumation. Finally the

authors present some of the unsolved issues related toexhumation such as: is there a relation between exhu-

mation rate and exhumation process? How well doultrahigh-pressure rocks preserve a record of their

exhumation? What is the relation between ultrahigh-pressure metamorphism and magmatism?

The ®rst section of the book includes ®ve papersthat deal with exhumation in subduction settings. Sed-

lock presents a discussion of the geology of a high-pressure, low-temperature terrain in Baja, Mexico. The

conclusion for this area is that extension dominatedfrom 97±30 Ma and normal faulting was the primary

mechanism responsible for exhuming the high-pressurerocks. It is not revealed why there is this extension in

a compressional subduction complex, though buoy-ancy is ruled out as a driving mechanism.

Ring and Brandon cover the exhumation within anaccretionary wedge in the Franciscan Complex of Cali-

fornia. Their method is strain analysis using mineral®bers. The resulting conclusion is that there has been

coaxial deformation with solution mass transfer as theprimary mechanism to accommodate this deformation.

The exhumation was dominantly due to erosion of a

forearc high.

In a paper focused on the high-pressure rocks ofCrete, Thomson, StoÈ ckhert and Manfred summarize

thermobarometric and thermochronologic data oflower plate rocks of a detachment system in a subduc-

tion setting. These authors conclude that retreating

subduction was responsible for the extension which,coupled with buoyancy, exhumed the high-pressure

rocks.

Rawling and Lister describe the tectonic setting ofNew Caledonia and the complex subduction history of

the region. Based on detailed structural analysis theypresent a model of alternating extension and com-

pression in New Caledonia that would have buried

continental and oceanic rocks and then brought themback to the surface.

Wintsch, Byrne and Toriumi worked in northwes-

tern Shikoku, Japan on the structure and metamorph-ism of the Sanbagawa blueschist belt. They found a

strike-parallel stretching lineation with a structurally

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underlying down-dip lineation. These structural obser-vations together with P-T-t determinations led theauthors to the interpretation of oblique northwardaccretion of Shimanto rocks synchronous with east-ward extrusion of the Sanbagawa rocks. The verticalcomponent of this extrusion exhumed these blueschistrocks.

The second section of the book (seven papers)focuses on exhumation in collisional settings. Thermaland mechanical modeling of P-T-t data from theLepontine region of the central Alps by Schluneggerand Willett addresses the area's exhumation history.Their numerical experiments suggest that for much ofthe last 40 million years convergence/accretion havebeen matched by erosional exhumation. The exceptionto this was the period 25±15 million years ago whentectonic exhumation dominated.

Vanderhaeghe, Burg and Teyssier compare corecomplex-type settings of the Canadian Cordillera andthe Variscides of the French Massif Central. They ana-lyze P-T estimates and sedimentologic/structural datain order to examine the metamorphic and erosionalhistories of each area. They deduce that erosionalexhumation dominated during compression and crustalthickening, but that each area then experienced heatingand melting of the deep crust which resulted in col-lapse of the orogen and tectonic exhumation of mig-matitic crustal rocks.

Calvert, Gans and Amato present new 40Ar/39Ardata for a region of the Seward Peninsula in Alaska.This region consists of amphibolite and granulite faciesgneissic rocks. The thermochronologic data suggestrapid cooling between 90 and 80 Ma. The authorsinvoke a model of diapiric emplacement for thesehigh-grade rocks. This idea contrasts with previous in-terpretations of exhumation due to compression orextension, but the author's base their interpretation ona lack of evidence for exhumation-related faults.

Glazner presents model results for numeric exper-iments relating pluton size, density, initial depth, anderosion rate. The goal of this modeling was to under-stand how and why dense plutons arrive at the earth'ssurface. Glazner's results indicate that the rise or fallof these intrusives are sensitive to these physical par-ameters and that this may explain why exposed ma®cintrusives are generally small compared to felsiccounterparts.

Miller, Gregory, Gray and Foster contribute new40Ar/39Ar ages for rocks from the Samail ophioliteregion of Oman. In this region a tectonic windowexposed a high-grade lower plate surrounded by ametamorphosed upper plate that is overlain by theSamail ophiolite. The 40Ar/39Ar ages for muscovitecome from both the lower plate and the upper plateand fall into three age categories: (1) 119±82 Ma forlower plate rocks; (2) 82±79 Ma for lower plate rocks

with lower-pressure mineral assemblages; and (3) 76±70 Ma for upper plate rocks. Though there is a possi-bility that older ages are the product of excess Ar, theauthor's favor a model where lower plate rocks wereexhumed (119±82 Ma) prior to thrust emplacement ofthe upper plate nappes (82±79 Ma). High level exten-sional activity ®nally exhumed upper and lower platestogether (76±70 Ma).

