Remagnetization of the Paleogene Tibetan Himalayan carbonate rocks in the Gamba area: Implications for reconstructing the lower plate in the India‐Asia collision

Abstract

The characteristic remanent magnetization (ChRM) isolated from Paleogene carbonate rocks of the Zongpu Formation in Gamba (28.3°N, 88.5°E) of southern Tibet has previously been interpreted to be primary. These data are pertinent for estimating the width of Greater India and dating the initiation of India‐Asia collision. We have reanalyzed the published ChRM directions and completed thorough rock magnetic tests and petrographic observations on specimens collected throughout the previously investigated sections. Negative nonparametric fold tests demonstrate that the ChRM has a synfolding or postfolding origin. Rock magnetic analyses reveal that the dominant magnetic carrier is magnetite. “Wasp‐waisted” hysteresis loops, suppressed Verwey transitions, high frequency‐dependent in‐phase magnetic susceptibility, and evidence that >70% of the ferrimagnetic material is superparamagnetic at room temperature are consistent with the rock‐magnetic fingerprint of remagnetized carbonate rocks. Scanning electron microscopy observations and energy‐dispersive X‐ray spectrometry analysis confirm that magnetite grains are authigenic. In summary, the carbonate rocks of the Zongpu Formation in Gamba have been chemically remagnetized. Thus, the early Paleogene latitude of the Tibetan Himalaya and size of Greater India have yet to be determined and the initiation of collision cannot yet be precisely dated by paleomagnetism. If collision began at 59 ± 1 Ma at ~19°N, as suggested by sedimentary records and paleomagnetic data from the Lhasa terrane, then a huge Greater India, as large as ~3500–3800 km, is required in the early Paleogene. This size, in sharp contrast to the few hundred kilometers estimated for the Early Cretaceous, implies an ever greater need for extension within Greater India during the Cretaceous.

Paleomagnetic and Geochronologic Results of Latest Cretaceous Lava Flows From the Lhasa Terrane and Their Tectonic Implications

“…Based on the position of the India‐Asia collision, the evolution of the TH, which is traditionally regarded as the northernmost continental unit of India, can be used to date the collision. There have been many paleomagnetic studies on Paleozoic‐Paleogene rocks from the TH, but many of these rocks have been remagnetized (e.g., Appel et al, ; Besse et al, ; Huang, van Hinsbergen, Dekkers, et al, ; Huang, Lippert, Dekkers, et al, ; Huang, Lippert, Zhang, et al, ; Klootwijk & Bingham, ; Liebke et al, ; Schill et al, ; Tong et al, ). Importantly, the paleomagnetic results from the late Cretaceous and Paleocene limestones reported by Patzelt et al () and Yi et al () have previously been regarded as primary remanences.…”

“…Importantly, the paleomagnetic results from the late Cretaceous and Paleocene limestones reported by Patzelt et al () and Yi et al () have previously been regarded as primary remanences. However, these results have been reevaluated for their primary origin and found to be unequivocally remagnetized during or after the limestones were folded (Huang, Lippert, Dekkers, et al, ; Huang, Lippert, Zhang, et al, ). Recently, Yi et al () argued that the associated secondary remanence was most likely acquired before the folding.…”

“…This discrepancy requires a ~1,800 km wide post‐Neotethyan Ocean (basin) opened within Greater India in the Cretaceous, leading to a two stage collision (van Hinsbergen et al, ). This two‐stage continental collision includes the first collision between the TH and the LT at ~59 Ma (DeCelles et al, ; Hu et al, ) and a final collision between the TH and the “remaining Greater India” at no later than ~39 Ma based on the latest initial collision age constructed in Figure a (Huang, Lippert, Dekkers, et al, ; Ma et al, ; van Hinsbergen et al, ; Yang, Ma, Bian, et al, ). Notably, considering potential challenges to different collision models described in Hu et al (), as well as debates on the shapes of the leading edges of the collisional units, especially the TH, more late Cretaceous and Paleocene paleomagnetic investigations from the TH and the LT are needed to further constrain the collision history and paleogeography of Greater India (Ali & Aitchison, , ; Ingalls et al, ; Jagoutz et al, ).…”

“…However, only paleomagnetism can quantitatively constrain the paleogeographic evolution of the precollisional leading edges of Asia and India (e.g., Lippert et al, ). Because India‐Asia convergence during the late Cretaceous and Palaeocene was initially north‐south directed, the overlapped paleolatitudes of the TH with the LT defines the collision in a paleomagnetic reference frame (e.g., Chen et al, ; Dupont‐Nivet et al, ; Huang, Van Hinsbergen, Lippert, et al, ; Lippert et al, ), if all the rocks preserve a primary remanence (Huang, Lippert, Dekkers, et al, ; Huang, Lippert, Zhang, et al, ).…”

Remagnetization of carbonate rocks in southern Tibet: Perspectives from rock magnetic and petrographic investigations

“…Poles 16, 17, and 22 were mistakenly used to constrain the size of Greater India and initiation of the India‐Asia collision and as discussed here and in Huang et al . [].…”