Research Article How the Mariana Volcanic Arc ends in the …rjstern/pdfs/SternTIA13.pdf(Bird 2003),...

Research ArticleHow the Mariana Volcanic Arc ends in the south

ROBERT J. STERN,1* YOSHIHIKO TAMURA,2 HARUE MASUDA,3 PATTY FRYER,4

FERNANDO MARTINEZ,4 OSAMU ISHIZUKA,5 AND SHERMAN H. BLOOMER6

1Geosciences Department, University of Texas at Dallas, Box 830688, Richardson, Texas 75083-0688, USA (email:[email protected]), 2Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Institute forResearch on Earth Evolution (IFREE), Yokosuka 237-0061, Japan, 3Department of Geosciences, Osaka CityUniversity, 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558-8585 Japan, 4Hawaii Institute of Geophysics and

Planetology, University of Hawaii, Honolulu, Hawaii, USA, 5Institute of Geoscience, Geological Survey of Japan/AIST, Tsukuba 305-8567, Japan, and 6Department of Geosciences, Oregon State University, 128 Kidder Hall,

Corvallis, Oregon 97331, USA

Abstract The southern Mariana Arc–Trench system is rapidly deforming, resulting inunusual interactions between arc and back-arc basin (BAB) magmatic systems. Newgeochemical data for volcanoes in this region are presented and explored. Tracey Sea-mount, an extinct submarine volcano about 30 km northwest of Guam, is the southernmoststratovolcano of the Mariana Arc. Tracey is built about 125 km above the subducted slaband has erupted a bimodal suite of typical arc mafic and felsic lavas as recently as0.527 � 0.023 Ma (40Ar/39Ar age). An unusual cluster of small basaltic volcanoes, informallytermed the Alphabet Seamount Volcanic Province (ASVP), is found about where the nextarc volcano to the southwest of Tracey Seamount should have grown. Samples from six ofthese volcanoes were studied here. At least two ASVP volcanoes were recently active, asshown by hydrothermal activity. The lack of magmatic focusing to build a single stratovol-cano where the ASVP is situated reflects strong extension in the BAB. Construction ofnorthern ASVP volcanoes is controlled by east–west extension accompanying opening ofthe Mariana Trough. In contrast, southern ASVP volcanoes are affected by north–southextension due to rapid rollback of a narrow slab of Pacific seafloor that is subducted alongthe east–west trending Challenger Deep segment of the Mariana Trench to the south.ASVP lava compositions are distinct from Tracey Seamount and other Mariana Arc lavas,instead showing affinities with Mariana Trough BAB basalt (BABBs): they are mafic,tholeiitic, low-K2O and LREE-depleted, with low 87Sr/86Sr, but show a subtle gradient fromsomewhat more arc-like lavas closer to the trench to BABBs farther west. The unusualtectonic setting of ASVP provides a unique perspective on how different arc magmabatches reflect melting of mantle with strong compositional gradients which are mixedtogether beneath long-lived arc volcanoes but here rise to form scattered small ASVPvolcanoes.

Key words: back-arc basin, basalt, Mariana Arc, Mariana Trough.

INTRODUCTION

Subduction zones are where altered oceanic crust,mantle lithosphere, and sediments sink into hotasthenospheric mantle, and progressively release

fluids and melts, which rise up into the overlyingmantle wedge to generate arc magmas. The Izu–Bonin–Mariana (IBM) arc system is a goodexample of an intraoceanic arc system constructedabove an active subduction zone (Stern 2010).Along most of IBM’s 3500 km length there is aregular arrangement of trench, about 200 km-wideforearc, active magmatic arc (volcanic front), and

*Correspondence.

Received 15 July 2012; accepted for publication 5 October 2012.

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Island Arc (2013) 22, 133–148

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back-arc region. As is typical of arc systems, theback-arc region may be extensional, forming riftsor a back-arc basin with seafloor spreading, or itmay have volcanic cross-chains, as is found behindthe Izu Arc. The approximately 1500 km-longMariana convergent margin has an activelyspreading back-arc basin (BAB; Fig. 1a) known asthe Mariana Trough. Elsewhere in the Marianas(and in other mature arc – back-arc basinsystems), the magmatic arc and back-arc basinspreading ridge are separated by several tens to100 or more kilometers and erupt lavas that arechemically and isotopically distinct. A third volca-nic style, arc cross-chains, consisting of alignedvolcanoes extending into the back-arc region, isalso seen in IBM. An unusual nest of small volca-noes that is different from all of these typical con-vergent margin volcanic styles is found in thesouthern Marianas (Fig. 1b) and is the focus of thisreport.

The southern Marianas is a region of complexdeformation and enhanced magmatic flux, reflect-ing the combined effects of upper plate extensionand subduction-related hydrous fluxing. This is alsowhere the trench separating the Pacific Plate sub-ducts orthogonally beneath the easternmost Phil-ippine Sea (Mariana Plate) at about 30 mm/year(Bird 2003), and where the Mariana Trench bendsto the west to form the ultra-deep Mariana Trenchsegment (Fig. 1b). Fryer (1995) first noted thatthe shoaling of the Mariana Trough south of about14°N reflected a fundamental change in the tecton-ics of the basin. Subsequent studies confirm thatthe southernmost Marianas (Fig. 1b) is the mostrapidly deforming part of the IBM arc system(Kato et al. 2003). Extension in the Mariana Troughsouth of Guam is accomplished by seafloor spread-ing along the Mariana Trough spreading axis anddiffuse extension east of the ridge. Modeling ofgravity and bathymetry data shows that the

(b)(a)

Fig. 1 Location of the study area. (a) Bathymetric map of the Mariana convergent margin in the Western Pacific, including Mariana Trench, Mariana Arc,and Mariana Trough (back-arc basin). Map was compiled from available bathymetric data including decimated one-minute grid for Mariana Trough (Kitadaet al., 2006). Swath-mapped bathymetry is recompiled and matched to predicted bathymetry from Sandwell and Smith (1997). Red line is back-arc basinspreading axis from Martinez and Taylor (2003). Islands are black, the largest and southernmost is Guam, USA. Dashed box outlines region shown in (b).(b) Bathymetry of the southern Mariana Trough, showing location of spreading ridge (inflated Malaguana–Gadao Ridge (MGR) in the south, axial rift ofMariana Trough spreading ridge (MTSR) farther north) and Fina Nagu Volcanic Chain (FNVC), also West Santa Rosa Bank Fault (WSRBF), SoutheastMariana Forearc Rift (SEMFR), Challenger Deep (CD), and Shinkai Seep (SS). The active magmatic arc southwest of Tracey Seamount is poorly constraineddue to the unknown age of arc-like features (e.g. FNVC), but active volcanism is present from Tracey Seamount in the northeast to Toto caldera. Most dataare from ‘Law of the Sea’ mapping carried out by the University of New Hampshire group with additional data from US-NGDC and JAMSTEC. Dashed boxshows study area in Fig. 2.

134 R. J. Stern et al.

© 2012 Wiley Publishing Asia Pty Ltd

Mariana Trough south of about 14°N has thickercrust (6–7 km) than the back-arc basin (BAB) to thenorth due to an enhanced magmatic flux (Kitadaet al. 2006). In this region the slowly and stablyopening central Mariana Trough BAB changessouthward into a rapidly and complexly opening,magma-rich system. The BAB spreading centerchanges from an axial rift in the north into theinflated Malaguana–Gadao Ridge (MGR; Fig. 1b).

