1. Rupturing Continental Lithosphere (RCL): styles of extension wrap-up lecture

2. Continental lithosphere ruptures in a variety of ways Styles of extension vary through space and time!

3. Variations: Low-angle to high-angle through time Change in location of main area of rifting Widely distributed faulting vs. focused/ localized rifting Little magmatism to robust magmatism

4. Variations: Low-angle to high-angle through time Change in location of main area of rifting Widely distributed faulting vs. focused/ localized rifting Little magmatism to robust magmatism

5. Northern Gulf: Upper Tiburon and Delfin Basins Location of next seismic line shown: extends across the Upper Delfin Basin to the Upper Tiburon Basin (González-Fernández et al., 2005)

6. Low angle detachment faulting in the Tiburon Basin (González-Fernández et al., 2005)

7. Northern Gulf: Upper Tiburon and Delfin Basins Localized crustal thinning begins ~6 Ma via a major low-angle detachment fault in the Upper Tiburon Basin Crust thinned from ~30 km to ~19 km (this includes ~8-10 km of added sediments) Extension in Upper Tiburon basin ended ~2-3 Ma

8. Northern Gulf: Upper Tiburon and Delfin Basins ~2-3 Ma the focus of extension shifted from the Upper Tiburon basin to the Upper Delfin basin This included – the change from low-angle detachment faulting in the Upper Tiburon basin to high-angle normal faults and the development of a narrow rift zone within the Upper Delfin basin – Some basaltic(?) magmatism, mostly intruded as sills into lower basin sediments

9. High-angle faulting and focused rifting in the Upper Delphin Basin (González-Fernández et al., 2005) (Martin-Barajas et al., 2013)

10. Variations: Low-angle to high-angle through time Change in location of main area of rifting Widely distributed faulting vs. focused/ localized rifting Little magmatism to robust magmatism

11. Westward migration of extension in the northern Gulf of California, Mexico: Rifting formed eastern transtensional pull-apart basins first (Aragon-Arreola and Martin-Barajas, 2007)

12. Westward migration of extension in the northern Gulf of California, Mexico: ~2-3 Ma, the locus of rifting shifted west, forming new transtensional pull-apart basins

13. Location of active (dark gray) and inactive (light gray) pull- apart basins (Aragon-Arreola and Martin-Barajas, 2007)

14. Active and inactive (abandoned) pull- apart basins with bathymetry shown (Bennett et al, 2013)

15. Why the westward shift? Not fully known but the current hypothesis is… Thick sediments deposited from the Colorado River may play a role in insulating the lithosphere and creating lateral heat flow, which in turn affects lithospheric strength and resulting strain Prior volcanism in Baja and mainland Mexico may have also affected lateral heat flow

16. Variations: Low-angle to high-angle through time Change in location of main area of rifting Widely distributed faulting vs. focused/ localized rifting Little magmatism vs. robust magmatism during rifting

17. Variations in space Different rift segments in the Gulf of California display wide vs narrow rift geometries and high vs low magmatism within the same time frame

18. The Gulf of California consists of numerous closely spaced rift segments Central and Southern Rift Segments: G = Guaymas C = Carmen F = Farallon (O needs an accent) P = Pescadero A = Alarcon Many rift segments: (Lizarralde et al., 2007)

19. Seismic Transects Seismic data was collected along northwest-southeast transects, parallel to the extension direction, within each of several segments to understand the geometry and composition of the crust from one rifted margin across to the other rifted margin The data revealed interesting patterns: lots of variation from segment to segment. This was not expected! (Lizarralde et al., 2007)

20. A big surprise from MARGINS work: The variation between wide rift zones with little magmatism and narrow rift zones with robust magmatism occurred over a short distance: much shorter than predicted and noticed elsewhere Required a different explanation than previously thought! Many rift segments: (Lizarralde et al., 2007)

21. Previous observations and assumptions: The width of rifts, narrow vs. wide, and the amount of magmatism (from almost none to 2-3 x the predicted amount) are thought to be controlled by: – Extension rate – The thickness of the crust or lithosphere – Heat flow – Lower crustal flow – The potential temperature of the mantle Some models suggest extension rates are the most important factor determining rift geometry: – For example, narrow rifts may result when “extension rates outpace thermal diffusion” and stretching and necking occurs (Lizzaralde et al., 2007, after England, 1983) – Wide rifts may form when extension rates are slow, allowing cooling of lithosphere which thus maintains strength and deformation is spread out, thus preventing necking (Lizzaralde et al., 2007; Hopper and Buck, 1996) Other models suggest that crustal thickness and heat flow are more important in controlling rift geometry: – Wide rifts may result from warm thin lithosphere, i.e, are a function of crustal thickness and heat flow (Hopper and Buck, 1996 and references therein) The temperature of the mantle is thought to control the amount of magmatism present during rifting In all of these models, the predicted controlling factors would all operate over large areas and thus all rift segments in a region would behave similarly Not the case in the southern Gulf of California!

