Seismic Attenuation, Temperature, H2O, Mantle Melting and Rock Uplift, Central North Island New ZealandMichelle Salmon, Martha Savage, Tim Stern michelle.salmon@vuw.ac.nz

AbstractWe use measurements of P-wave attenuation (Qp-1) to determine constraints on temperature, melt and H

2O in the mantle wedge below the Central Volcanic Region (CVR)

and the North-West North Island (NWNI) of New Zealand.

A region of high attenuation extending to depths of ~180 km correlates, spatially, with the region of back-arc extension, volcanism and high heat flow (CVR). In this region the path-averaged Qp-1 is 4.0x10-3±0.3x10-3 and shows little variation with depth. West of the CVR, the NWNI Qp-1 is lower (path-averaged Qp-1 1.4x10-3±0.2x10-3). Here attenuation increases with depth until it reaches similar values as the CVR mantle at ~80 km. South of the Taranaki-Ruapehu line attenuation decreases further (path-averaged Qp-1 ~1.0x10-3).

We use empirical relationships between temperature and seismic attenuation to convert our Qp-1 model to homologous temperatures (T/Tm where Tm is the melting temperature). Temperatures at 30 km below the CVR are elevated to just above the melting temperature (T=1.02 Tm) while to the west temperatures are just below the solidus (T ~0.97 Tm). At 90 km depth, Qp-1 indicates that temperatures for both regions are just above the solidus. To reconcile temperatures calculated from heat flow measurements in the NWNI with those calculated from our Qp-1 model, melting temperatures must be lowered by the presence of water.

Four observations point to elevated temperatures and melts in the mantle wedge beneath the central North Island.

1.The surface expression in the CVR of volcanoes and geothermal activity. The CVR is associated with high heatflow (~800 mWm-2). The NWNI is also associated with elevated heatflow (70-80 mWm-2) north of the Taranaki-Ruapehu line (TRL).

2.Pn and Sn wave speeds are up to 10% lower than normal below the CVR [Haines, 1979].

3.Strong reflections within the mantle beneath the region of concentrate d extension are interprete d as melt bodies [Stratford and Stern, 2004].

4.Geologica l records show that the North Island has undergone ~2.5 km of broad wavelength (~400 km) rock uplift since 5 Ma (figure 3) [Pulford and Stern, 2004].

To explain the rock uplift a density contrast through the upper mantle consistent with elevated temperatures and the presence of partial melt below the CVR and the NWNI is required.

Figure 1. Central North Island location map. Seismographs from two passive arrays used for this attenuation study are shown as triangles . The northern array (yellow triangles ) is situated across the western boundary of the CVR and the southern array (red triangles) crosses the Taranaki-Ruapehu line (TRL). This line marks a change in crustal thickness, heatflow, and a change from positiv e isostatic gravity anomalies in the north to negative in the south.

1. Tectonic Setting

Figure 1

Back-arc basins of the western Pacific are elevated 1-2 km above the adjacent oceanic floor. Where oceanic back-arc basins propagate into continental lithosphere we also see an uplift signal, which can be mapped and evaluated with geological methods. New Zealand’s North Island is one such place. To try to understand the driving force for uplift we use seismological methods to quantify temperatures, and therefore buoyancy, in the upper mantle.

The CVR is the apparent extension of the Lau-Havre Trough back-arc spreading center into continental New Zealand (figure 2). The CVR is undergoing active extension of ~ 10mm/yr and is the site of voluminous rhyolitic volcanics. A manifestation of spreading is the observation from explosion seismology that the original crust beneath the CVR is now about half (16 km) the thickness of regular continental crust.

Ker

mad

ec T

renc

h

Hikurangi T

renc

h

Pacific Plate

Australian Plate

Hav

re T

roug

h

45mmCVR

NIWA

Figure 2.

-40

-38

174 176 178

CVR

0 1000 2000 3000 mRock uplift

Figure 3.

TRL

NWNI

West East

20

40

60

80

100

0

Subd

ucted

pacific

plate

Roc

k up

lift (

m)

1000150020002500

CVR

Rock Uplift

A A'

Buoyant volatile rich mantleasthenosphere with

~ 2% partial meltQp-1 ~ 4x10-3

7.6 km/s

7.9 km/s

7.4 km/s

Buoyancy Pressure

Bu

oya

ncy

P

ress

ure

(M

Pa

)

65

35

100 km0

0

NWNI

1.02 Tm0.97 Tm

Thinned crust

Qp-1~1.4x10-3

Conclusions1.Seismic attenuation is high through the full

depth of the mantle wedge below the CVR.

2.There is a region of high attenuation below 80 km depth below the NWNI.

3.Mantle temperatures below the CVR are ~1.02 times the solidus temperature consistent with the presence of ~2% partial melt.

4.At 30 km below the NWNI temperatures are just below the solidus.

5.At 90 km below the NWNI temperatures are above the solidus.

6.Water must be present in the mantle wedge below both the NWNI and the CVR.

10-3

10-2

10-1

0.8 0.85 0.9 0.95 1 1.05 1.1 1.15T/Tm

Qs-1

at 1

Hz

NWNI

CVR

Solidus

[Jackson et al. 2004]

4. Temperature, H20 and MeltSeismic attenuation can be used as a proxy for temperature. Laboratory studies of olivine aggregates give empirical relationships between S-wave attenuation and temperature. We convert Qp-1 to Qs-1 assuming that bulk attenuation is negligible [Anderson and Hart 1978]. We then correct for frequency dependence to get Qs-1 at 1 Hz [Jackson et al. 2004]. These values can then be converted to homologous temperatures (T/Tm).

