Fig. 1. Taken from Cloos et al. (2005)
Colin Sturrock
A central concern for any model of Cordilleran assembly in North America is subduction polarity. Though eastward convergence onto the western margin of North America since the late Devonian is hypothesized in many Cordilleran models (e.g. Dickinson 2004), Hildebrand (2009, 2013) argues for a passive western margin and west dipping subduction until 85-75 Ma. As convergence slowed, the slab flattened, shutting down magmatism, then broke off as subduction switched to east dipping at 53 Ma (Hildebrand 2009, 2013). Slab breakoff has been suggested as a mechanism to flip subduction polarity (Teng et al. 2000), which is essential for Hildebrand’s collisional Cordilleran model.
However, slab break-off does not necessarily imply that subduction polarity had to have flipped, as decoupling of mantle and crust can occur whenever transitional crust (15-20km in thickness) enters a subduction zone (Cloos et al 2005). This would be consistent with any thick crust entering a subduction zone regardless of polarity, and would not unequivocally favor the hypothesis of Hildebrand (2009). In arc-continent collisions, subduction polarity reversal is possible without slab break-off, though it is unclear whether this applies to continent-continent collisions (e.g. Clift et al. 2003; von Hagke et al. 2016). Models of continent-continent collision often assume slab break-off as a natural consequence (e.g. Davies and Blanckenburg 1995). Intuitively, it seems that slab break-off is a natural consequence of subduction polarity reversal during a continent-continent collision, but this may be the product of thinking in a simplistic 2-D “cross-sectional” mindset. The following sections will discuss Hildebrand’s claim of slab break-off magmatism occurring in the Coast Mountains batholith, British Columbia and its resemblance to other instances of slab break-off.
Coast Mountains Batholith
The Coast Mountains batholith (CMB) are a 1600 km belt of Jurassic-Tertiary plutons along the British Columbian and southern Alaskan coast (Rusmore et al 2013). Straddling the Insular and Intermontane Superterranes, the CMB has been interpreted to represent arc magmatism from subduction of the Pacific plate, at a time when the margin was transitioning from convergent to dextral transform (Monger et al. 1982). The portion of the CMB hypothesized to have resulted from slab break-off magmatism is late Cretaceous–early Tertiary in age. Hildebrand (2009) posits a few criteria that argue for slab break-off magmatism in the CMB: (1) linear belts of magmas, (2) wide range of compositions, (3) rapid uplift with a strong, linear focus, (4) voluminous bloom of magmatism, (5) colocation of the edge of the craton, (6) porphyry copper deposits. Of these, (1) and (2) would be indistinguishable from arc magmatism, and it is unclear how (5) is significantly different from most instances of arc magmatism. This leaves (3) rapid uplift, (4) voluminous magmatism and (6) porphyry deposits.
Rapid Uplift
Rapid uplift is a clear physical consequence of slab break-off (Davies and Blanckenburg 1995). Between 60-50 Ma, the CMB experienced 15 km of tectonic exhumation (Hollister and Andronicos 2006), interpreted to result from slab-breakoff (Hildebrand 2009, 2013). In New Guinea, slab break-off magmatism is thought to have produced 2.5 km of uplift in <2 m.y. (Cloos et al. 2005). In the Tibetan plateau, 4 km of uplift is also thought to have occurred in 5- 10 Ma after slab breakoff (Lee et al. 2009). If true, this supports slab break-off in the CMB, as a simple arc model would not predict the sudden initiation of a large, focused uplift.
Voluminous Magmatism
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Fig. 1. Taken from Cloos et al. (2005) |
Slab break-off magmatism is thought to result from hot asthenosphere rising up through the opening left by the torn or broken oceanic lithosphere. This would result in adiabatic decompression melting, though the volume produced depends on the amount the lithosphere has stretched prior to tearing and the rate at which it tears (Fig. 1) (Cloos et al. 2005). Melt column thickness increases with greater amount of stretching and more rapid tearing. Without taking these variables into account, it seems difficult to determine exactly what constitutes “voluminous magmatism” and if it is a unique result of slab break-off.
An attempt was made to quantify the magmatic flux in the CMB areas denoted as slab breakoff magmas. The map units denoted “slab failure magmatism” in Figure 13 of Hildebrand (2009) were selected in Photoshop. Pixel counts for the selected areas were divided by the length in pixels of the plate boundary, drawn from end to end of the distributed magmas, and then converted to km2/km using the scale bar. The resulting value is 1.12 km2/km. During what is presumed to be the slab failure period (58-50 Ma) (Hollister and Andronicos 2006; Hildebrand 2009), magmatic fluxes in the CMB were estimated at 35-50 km3/km, which is comparable to my value when multiplied by the same paleodepth of 25 km (Gehrels et al. 2009). The same procedure was performed on the Linzizong volcanics and Gangdese batholith in the Tibetan plateau, resulting in a value of 10.64 km2/km (Lee et al. 2009). This value is much greater than that of the CMB likely because it includes volcanic units, whereas the CMB is only the plutonic core. Comparisons to Taiwanese magmatic distributions would be similarly problematic.
