N

0 50

kilometers

100

40

35

120

C A S C A D ES &M O D O C

P L AT E AU

K L A M AT HM O U N TA I N S

BA

SIN

& R

AN

GE

SIE

RR

A N

EV

AD

A

SA

LINIA

G

RE

AT

VA

LLE

Y

Zeolite

Lws-Ab-Chl

Blueschist

Franciscan Complexdominant MetamorphicFacies

Other Units

Granitic Plutons

Ophiolitic Rocks

Pre-batholithic rocksof Sierra Nevada

Salinia

Great Valley Sequence

JennerJennerJenner

Ward CreekWard CreekWard Creek

Mount HamiltonMount HamiltonMount Hamilton

San SimeonSan Simeoncoast mélangecoast mélangeSan Simeoncoast mélange

City CollegeCity Collegeshear zoneshear zoneCity Collegeshear zone

Hunters PointHunters Pointshear zoneshear zoneHunters Pointshear zone

South Fork Mountain schistSouth Fork Mountain schist(Tomhead Mtn.)(Tomhead Mtn.)South Fork Mountain schist(Tomhead Mtn.)

Teham

a-C

olu

saTe

ham

a-C

olu

sam

éla

nge

méla

nge

Teham

a-C

olu

sam

éla

nge

Willow SpringsWillow SpringsWillow Springs

Tiburon

Tiburon

mélange

mélange

Tiburon

mélange

E�������� ���������� �� ����-����� ������ �� ��� ����� F��������� ���������� ����

The metamorphic history of high-grade rocks provides invaluable insight into the thermomechanical processes of subduction zones. While subduction in most orogens is terminated by continent collision entailing variably strong overprint of related units, the Franciscan Complex of California allows studying a >150 Myr long subduction history that started at ~175 Ma and ended by transformation into a transform plate boundary (San Andreas fault) without significant metamorphic overprint. The highest grade metamorphic rocks of the Franciscan Complex of California are found as blocks in serpentinite and shale matrix mélanges. They include amphibolites, eclogites, blueschists, and blueschist facies metasediments. These Franciscan mélanges inspired the subduction channel return-flow model, but other processes e.g., buoyancy-driven serpentinite diapirism have been argued to be concordant with our current understanding of their metamorphic history, too.

Introduction

We find: (i) Exhumation from the eclogite-amphibolite facies only occurred at 165–155 Ma with an apparent southward younging trend. (ii) Exhumation of the blocks was uniform and fast in the eclogite-amphibolite facies with rates of 2–8 km/Myr. In the blueschist facies exhumation of the blocks was less uniform and slowed by an order of magnitude. (iii) The age of amphibole in a metasomatic reaction zone indicates that at least one amphibolite was enclosed in a serpentinite matrix at ~155 Ma already. Considering the entire subduction zone system, the high-grade exhumation temporally correlates with a significant pulse of magmatism in the respective magmatic arc (Sierra Nevada) and termination of forearc spreading (Coast Range Ophiolite).Our findings do not support a steady-state process that is continuously exhuming high-grade rocks. Instead the subduction zone system appears to have changed with an eventlike character resulting in exhumation of high-grade rocks enclosed in serpentinite.

Figure 1. Early proposed model of subduction channel return flow (Cloos 1982). Main prediction of this continuous process model: Prograde and retrograde metamorphism occur at all grades at all times.

Study Setup

We sampled high-grade blocks f rom serpentinite and serpentinite-shale mélanges. We focused on eclogite-amphibolites that were previously investigated petrologically and geochronologically (primarily Lu-Hf garnet). We dated amphibole, phengite, and

40 39zircon using the Ar/ Ar and U-Pb systems, respectively.

Figure 4 (↑): Tectonic and metamorphic overview map of northern and central California. Investigated locals in the Franciscan complex are color coded in conjunction with figure 5. Modified from Mulcahy et al. (2018).

Figure 3 (↑): Exemplary high-grade block TIBB in the field (a) and in thin section (b).

500 µm

phengitephengitephengite

hornblendehornblendehornblende

Na-amphiboleNa-amphiboleNa-amphibole

a

b

We investigate a suite of metabasite blocks from serpentinite and shale matrix mélanges of the Califonia Coast Ranges. Our new dataset consists of U-Pb dates of metamorphic zircon and 40Ar/39Ar dates of calcic amphibole and white mica. Combined with published geochronology, particularly prograde Lu-Hf garnet ages we can reconstruct the timing and time scales of prograde and retrograde metamorphism of individual blocks.

