Structural record (N to S) along the western side of the Gulf: results and significance

1. Structural record (N to S) along the western side of the Gulf: results and significance Paul Umhoefer Northern Arizona University Colleagues: B. Dorsey, L. Mayer, A. Stone, P. Falk, D. Semmans, K. Vincent, S. Kidwell T. Dixon, L. Mayer, S. Willsey, A. Vlad, CICESE T. Schwennicke, M. Puchalski, M. Del Margo, L. Skinner, J. Macy, UABCS D. Lizarralde, G. Kent, A. Harding, S. Holbrook, G. Axen, J. Fletcher, A. Gonzalez P. Lonsdale, J. Hawkins, J. Fletcher, B. Eakins, W. Drake, M. McTeague R. Arrowsmith, G. Martinez, J. Fletcher, G. Kent, N. Driscoll, T. Dixon, B. Buchanan {and many others….} Much thanks to NSF

2. Corollaries of Plate Tectonics and Plate-scale questions • Do transform faults initiate from rift segment boundaries? • Is the simple hypothesis for rift to drift transition correct? {Rifting ceases when oceanic spreading initiates} • If not, why not and what drives continued rifting? • Are there fundamental asymmetries across divergent margins? • Are there asymmetries when rifting a narrow continental sliver? • Do rifts behave as one coherent belt or are there fundamental changes along the trend of a rift? Controls on changes?

3. 2 fundamental structural divisions of Gulf of California Southeast versus Northwest • SOUTHEAST side: Simple rift to drift b) Transition at 3-4 Ma? Timing only guess in South Posters… Manuel Aragon talk North to South along West side NOT a coherent, simple belt: Faulting and geology suggests 3-4 major domains…

4. I) RIFT - DRIFT: NORTHWEST side still obliquely rifting despite well developed(?) transform-ridge system in southern Gulf 1) Since 6.1 Ma 85% plate motion in Gulf (Oskin et al) 2) Alarcon spreading since 3.6 Ma (Lonsdale) 3) Other southern “basins” since 2 - 3 Ma?

5. Geology of Baja margin: Change from high to low rates faulting 2.5 - 2 Ma (Umhoefer et al) a) Loreto basin: New tuff ages 2.50 and 2.21 Diminished faulting: • Loreto fault • Punta Colorado fault • San Jose Cabo fault b) Punta Colorado basin (San Jose Island) • mudstone = 2 - 2.4 Ma = major changes in basin • abrupt uplift of basin & jump of faulting to east c) Perico basin (Carmen Island) Similar to San Jose Island Forams 3-3.5 Ma (younger?) 2 km

6. 3-4 systems active faults across any latitude in south (many datasets) Harvard earthquake catalog: plate boundary quakes since 2000 Fit transforms well E marginal quakes = oblique faults W marginal quakes = normal faults: since 1976 & 1969 (Molnar, 1973) Ongoing paleoseismology: Wide lines = known Quaternary faults based on scarps & terraces Dashed lines = possible faults

7. Earthquakes common along transforms Along spreading centers And a few normal faults in margin Uplifted marine terraces & fault scarps Indicate other normal faults Bathymetry from: Lonsdale, Eakins and others 2004 cruise

8. INTERLUDE: Instantaneous Extension More westerly KINEMATIC PARTITIONING (from Teyssier et al, 1995)

9. TRANSTENTIONAL STYLES: Loreto Homogeneous (Partitioned in Fault type only) Extension - Dominated Wrench - Dominated “Partitioned Transtension” (De Paola et al) Homogeneous Strike-slip (Kinematic) Partitioning La Paz - Cabo 90 45 Gulf of California  Extension - dominated 0 20 90  Wrench - dominated

10. II) Despite coherent mid K Batholith, (Aeromagnetics: Langenheim & Jachens, 2003) 3 - 4 RIFT DOMAINS along Northwest side of Gulf: Based on: Width of plate boundary Style of faulting Strain partitioning Developmental stage main plate boundary north central south

11. Northern Domain: (Gary Axen review) {Delphin to Salton trough & Agua Blanco fault to W Transverse Ranges} • ~300 km wide active zone • strike-slip & detachment faults • faults mechanically linked • little or no strike-slip (kinematic) partitioning at present • partitioned in past?? • spreading centers not developed: * step-over zones between transforms * linked half graben north central south