The volcanic group of interest lies between theMariana Trough spreading ridge and the southernend of the magmatic arc at Tracey Seamount, in aregion that is affected by diffuse extension (Fig. 2).We informally term this the ‘Alphabet SeamountVolcanic Province’ (ASVP) because several of thesevolcanoes have only a letter for their name. Here wepresent new geochemical data for six ASVP volca-noes along with data for the southernmost largevolcano of the Mariana Arc (Tracey Seamount). Wediscuss the implications of these data for under-standing arc and back-arc basin igneous activity ingeneral and the magmato-tectonic evolution of thesouthern Mariana Trough in particular.

TRACEY SEAMOUNT AND THE ALPHABETSEAMOUNT VOLCANIC PROVINCE

The study area lies between 13°40′N and 13°N,between the MGR and a major north–south fault,

the West Santa Rosa Bank Fault (WSRBF).WSRBF can be traced as a >5 km scarp south of13°N, diminishing in relief northwards until it isreplaced by a northeast-trending fault scarp southof Tracey Seamount (Fig. 1b). Tracey Seamount isthe southernmost arc volcano of the Mariana Arcand is part of the Southern Seamount Province(SSP) of Bloomer et al. (1989). It was the first sub-marine feature in the Mariana Arc to be studied(Tracey et al. 1964) and was investigated using R/VNatsushima and ROV HyperDolphin duringNT0902 in 2009 (Fig. 3).

Seamounts A, B, C, and X were first studied byFryer et al. (1998), who noted that in terms ofNa2O and TiO2 vs MgO, the western seamounts (Aand B) are more like back-arc basin basalts(BABB-like) compared to the eastern seamounts(X and C), which erupt more arc-like compositions.Gamo et al. (1997) named Seamount B ‘Forecast’and documented its hydrothermal activity. Bakeret al. (2008) named seamounts Z and Y andreported the presence of a hydrothermal plumeabove Seamount X. Seamounts Z and Y weredredge-sampled during the Cook 7 expeditionaboard R/V Melville in March–April 2001. Study-ing samples from these six volcanoes is a usefulstarting point for understanding the ASVP. TheMariana Trough back-arc basin spreading axis inthe study area (Fig. 2a) was sampled in threeplaces by wax-coring during Cook 7. Pearce et al.

(b)(a)

Fig. 2 (a) Multibeam bathymetry and (b) HMR-1 sidescan sonar maps of the study area showing sample localities. The bathymetry is largely from theUniversity of New Hampshire ‘Law of the Sea’ project with some additional data from NGDC. Locations of studied Seamounts A, Forecast, C, Z, Y, and Xare shown. Sites of known hydrothermal activity (red stars) are from Baker et al. (2008). Location of mafic glasses (GL-1, GL-2, and GL-3) from MarianaTrough spreading axis collected during Cook 7 (C7) expedition reported by Pearce et al. (2005) are shown with red dots. MGR marks northern end ofinflated Malaguana–Gadao Ridge; line ‘MC’ marks location of seismic profile over magma chamber of Becker et al. (2010). Dashed line approximate depths(in km) to top of subducted Pacific Plate, after Syracuse and Abers (2006). HMR-1 sonar side-scan data were collected in 1997 on R/V Moana Wave cruiseMW9719 with high backscatter indicated by darker shades. Note that linear features on Tracey Seamount are likely multibeam artifacts.

How the Mariana Volcanic arc ends 135


(2005) reported major and trace element composi-tions of these mafic glasses, and isotopic data werereported by Woodhead et al. (2012); these data arelisted for comparative purposes in Table 1. Sea-mount X, the largest ASVP volcano, is a simplecone that rises to < 1300 m summit from anapproximately 10 km-diameter base. It is some-what elongated east–west but mostly shows subtlestructural control limited to its western flank,where its smooth slopes are disrupted by north–south and NNW–ESE structures. It was sampledduring Shinkai 6500 Dive 186. Seamount C is anunusual ‘bird-like’ edifice with a shield-like mor-phology and well-developed crater; it rises to< 2700 m water depth. It is about 6 km in diameterfrom north to south but has approximately 6 km-long ridges that extend from the shield westtowards the spreading ridge and east into a zone ofESE-trending ridges generated in response to anapproximately north–south directed extension. Itwas sampled during Shinkai 6500 Dive 189. Sea-mounts C and X lie 50 and 70 km, respectively,southwest from Tracey Seamount, about where thenext arc volcano should be located along the volca-nic front (Bloomer et al. 1989). Seamount A is thesmallest edifice considered here. It lies a few kilo-

meters east of the spreading ridge, is about 1 kmacross and 500 m tall, and is slightly elongatedENE–WSW. It was sampled during Shinkai 6500Dive 160. Forecast (Seamount B of Fryer et al.1998) lies athwart a major NNE-trending struc-ture, which appears to be a fault scarp to the southand a ridge-like construction to the north, whereit connects Forecast and Seamount Z. Forecast isconical, with a base that is about 7 km across,rising to < 1600 m water depth. It was sampledduring Shinkai 6500 Dive 161. Seamount Z is thesecond smallest edifice studied, a conical volcanowith a relatively undisturbed eastern flank butfaulted western slope. It is about 8 km across itsbase and rises to < 2400 m water depth. It wassampled by dredging during Cook 7 D1. Sea-mount Y is a complex volcano. Its older part has acircular outline, about 7 km in diameter, with steepsides that rise about 500 m from the seafloor. Thiscircle is cut by an approximately north–south ridgethat rises to about 2300 m and by associated faults.This volcano is clearly affected by strong east–west directed extension. It was sampled by dredg-ing during Cook 7 D2.

The study area is underlain by the northwest-dipping subducted Pacific Plate, about 50 km deep

Fig. 3 Bathymetric maps of Tracey Seamount, from Mariana Bathymetric Compilation (Susan Merle, PMEL/NOAA, compiler). (a) Map of entire edifice,with location of detail shown in (b) in dashed box. Location of Tunes 7 Dredge 84 (R/V T.G. Thompson, 1992) also shown (dotted line). (b) Tracey Seamountsummit region, with track of HyperDolphin Dive 949 and location of samples reported in Table 2. Note that samples R01 to R07 were collected from westslope of cone in summit crater and samples R08 to R15 were collected during traverse up crater wall to summit.



Tabl

e1

Geo

chem

istr

yof

Alp

habe

tSo

upV

olca

nic

Pro

vinc

ean

dne

arby

Mar

iana

Tro

ugh

spre

adin

gax

isla

vas

Fea

ture

Smt

AF

orec

ast

Smt

XSm

tC

Smt

ZSm

tY

Spre

adin

gA

xis

glas

ses

Sam

ple

160-

216

1-6

186-

318

9-1

189-

6-2

C7

D1-

2C

7D

2-1

Coo

k7

GL

-1C

ook

7G

L-2

Coo

k7

GL

-3

Lat

itud

e(°

N)

13°1

4.12

′N13

°23′

N13

°14′

N13

°09′

N13

°08′

N13

°28.

5′N

13°3

0.54

′N13

°19.

8′N

13°2

8.8′

N13

°38.

4′N

Lon

gitu

de(°

E)

143°

45.3

5′E

143°

56′E

144°

01′E

144°

48′E

144°

01′E

144°

03.9

′E14

3°58

.44′

E14

3°45

′E14

3°48

′E14

3°52

.2′E

Dep

th(m

)33

0016

00~1

300

~320

0~2

600

3000

2825

3220

3322

3346

Slab

dept

h(k

m)

155

165

130

130

130

150

180

170

200

210

SiO

253

.753

.44

56.0

257

.92

53.5

48.3

152

.34

55.9

550

.69

51.4

TiO

21.

