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Terrane Sta)ons: seismic tomography explains where the North American Cordillera came from Karin Sigloch, University of Munich (LMU) Work with Mitch Mihalynuk, Geol. Survey of Bri)sh Columbia The proto-Pacific in Jurassic/Cretaceous times? Moores 1998: “Whimsical tectonic map,” from Simkin et al. 1989. 70% of the North American Cordillera is made up of ‘suspect terranes’ A cratonic N es Coney et al. 1980 accreted terran • Terranes accreted to western NA margin between ～200 Ma and 50 Ma. • “…most of them display sedimentary and volcanic rock sequences that are of oceanic affinity rather than con)nental.” • Rela)vely young rocks (middle Paleozoic and younger). Standard model: Pacific basin 140 Myr ago… F YS Engebretson et al. 1985 …and 80 Myr ago… YS F Engebretson et al. 1985 …and today. This scenario is constrained by marine magnetic anomalies only. F YS Engebretson et al. 1985 150 million years of simple, Farallon-beneathcontinent subduction? • North America moved west as the Atlantic opened (constrained by magnetic stripes on Atlantic seafloor). • But was there always a trench along the continent’s west coast? Reconstructed western margin of North America over time (Ren et al. 2007). Andean-‐style margin or not – whence the uncertainty? A cratonic N es Coney et al. 1980 accreted terran • Very complex geology, every new accre)on obscured the previous one. • Ambiguity because arc plutons did not intrude into craton. • Terranes were not accreted in their current posi)ons. Displacements of 500 km or 3000 km? (“Baja-‐BC controversy”) Slabs subducted into a west-coast trench? Westward mo)on of NA margin is well constrained by Atlan)c spreading… but its trajectory does not fit the shape of imaged slabs (= paleo-‐ trenches). slabs at 1500 km depth Sigloch & Mihalynuk, in review Contribu)on from seismology: Whole-‐mantle tomography under North America Discussion is based on my most recent waveform inversion of teleseismic P-‐waves (mul)-‐frequency tomography, Sigloch 2011 G-‐cubed). Sensitivity: what is being measured? predicted observed Sigloch & Nolet 2006 Example: a )me delay between predicted and observed P-‐ waveform. Need to compute the sensi)vity of the chosen observable: Which mantle region(s) could have caused the delay? time in sec Nissen-‐Meyer, 2007 Waveform tomography: proper modeling of wave propagation Waveform methods displacement / AU displacement / AU Ray theory 0 P 500 ◆tP ◆tPP 0 PP time / s 1000 Traveltime „measurement“ over a fictitious, infinitely short time window. 500 time / s Traveltime (or other) measurement over a finite time window. dominant period 20 s Measurement sensitivity confined to an infinitely narrow ray. Sensitivity of finite spatial support (the Fresnel zone) 1000 Resul)ng velocity models: ray-‐theory versus mul)-‐frequency tomography Ray-‐theore)cal tomography Finite-‐frequency tomography Comparing a ver)cal east-‐west sec)on through North America at 40°N (la)tude of northern California). Finite-frequency result, rendered in 3-D Velocity anomalies in the mantle are resolved down to 1500-‐2000 km depth under North America. Conven)onal rendering of a tomographic model: 2-‐D sec)on at a fixed depth (600 km). My renderings: 3-‐D isosurface enclosing all FAST structure (=subducted slabs) at and below 600 km depth. Seismically fast domains in the lower mantle Figure S3 Grand 1994 1600-‐1750 km depth Sigloch et al. 2008 Slab walls under North America 3-‐D isosurface rendering of all seismically fast structure at and below 800 km depth, dVp/Vp=0.35% Slab walls under North America 3-‐D isosurface rendering of all seismically fast structure at and below 400 km depth, dVp/Vp=0.35% Slab walls