12. Central Domain: {Alarcon ridge to Ballenas transform: Pescadero, Farallon, Carmen, Guaymas basins} • narrow west margin Perpendicular to motion: ~50 - 100 km wide active zone • N-S normal faults at margins • faults mechanically linked • no partitioning • transforms well developed • most spreading centers basins rather than ridges • 2 subdomains…. north central south

13. Farallon basin - typical of south-central spreading centers Alarcon spreading center - good axial ridge Lonsdale, Eakins and others, 2004 cruise

14. Central Subdomains & boundaries: • north-central basement at 1-2 km elevation • north-central basement exhumed since latest Cretaceous? • south-central basement at or below sea level • local basement highs in footwall of normal faults • south-central = Guadalupe + Magdalena microplates? • N boundary = Agua Blanco fault =? old Mesozoic transform fault

15. Southern Domain: {East Pacific Rise to Tamayo transform} • wider margin perpendicular to motion: ~100 km wide active zone • N & NNW normal faults at margins • faults not mechanically linked • major kinematic partitioning • Tamayo transform well developed • N EPR spreading center well developed ridge • high basement - exhumed in Late Cretaceous to Early Cenozoic

16. South-central vs southern domains: Loreto vs La Paz Differences N & S of San Jose Island: East Pacific Rise segment versus Alarcon - Pescadero - Farallon - Carmen Key exposed basins: Black transtensional; gray rift

17. East Pacific Rise Alarcon-Pescadero-Carmen Partitioning major kinematic no Structural evolution 1 stage 2 stage Basins rift transtensional Active faults onshore & offshore mainly offshore Seismicity moderate quakes: 1969, 1995 little historically (quakes along EPR - Alarcon zone boundary)

18. Hypothesis: transform faults form from older en echelon strike-slip Faults (Lonsdale, 1989) (project to rift accommodation zones) Alarcon-Pescardero-Farallon-Carmen: Most of margin in “Transform” zone Linked to early formation of transforms Zone of no partitioning black = transtensional basins at margin EPR - Cabo zone: Includes major area outside Transform zone Major partitioning

19. Controls on Rifting style & Partitioning Observation from Seismic transects: Guaymas & N EPR = abrupt rifting; narrow margin Alarcon = slow rifting; wide margin 2002 Ewing cruise

20. Guaymas transect = narrow rifting (similar to north EPR transect) Note Differences Uppermost crust NOTE: significant ENE-WSW to E-W extension out of plane Alarcon transect = wide rifting

21. Controls on Rifting location & style & Partitioning LOCATION: Rift escarpment along Suture in middle Cretaceous Batholith (ONE belt from aeromag. Langenheim & Jachens, 03) Main rift on East side batholith STYLE & PARTITIONING: Crustal strength & Microplate interactions 2002 Ewing cruise

22. Controls on Rifting style & Partitioning MODEL: narrow rifting (major partitioning): = north-central & southern domains CAUSE: Strong crust (high basement) wide rifting (no partitioning): = south-central domain CAUSE: Weak crust (low basement) and microplate interactions: • Guadalupe & Magdalena off south- central domain only until 12 - 8 Ma North North- central South- central South

23. HOW TO TEST HYPOTHESES? • More seismic reflection - determine upper crustal faulting • Drilling edges of basins - determine age of offshore basins/faults • 2 more crustal scale transects across Baja California peninsula within north-central and south-central domains • Quantification of late Quaternary - modern fault patterns/rates: • Document deformation rates over many timescales • Fault localization • How and why rifting continuing across margins • Modeling of fault interactions, fault growth, and how fault arrays & surface processes produce bathymetry/topography • Ongoing in Southern domain

25. Southern - south-Central domains Boundary = Tamayo fracture zone: To NW 20-km wide zone Projects to old rift boundary (SJI) Quakes = partitioning Alarcon-Pescadero-Farallon zones: N faults near peninsula NNE to NE faults closer to ridge No partitioning Local blocks rotating clockwise?? (3 islands) F P A C