Terrane accretion

1. Terrane accretion GEOS 304 - 2009

2. Terrane • Out of place large piece of lithosphere • Bounded by major plate margin faults • Ophiolites present if the motin was not transform • Geology different from adjacent lithosphere • Identifiable source

3. Present day config (Miller et al., 2006) 90 Ma config no trans (Miller et al., 2006)

4. Present day config 90 Ma config N option (Miller et al., 2006)

5. Present day config 90 Ma config S option (Miller et al., 2006)

6. Present day config 90 Ma config (Butler et al., 2001)

7. Present day config 247.5-120 Ma config (Dickinson, 2004)

8. Present day config 120-34 Ma config (Dickinson, 2004)

9. Present day config Present day config (DeCelles, 2004)

10. Present day config 100 Ma config (Wyld et al., 2006)

11. Gehrels reconstruction

12. Cowan et al., 1997, Miller et al., 2006 • Problem: - where were the Insular and Intermontane terranes at ~90 Ma? 1) < 1000 km south, with tilt 2) >2400 km south, without tilt

13. Butler et al., 2001; Butler et al., 2006 • 1) <1000 km south, with tilt • “geologic model”

14. Maxson and Tikoff, 1996 • 2) >2400 km south, without tilt • “paleomagnetic model” Miller et al., 2006

15. Cowan et al., 1997 • “Major diff between two models is position of Insular and Intermontane terranes relative to Franciscan and SNB” Paleomag and Geology says SNB/Franciscan are within several hundred km of position (relative to NA) at 90 Ma (but not prior) Franciscan - subduction from Early Jurassic to Early Tertiary

16. Cowan et al., 1997 • BC: • Insular and Intermontane definitely together by mid-Cretaceous • Does Baja/BC include Intermontane, or just Insular? • Dilemma: was Insular terrane south of Sierras (i.e., >2500 km) or north (i.e., ~1000 km)

17. BC: • Thrusting/crustal thickening W of CSZ was done by ~85 Ma • Thrusting E of CSZ (Sheemahant) is between 91 and 54 Ma, likely pre-dating CSZ motion from ~62-56 Ma (not transpressive)

18. Paleomag model • Mean “good” paleomag results suggest 3000 km displacement for Insular • Paleomag from W Intermontane suggest 1530+770 km of displacement (attenuated displ within boundary zone?) • Paleomag from E Intermontane suggest no displacement Cowan et al., 1997

19. Paleomag model • Prior to ~100 Ma, Franciscan, GV, SNB were together, but ~650 km south of current position, in Mexico • Salinia was therefore ~1000 km S adding in SAF mvmt • PRB was ~1100 km (or 300-350 km according to Butler, Dickinson, and Gehrels, 1991, based on geology) Cowan et al., 1997

20. Paleomag model • “Collision” of Baja/BC occurred from ~100 to ~83 Ma with ongoing magmatism, S end of Franc • Caused “extinction of magmatism” at ~90 Ma • What about Laramide (Maxson and Tikoff, 1996)? • Insular began to move Nward no earlier than 85 Ma - Concordant paleomag by 50 Ma • 38.4ºN Lat based on paleoflora (2200 km) • W Intermontane began to move Nward no earlier than 70 Ma Cowan et al., 1997

21. Paleomag model • They suggest Insular was displaced at 13 cm/yr (Farallon/Kula PM) for 2000 km from 85 to 70 Ma (geologically reasonable???) - if constant displacement, would be 8.5 cm/yr • Met up with W Intermontane, then moved at 5 cm/yr from 70 to 50 Ma Cowan et al., 1997

22. Paleomag model • Fault was likely a continental transform • Near margin • Outboard of PRB, Salinia, and Franciscan complex, Intermontane • Another fault within Intermontane, to E? Cowan et al., 1997

23. Paleomag model • Fault was likely a continental transform • Near margin • Outboard of PRB, Salinia, and Franciscan complex, Intermontane • Another fault within Intermontane, to E? Cowan et al., 1997

24. Maxson and Tikoff, 1996 Paleomag model • Fault was likely a continental transform • Near margin • Outboard of PRB, Salinia, and Franciscan complex, Intermontane • Another fault within Intermontane, to E? Cowan et al., 1997

25. Paleomag model • Tests basically include: • Amount, timing of slip of subject faults • Provenance of seds on Baja/BC from 90-50 Ma • Tectonic motions, I.e. subduction and transform faults Cowan et al., 1997

26. Geologic/tilt model • Evidence for small displacements (?): • Dz links of terranes to margin (e.g., Gehrels, 2002) Cowan et al., 1997

27. Geologic/tilt model • Instead of large displacement, they call for tilt (28-40°) and smaller displacement • Tilt is indicated by regional thermobarom (Al in hbl and met rocks)

28. Geologic/tilt model • Butler’s Al in hbl data suggest ~40° NE side up tilt (10 km in W, 22 km in E) Butler et al., 2002

29. Geologic/tilt model • thermobarom My data, Himmelberg et al., 2004, Butler et al., 2002, 2006, Ducea, etc.

30. Geologic/tilt model • Discrepancies among thermochronometers suggest tilt Butler et al., 2002 Butler et al., 2006

31. Geologic/tilt model • Paleomag discrepancies among diff plutons Butler et al., 2006

32. Geologic/tilt model • Seismic evidence for Neogene tilt Butler et al., 2006

33. “accounted for” displacement • ~450-900 km displacement vs. “1100-3000 + 500-900 km” Wyld et al., 2006

34. Colpron et al., 2007 • “intact” geological history of NW NA Cordillera since Late Pz

35. No fault! Rusmore et al., 2000

36. New data show discordant paleomag in E Intermontane terrane and S Rocky Mtns. - entire W Cordillera prob? • Enkin et al., 2006 • How does Wetmore et al. play into this? Miller et al., 2006 map

37. Kelley, 2003 map

38. Geologic/tilt model • Problems include: • Butler et al., 2006 says much of tilt is on Neogene faults, but at ~50 Ma paleomag (according to Cowan) is concordant, and discordant stuff is older • Older faults are all overprinted and intruded by plutons • More deeply exhumed rocks near CSZ • Subjective correction for compaction shallowing/pluton tilt