• Along-strike diachroneity in deposition of the Kailas Formation in central southern Tibet: Implications for Indian slab dynamics

      Leary, Ryan; Orme, Devon A.; Laskowski, Andrew K.; DeCelles, Peter G.; Kapp, Paul; Carrapa, Barbara; Dettinger, Matt; Department of Geosciences, University of Arizona (Geological Society of America, 2016-08)
      The Oligo-Miocene Kailas Formation is exposed along strike for ~1,300 km within the southernmost Lhasa terrane. In this study, we document the sedimentology, structure, and age of this unit exposed between 87˚ and 90˚ E. Within this region, the Kailas Formation is composed of continental deposits dominated by conglomerate and sandstone, with lesser volumes of siltstone and paleosols. These rocks were deposited nonconformably on Gangdese Batholith and related volcanic rocks along their northern boundary, whereas to the south, the south-dipping Great Counter Thrust places them in contact with Xigaze Forearc and mélange units. We interpret the Kailas Formation to have been deposited in alluvial fan and fluvial environments with sediment principally derived from the north. Based on sedimentology and structural relationships, we interpret these rocks to have formed in a north-south extensional setting. New zircon U-Pb ages from volcanic tuffs and flows show that Kailas Formation deposition younged to the east: deposition in western Tibet (81˚E) occurred between 26-24 Ma, 25-23 Ma north of Lazi (87.8˚E), 23-22 Ma near Dazhuka (89.8˚E), and as late as 18 Ma southwest of Lhasa (92˚E). Overall, basin development propagated eastward at a rate of ~300 mm/yr. This pattern and rate of propagation are similar to that of the temporal-spatial distribution of adakitic and ultrapotassic magmatism within the Lhasa terrane to the north, which has been interpreted as a record of slab breakoff. Magmatism lags several million years behind Kailas basin development at most locations. We interpret the Kailas basin to have formed as the result of Indian slab shearing and breakoff, which began in western Tibet around 26 Ma and reached eastern Tibet by ~18 Ma.
    • Identification of seasonal varves in the lower Pliocene Bouse Formation, lower Colorado River Valley, and implications for Colorado Plateau uplift

      Spencer, Jon E.; Constenius, Kurt N.; Dettman, David L.; Domanik, Kenneth J.; Department of Geosciences, The University of Arizona; Lunar and Planetary Laboratory, The University of Arizona (Geological Society of America, 2021-11-01)
      The cause of Cenozoic uplift of the Colorado Plateau is one of the largest remaining problems of Cordilleran tectonics. Difficulty in discriminating between two major classes of uplift mechanisms, one related to lithosphere modification by low-angle subduction and the other related to active mantle processes following termination of subduction, is hampered by lack of evidence for the timing of uplift. The carbonate member of the Pliocene Bouse Formation in the lower Colorado River Valley southwest of the Colorado Plateau has been interpreted as estuarine, in which case its modern elevation of up to 330 m above sea level would be important evidence for late Cenozoic uplift. The carbonate member includes laminated marl and claystone interpreted previously in at least one locality as tidal, which is therefore of marine origin. We analyzed lamination mineralogy, oxygen and carbon isotopes, and thickness variations to discriminate between a tidal versus seasonal origin. Oxygen and carbon isotopic analysis of two laminated carbonate samples shows an alternating pattern of lower δ18O and δ13C associated with micrite and slightly higher δ18O and δ13C associated with siltstone, which is consistent with seasonal variation. Covariation of alternating δ18O and δ13C also indicates that post-depositional chemical alteration did not affect these samples. Furthermore, we did not identify any periodic thickness variations suggestive of tidal influence. We conclude that lamination characteristics indicate seasonal genesis in a lake rather than tidal genesis in an estuary and that the laminated Bouse Formation strata provide no constraints on the timing of Colorado Plateau uplift.
    • Labyrinth patterns in Magadi (Kenya) cherts: Evidence for early formation from siliceous gels