981.

271.

011.

640.

740.

661.

271.

950.

961.

9A

l 2O

313

.86

14.3

614

.34

13.0

1816

.121

.09

16.5

414

.06

16.3

315

.13

FeO

*11

.65

10.2

610

.62

11.5

28.

557.

349.

5311

.45

8.32

10.5

2M

nO0.

240.

230.

230.

20.

170.

140.

180.

220.

170.

19M

gO3.

695.

314.

062.

015.

56.

145.

052.

876.

725.

66C

aO7.

659.

888.

826.

8210

.610

.82

10.3

36.

6812

.47

10.4

8N

a 2O

3.77

2.69

2.51

1.67

2.1

2.05

3.21

3.68

2.21

3.37

K2O

0.3

0.31

0.42

0.48

0.18

0.3

0.55

0.34

0.18

0.15

P2O

50.

250.

20.

190.

270.

060.

080.

130.

190.

080.

17To

tal

97.0

997

.95

98.2

295

.548

97.5

96.9

399

.13

97.3

998

.13

98.9

7M

g#35

.847

.740

.223

.553

.159

.648

.330

.658

.748

.6F

eO*/

MgO

3.16

1.93

2.62

5.73

1.55

1.20

1.89

3.99

1.24

1.86

Ti(

ppm

)11

868

7612

6054

9830

4436

3956

7612

1168

857

5411

389

K(p

pm)

2490

2573

3486

3984

1494

2490

4565

2822

1494

1245

Trac

eel

emen

ts(p

pm)

Sc33

38.7

40.5

33.7

36.7

34.6

28.8

37.3

35.7

V39

631

438

933

429

430

035

323

929

7C

r9.

123

9.5

9.5

614

7816

411

0C

o33

.531

33.4

31.8

36.6

38.2

30.5

35.6

35.3

Ni

10.7

19.6

12.9

14.3

2341

6512

095

Cu

46.4

116

152

63.7

123

7047

8951

Ga

18.4

15.8

15.8

16.2

14.7

17.6

18.4

13.9

16.4

Rb

4.29

4.73

6.09

5.24

3.4

5.7

4.1

2.6

1.8

Sr15

7.5

194

194

170

137.

818

814

716

613

6Y

3524

.523

.329

.819

.927

39.5

19.8

37.1

Zr

109

5947

.467

.539

.678

118

49.3

117.

5N

b2.

561.

380.

861.

230.

561.

92.

460.

981.

95B

a39

.767

.410

276

.144

4754

3319

La

4.31

3.28

2.82

3.34

1.73

3.53

4.91

2.65

4.11

Ce

12.0

28.

336.

888.

64.

5410

.113

.83

6.95

12.8

1P

r2.

071.

441.

181.

520.

831.

722.

401.

222.

27N

d10

.86

7.59

6.29

8.26

4.73

8.8

12.3

16.

2211

.88

Sm3.

662.

522.

182.

831.

742.

824.

032.

063.

88E

u1.

270.

920.

801.

000.

661.

051.

390.

791.

39G

d4.

973.

413.

073.

992.

573.

525.

052.

674.

99T

b0.

930.

640.

590.

750.

50.

630.

920.

470.

88D

y5.

663.

933.

674.

653.

14.

166.

243.

195.

92H

o1.

260.

880.

841.

050.

720.

91.

360.

671.

28E

r3.

472.

432.

352.

921.

992.

583.

881.

943.

64T

m0.

40.

630.

30.

57Y

b3.

442.

432.

422.

982.

082.

573.

961.

983.

59L

u0.

540.

390.

390.

470.

330.

40.

630.

30.

55H

f2.

821.

71.

462.

011.

242.

023.

081.

362.

92Ta

0.14

0.17

60.

071

0.15

1P

b1.

061.

261.

911.

511.

050.

861

1.2

1.1

Th

0.29

0.26

0.28

0.30

0.12

0.24

0.33

0.23

0.19

U0.

120.

130.

160.

170.

070.

170.

181

0.1

0.10

287

Sr/86

Sr0.

7029

30.

7028

30.

7033

60.

7032

40.

7032

40.

7030

110.

7031

60.

7027

5114

3 Nd/

144 N

d0.

5131

000.

5130

860.

5131

060.

5130

910.

5130

950.

5131

070.

5131

000.

5131

21eN

d9.

18.

89.

28.

99

9.2

9.1

9.4

206 P

b/20

4 Pb

18.5

0518

.839

18.8

6518

.856

18.7

3618

.494

18.6

2718

.131

207 P

b/20

4 Pb

15.5

5615

.546

15.5

4815

.534

15.5

3215

.495

15.4

7915

.459

208 P

b/20

4 Pb

38.2

1338

.418

38.3

8538

.361

38.2

5438

.094

38.1

4337

.782



beneath the southeast part of the study area,> 200 km deep in the northwest, beneath thenorthern part of the Mariana Trough spreadingaxis (Fig. 2a).

ASVP edifices developed in response to complexextensional stresses in the region. Forecast andSeamount Z have rifts aligned approximatelynorth–south sub-parallel to the spreading centerbut rifts on Seamount C are approximately east–west, orthogonal to this trend. Seamount C showswell the junction separating the approximatelyeast–west Mariana–Philippine opening to thenorth from the more complex and tectonicallyactive region to the south. These differing struc-tural controls suggest that stress in this part of theMariana Trough changes from east–west exten-sion in the north to north–south extension to thesouth. Deformation is limited to the Mariana Plate,east of the spreading ridge, and continues all theway to the Challenger Deep segment of theMariana Trench. The Philippine Plate to the westis unaffected by this deformation apparentlybecause the spreading axis decouples it fromstresses affecting the Mariana Plate (Martinezet al. 2000). So Seamount C is near a triplejunction-like setting except that the Mariana‘plate’ to the south is pervasively deforming and isnot really a rigid plate. Shearing deformation isindicated by the seafloor fabric as well as the dis-tribution of seismicity and the abundance of strike-slip focal mechanisms.

There are no radiometric ages for ASVP lavas.Active hydrothermal vents on Forecast and Sea-mount Z implies that these are active or dormantvolcanoes. The generally high sonar backscatterover the ASVP (Fig. 2b) indicates that there islittle sediment cover, implying recent volcanism.Lighter backscatter areas with a fine radialpattern over the summit of Seamount X suggestvolcaniclastic eruptions. ASVP must be youngerthan the BAB crust on which it is built. Theopening of the Mariana Trough began before 5 Maat 18°N (Scott et al. 1981; Masuda et al. 1994), butmagnetic data suggest that opening began morerecently in the south (~3.0 Ma at 13°N; Ishiharaet al. 2001); therefore ASVP lavas must beyounger than 3–5 Ma.

Tracey Seamount is a simple cone that risesfrom a base approximately 3 km deep to a summit< 600 m deep (Fig. 3). It is by far the largestvolcano discussed in this report, but with a volumeof about 250 km3, about 10% of the size of largeMariana Arc volcanoes like Pagan and Agrigan(Bloomer et al. 1989). Its summit region contains a

small crater, which hosts a felsic plug or resurgentcone, apparently Tracey Seamount’s last activity.