Porphyry Copper
Hildebrand (2009, 2013) notes the correlation of porphyry copper deposits in the CMB with proposed slab break-off magmatism. The link between break-off and major mineral deposits is based on an instantaneous heat flux for magmatic and hydrothermal activity coincident with transition from regional extension to compression (Vos et al. 2007). Porphyry copper exists in hypothesized break-off magmas in the CMB and Sonoroa (Hildebrand 2009, 2013), the Alps (de Boorder et al. 1998) and in New Guinea (Cloos et al. 2005). However, other areas of supposed slab break-off do not contain porphyry deposits (Taiwan) or the deposits are much younger than the period of break-off (Tibet) (Teng et al. 2000; Lee et al. 2009).
Conclusions
Based on conventional models of continent-continent collisions, slab break-off is virtually required for subduction polarity to reverse on the western margin of North America from west to east in the late Cretaceous. When considering other areas of hypothesized slab break-off, the Coast Mountains Batholith compares favorably in that it shows rapid uplift coincident with magmatism in the late Cretaceous-early Tertiary, and contains porphyry copper deposits. Comparisons of magmatic flux prove to be problematic, both in attempts to quantify magmatic flux and significant potential differences in what amount of magmatic flux an individual case of slab break-off should produce. A truly quantitative comparison may be possible in a more rigorous study. There appears to be enough of a case here for slab break-off to avoid discarding the collisional hypothesis on the basis of its absence. However, even if slab break-off were to have produced the CMB, it would not be inconsistent with models of terrane accretion at an east dipping subduction zone.
References
Cloos, M., Sapiie, B., van Ufford, A. Q., Weiland, R. J., Warren, P. Q., & McMahon, T. P. (2005). Collisional delamination in New Guinea: The geotectonics of subducting slab breakoff. Geological Society of America Special Papers, 400, 1-51.
Clift, P. D., Schouten, H., & Draut, A. E. (2003). A general model of arc-continent collision and subduction polarity reversal from Taiwan and the Irish Caledonides. Geological Society, London, Special Publications, 219(1), 81-98.
Davies, J. H., & von Blanckenburg, F. (1995). Slab breakoff: a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens. Earth and Planetary Science Letters, 129(1), 85-102.
Dickinson, W. R. (2004). Evolution of the North American cordillera. Annu. Rev. Earth Planet. Sci., 32, 13-45.
Gehrels, G., Rusmore, M., Woodsworth, G., Crawford, M., Andronicos, C., Hollister, L., ... & Davidson, C. (2009). U-Th-Pb geochronology of the Coast Mountains batholith in north-coastal British Columbia: Constraints on age and tectonic evolution. Geological Society of America Bulletin, 121(9-10), 1341-1361.
Hildebrand, R. S. (2009). Did westward subduction cause Cretaceous–Tertiary orogeny in the North American Cordillera? Geological Society of America Special Papers, 457, 1-71.
Hildebrand, R. S. (2013). Mesozoic assembly of the North American cordillera (Vol. 495). Geological society of America.
Hollister, L. S., & Andronicos, C. L. (2006). Formation of new continental crust in Western British Columbia during transpression and transtension. Earth and Planetary Science Letters, 249(1), 29-38.
Lee, H. Y., Chung, S. L., Lo, C. H., Ji, J., Lee, T. Y., Qian, Q., & Zhang, Q. (2009). Eocene Neotethyan slab breakoff in southern Tibet inferred from the Linzizong volcanic record. Tectonophysics, 477(1), 20-35.
Magni, V., Faccenna, C., Hunen, J., & Funiciello, F. (2013). Delamination vs. break‐off: the fate of continental collision. Geophysical Research Letters, 40(2), 285-289.
Monger, J. W. H., Price, R. A., & Tempelman-Kluit, D. J. (1982). Tectonic accretion and the origin of the two major metamorphic and plutonic welts in the Canadian Cordillera. Geology, 10(2), 70-75.
Rusmore, M. E., Bogue, S. W., & Woodsworth, G. J. (2013). Paleogeography of the Insular and Intermontane terranes reconsidered: Evidence from the southern Coast Mountains Batholith, British Columbia. Lithosphere, 5(5), 521-536.
Teng, L. S., Lee, C. T., Tsai, Y. B., & Hsiao, L. Y. (2000). Slab breakoff as a mechanism for flipping of subduction polarity in Taiwan. Geology, 28(2), 155-158.
Vos, I. M., Bierlein, F. P., & Heithersay, P. S. (2007). A crucial role for slab break-off in the generation of major mineral deposits: insights from central and eastern Australia. Mineralium Deposita, 42(5), 515-522.
Von Hagke, C., Philippon, M., Avouac, J. P., & Gurnis, M. (2016). Origin and time evolution of subduction polarity reversal from plate kinematics of Southeast Asia. Geology, 44(8), 659-662.