Figure 10 shows the results for the NWNI and CVR at 30 km depth. CVR temperatures are just above melting (1.02 Tm). This homologous temperature is consistent with ~2% melt. NWNI temperatures are just below melting (0.97 Tm) at this depth.

Temperatures below the NWNI calculated from heat flow are estimated to be between 900 and 1000 oC at 30 km depth [Pandey 1981]. This is below the temperature calculated for attenuation if we use the dry solidus. To reconcile this difference, melting temperatures must be reduced within the mantle wedge to the wet solidus (~1000 oC) indicating the presence of water.

At 90 km depth temperatures for both the CVR and NWNI are both above the solidus.

Figure 10.

174 175 176

-38

-39

Taranaki-Ruapehuline

North-west North Island

(NWNI)

CVR

Figure 4 shows two seismograms recorded at a station west of the CVR (A and B). Expected P and S arrival times calculated from AK135 are shown. A) is from an earthquake with a hypocenter that is also west of the CVR and clearly shows both a P and S arrivals. B) is from an earthquake with a hypocenter on the eastern edge of the CVR; here the S wave has been attenuated to amplitudes less than that of the P wave coda. Preferentia l attenuation of S-waves for raypaths that cross the CVR indicate the presence of fluids.

Data from the north-south line across the TRL are shown in figure 6. Qp-1 values for the western North Island are twice as high north of the TRL.

Estimates of P-wave attenuation (Qp-1) have been calculated using a spectral decay method. Qp-1 values for the seismic array along the western boundary of the CVR are shown in figure 5. Qp-1 for station-event pairs are plotted along the raypath. A high attenuation azimuth range correlates spatially with the CVR. In this region high attenuatio n persists from the near surface to depths of 150 km.

2. Seismic Attenuation

Spectra for these two events are shown in C and D. The P signal is shown in blue and the noise is the green dashed line. The solid red line shows the fit to the spectra for the path averaged Qp-1 calculated for that event-station pair. For comparison the dotted red line shows the predicted spectra calculated using the Qp-1 from the other station-event pair.

Figure 11. Cross section A-A’. A hot volatile rich mantle with ~2% partial melt below the CVR and the NWNI provides the buoyancy force required to explain the ~ 2.5 km uplift of the North Island.

Event: 1735847 Sta: ALL -37.83N 177.36E 108km

Event: 1711932 Sta: ALL -38.68N 175.30E 214kmFigure 4.

TRL

174 175 176 177 178

-41

-40

-39

-38

-37

xx

x

x

x

x

x xx

x

x

x

x

x

x

x

x

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x

x

x

x

x

<0.7x10-3

0.8x10-3

1.0x10-3

1.3x10-3

2.0x10-3

4.0x10-3

10.0x10-3

Normalised raypath Qp-1

x

0-25km25-75km75-125km125-175km>175km

earthquakedepth

High attenuationazimuth range

Mt Ruapehu

increasing attenuation

TRL

Figure 5.

Latit

ude

-40.0

-39.8

-39.6

-39.4

-39.2

-39.0

-38.8

0.8 1.2 1.6 2.0path-averaged Qp-1 (x10-3)

Figure 6.NWNI

CVR

Pn S

10 20 30 40

E

N

Z

Seconds

dis

pla

cem

ent

(mx1

0-7

)

0

0

0

1

1

1

-1

-1

-1

A)

Pn Sn E

N

Z

10 20 30 40Seconds

dis

pla

cem

ent

(mx1

0-6

)

0

0

0

2

2

2

-2

-2

-2

B)

A

A‘

Figure 7 shows path-averaged Qp-1 (and Qp) values for the northern array plotted against earthquake depth. Qp-1 for CVR ray paths are shown as solid red triangles. Blue symbols are for ray paths outside the CVR for different stations. Qp-1 for CVR ray paths show little depth variation.

Lines show Qp-1 for layered models (figures 8 and 9). The solid blue line is our preferred NWNI model. The dashed line is for a constant mantle Q-1 and does not fit data for earthquakes from 50-100 km depth for the NWNI. The solid red line is for a constant mantle Q-1 and a high crustal Q-1 indicated for CVR ray paths. Low Qp-1 values for earthquakes below 300 km (highlighted in blue) are for slab paths and reflect the low Q-1 of the slab. Straight-line ray paths cannot be assumed for these earthquakes.

Depth vs path-averaged Qp-1 for the NWNI and CVR

10-2

10-3

0

200

400

600

800

1000

1200

1400

1600

18000 50 100 150 200 250 300 350

Depth (km)

Qp

Pat

h-av

erag

ed Q

p Qp-1

Slab

raypaths

Figure 7.

Velocity (km/s)

5 6 7 8 90 1 2 3 4 5 6

Qp-1 x10-3

0

50

100

150

200

250

300

Dep

th (

km)

Layered Qp-1

model Vp Model

Figure 9.Figure 8.

Slab

raypaths

Slab

raypaths

NWNI CVR

CVR

NWNI

3. Attenuation Model

D)

Frequency (Hz)10 20 30 40 50

dis

p. a

mp

litu

de

10 -10

10 -8

10 -6

10 -4 Qp-1=3.8x10-3

Qp-1=2.0x10-3

C)

10 20 30 40 50Frequency (Hz)

dis

p. a

mp

litu

de

10 -10

10 -8

10 -6 Qp-1=2.0x10-3

Qp-1=3.8x10-3