22. Southern Gulf of California: several segments were imaged The width or distance over which MOHO relief is observed varies greatly from segment to segment MOHO found between mantle shown in blue and crust shown in warm colors (Lizarralde et al., 2007)

23. Variations in the G of C rift segments: Guaymas: narrow, robust magmatism Alarcon: wide with minor magmatism Cabo-PV: narrow, “neither magma-rich or magma-poor”

24. How to explain the variation over such short distances?? The authors examined the geologic map and volcanic record of Baja and mainland Mexico and noticed a unique pattern

25. When you restore Baja CA back to mainland Mexico, some rift segments formed in an area that experienced significant Miocene volcanism (Alarcon) and some did not (Guaymas and Cabo-PV)! Red boxes show areas of significant volcanism prior to rifting! (Lizarralde et al., 2007)

26. Lizzaralde et al. 2007 conclusions: Mantle depletion from volcanism prior to extension leads to wide, magma-poor segments A lack of volcanism just prior to rifting helps maintain mantle fertility which leads to magmatism during rifting Sedimentary insulation may also contribute to rift and post-rift magmatism by insulating the lithosphere and increasing melting (this idea is still being discussed and debated)

27. Other authors have different interpretations for the cause of the variations in the width and amount of magmatism from one segment to another

28. Sutherland et al., 2012… Suggest that a tear in the subducting slab between the north and south GOC, just north of the Alarcon segment, may be responsible for the differences in extensional styles Note that low-angle detachment faulting/ductile deformation is found in the north (Upper Tiburon) whereas the southern rift segments are symmetric and display brittle deformation Suggest the tear in the slab created different thermal regimes north and south of the tear which changed the strength of the lithosphere

29. Summary of Evolution of Gulf of California First interpretation: – Pre-Gulf Stage: Subduction, Arc Volcanism and Back- arc east-west (ENE-WSW) extension present up to 12 Ma – Proto-Gulf Stage: 12-6 Ma change to Pacific-North American dextral transform boundary Transform faults west of Baja Proto-Gulf east of Baja with continued ENE-WSW extension – Gulf of CA stage: 6-present P-NM boundary shifts to Gulf around 6, Baja mostly coupled to Pacific plate Gulf opens via a series of oblique transtensional faults, transform faults and normal faults

30. New Interpretations from MARGINS: revising the Proto-Gulf Stage: Some dextral slip west of Baja microplate and some east during this stage, 12-6 Southern GOC : transtensional began around 12 Ma (Sutherland et al., 2012) Northern GOC: dextral shear is recorded onshore in mainland Mexico, east of and prior to the opening of the northern GOC and may have started as early as 11.5 Ma but certainly by 8 Ma (Bennett et al., 2013),

31. Pre-Gulf stage Proto- Gulf stage From Bennett et al, 2013

32. Proto- Gulf stage 12.5-6 Ma Modern Gulf 6-0 Ma From Bennett et al, 2013

33. References Aragón-Arreola, M. and Martín-Barajas, A., 2007, Westward migration of extension in the northern Gulf of California, Mexico, Geology, v. 35, p. 571-574. Bennett, S.E., Oskin, M.E, and Iriondo, A., 2013, Transtensional Rifting in the Proto-Gulf of California 1 Near Bahía Kino, Sonora, México, Geolgical Society of America Bulletin, v. 125; no. 11/12; p. 1752–1782; doi: 10.1130/B30676.1. Dorsey, R.J., 2010, Sedimentation and crustal recycling along an active oblique-rift margin: Salton Trough and northern Gulf of California, Geology, v. 38, p. 443-446. England, P.C., 1983, Constraints on extension of continental lithosphere, Journal of Geophysical Research, v. 88, 1145-1152. González-Fernández, A., Danobeitia, J.J., Deldago-Argote, L., Michaud, F., Cordoba, D., and Bartolome, R., 2005, Mode of extension and rifting history of upper Tiburón and upper Delfin basins, northern Gulf of California, Journal of Geophysical Research, v. 110, p. 1-17. Hopper, J.R. and Buck, W.R., 1996, The effect of lower crustal flow on continental extension and passive margin formation, Journal of Geophysical Research, v. 101, No. B9, p. 29,175-29,194. Lizarralde, D., Axen, G.J., Brown, H.E., Fletcher, J.M., Antonio González-Fernández, A., Harding, A.J., Holbrook, W.S., Kent, G.M., Paramo, P., Sutherland, F. & Umhoefer, P.J. (2007) Variation in styles of rifting in the Gulf of California, Nature, 448, 466-469. Martín-Barajas, A., M. González-Escobar, J. M. Fletcher, M. Pacheco, M. Oskin, and R. Dorsey (2013), Thick deltaic sedimentation and detachment faulting delay the onset of continental rupture in the Northern Gulf Of California: Analysis of seismic reflection profiles, Tectonics, 32, doi:10.1002/tect.20063.