COMPARISON OF DIKE AZIMUTHS IN THE INDEPENDENCE DIKE SWARM, CALIFORNIA, TO PALEOMAGNETIC DATA—IMPLICATIONS FOR CENOZOIC VERTICAL AXIS ROTATIONS

R. F. HopsonGeological Information Services

Reno, Nevada

J. W. Hillhouse and K. A. HowardU.S. Geological SurveyMenlo Park, California

INTRODUCTIONINTRODUCTION

Geographic extent—Eastern Transverse Ranges to the central Sierra Nevada; 500 km

Host rocks—PC to Jurassic sedimentary, igneous, metamorphic

Ages—139–159 Ma, peaking ~148 Ma

Composition—Mafic to silicic

Texture—Microcrystalline to medium-grained, some porphyritic

Other—Cluster in domains (most dikes exhibit a preferred orientation)

PURPOSEPURPOSETo evaluate the consistency of IDS trends

with vertical axis rotations based on published paleomag results for Miocene volcanic rocks

METHODSMETHODS• Determined azimuths of ~3800 dikes in 40

domains by measuring orientations on geologic maps and aerial photographs

• Dike orientations analyzed with Stereoplot and StereoWin

COMPARISON OF DIKE AZIMUTHS COMPARISON OF DIKE AZIMUTHS TO PALEOMAGNETIC ROTATIONSTO PALEOMAGNETIC ROTATIONS

Sierra Nevada and ranges to the east

Northeastern Mojave Desert

COMPARISON OF DIKE AZIMUTHS COMPARISON OF DIKE AZIMUTHS TO PALEOMAGNETIC ROTATIONSTO PALEOMAGNETIC ROTATIONS

Eastern Mojave Desert Southern Mojave Desert and Eastern Transverse Ranges

DISCUSSIONDISCUSSIONMean domain azimuths of Independence dikes as measured today

Mean domain azimuths of Independence dikes restored for paleomagnetic rotations where paleomagnetic data available

0

2

4

6

8

10

12

275 295 315 335 355 15 35 55 75

Azimuths

Do

mai

ns

Fre

qu

ency

East of Sierra

Sierra Nevada

Mojave Desert

ETR

0

2

4

6

8

10

275 295 315 335 355 15 35 55 75

Azimuths

Do

ma

ins

Fre

qu

en

cy

East of Sierra

Sierra Nevada

Mojave Desert

ETR

DISCUSSIONDISCUSSION

Total Independence dikes azimuths restored for paleomagnetic rotations where paleomagnetic data available

Total Independence dikes azimuths as measured today

0100

200300

400500

600700

275 295 315 335 355 15 35 55 75

Azimuths

Dik

e F

req

ue

nc

y

East of Sierra

Sierra Nevada

Mojave Desert

ETR

0

200

400

600

800

275 295 315 335 355 15 35 55 75

Azimuths

Dik

e Fr

eque

ncy

East of Sierra

Sierra Nevada

Mojave Desert

ETR

DISCUSSIONDISCUSSION

Large deviations from the main trend of IDS may reflect:

1. Dike intrusions of other ages

2. Crack filling at angles oblique or perpendicular to the swarm

3. Pre-Miocene rotations

4. Unrecognized domain boundaries between sites with paleomagnetic determinations and the dikes.

CONCLUSIONSCONCLUSIONS

• In most cases, local orientations of IDS are consistent with Miocene and younger rotations

• Several domains where dikes do not line up with the swarm

• More age dates are needed, especially where dikes vary from the regional trend

• IDS has limited usefulness for constraining late Cenozoic rotations

THE ENDTHE END