We sampled high-grade blocks from serpentinite and serpentinite-shale mélanges. We focused on eclogite-amphibolites that were previously invest igated pet ro log ica l ly and geochronologically (primarily Lu-Hf garnet). We dated amphibole, phengite,

40 39and zircon using the Ar/ Ar and U-Pb systems, respectively.

Study Setup

IntA = 163.16 ± 0.74 MaIntA = 163.16 ± 0.74 Ma

159.34 ± 0.67 Ma159.34 ± 0.67 Ma

Heart rock, HR-2 and HR-3Heart rock, HR-2 and HR-3

100100

120120

140140

160160

180180

200200

IntA(18057-01) = 140.4 ± 1.5 MaIntA(18057-01) = 140.4 ± 1.5 Ma

140.37 ± 0.71 Ma (vein)140.37 ± 0.71 Ma (vein)

IntA(18057-02) = 141.4 ± 1.0 MaIntA(18057-02) = 141.4 ± 1.0 Ma

141.86 ± 0.61 Ma (vein)141.86 ± 0.61 Ma (vein)

IntA(18220-01) = 149.7 ± 1.9 MaIntA(18220-01) = 149.7 ± 1.9 Ma

148.4 ± 1.8 Ma (matrix)148.4 ± 1.8 Ma (matrix)

00

22

44

6060

120120

140140

160160

180180

200200

158.13 ± 0.51 Ma158.13 ± 0.51 Ma159.9 ± 2.0 Ma159.9 ± 2.0 Ma

156.39 ± 0.70 Ma156.39 ± 0.70 Ma

154.8 ± 1.3 Ma154.8 ± 1.3 Ma

00

2020

00

6060

RM-8RM-8

120120

140140

160160

180180

200200

156.1 ± 1.1 Ma156.1 ± 1.1 Ma

IntA(18054-01) = 154.6 ± 1.5 MaIntA(18054-01) = 154.6 ± 1.5 Ma

154.79 ± 0.66 Ma154.79 ± 0.66 Ma

00

2020

00

6060

matrix

vein vein

120120

140140

160160

180180

200200

IntA = 152.9 ± 1.3 MaIntA = 152.9 ± 1.3 Ma

155.3 ± 1.3 Ma155.3 ± 1.3 Ma

5050

00

6060

Sunset boulders Je2 and Je1rSunset boulders Je2 and Je1r

matrix

rind

100100

00

Je3dJe3d

100100

120120

140140

160160

180180

200200

IntA = 137 ± 44 MaIntA = 137 ± 44 Ma

156 ± 17 Ma156 ± 17 Ma

4040

00

6060

IntA(18041-03) = 156.19 ± 0.55 MaIntA(18041-03) = 156.19 ± 0.55 MaIntA(18041-04) = 159.8 ± 2.6 MaIntA(18041-04) = 159.8 ± 2.6 Ma

IntA(18059-01) = 156.6 ± 1.6 MaIntA(18059-01) = 156.6 ± 1.6 MaIntA(18230-03) = 157.4 ± 1.7 MaIntA(18230-03) = 157.4 ± 1.7 Ma

00 0.020.02 0.040.0400

0.50.5

1.01.0

1.51.5

2.02.0

2.52.5

Age = 147.3 ± 3.0 MaAge = 147.3 ± 3.0 Ma40 36Ar/ Ar Int. = 404 ± 9240 36Ar/ Ar Int. = 404 ± 92MSWD = 4.5, n = 10MSWD = 4.5, n = 10

00 0.020.02 0.040.04 0.060.0600

11

22

33

IIA = 157.6 ± 1.6 MaIIA = 157.6 ± 1.6 Ma40 36Ar/ Ar Int. = 313.8 ± 5.040 36Ar/ Ar Int. = 313.8 ± 5.0MSWD = 15, n = 11MSWD = 15, n = 11

PG23/TIBBPG23/TIBB

00 0.020.02 0.040.0400

0.50.5

1.01.0

1.51.5

Age = 149.9 ± 2.9 MaAge = 149.9 ± 2.9 Ma40 36Ar/ Ar Int. = 690 ± 18040 36Ar/ Ar Int. = 690 ± 180MSWD = 2.4, n = 14MSWD = 2.4, n = 14

vein

IntA(18046-13) = 154.0 ± 1.6 Ma

36

40

-3A

r/A

r (1

0)