The arc magmatic front defined by Tracey Sea-mount lies about 120–130 km above the subductedslab. This is indistinguishable from the approxi-mately 135 km slab depth beneath central andsouthern Mariana Volcanic Front (Syracuse &Abers 2006). Seamounts X and C and the MGR lieabout 120–130 km above the subducted slab. Sea-mounts A, Forecast, and Z lie about 150–160 kmabove the slab, and Seamount Y lies about 180 kmabove the slab. As a result, the MGR captures thearc magmas that usually rise beneath Mariana arcvolcanoes (slab depth ~100–150 km; Syracuse &Abers 2006), resulting in a well-defined ridge axiswith intense magmatism and hydrothermal activ-ity (Fryer et al. 1998; Baker et al. 2008), and aseismically-imaged axial magma chamber (Beckeret al. 2010).

ANALYTICAL METHODS

Samples have been analyzed by several differentlaboratories. C7 D1-2 and C7 D2-1 (Table 1) wereanalyzed for major elements by ACTLABS and fortrace elements using inductively coupled plasma–mass spectrometry (ICP-MS) facilities at Univer-sity of Texas, Dallas (UTD) following proceduresoutlined in Stern et al. (2006). Tracey Seamountsamples (HPD949 samples in Table 2) were ana-lyzed at JAMSTEC by XRF for major andselected trace elements and isotopic compositionsof using procedures outlined in Tamura et al.(2011). Major and trace element analyses ofsamples 160-2, 161-6, 186-3, 189-1, and 189-6-2were analyzed at the University of Hawaii usingstandard inductively coupled plasma–atomic emis-sion spectroscopy (ICP-AES) and ICP-MS analyti-cal procedures. Isotopic compositions of Sr, Nd,and Pb for ASVP samples in Table 1 were analyzedat UTD using procedures outlined by Stern et al.(2006). 87Sr/86Sr are adjusted to a value of 0.70800(multiple analyses yielded 0.70806 � 3 (1 s) forthe Eimer and Amend (E&A) standard, multiple143Nd/144Nd standard analyses yielded a meanvalue of 0.511863 � 12, and multiple analyses ofNBS-981 Pb standard yielded mean values of206Pb/204Pb = 16.944 � 8, 207Pb/204Pb = 15.500 � 9,and 208Pb/204Pb = 36.743 � 31). Total processingblanks were negligible (< 0.1 ng Sr, < 0.3 ng Nd,and < 0.3 ng Pb).

One whole-rock sample of dacite from TraceySeamount (HPD949R7) was analyzed for 40Ar–39Ar



Tabl

e2

Geo

chem

istr

yof

lava

sfr

omsu

mm

itof

Tra

cey

Seam

ount

Sam

ple

HP

D94

9R01

HP

D94

9R03

HP

D94

9R04

HP

D94

9R05

HP

D94

9R06

HP

D94

9R07

HP

D94

9R08

HP

D94

9R10

HP

D94

9R11

HP

D94

9R12

Lat

itud

e(°

N)

13°3

8.07

′N13

°38.

05′N

13°3

8.05

′N13

°38.

05′N

13°3

8.03

′N13

°38.

02′N

13°3

7.95

′N13

°37.

96′N

13°3

7.97

′N13

°37.

98′N

Lon

gitu

de(°

E)

144°

21.5

69′E

144°

23.6

13′E

144°

23.6

17′E

144°

23.6

17′E

144°

23.6

48′E

144°

23.6

72′E

144°

24.1

23′E

144°

24.1

75′E

144°

24.2

10′E

144°

24.5

29′E

Dep

th(m

)13

4212

8412

7412

455

1212

1190

1242

1180

1135

1098

Slab

dept

h(k

m)

130

130

130

130

130

130

130

130

130

130

SiO

254

.94

70.2

570

.37

70.6

169

.53

68.7

354

.33

54.8

454

.94

52.7

2T

iO2

0.7

0.36

0.36

0.36

0.36

0.37

0.71

0.71

0.7

0.72

Al 2

O3

17.4

914

.48

14.6

14.5

14.3

614

.64

17.9

817

.59

17.5

418

.29

FeO

*7.

342.

82.

862.

852.

93.

187.

417.

487.

417.

61M

nO0.

150.

10.

10.

10.

10.

10.

150.

180.

150.

14M

gO4.

360.

991.

030.

930.

951.

184.

494.

44.

264.

94C

aO9.

413.

563.

73.

553.

563.

939.

669.

529.

299.

74N

a 2O

2.65

4.27

4.17

4.24

4.11

4.19

2.55

2.59

2.55

2.01

K2O

0.48

1.17

1.21

1.17

1.34

1.04

0.46

0.46

0.49

0.45

P2O

50.

090.

080.

080.

080.

080.

080.

110.

090.

090.

09To

tal

97.6

198

.06

98.4

898

.39

97.2

997

.44

97.8

597

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97.4

296

.71

Mg#

51.1

38.4

38.8

36.5

36.6

39.5

51.6

50.9

50.3

53.3

FeO

*/M

gO1.

682.

832.

783.

063.

052.

691.

651.

701.

741.

54T

i(pp

m)

4196

2158

2158

2158

2158

2218

4256

4256

4196

4316

K(p

pm)

3984

9711

1004

397

1111

122

8632

3818

3818

4067

3735

Trac

eel

emen

ts(p

pm)

Sc V27

041

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625

525

829

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r21

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83.

94.

34.

24

23.2

19.5

19.2

20.8

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Ni

10.4

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2.4

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81.

613

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98.

317

Cu

90.4

9.8

14.7

16.8

25.2

14.5

76.2

89.9

83.7

71.9

Ga

Rb

7.5

13.8

21.2

20.5

22.8

16.3

6.6

77.

79.

5Sr

236

194

186

187

192

211

248

228

233

241

Y21

.833

.831

.433

.132

.732

.821

.221

23.9

22Z

r55

.813

413

213

513

314

056

.554

.556

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b0.

751.

551.

521.

621.

581.

730.

780.

740.

790.

78B

a14

434

835

135

434

435

612

514

014

113

2L

a3.

337.

467.

427.

387.

397.

373.

533.

373.

863.

36C

e8.

5217

.57

17.5

517

.36

17.3

917

.39

8.67

8.64

9.19

8.4

Pr

1.25

2.39

2.29

2.39

2.33

2.33

1.26

1.24

1.35

1.25

Nd

6.54

11.3

110

.84

11.2

511

.32

11.0

56.

566.

537.

046.

5Sm

2.1

2.95

2.87

3.01

2.99

2.97

2.07

2.03

2.24

2.02

Eu

0.71

60.

852

0.85

30.

903

0.87

10.

917

0.75

0.72

30.

773

0.74

9G

d2.

543.

383.

233.

543.

463.

392.

742.

652.

932.

76T

b0.

454

0.59

80.

608

0.61

30.

616

0.61

80.

490.

467

0.51

30.

482

Dy

3.03

4.09

3.9

3.99

4.09

4.05

3.25

3.17

3.4

3.13

Ho

0.68

70.

921

0.90

10.

907

0.94

30.

927

0.72

40.

705

0.73

90.

713

Er

2.08

2.8

2.71

2.88

2.91

2.86

2.16

2.08

2.24

2.1

Tm

0.33

40.

485

0.46

70.

486

0.49

10.

475

0.35

20.

336

0.36

60.

328

Yb

2.12

3.23

3.1

3.24

3.28

3.22

2.19

2.13

2.29

2.2

Lu

0.33

20.

520.

514

0.52

30.

525

0.52

50.

334

0.33

70.

357

0.33

7H

f1.

513.

293.

183.

453.

363.

461.

61.

521.

611.

53P

b1.

783.

44.

156.

214.

443.

682.

061.

932.

131.

57T

h0.

320.

955

0.93

90.

