Dextral shear as a catalyst for lithospheric rupture in the northern Gulf of California: Kinematic and paleomagnetic evidence from coastal Sonora, México

UNC Martin Fund

Scott BennettMike OskinUniversity of North Carolina Chapel Hill

Paper No 123-9

Graduate Student Research Grant

OUTLINEMOTIVATION / DEBATE

- 2 End-Members

METHODS / RESULTS

- Paleomagnetism

- Structural Mapping

CONCLUSIONS

- Timing and Mechanism for Gulf of California Rupture

INTRODUCTION & MOTIVATIONPre-30 Ma Subduction Modern Gulf

TIME

?

INTRODUCTION & MOTIVATION

Distributed Transtension

6-0 MaPre-ProtoGulf

Proto-Gulf (12.5 - 6 Ma)

StrainPartitioningOR

TIME

INTRODUCTION & MOTIVATION

Sacrificio Fault

METHODS: StratigraphyTuff of San Felipe (Tsf) 12.50 Ma

METHODS: Stratigraphy

Tsf marks the beginning of Proto-Gulf

Tuff of San Felipe (Tsf) 12.50 Ma

METHODS: StratigraphyTuff of Mesa Cuadrada (Tmc) 6.39 Ma

Tmc marks the end of Proto-Gulf

METHODS: Stratigraphy

METHODS: Stratigraphy

RESULTS: Stratigraphy

RESULTS: Stratigraphy

Dips Shallow

with age

METHODS: Paleomagnetism

PaleomagneticTests Within Gulf

ExtensionalProvince

Pro

to-G

ulf

‘PC’ = Punta Chueca‘CT’ = Cerro Tordillos

RESULTS: Paleomagnetism

PC

CT

2 Pmag sites with exposures of both tuffs

RESULTS: Paleomagnetism

PC

CT

Rotated Sites:Tsf & TmcDeformed

Coastal Sonora

Reference Site:Tsf & Tmc

StableBaja CA

RESULTS: Paleomagnetism

PC

CT

‘CT’Tsf not rotated

GSA (1999)

2 of 15 cores

RESULTS: Paleomagnetism

PC

CT

‘CT’Tsf not rotatedTmc unable to determine

GSA (1999)

2 of 18 cores

RESULTS: Paleomagnetism

PC

CT

‘PC’Tsf rotated 42 ±4° CW Tmc rotated 46 ±11° CW

GSA (1999)

2 of 18 cores

RESULTS: Paleomagnetism

PC

CT

Reference Site:Tsf & TmcExposures

in stableBaja CA

‘CT’Tsf not rotatedTmc unknown rotation

‘PC’Tsf &Tmc same CW rotation

METHODS: Structural Mapping

KinematicTests Within Gulf

ExtensionalProvince

Pro

to-G

ulf

RESULTS: Deformation

Strike-Slip faults:NW-dextralNNE-sinistral

Normal faults:N-S low-angleN-S high-angle

Reactivation of fault surfaces

RESULTS: Fault Kinematics

Fault Striae Post-6.4 Ma (n=8)

Fault Striae 12.5-6.4 Ma (n=24)

Tmc (6.4 Ma) divides Proto-Gulf from modern Gulf fault kinematics

Tmc (6.4 Ma)

CONCLUSIONS(1) Paleomagnetics

suggest transition from pure extension to transtension after 6.4 Ma

(2) Fault Kinematics suggest transition from E-W extension to NW-directed transtension after 6.4 Ma

(3) 2 Methods both support Strain Partitioning for the proto-Gulf of Calfornia

CONCLUSIONS

Modern GulfPre-Proto Gulf

Proto-Gulf (12.5 - 6 Ma)

Strain Partitioning

TIME

ACKNOWLEDGMENTS

Alex Iriondo- UNAM Juriquilla (40Ar/39Ar geochronology)

Joseph Kirschvink- CalTech Paleomagnetics Laboratory

Tadeo Pfister- Kino Bay Center, Prescott College

Michael Joseph Tappa- UNC Chapel Hill

“Dextral Displacement in Gulf of CA”modified from Oskin & Stock (2003)`

CONCLUSIONS

Proto-GulfTime (Ma)

modified from Oskin & Stock (2003)

volcanic ventfor 6.4 Ma

Tmc tuff

Tsf

TmcWithin

theGulf