0

36

40

-3A

r/A

r (1

0)

Appare

ntA

ge

(Ma)

Appare

ntA

ge

(Ma)

40

%A

r*40

%A

r*C

a/K

Ca/K

Appare

ntA

ge

(Ma)

Appare

ntA

ge

(Ma)

40

%A

r*40

%A

r*C

a/K

Ca/K

Jenner

Tiburon Mélange (Ring Mountain)

36

40

-3A

r/A

r (1

0)

39 40Ar / Ar*K

39 40Ar / Ar*K

00 0.20.2 0.40.4 0.60.6 0.80.8 1139cum. Ar releaseK

39cum. Ar releaseK

00 0.20.2 0.40.4 0.60.6 0.80.8 1139cum. Ar releaseK

39cum. Ar releaseK

00 0.20.2 0.40.4 0.60.6 0.80.8 1139cum. Ar releaseK

39cum. Ar releaseK

00 0.20.2 0.40.4 0.60.6 0.80.8 1139cum. Ar releaseK

39cum. Ar releaseK

00 0.20.2 0.40.4 0.60.6 0.80.8 1139cum. Ar releaseK

39cum. Ar releaseK

Appare

ntA

ge

(Ma)

Appare

ntA

ge

(Ma)

40

%A

r*40

%A

r*C

a/K

Ca/K

40 39Figure x: Ar/ Ar geochronology results. Apparent age spectra with weighted mean ages and integrated gas ages

(K-Ar equivalent) are reported for all samples and aided by inverse isochron plots for samples with complex 40 36Ar/ Ar composition. Results from amphibole and white mica are in red and green tones, respectively. Uncertainties

are plotted at the 1σ level, but calculated results are at the 2σ level for ease of comparability with other methods.

Further analytical detail in Table x and Table Sx.

2020

MH-3MH-3

100100

120120

140140

160160

180180

200200

IntA(18216-01) = 144.0 ± 2.6 MaIntA(18216-01) = 144.0 ± 2.6 Ma

151.4 ± 2.2 Ma151.4 ± 2.2 Ma

IntA(18216-02) = 141.9 ± 2.5 MaIntA(18216-02) = 141.9 ± 2.5 Ma

153.6 ± 2.2 Ma153.6 ± 2.2 Ma

IntA(18217-01) = 148.3 ± 1.8 MaIntA(18217-01) = 148.3 ± 1.8 Ma

148.2 ± 1.7 Ma148.2 ± 1.7 Ma

1010

00

6060

00

Mount Hamilton

00 0.20.2 0.40.4 0.60.6 0.80.8 1139cum. Ar releaseK

39cum. Ar releaseK

40

%A

r*40

%A

r*C

a/K

Ca/K

Appare

ntA

ge

(Ma)

Appare

ntA

ge

(Ma)

39 40Ar / Ar*K

BH-32BH-32

100100

120120

140140

160160

180180

200200

Appare

ntA

ge

(Ma)

Appare

ntA

ge

(Ma)

IntA(18044-14) = 159.3 ± 2.0 MaIntA(18044-14) = 159.3 ± 2.0 Ma

159.4 ± 1.2 Ma159.4 ± 1.2 Ma

IntA (18056-01) = 145.5 ± 1.5 MaIntA (18056-01) = 145.5 ± 1.5 Ma

145.69 ± 0.68 Ma145.69 ± 0.68 Ma

00

2020

00

6060

BR-5BR-5

100100

120120

140140

160160

180180

200200

IntA = 150.6 ± 1.5 MaIntA = 150.6 ± 1.5 Ma

149.90 ± 0.75 Ma149.90 ± 0.75 Ma

00

22

6060

100100

120120

140140

160160

180180

200200

IntA(18222-01) = 158.5 ± 3.9 MaIntA(18222-01) = 158.5 ± 3.9 Ma

158.1 ± 2.6 Ma158.1 ± 2.6 Ma

IntA(18222-02) = 160.4 ± 3.5 MaIntA(18222-02) = 160.4 ± 3.5 Ma

160.0 ± 2.2 Ma160.0 ± 2.2 Ma

IntA (18223-01) = 147.2 ± 1.9 MaIntA (18223-01) = 147.2 ± 1.9 Ma

146.6 ± 1.8 Ma146.6 ± 1.8 Ma

00

2020

4040

6060

PG14PG14

100100

120120

140140

160160

180180

200200

Appare

ntA

ge

(Ma)