995

0.98

21.

040.

327

0.33

80.

357

0.32

7U

0.63

90.

485

0.46

60.

485

0.51

40.

499

1.3

0.75

50.

496

0.41

887

Sr/86

Sr0.

7035

170.

7035

3320

6 Pb/

204 P

b18

.879

18.8

7920

7 Pb/

204 P

b15

.553

15.5

5420

8 Pb/

204 P

b38

.441

38.4

37



at the Geological Survey of Japan/AIST on a VGIsotech VG3600 noble gas mass spectrometerfitted with a Balzers electron multiplier. Theseresults are summarized in Table 1 and completeanalytical results are listed in Table S1.

RESULTS

Data for ASVP samples are reported in Table 1(along with BAB glass analyses) and data forTracey Seamount lavas are reported in Table 2.Analyses of Cook 7 BAB glasses are included inTable 1 for comparison (original data from Pearceet al. 2005 and Woodhead et al. 2012). Samples ofASVP are basalts, basaltic andesites, and andesite,containing variable amounts of olivine, calcic pla-gioclase, and clinopyroxene. MGR lavas arebasalts and basaltic andesite. Tracey Seamountlavas are compositionally bimodal, comprisingbasaltic andesite and dacite (Le Bas et al. 1991).Tracey Seamount lavas contain plagioclase, cli-nopyroxene, and orthopyroxene; dacites alsocontain hornblende. Electron probe microanalysis(EPMA) shows that basaltic andesites havereversely zoned pyroxenes and disequilibrium tex-tures observed in dacite plagioclase phenocrystsalso suggest some magma mixing. All lavas fromASVP, Tracey, and MGR define a mostly low-Ksuite that contrasts with most other Mariana Arclavas, which generally define a medium-K suiteexcept in the far north, where very enriched shos-honites are found (Fig. 4a). Lavas from TraceySeamount define a Low-Fe, calc-alkaline suite thatcontrasts with Mariana Trough spreading ridgeglasses and ASVP samples, which are tholeiitic,either medium- or high-Fe suites (Fig. 4b).

Most samples are quite fractionated, with Mg#(= 100Mg/Mg + Fe) << 65, expected for melts inequilibrium with mantle peridotite. This is alsotrue for Tracey lavas. A similar conclusion isreached from considering Ni and Cr contents(Tables 1 and 2).

One sample of dacite from the small cone in thecrater near the summit of Tracey (HPD#949R7)was analyzed for 40Ar/39Ar age. This yielded a goodplateau age of 0.527 � 0.023 Ma (Table 3; Fig. 5;Table S1), which we take as the last significanteruption from this volcano. On this basis, we con-sider Tracey Seamount to be extinct, which is con-sistent with the absence of evidence forhydrothermal activity on its summit (Baker et al.2008 describe this volcano as ‘inactive’).

Ti–V relationships are thought to be sensitiveto mantle oxidation state and thus tectonic setting

(Shervais 1982). Figure 6 plots compositions ofthe sample suite, excluding felsic Tracey Sea-mount lavas. As expected, Tracey lavas have lowTi/V and plot in the ‘arc’ field whereas lavas fromthe Mariana Trough spreading ridge have higherTi/V and plot in the field for BABB and mid-oceanic ridge basalt (MORB). Two ASVP samplesplot in the ‘arc’ field, the sample from Seamount Xand the more mafic sample from Seamount C,whereas other ASVP samples plot in the BABB/MORB field. The more arc-like character of Sea-mount X and C samples is consistent with theposition of these volcanoes closest to the trench(Fig. 2a).

The ratio of light rare earth element (LREE) Lato high field strength element (HFSE) Nb is sen-sitive to tectonic environments, with La/Nb >1.4

Tholeiitic

Calc-Alkalic

High-Fe

Med.-Fe

Low-FeFe

O*/

Mg

O

Low-K (tholeiitic) series

Med.-K (calc-alk.) series

High-K (calc-alk.) series

Mar

iana

Sho

shon

ite Province

Mariana Arc Lavas

Basalt B - A

K

O2

And. Dacite & Rhyolite

Shoshonitic series

Tracey FelsicTracey MaficBAB glasses

Smt AForecastSmt XSmt C 189-1

Smt ZSmt Y

Smt C 189-6-2A)

B)

Fig. 4 Major element characteristics of samples. (a) K2O-SiO2

diagram, showing that lavas from Tracey Seamount, Alphabet SeamountVolcanic Province, and glasses from the Mariana Trough spreading ridgedefine a low- to medium-K suite that is depleted relative to most MarianaArc lavas (grey field encompasses mean compositions for individualvolcanoes; dashed field encompasses mean compositions for volcanoesfrom the Shoshonitic Province, both from Stern et al. (2003). (b) FeO*/MgO vs SiO2 diagram, distinguishing both tholeiites and calc-alkalinesuites (dashed line after Miyashiro 1974) and high-, medium-, andlow-Fe suites of Arculus (2003). Note that Tracey lavas are low-Fe,calc-alkaline lavas, Mariana Trough spreading ridge glasses and ASVPsamples are tholeiitic, either medium- or high-Fe suites.



expected for convergent margin magmatic suitesand lower ratios expected for oceanic basalts fromnon-convergent margin tectonic environmentssuch as MORB and oceanic plateau (Condie 1999).La/Nb for the sample suite is plotted against depth

to the subducted Pacific Plate in Figure 7; allsamples are > 1.4 although samples from TraceySeamount has higher La/Nb (4.59 � 0.22; 1s) thanBAB or ASVP samples. The wide range of SiO2

contents for Tracey Seamount lavas allows us to

Table 3 Result of stepwise-heating analyses of groundmass of volcanic rock from Tracey Seamount

Analysis no. Sample no. Total age (� 1s)integrated age†

(Ma)

Plateau age (� 1s)

Weightedaverage

(Ma)

Inverse isochronage (Ma)

40Ar/36Arintercept

MSWD Fractionof 39Ar

(%)

U10199 HPD949R7 0.583 � 0.021 0.527 � 0.023 0.43 � 0.04 301.7 � 1.8 0.72 100.0

MSWD, mean square of weighted deviates ((SUMS/(n–2))∧0.5) in York (1969).†Integrated ages were calculated using sum of the total gas released.lb = 4.962x10-10/y, le = 0.581x10-10/y, 40K/K = 0.01167% (Steiger & Jager 1977).

0

1

2

3

4

5

0 20 40 60 80 100

Tracey Seamount Rhyodacite

HPD#949R7

0.527±0.023 Ma (98.1% of 39Ar released)

% of 39Ar released

Ma

Fig. 5 40Ar/39Ar age spectra for Tracey dacite sample HPD#949R7.

0 4,000 8,000 12,000 16,000 20,000 24,000Ti (ppm)

0

100

200

300

400

500

600

V (

ppm

)

Ti/V = 1020

50

100 Tracey MaficBAB glasses


Smt Y

Smt C 189-6-2

BABB MORB

OIB

Arc

Fig. 6 Ti-V geochemical discriminant diagrams, after Shervais (1982),showing compositions of ASVP, BABB, and Tracey lavas (mafic only).Lines of constant Ti/V distinguish arc lavas (Ti/V < 20), back-arc basinbasalt (BABB) and mid-ocean ridge basalt (MORB) (> 20, < 50), andocean island basalt (OIB).