      Leet, Kennie; Lowenstein, Tim K.; Renaut, Robin W.; Owen, R. Bernhart; Cohen, Andrew; Department of Geosciences, University of Arizona (Geological Society of America, 2021-06-03)
      Sedimentary cherts, with well-preserved microfossils, are known from the Archean to the present, yet their origins remain poorly understood. Lake Magadi, Kenya, has been used as a modern analog system for understanding the origins of nonbiogenic chert. We present evidence for synsedimentary formation of Magadi cherts directly from siliceous gels. Petrographic thin-section analysis and field-emission scanning electron microscopy of cherts from cores drilled in Lake Magadi during the Hominin Sites and Paleolakes Drilling Project in 2014 led to the discovery of two-dimensional branching “labyrinth patterns” in chert, which are a type of fractal “squeeze” pattern formed at air-liquid interfaces. Labyrinth patterns preserved in chert from Lake Magadi cores indicate invasion of air along planes in dewatering gels. These patterns support the precipitation of silica gels in the saline-alkaline Lake Magadi system and syndepositional drying of gels in contact with air as part of chert formation. Recognizing cherts as syndepositional has been critical for our use of them for U-Th dating. Identification of labyrinth patterns in ancient cherts can provide a better understanding of paleoenvironmental and geochemical conditions in the past © 2021 Geological Society of America.
    • Quantifying the growth of continental crust through crustal thickness and zircon Hf-O isotopic signatures: A case study from the southern Central Asian Orogenic Belt

      Wang, Yujian; Zhu, Dicheng; Lin, Chengfa; Hu, Fangyang; Liu, Jingao; Department of Geosciences, University of Arizona (Geological Society of America, 2021-12-23)
      Accretionary orogens function as major sites for the generation of continental crust, but the growth model of continental crust remains poorly constrained. The Central Asian Orogenic Belt, as one of the most important Phanerozoic accretionary orogens on Earth, has been the focus of debates regarding the proportion of juvenile crust present. Using published geochemical and zircon Hf-O isotopic data sets for three belts in the Eastern Tianshan terrane of the southern Central Asian Orogenic Belt, we first explore the variations in crustal thickness and isotopic composition in response to tectono-magmatic activity over time. Steady progression to radiogenic zircon Hf isotopic signatures associated with syn-collisional crustal thickening indicates enhanced input of mantle-derived material, which greatly contributes to the growth of the continental crust. Using the surface areas and relative increases in crustal thickness as the proxies for magma volumes, in conjunction with the calculated mantle fraction of the mixing flux, we then are able to determine that a volume of ~14–22% of juvenile crust formed in the southern Central Asian Orogenic Belt during the Phanerozoic. This study highlights the validity of using crustal thickness and zircon isotopic signatures of magmatic rocks to quantify the volume of juvenile crust in complex accretionary orogens. With reference to the crustal growth pattern in other accretionary orogens and the Nd-Hf isotopic record at the global scale, our work reconciles the rapid crustal growth in the accretionary orogens with its episodic generation pattern in the formation of global continental crust.
    • 'Taters versus sliders: Evidence for a long-lived history of strike-slip displacement along the canadian arctic transform system (CATS)

      McClelland, W.C.; Strauss, J.V.; Colpron, M.; Gilotti, J.A.; Faehnrich, K.; Malone, S.J.; Gehrels, G.E.; Macdonald, F.A.; Oldow, J.S.; Dept. of Geosciences, University of Arizona (Geological Society of America, 2021)
      Recent field-based studies indicate that the northern margin of North America is best interpreted as a tectonic boundary that experienced a long, complex history of strike-slip displacement. Structures juxtaposing the Pearya and Arctic Alaska terranes with North America are linked and define the Canadian Arctic transform system (CATS) that accommodated Paleozoic terrane translation, truncation of the Caledonian orogen, and shortening within the transpressional Ellesmerian orogen. The structure was reactivated during Mesozoic translational opening of the Canada Basin. Land-based evidence supporting translation along the Canadian Arctic margin is consistent with transform structures defined by marine geophysical data, thereby providing a robust alternative to the current consensus model for rotational opening of the Canada Basin. © 2021 Geological Society of America. All rights reserved.