Degrees from epicenter

tim

e in

min

ute

s

UT

53

UT

52

UT

51

AZ

49

AZ

48

AZ

47

AZ

46

NM

43

NM

42

NM

40

NM

39

NM

38

NM

37

NM

33

NM

32

NM

31

NM

30

NM

27

NM

26

NM

24

NM

23

NM

22

NM

20

NM

17

NM

15

NM

10

NM

08

TX

06

TX

05

TX

03

TX

01

2.5 km/s

3 km/s

4 km/s

5 km/s

8 km/s

35 36 37 38 39 40 41 42

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34

36

38

Northwest Southeast

Rio

Gra

nd

e R

ift

112° 108° 104° 100°W

30°

32°

34°

36°

38°

40°

Great

Plains

Colorado

Plateau

Basin and

Range

Rocky

Mtns.

114°

#54

#01

TX

CO

NM

AZ

UT

Mexico

Figure 1 Tectonic prov-

inces of the southwestern

U.S. The Great Plains

mark the western edge of

the stable North American

craton. Cenozoic exten-

sion is evident throughout

the region, particularly in

the Basin and Range.

Despite this regional

extension, the Colorado

Plateau has remained

relatively undeformed as

it has rifted away from the

Great Plains. The result is

a concentrated extension

in the Rio Grande Rift.

The RISTRA project

examined a 950 km

transect across these

regions using 57 broadband PASSCAL seismic stations. The strike of the array is

in line with earthquakes in both directions from the Pacific Rim.

Figure 2 Seismic record section from the Mw 6.0 event in Kodiak, Alaska on

2000/11/06. Rayleigh waveforms vary considerably across the RISTRA array

suggesting complex structure. Phase velocities are calculated for a moving

window of vertical component seismograms. Blue box indicates the bin of

stations stacked in Figure 3.

Figure 3 Phase velocity stack function for four events. For each event, a bin of

vertical component wavefields are slant-stacked over a range of slownesses in

the fourier domain similar to the approach of McMechan & Yedlin and Herrmann

& Ammon. Contour interval is 0.1. Maximum stack value is 1. Events A and B

stack coherently from 10 to ~100 seconds. C and D stack well for a limited range

of periods. To make use of all information we sum the contour maps for 29

events before extracing phase velocities. This multiple event approach also

provides error estimates.

Background & Data

10 20 30 50 70 100 1502.5

3

3.5

4

4.5

5

Period (s)

2000/11/06 - 11:40

2.5

3

3.5

4

4.5

52000/05/12 - 18:43

2.5

3

3.5

4

4.5

52001/04/09 - 09:00

2.5

3

3.5

4

4.5

5

Phase v

elo

city (

km

/s)

1999/10/13 - 01:33

A B C D

10 20 30 50 70 100 150Period (s)

10 20 30 50 70 100 150Period (s)

10 20 30 50 70 100 150Period (s)

1 New Mexico State Univ., Dept. of Phys., Las Cruces, NM 88003

2 Los Alamos National Laboratory, EES-1, Los Alamos, NM 87545

3 New Mexico Insitute of Mining And Tech., Socorro, NM 87801

4 Univ. of Texas, Austin, Dept. of Geo. Sciences, Austin, TX 78712

5 Lawrence Livermore Natl. Lab., Earth Sci. Div., Livermore, CA 94550

6 Dine College, Division of Natural Sciences, Shiprock, NM 87420

Contact: west@nmsu.edu

S61A-1116

Structure of the Uppermost Mantle Beneath the

Colorado Plateau, Rio Grande Rift and Great PlainsMichael West

1, James Ni

1, Scott Baldridge

2, Dave Wilson

3, Wei Gao

4, Steve Grand

4, Rick Aster

3, Rengin Gok

5,Steve Semken

6, John Schlue

3

20 40 60 80 100 120 140 160 1803.2

3.4

3.6

3.8

4

4.2

4.4

4.6

Period (s)

Ph

ase

ve

locity (

km

/s)

Colorado Plateau

Rio Grande Rift

Great Plains

= 1 σ

Figure 4 Interstation

phase velocity in each

province along the array.

Curves show the mean

velocity across a third of

the array corresponding

roughly to the Colorado

Plateau, Rio Grande Rift

and Great Plains.

Bootstrap errors are

estimated from 29 events.

Variations of 0.3 km/s are

observed between

regions. The Rio Grande Rift is slower than the cratonic Great Plains at all

periods. The Colorado Plateau is similar to the Great Plains at < 80 seconds.

Long periods however show velocities more in common with the rft.