100 125 150 175 200 225 250Slab Depth (km)

0

1

2

3

4

5

La/

Nb

Tracey FelsicTracey MaficBAB glasses


Smt ZSmt Y

Smt C 189-6-2

La/Nb = 1.4

Fig. 7 La/Nb vs depth to subducted Pacific Plate. Dashed line sepa-rates arc basalts (La/Nb > 1.4) from MORB and oceanic plateau basalts(Condie 1999).



examine whether or not fractionation affectsLa/Nb. Five mafic samples (SiO2 < 55%) havemean La/Nb = 4.57 � 0.19 whereas five felsicsamples (SiO2 > 68%) have indistinguishable meanLa/Nb = 4.64 � 0.19; we conclude that the modestfractionation experienced by BAB and ASVPsamples has no significant effect on La/Nb andthese values reflect that of the parental magmas.BAB glasses erupt over different depths to thesubducted slab, 170–210 km, but La/Nb varieslittle, 2.0–2.7. ASVP lavas have a meanLa/Nb = 2.5 � 0.65, with higher values from Sea-mounts C and X (La/Nb = 2.7–3.1), which lie abovethe shallowest parts of the subduction zone andlower values for Seamounts A and Y (La/Nb = 1.7–1.9), which lie above the deepest parts of the sub-duction zone.

REE patterns are shown in Figure 8. ASVPlavas are all depleted in light REE, with (La/Yb)n

(= chondrite-normalized La/Yb) ranging from0.52 to 0.86, indistinguishable from those of thenearby Mariana Trough spreading axis ((La/Yb)n = 0.72–0.84). These contrast with REE pat-terns for Tracey Seamount lavas, which arealmost flat for mafic lavas ((La/Yb)n = 0.95–1.05)

and slightly LREE-enriched ((La/Yb)n = 1.4–1.5)for felsic samples). ASVP lavas have slightlynegative to slightly positive Eu anomalies (Eu/Eu* = 0.93 to 1.04), similar to nearby BAB maficglasses (Eu/Eu* = 0.96 to 1.06). Tracey Seamountmafic lavas have a very slightly negative Euanomaly (Eu/Eu* = 0.95–0.99) whereas felsicsamples have a small anomaly (Eu/Eu* = 0.84–0.90). These features of Tracey Seamount felsiclavas along with low Sr/Y (5.6–5.9) indicate thesemelts formed by low-P fractionation or anatexis.It is still unclear whether felsic melts formed byfractionation of mafic melts or anatexis of olderarc crust.

Figure 9 shows trace element incompatibilitydiagrams (spider diagrams) for ASVP lavas andregional Mariana Trough back-arc basin (BAB)spreading ridge glasses and arc lavas (mafic andfelsic) from Tracey Seamount. Fields for BABglasses and mafic Tracey Seamount lavas areshown in Figure 9a for comparison. All samplesshow convergent margin melt compositions,including enrichments in large ion lithophile ele-ments (LILEs), especially Rb, Ba, (Th) U, K, Pb,and Sr. Comparing arc and BAB end-members,ASVP trace element patterns are more similarto BAB trace element patterns, specifically insubdued U anomaly and in HREE abundances,indicating broadly similar melt fraction and mantlesource depletion.

Isotopic compositions of Sr show lower 87Sr/86Srin the western ASVP (<0.7030 for SeamountsA and X), similar to values for the adjacentBAB spreading axis (87Sr/86Sr = 0.70275-0.70316;Table 1). Seamounts in the east (X and C) showhigher 87Sr/86Sr (0.70324 to 0.70336), closer tocompositions of Tracey lavas (87Sr/86Sr = 0.7035;Table 2), which are typical for the Mariana ArcSouthern Seamount Province (Stern et al. 2003).In contrast, there is no significant distinction in143Nd/144Nd between BABB glasses (0.51310 to0.51312; eNd = +9.1 to +9.4) and any of the ASVPsamples (0.51309 to 0.51311; eNd = +8.8 to +9.2).These are significantly more radiogenic thantypical arc lavas of the Mariana Central IslandProvince (143Nd/144Nd = 0.51295 to 0.51305;eNd = +6.1 to + 7.2 Elliott et al. 1997).

BAB glasses and Tracey Seamount arc lavas canalso be distinguished by their Pb isotopic compo-sitions; the former have lower 206Pb/204Pb (<18.5;Table 1) whereas the latter have higher 206Pb/204Pb(>18.8; Table 2). ASVP lavas are slightly higherthan BAB samples and overlap arc lava composi-tions, with 206Pb/204Pb = 18.51 to 18.87.

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

10

20

30

10

20

30

a) Alphabet Seamount Volcanic Province

b) Tracey Seamount and Backarc Basin

Sam

ple/

Cho

ndri

te


Smt Y

Smt C 189-6-2

BAB glassesTracey Felsic

Tracey Felsic

Tracey Mafic

BABB lavas

Tracey Seamount mafic lavas

Fig. 8 Chondrite-normalized Rare Earth Element (REE) patterns for (a)ASVP lavas and (b) regional Mariana Trough back-arc basin (BAB)spreading ridge glasses and lavas (mafic and felsic) from Tracey Sea-mount. Fields for BAB glasses and mafic Tracey Seamount lavas areshown in (a) for comparison. Note the similarity of ASVP REE patterns toBAB REE patterns.



DISCUSSION

Igneous and tectonic activity in the southernMariana convergent margin is unusual and pro-vides some useful perspectives on convergentmargin magma genesis. In this section, we discusstwo insights that these perspectives provide: (i)how the unusual tectonics of the southern MarianaTrough have affected development of a normalmagmatic arc; and (ii) how these tectonics arereflected in melt compositions.

HOW HAS SOUTHERN MARIANA TROUGH EXTENSIONAFFECTED DEVELOPMENT OF THE MAGMATIC ARC?

Usually, intra-oceanic convergent margins withactive BAB spreading have two distinct magmaticsystems: (i) point-source arc volcanism that overhundreds of thousands of years constructs largecentral volcanoes separated by several tens of kilo-meters; and (ii) line-source volcanism associatedwith BAB spreading. Sometimes there are linear

arrays of smaller volcanoes extending at highangles away from the magmatic front, known as‘cross-chains’, but even here, the distinctionbetween arc point-source and BAB line sourcestyles of volcanism is clear. The IBM volcanic arc iswell defined for about 3500 km south of Japan,with volcanic islands and seamounts variablyspaced at 75 � 40 km (de Bremond d’Ars et al.1995), but a well-developed volcanic arc does notexist above the subducted Pacific Plate southwestof Tracey Seamount (Fig. 2a). Instead, ASVP (acomplex nest of small volcanoes 15–20 km apart,showing no sign of concentrating magmatic activ-ity around a single vent) volcanism sits aboutwhere the next major Mariana arc edifice shouldbe, 50–70 km southwest of Tracey Seamount and130–165 km above the subducted Pacific Plate.What is responsible for this unusual style of con-vergent margin volcanism?