In order to investigate the exhumation history adja-cent to the Alpine fault of the Southern Alps, Batt,Kohn, Braun, McDougall and Ireland used U±Pb,Rb±Sr, K±Ar, 40Ar/39Ar, and ®ssion track methods onpegmatitic rocks to constrain the cooling history of aportion of the Southern Alps. The result was inferredemplacement of the pegmatites at 068±81 Ma, fol-lowed by slow cooling until 010 Ma when the rateaccelerated to 0188C/Ma until 05 Ma. At 5 Ma mod-elling of K-feldspar 40Ar/39Ar suggests a reduction incooling rate until 2 Ma and then an increase to03508C/Ma. In order to bring samples to the surface(00±208C) at the present there must have been anotherreduction in cooling rate during the last 1±1.5 Ma.The interpreted cooling path is consistent with anexhumation history resulting from increased plate con-vergence, surface uplift, and erosion during the last 5±10 Ma.

Garver, Brandon, Roden-Tice and Kamp use ®s-sion-track methods on samples from basin settings toexamine orogenic exhumation. They summarize how®ssion tracks are created and some of the consider-ations that are important when applying ®ssion trackmethods to detrital grains. The authors then outlinefour studies in di�erent mountain belts where ®ssiontrack grain ages were used to estimate timing of exhu-mation or rates of exhumation. These case studies aregiven very brief coverage, but they do illustrate, how-ever, that there are di�erent ways that exhumation in-formation can be gleaned from detrital ®ssion trackages and that there are limitations to the method aswell.

The third section of this book includes three papersthat discuss exhumation in extensional settings. Forsterand Lister present details of their mapping of exten-sional faults and shear zones on the island of Ios inthe Cyclades region of Greece. The authors documentseveral phases of extensional deformation followed bya doming of these structures much like many of thedetachment faults of the western U.S. Because therehave been several generations of extensional faultswith anastamosing geometries the resulting map pat-tern is quite complex. Forster and Lister suggest thatthe faults on Ios are part of the same system exposedon other Cyclades islands.

Goodwin takes an in-depth look at pseudotachylitefrom a well-characterized metamorphic core complex(South Mountains) in south-central Arizona. The goal

Book review / Journal of Structural Geology 22 (2000) 1523±15251524

of Goodwin's work is an understanding of the for-mation of this particular pseudotachylite and an evalu-ation of existing models of pseudotachylite formation.Evidence for deformation under greenschist facies con-ditions and the presence of cataclasite adjacent topseudotachylite led Goodwin to conclude that pseudo-tachylite formed above the brittle±ductile transition.Close inspection of the pseudotachylite (TEM andBSE), however, shows that the pseudotachylite waslocally overprinted by ductile processes. Goodwinsuggests that the reduced grain size in the pseudota-chylite region may have localized strain and favoredductile deformation in an otherwise brittle rock. Thisstudy also emphasizes rock composition and fabricorientation as important components of pseudotachy-lite formation.

Foster and John utilize new and existing thermo-chronologic data from several core complexes in orderto gain insight on the timing of initial extension, theslip rates, the paleo-geothermal gradient and the initialdepths of detachment faults in the Colorado Riverextensional corridor of the southern Basin and Rangeprovince. Temperature-time plots for 40Ar/39Ar and®ssion track ages for a variety of minerals indicate theonset of extensional exhumation at 022 Ma. A plot ofmineral age versus distance in slip direction also

suggests 20±22 Ma for the onset of exhumation. Theslope of age versus distance data for low-closure tem-perature minerals was used to determine slip rates.Resultant slip rates were on the order of 3±9 km/Mathough uncertainties were generally 1±4 km/Ma. Over-all these rates are consistent with independent geologicestimates. Apatite-zircon mineral pairs yielded calcu-lations of approximate geothermal gradients at theonset of extension of 17±258C/km. The authors madea contour map of paleotemperatures at 22 Ma basedon ®ssion track and 40Ar/39Ar data and by assuming ageothermal gradient they were able to determine thepaleodip on the Chemcheui detachment fault. Thispaper is a comprehensive look at thermochronologicdata and many of its uses in an extended terrain.

Overall, this book nicely covers a spectrum of exhu-mation settings. The papers included also present avariety of techniques that can be used to understandthe exhumation process. For anyone with an interestin exhumed rocks, this book should ®nd a place ontheir bookshelf!

Mary HubbardDepartment of Geology, Kansas State University,

Manhattan, KS 66506, USA

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