Phase velocities

Figure 5 Phase velocity as a function of distance along the array. For periods

up to 40 s, velocities are determined from a moving bin of five stations. At

periods greater than 40 s, nine stations are used. An increased apperture for

long periods is required to minimize errors. The bin widths impose a maximum

resolution of ~75 km for short periods and ~150 km for long periods. Red lines

mark the three regions averaged in Figure 4.

0 0.05

0

50

100

150

200

250

300

3500 0.05 0 0.05 0 0.05

De

pth

(km

)

Sensitivity

20 s 50 s 90 s 150 s

400 300 200 100 0 100 200 300 400

20

40

60

80

100

120

140

160

180

NM

41

3.6

Phase v

elo

city (

km

/s)

NM

27

UT

54

4.2

UT

53

4.0

UT

52

NM

26

UT

51

TX

01

AZ

50

4.4

AZ

49

NM

25

AZ

48

NM

28

AZ

47

NM

07

AZ

46

TX

06

AZ

45

TX

05

NM

44

TX

04

NM

43

TX

03

NM

42

NM

29

3.8

NM

15

NM

40

NM

14

NM

39

NM

13

NM

38

NM

12

NM

37

NM

11

NM

36

NM

10

NM

35

NM

09

NM

34

NM

08

NM

33

TX

02

NM

32

NM

24

NM

31

NM

16

NM

30

NM

23

NM

22

NM

21

NM

20

NM

19

NM

18

NM

17

Period (

s)

3.4

3.5

3.6

3.7

3.8

3.9

4

4.1

4.2

4.3

4.4

4.5

4.6

4.7

Distance along array (km)

Northwest Southeast

Figure 6 Sensitivity kernels showing the

dependence of phase velocity on shear

velocity structure. 20 s waves are most

sensitive to the mid-crust while 150 s

waves are sensitive to depths of 200-300

km. The layers reflect the model

parameterization which has increasing

layer thicknesss with depth. The phase

velocity profile at each station (Figure 5)

was inverted using a damped least-

squares approach and kernels similar to

this example. Crustal thickness and depth

of sedimentary basins were defined a

priori using the receiver function results of

Wilson et al. (see poster S61A-1113).

Figure 7 Shear velocity structure of the crust and uppermost mantle. Velocity

structure obtained by inverting phase velocities in Figure 5. In the crust, velocities

are generally less near the rift where the crust is thinnest. The lower crust

beneath the Great Plains is very fast (> 4.0 km/s) in agreement with prior studies.

The mantle beneath the Great Plains is also fast reflecting its cratonic history. No

clear asthenospheric channel is present. Beneath the Colorado Plateau, the top

of the mantle has velocities of 4.55 km in agreement with Pn velocities of 8.1 km

from this and previous studies. However at depths of 150-250 km, the Colorado

Plateau is underlain by material that is almost 10% slower than at comparable

depths beneath the Great Plains. The lowest mantle velocities (~4.2 km/s) are

found beneath the rift just under the Moho. Low velocities beneath the rift do not

extend to depth.

400 300 200 100 0 100 200 300 4000

100

200

300

400

Distance (km)

Depth

(km

)

4.2 4.4 4.6 4.84.2<3

shear velocity in crust (km/s) shear velocity in mantle (km/s)

Mantle structure

Northwest Southeast

100

200

300

0

A

E

D

C

B

crust

lithospheric

mantle

asthenosphere

Colorado

Plateau

Rio Grande

Rift

Great

Plains

Figure 8 Intepreted section.

A: High velocities of 4.6-4.7 km/s beneath the Great Plains indicate a thick

shield-like lithosphere of at least 200 km.

B: Great Plains asthenosphere velocities are faster than anywhere else along the

array indicating a sharp edge to the tectonically stable region of North America.

C: The lithosphere beneath the Colorado Plateau is ~140 km thick. The fast

velocities indicate cold lithosphere relative to the adjacent Basin and Range

province and Rio Grande Rift. The inherited strength of the lithosphere has

allowed the Colorado Plateau to remain undeformed despite regional extension.

D: Under the Colorado Plateau below 150 km, a low velocity zone (4.2 km/s)

indicates warm asthenosphere. We propose that low density associated with this

mantle anomaly provide a buoyant force which partially supports the plateau's

present high elevation.

E: Beneath the rift, the thermal lithosphere shallows to within about 20 km of the

base of the crust (thinned as well). Sub-Moho velocities of 4.2 km/s are

consistent with Pn observations and indicate temperatures elevated by a few

hundred degrees and/or small amounts of partial melt.

F: Asthenospheric velocities beneath the rift do not connect to a broad deep

plume or sheet-like upwelling. However small convective cells could exist at

scales smaller than the resolution of the surface waves.

F

low velocity zone