Part of the answer must be that ASVP activityoccurs on thin, weak lithosphere. These edifices liein a region of Mariana Trough south of about 14°N

0.1

1

10

0.20.30.4

234

203040

0.1

1

10

0.20.30.4

234

203040

Rb Ba Th U Nb K La Ce Pb Pr Sr NdSm Zr Hf Eu Ti Gd Tb Dy Y Ho Er TmYb Lu

a) Alphabet Seamount Volcanic Province


Smt Y

Smt C 189-6-2




Sam

ple/

N-M

OR

B

More Incompatible More Compatible

BABB glasses

0.1

1

10

0.20.30.4

234

203040

0.1

1

10

0.20.30.4

234

203040

Rb Ba Th U Nb K La Ce Pb Pr Sr NdSm Zr Hf Eu Ti Gd Tb Dy Y Ho Er TmYb Lu

a) Alphabet Seamount Volcanic ProvinceVV


Smt Y

Smt C 189-6-2




Sam

ple/

N-M

OR

B

More Incompatible More Compatible

BABB glasses

Fig. 9 Spider diagrams for (a) ASVP lavas and (b) regional Mariana Trough back-arc basin (BAB) spreading ridge glasses and arc lavas (mafic andfelsic) from Tracey Seamount. Fields for BAB glasses and mafic Tracey Seamount are shown in (a) for comparison. Note the similarity of ASVP REE patternsto BAB trace element patterns. NMORB-normalized spider diagrams, element sequence and normalizing values from McDonough and Sun (1995).



that shows complex deformation patterns on sonarbackscatter images, with north–south structuresdominating north of 13°10′N, east–west structuresdominating between 12°30′ and 13°10′N, andnortheast- and northwest-trending structuressouth of 12°30′N (Martinez et al. 2000). Thiscomplex deformation is reflected in the unusualshape of ASVP volcanoes, elongated east–west inthe south (Seamount C) and north–south in thenorthwest (Forecast and Seamount Y). This defor-mation is relatively recent as shown by thin tononexistent sedimentary deposits except insediment-filled rift basins in the easternmost partof the basin (Fig. 2a,b), which formed during earlyBAB rifting ca 3–5 Ma. The presence of wide-spread hydrothermal activity in the region isfurther evidence of youthful magmato-tectonicactivity (Baker et al. 2008).

Rapid and complex deformation in this regionmay be caused by rapid rollback of the subductingPacific Plate beneath the east–west trendingMariana Trench to the south (Fig. 1a,b; Fryer et al.2003; Gvirtzman & Stern 2004). GPS measure-ments indicate that the Mariana Trough at thelatitude of Guam is opening at about 45 mm/year(Kato et al. 2003) and that BAB extensiondecreases northwards. Magnetic anomaly patternsare not clear enough to constrain tectonic models.Rapid rollback of the Pacific Plate helps explaineast–west-trending extensional structures in thesouthern Mariana Trough (including the seismi-cally active 14°40′N discontinuity; Heeszel et al.2008) and by GPS results indicating Marianaislands are moving apart north–south (Kato et al.2003). The Mariana Trough is everywhere widen-ing east–west but it is also southwards.

Complex extensional deformation in the south-ern Mariana Trough is responsible for the ASVP,in the sense that no large arc volcanoes exist atthe expected position. This disorganization occurson weak BAB lithosphere, west of the West SantaRosa Bank Fault (WSRBF), a major north–southfault. The WSRBF can be traced as a >5 km scarpsouth of 13°N, diminishing in relief northwardsuntil it is replaced by a northeast-trending faultscarp south of Tracey Seamount (Fig. 1b). Thesefaults formed during rifting to open the MarianaTrough and have localized subsequent deformation.Evidence for this opening is beautifully preservedin the eastern part of the study area, where theWSRBF scarp is >2 km high and flanked bysediment-filled early rift basins (Fig. 2a). Theregion to the west is characterized by young, thinoceanic lithosphere that continues to be vigorously

deformed. This WSRBF can be traced south intoa north–south structural ridge along 144°10′Eexposing exhumed mantle peridotites and met-agabbros (Michibayashi et al. 2009). At Tracey Sea-mount and northwards, these rift faults controlwhere arc volcanoes re-established after rifting butin the south the locus of arc magmatism and earlyrift structures diverge as the trench and the sub-duction zone turns to the west. The absence of awide, cold, stable forearc separating the southernMariana Trough and the trench west of approxi-mately 144°10′E is a further indication of thecomplex tectonomagmatic style of the region southof 14°N. In this region all volcanism including theMGR lie <140 km from the trench, instead of themore typical 200–250 km from the trench to thesubaerial Mariana arc volcanoes and the approxi-mately 350 km from trench to BAB spreading axis.

The scattering of small ASVP volcanoes aboutwhere there would otherwise be an arc stratovol-cano partly reflects the disruption of the Marianaarc by continued opening of the southern MarianaTrough. This deformation is complex, with bothnorth–south faults due to opening of the MarianaTrough and east–west faults due to rapid slabretreat along the Challenger Deep portion of theMariana Trench.

The subducted Pacific Plate is clearly presentbeneath Tracey, ASVP, and MGR where it is stillcausing arc-related melts to rise to the surface.The ASVP reflects interaction of stable, deepmantle upwellings and weak magmatic focusing bythin lithosphere. Globally, arc volcanoes distributerandomly along the volcanic front but these loca-tions, once established, are maintained for hun-dreds of thousands of years. Once a volcanic edificebegins to grow, eruptions increasingly concentratearound the main vent, building a stratovolcano.Hildreth (2007) concludes that modern CascadeArc stratovolcanoes have been magmatic centersfor as long as 600 ka. What controls this spacing ofvents? Vogt (1974) argued that volcanoes are gen-erally spaced at approximately the thickness of theunderlying lithosphere. This relationship seemsnot to hold above subduction zones, where volcanospacing is controlled at greater depth, near thesubducted slab, by gravitational instability of athin, low-density and low-viscosity layer (ten Brink1991). Such long-term magma focusing may becontrolled by vertical pathways, which favor dia-piric ascent in the underlying mantle wedge(Fig. 10a; de Bremond d’Ars et al. 1995).

Another part of the reason for the ASVP is thatthin lithosphere does not focus melts. Normally,



sub-arc lithosphere thickens with time so thatmagma pathways focus rising melts packets intothin lithosphere beneath large arc volcanoes(Fig. 10a). Distributed extension in the southernMariana Trough has thinned this lithosphere, sothat magma batches are not funneled into a long-lived system of magmatic plumbing. Faults maycause some localization, but for the most partmagmas erupt above where these impinge on thebase of the lithosphere (Fig. 10b). Magmatic focus-ing and establishment of a large arc stratovolcanolike Tracey Seamount cannot yet be establishedbut is likely once the region stops stretching.

HOW HAS SOUTHERN MARIANA TROUGH EXTENSIONAFFECTED ASVP LAVA COMPOSITIONS?

Given the unusual style of ASVP volcanism (amixture of arc and BAB styles) it might beexpected that compositions of ASVP lavas wouldbe similarly mixed. This is true to a certain extent;ASVP lavas show compositions that have clearsupra-subduction zone characteristics, includingLILE enrichments and HFSE depletions. It isless clear whether ASVP are arc-like or BAB-like;

they are broadly intermediate between the two.Tracey Seamount lavas are clearly arc-like andinclude a significant proportion of felsic lavas,which are common in arc and rarely encounteredamong BAB lavas. ASVP lavas define a low- tomedium-K suite, more like Mariana BAB thanMariana Arc lavas, and also are tholeiitic, showingFe-enrichment trends that are more similar toBABB than to Tracey lavas (Fig. 4). Similaritieswith BAB igneous rocks vs arc lavas include higherTi/V (Fig. 6), moderate La/Nb (Fig. 7), REE abun-dances that are mostly greater than in Tracey Sea-mount mafic lavas and that show BABB-likeLREE-depletion (Fig. 8a), and trace element pat-terns showing relatively modest LILE enrich-ments and HFSE depletions (Fig. 9a).