Appare

ntA

ge

(Ma)

IntA(18045-01) = 147.2 ± 3.1 MaIntA(18045-01) = 147.2 ± 3.1 Ma

154.1 ± 1.5 Ma154.1 ± 1.5 Ma

IntA(18045-02) = 147.12 ± 0.59 MaIntA(18045-02) = 147.12 ± 0.59 Ma

00

2020

4040

00

6060

Ca/K

Ca/K

Ca/K

Ca/K

00 0.020.02 0.040.04 0.060.0600

0.50.5

1.01.0

1.51.5

2.02.0 IIA = 148.0 ± 1.4 MaIIA = 148.0 ± 1.4 Ma40 36Ar/ Ar Int. = 495 ± 8840 36Ar/ Ar Int. = 495 ± 88

MSWD= 1.3, n = 13MSWD= 1.3, n = 13

DRM-1DRM-1

00 0.020.02 0.040.0400

0.50.5

1.01.0

1.51.5

2.02.0

2.52.5

IIA = 146.3 ± 2.3 MaIIA = 146.3 ± 2.3 Ma40 36Ar/ Ar Int. = 357 ± 7440 36Ar/ Ar Int. = 357 ± 74MSWD = 4.2, n = 11MSWD = 4.2, n = 11

00 0.020.02 0.040.04 0.060.0600

0.50.5

1.01.0

1.51.5

2.02.0

2.52.5

IIA = 144 ± 10 MaIIA = 144 ± 10 Ma40 36Ar/ Ar Int. = 355 ± 8940 36Ar/ Ar Int. = 355 ± 89MSWD = 7, n = 13MSWD = 7, n = 13

IIA = 133 ± 21 MaIIA = 133 ± 21 Ma40 36Ar/ Ar Int. = 480 ± 26040 36Ar/ Ar Int. = 480 ± 260MSWD = 200, n = 8MSWD = 200, n = 8

36

40

-3A

r/A

r (1

0)

0

36

40

-3A

r/A

r (1

0)

36

40

-3A

r/A

r (1

0)

39 40Ar / Ar*K

39 40Ar / Ar*K39 40Ar / Ar*K

39 40Ar / Ar*K

39 40Ar / Ar*K

40

%A

r*40

%A

r*C

a/K

Ca/K

00 0.20.2 0.40.4 0.60.6 0.80.8 1139cum. Ar releaseK

39cum. Ar releaseK

000 0.20.2 0.40.4 0.60.6 0.80.8 11

39cum. Ar releaseK

39cum. Ar releaseK

00 0.20.2 0.40.4 0.60.6 0.80.8 1139cum. Ar releaseK

39cum. Ar releaseK

00 0.20.2 0.40.4 0.60.6 0.80.8 1139

cum. Ar releaseK

39cum. Ar releaseK

40

%A

r*40

%A

r*

Appare

ntA

ge

(Ma)

Appare

ntA

ge

(Ma)

40

%A

r*40

%A

r*C

a/K

Ca/K

Appare

ntA

ge

(Ma)

Appare

ntA

ge

(Ma)

40

%A

r*40

%A

r*

207

206

Pb/

Pb

48

14

65

72

74

70

64

66

63

67

71

79

68

76

0.05

0.06

0.07

0.08

33 35 37 39 41

207

206

Pb/

Pb

238 206U/ Pb

6

70.1

0.2

30

40

52

12_5

3

28

1

35

21

4755

16

15 51

18

19

38

0.05

0.06

0.07

0.08

0.09

160 Ma160 Ma160 Ma

170 Ma170 Ma170 Ma

180 Ma180 Ma180 Ma

190 Ma190 Ma190 Ma

160 Ma160 Ma160 Ma170 Ma170 Ma170 Ma180 Ma180 Ma180 Ma190 Ma190 Ma190 Ma

rim and core analyses:7CA range 154±4.7 - 168.1±4.7 Manon-anchored discordia intercepts:

154.4 ± 5.6 Ma & 3255± 740MSWD = 2.9

PG23/TIBB

BH-32

rims:7CA range 168.1±4.7 - 154±4.7 Ma

discordia intercepts:157.6 ± 2.4 Ma & 4640± 260

MSWD = 1.3

cores:7CA range 175.0±5.0 - 160.5±4.6 Ma

discordia intercepts:163.7±2.7 Ma & 3218± 630

MSWD = 1.6

anchored: 159.9±2.9 MaMSWD = 4.3

anchored: 166.7±1.6 MaMSWD = 1.9

cores

rims

cores

rims

130 140 150 160 170 180

white micagarnet

hornblende zircon

Lawsonite

progressionretrogressionPg31; eclogite; A2004

Je1r&Je2; grt-amphibolite; this study

Je3; amphibolite-blueschist-eclogite; this study

WC-1; grt-laws-blueschist/eclogite; M2014

BR-5; grt-blueschist, hbl relics; M2014, this study

T-90-3AG&AB; eclogite; CS2003

T-90-2; grt-amphibolite; CS2003

PG23, TIBB, TIBBvein; eclogite/grt-amphibolite; A2004, this study

RM-8; grt-epi-amphibolite; M2014, this study

Turtle Rock; lws-blueschist; M2009

RR-1; lws-blueschist; M2014

EA, DRM-1; grt-amphibolite; M2018, this study

PG14; grt-amphibolite; A2004, this study

BH-32; grt-amphibolite; M2018, this study

MP-1; epi-amphibolite; WD2007

HPSZ Meta; grt-amphibolite; WD2007

MH-90-01, MH-3, eclogite/grt-amphibolite; CS2007, this study

PnP; hornblendite block, P2014

KA3839; amphibolite; S2011

AldSp1; amphibolite; S2011

WbSp; amphibolite; S2011

04-EU-SS-45; blueschist; U2012

04-EU-SS-48; blueschist; U2012

04-EU-SS-49; blueschist; U2012

04-EU-SS-60; blueschist; U2012

WC-1; metachert; WD2007

Tomhead Mtn. samples ; terrane; DW2007, D2010

120110

PG80, PP13, PP1; Hermes Block/Antelope Creek slab; grt-amphibolite; RS1988, A2004, this study

95Diab-107, PG76; eclogite; C2011; WD2007

95Diab-41; epi-blueschist; WD2007

PG73, glaucophane schist; WD2007

Ap

tian

Ba

rre

mia

n

Ha

ute

rivi

an

Va

lan

gin

ian

Be

rria

sia

n

Tith

on

ian

Kim

me

rid

gia

n

Oxf

ord

ian

Ca

llovi

an

Ba

tho

nia

nB

ajo

cia

n

Aa

len

ian

Tehama-Colusa mélange

Ward Creekslab

Jenner

Tiburon mélange(Ring Mountain,Berkeley Hills,and Big Rock

Ridge)

Hunters Pointshear zoneCity Collegeshear zone

Mount Hamilton

San Simeoncoast mélange

North (strike-slip restored)

South (strike-slip restored)

HR2/3; Jenner blueschist; this study

Willow Springsimbrication

To

arc

ian

Alb

ian

0

South ForkMountain schist

retrogression

retrogression

hydrothermal

matrixvein

matrixvein

40 39Ar/ Ar

Lu-Hf

U-Pb

rims cores

ocean floor formation

sedimentation

2nd phase plutonism

volcanopelagicsedimentation

?

in garnetin matrix

matrixrind

matrixrind

BH-1; grt-blueschist; M2014

matrixrind

43

(1

0 k

m/M

yr)

2

3

1

Sierra Nevada magmatism

Coast Range ophiolite

Great Valley group

magma addition rate (PD2015)

JurassicCretaceousSymbology

>/< minimum/maximum age

*>

*>

ac�nolithe * low-confidence

blueschist facies grt

blueschist facies grt

(H2008)

Franciscan Complex sedimentary units

coherent unit

coherent unit

< 131 Ma det. zrc.coherent unit

onset of accretion (D2010)

PG23, TIBB, TIBBvein; eclogite/grt-amphibolite; A2004, this study

Results 1 - Timescales of Metamorphism

40 39Figure 6 (→): Published and new Ar/ Ar amphibole and white mica ages from high-grade blocks and eclogite-amphibolite facies coherent units displayed in histograms, probability density functions (line), and kernel-density estimate curves (area).