at and below 700 km depth pronounced eastward promontory depth (km) Sigloch 2011 G-‐cubed Slab walls at and below 700 km depth Farallon, too?? Definitely Farallon! depth (km) Sigloch et al. 2008 NatGeo Sigloch 2011 G-‐cubed Slab walls at and below 700 km depth Width of slab walls: 400-‐600 km (i.e., 4-‐6 )mes lithospheric thickness) Sigloch 2011 G-‐cubed Did North America override an archipelago of island arcs during Cretaceous )mes? North Farallon slabs (Cascadia) South Farallon Mezcalera slab Figure 1 All slabs at and below 900 km depth (i.e., deposited before ~90 Ma?) Plate reconstruc)ons (O’Neill et al. 2005) and interpreta)on. How to deposit a massive, vertical slab wall: a long-lived, stationary trench – and a slab that buckles beneath it. Ribe et al. 2007 EPSL Stationary trench deposits a vertical slab wall. Sta)onary trench? Only possible if trench was intra-‐ oceanic (west of westward-‐moving North America). island arc terrane Farallon ocean WEST WEST Mezcalera ocean NA EAST Sigloch & Mihalynuk, in review Stages of island arc override a: Oceanic trench ac)ve. b: Shortly aper override. sharp upward trunca)on of slab wall c: Long aper override. New Andean-‐style margin produces smeared out upper-‐mantle slab. Slab wall con)nues to sink. accreted arc terrane Stages of island arc override The observa)on of steep slab walls strongly suggests ver)cal sinking. If slab wall sank ver)cally: ⇒slab s)ll located in the absolute x-‐y posi)on where its trench was. ⇒”slab wall” and hotspot reference frames are iden)cal. ⇒plate reconstruc)on predicts arrival )me of con)nental margin at slab (= paleo-‐trench). Quantification of slab sinking rates and ages Using sharply imaged upper trunca)ons of the slab walls. Plate reconstruc)on: “Margin was here at )me T.” D (from tomography) TODAY Figure 2 Sinking rate = D/T ≈ (10 ± 2) mm/yr ⇒ Slab age, example: D = 1000 km depth ≙ T= 100 Ma since subduc)on Geophysics predicts paleogeography If slab wall sank ver)cally: ⇒slab s)ll located in the absolute x-‐y posi)on where its trench was. ⇒plate reconstruc)on predicts arrival )me of con)nental margin at slab (= paleo-‐trench). This is also the predicted )me of terrane accre)on. Hence geophysical predic)ons can be tested using terrane collision observa)ons. Sigloch & Mihalynuk, in review Sigloch & Mihalynuk, in review Before 140 Ma: Stationary oceanic trenches acted as “terrane stations” Slabs already deposited at 140 Ma, based on 10 mm/yr sinking. Inferred terrane assemblage before override of archipelago The Mezcalera and Angayucham oceans will gradually close. First con)nent-‐arc collisions predicted and observed in Pacific Northwest (A1) Sigloch & Mihalynuk, in review 110 Ma: Oceanic arcs are being overridden along much of the NA margin. Causes Sevier and Canadian Rocky Mountain orogenies (B2). Prolonged override of MEZ promontory à Omenica magma)c belts. Intermontane superterrane (purple) collapsed onto NA margin. Sigloch & Mihalynuk, in review 75 Ma: Coupled to Farallon, accreted terranes get shuffled north. Intermiwent strong coupling of terranes to Farallon plate, due to accre)on of a buoyant oceanic plateau (Shatsky Rise conjugate, B3). A corresponding slab window is clearly imaged, explains Tarahumara ignimbrite province (85±5 Ma, A4). Buoyant plateau accre)on also causes Laramide orogeny (B3). Sigloch & Mihalynuk, in review 55 Ma: Archipelago override complete, last accretions along Cascadia margin. In Pacific Northwest, North Farallon trench Figure turns con)nental. Accre)on 2 of associated terranes (Siletzia, Pacific Rim). End of northward shuffle along margin. Red Angayucham terranes now make up interior Alaska. Intermontane/Insular superterranes now in B.C. Conclusion