Overall, ASVP lavas show compositions that areintermediate between arc and BAB lavas. In addi-tion, there is a compositional gradient within theASVP, with more arc-like compositions in the eastand more BABB-like compositions in the west. Forexample, Seamounts X and C, which lie above theshallowest parts of the subducted slab (~130 kmdeep), have low (arc-like) Ti/V (Fig. 6) and have thehighest La/Nb of ASVP lavas. In contrast, the

BAB Crust

Mantle

lithosphere

Asthenosphere

upw

ellin

g hy

drat

ed m

antle

sea levelspreading axis

** * *

* regions of shallowdecompression melting

distributed extension and volcanism

*

(a) (b)arc volcano

Arc Crust

*

Fig. 10 Magmagenetic cartoon illustrating differences between normal situation for magmatic arc associated with (a) back-arc basin (BAB) spreadingridge and (b) the situation for Alphabet Seamount Volcanic Province. (a) Cartoon shows the typical situation for the southern Mariana Arc (where the activevolcanic arc is built on BAB crust (see fig. 8 of Bloomer et al. 1989), characterized by coexisting but distinct and spatially separated BAB spreading andarc volcanism. BAB volcanism reflects decompression melting of shallow mantle as a result of focused extension. Arc volcanism reflects a broadlycylindrical zone of upwelling asthenospheric mantle, which is relatively low density because it is hydrated by fluids and melts released from the subductedslab. Upwelling hydrated mantle melts due to hydrous fluxing and decompression. Melt packages beneath the arc volcano are funneled by thin lithosphereinto the shallow arc magma plumbing system and so erupt from the volcano, helping to further build the stratovolcano; this focusing also results in thinlithosphere beneath arc volcano. (b) Cartoon shows what happens when distributed extension keeps lithosphere thin over the region of upwelling hydratedmantle. Melt packages are not funneled into a single volcano’s magmatic plumbing but erupt over a broad region. Extension also results in enhanceddecompression melting of shallow mantle (red *).



other four edifices (Seamounts A, Y, Z, and Fore-cast) lie over deeper parts of the subducted slab(and closer to the BAB spreading axis), and haveBABB-like Ti/V and La/Nb. Consistent with thisvariation, lower 87Sr/86Sr in the western ASVP vol-canoes is BABB-like and higher 87Sr/86Sr in theeastern ASVP is arc-like.

The small-scale variations in melt compositionsindicate that volcanoes in the ASVP lavas tappedheterogeneous sources through a range of pro-cesses, from extension-related decompressionmelting to flux melting, as shown in Figure 10.ASVP mantle sources change systematically frommore arc-like above the shallower part of the sub-ducted slab in the east to more BABB-like abovethe deeper part of the subducted slab in the west.Processes forming ASVP melts thus change frommore shallow decompression in the west to moredeep hydrous fluxing in the east.

CONCLUSIONS

Tracey Seamount is the southernmost discretestratovolcano of the approximately 3000 km-longIBM Arc and so defines its southern termination, inspite of the fact that a well-defined subducted slabis found beneath the region to the southwest. TheAlphabet Seamount Volcanic Province (ASVP)occurs 50–70 km southwest of Tracey Seamountand about 130 km above subducted slab, aboutwhere the next major Mariana Arc edifice shouldbe, but contrasts with IBM Arc volcanic style andlava compositions. ASVP is a complex of small vol-canoes 15–20 km apart; we studied samples fromsix of these (Seamounts A, C, X, Y, Z, and Forecast).ASVP volcanoes are scattered about where therewould otherwise be an arc stratovolcano because ofcomplex extension in the southern Mariana Troughback-arc basin. Lithospheric stretching is causedby rapid rollback of the Pacific Plate to the southalong the Challenger Deep portion of the MarianaTrench, causing north–south extension) andMariana Trough opening (causing east–west exten-sion), resulting in both north–south and east–westfaults and magmatic fabrics. ASVP edifice mor-phologies indicate formation in response to strongextensional stress, which changes from east–westin the northern part of the study area, nearest thespreading ridge, to north–south in the south,nearest the Mariana Trench.

The region occupied by Tracey Seamount andthe ASVP encompasses a fundamental lithos-pheric transition that is reflected in different vol-

canic styles and magma compositions. TraceySeamount is flanked to the east by the older(Paleogene), thicker, and relatively stable Marianaforearc. Here (and along the IBM magmatic frontto the north) the locus of arc volcanism controlledthe development of and subsequently exploited thefault between forearc and younger back-arc basincrust. In contrast, the ASVP is built on younger(Late Neogene), thinner, unstable crust of theMariana back-arc basin. Tracey Seamount is anextinct stratovolcano, with a summit region thatlast erupted 0.527 � 0.023 Ma. Summit regionlavas are compositionally bimodal, comprisingbasaltic andesites and dacites with typical arc com-positions. In contrast, ASVP lavas are basalts,basaltic andesites, and andesite with mixed affini-ties between arc and BAB igneous rocks. TraceySeamount lavas define a low-Fe, calc-alkaline suitethat contrasts with Mariana Trough spreadingridge glasses and ASVP samples, which are tholei-itic medium- or high-Fe suites. Tracey Seamountfelsic samples have minor Eu anomalies and Sr/Yindicating melt formation due to low-P fraction-ation or anatexis. Tracey lavas have low, arc-likeTi/V whereas nearby Mariana Trough BABB havehigher Ti/V and plot in the field for BABB andMORB. Easternmost ASVP volcanoes erupt lavaswith arc-like Ti/V, whereas other ASVP samplesare BABB-like. Tracey lavas have high, arc-likeLa/Nb (>4) whereas Mariana Trough BABBs havelower La/Nb (2–2.7); ASVP lavas are intermediate.Tracey Seamount lavas have REE patterns thatare flat for mafic lavas ((La/Yb)n = 0.95–1.05) andslightly LREE-enriched ((La/Yb)n = 1.4–1.5) forfelsic samples, in contrast to ASVP REE patterns,which are LREE-depleted and BABB-like. TraceySeamount lavas show arc-like Sr, Nd, and Pb com-positions whereas ASVP lavas have BAB-like Ndisotopic compositions but arc-like Pb isotopes.

The ASVP reflects interaction of stable, deepmantle upwellings and weak magmatic focusing bythin lithosphere, which has not yet led to a lithos-pheric structure that funnels melts. Continuedextension allows melts to rise to the surface abovethe mantle region where they formed, preservingsmall-scale variations in melt compositions. Thesevariations capture metasomatic gradients in theunderlying mantle wedge, which changes frommore arc-like above the shallower part of the sub-ducted slab in the east to more BABB-like abovethe deeper part of the subducted slab in the west.As a result of regional extension, ASVP volcanoestap heterogeneous subduction-modified mantlesources by a range of processes, from dominantly



decompression melting in the west to dominantlyflux melting in the east. A similar range of magmabatches may feed arc volcanoes like TraceySeamount but these batches are mixed andfractionated in the long-established magmaticplumbing systems developed beneath most arcstratovolcanoes.

ACKNOWLEDGEMENTS

This work was supported in part by the JSPSGrant-in-Aid for Scientific Research (B) 23340166to Tamura, NSF grant 961352 to Stern. We thankM. Narui and M. Yamazaki at the InternationalResearch Center for Nuclear Materials Science,Institute for Materials Research, Tohoku Univer-sity for providing opportunities of neutron irradia-tion of samples at the JRR3 reactor. Constructivereviews by P Castillo and R Maury are gratefullyacknowledged. This is UTD Geosciences contribu-tion #1239.

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SUPPORTING INFORMATION

Additional Supporting Information may be foundin the online version of this article:

Table S1 Result of isotopic analysis for 40Ar/39Ardating of Tracey Seamount lava.



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