Figure 5 (↑). Summary of geochronologic data for Franciscan high-grade bocks and coherent units and comparison with chronology of accretion of Franciscan Complex sedimentary units, Coast Range ophiolite genesis, Great Valley group sedimentation, and Sierra Nevada magmatism. Samples described in black font experienced eclogite-amphibolite facies and variable blueschist facies overprint while those in blue font experienced only blueschist facies metamorphism. Abbreviations: A2004 - (Anczkiewicz et al., 2004); CS2007 - (Catlos & Sorensen, 2003); C2011 - (Cooper et al., 2011); D2010 - (Dumitru et al., 2010); H2008 - (Hopson et al., 2008); M2009 - (Mulcahy et al., 2009); M2014 - (Mulcahy et al., 2014); M2018 - (Mulcahy et al., 2018); P2014 - (Page et al., 2014); PD2015 - (Paterson & Ducea, 2015); RS1988 - (Ross & Sharp, 1988); S2011 - (Shervais et al., 2011); U2012 - (Ukar et al., 2012); WD2007 - (Wakabayashi & Dumitru, 2007).

Hypothesis 1: Continuous Subduction Channel Return Flow (Cloos 1982)

(n=10+3)

(n=24)

40 39Ar/ Ar amphibole

num

ber

num

ber

40 39Ar/ Ar white mica

130 135 140 145 150 155 160 165 170 175

0

1

2

3

4

5

6

7

0

1

2

3

4

5 (n=10)

Teha

ma-

Colus

a m

élan

ge

Teha

ma-

Colus

a m

élan

ge

(Nor

ther

n Califo

rnia)

(Nor

ther

n Califo

rnia)

Teha

ma-

Colus

a m

élan

ge

(Nor

ther

n Califo

rnia)

Cen

tral C

alifo

rnia

Cen

tral C

alifo

rnia

Cen

tral C

alifo

rnia

Cen

tral C

alifo

rnia

Cen

tral C

alifo

rnia

Cen

tral C

alifo

rnia

(n=3)

Age (Ma)

Initiation (Anczkiewicz et al., 2004)Hypothesis 2: Cooling of the Subduction Zone after Subduction

Figure 2. Geothermometric estimates vs. age diagram for analyzed samples. Slope of the regression line suggests ca. 15 °C/Ma cooling rate for the Franciscan subduction (Anczkiewicz et al., 2004).

Lu-Hf garnet

Results 2 - Thermokinetic Evolution of the Subduction zone

Undifferentiated

800

700

600

500

400

300

peak-

T (

°C)

0

10

20

30

40

50

60

70

80~160 Ma (PG23/TIBB)

~158 Ma (PG31)

~121 Ma (SFMS)

blockscoherent

0

0.5

1

1.5

2

2.5

200 300 400 500 600 700

~147 Ma (PG73/WSs)

~158 Ma (PG76/WSs)

depth

(km

)

pre

ssu

re (

GP

a)

125 135 145 155 165 175

800

~169 Ma (PG80/ACs)

max

max

~163 Ma (PG14)

max

14°C/km

7°C/k

m

Ring Mountain

Berkeley Hills

Jenner

temperature (°C)

age (Ma)

white micagarnethornblende

zircon

Lawsonite40 39Ar/ Ar Lu-Hf

U-Pb

Symbology

ac�nolithe

Figure 7. (a) Tt diagram showing dates of geochronometers of individual blocks as a function of the peak-T determined for the block.(b) PT diagram showing peak-PT data of blocks and coherent units along with the approximate age of peak metamorphism, estimated between prograde Lu-Hf garnet

40 39or U-Pb zrc and retrograde Ar/ Ar hornblende or phengite dates. The arrows indicate maximum pressure estimates.

Ÿ Geothermal gradients ranged 7-14°C/km.

Key Findings:

Ÿ Younger metamorphism was rather hotter than cooler.

Key Findings:Ÿ Prograde metamorphism appears temporally variable, but geochronometers may date

variable PT conditions40 39

Ÿ Ar/ Ar amphibole ages cluster at 165-159 Ma with a N-S younging trend

became more variable

- high-T retrograde metamorphism of eclogite-amphibolite was fast and uniform

- Exhumation rates of eclocite-amphibolite slowed in the blueschist facies and

40 39Ÿ Duration between Ar/ Ar amphibole and white mica dates ranges 2–14 Myr

1,2,3 4 5 1,2Daniel Rutte , Joshua Garber , Andrew Kylander-Clark , Paul R. Renne1 Earth and Planetary Science, University of California, Berkeley2 Berkeley Geochronology Center

5 Department of Earth Science, University of California, Santa Barbara

3 University of Bonn4 Penn State University

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b

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