Comment on “Remagnetization of the Paleogene Tibetan Himalayan carbonate rocks in the Gamba area: Implications for reconstructing the lower plate in the India‐Asia collision” by Huang et al.

Abstract

The recent publication of “Remagnetization of the Paleogene Tibetan Himalayan carbonate rocks in the Gamba area: Implications for reconstructing the lower plate in the India‐Asia collision” by Huang et al. [J. Geophys. Res. Solid Earth, 122, doi:10.1002/2016JB013662] argued for a complete chemical remagnetization on the characteristic remanence of carbonate rocks from the Zongpu Formation (Fm) at Gamba, southern Tibet, and discussed its implications for the reconstruction of India‐Asia collision. To support their conclusion, the authors performed nonparametric fold tests on the paleomagnetic data obtained from the Zongpuxi section, we previously published in Yi et al. [Earth Planet. Sci. Lett., 2011, 309 (1–2):153–165] and argued for a synfolding or postfolding origin. In this comment, we demonstrate that their reinterpretation of the fold tests is invalid, and a prefolding origin of the ChRMs from the Zongpu Fm is still supported by the data. We agree that the new rock magnetic and SEM results are robust evidence for a secondary origin of the magnetite fraction in the Zongpu Fm; however, the associated secondary remanence was most likely acquired shortly after deposition of the carbonates, or even in an early diagenetic stage. We conclude that the paleomagnetic results from Gamba area can still be used for reconstructing the precollisional extent of Greater India.

Reply to comment by Z. Yi et al. on “Remagnetization of the Paleogene Tibetan Himalayan carbonate rocks in the Gamba area: Implications for reconstructing the lower plate in the India‐Asia collision”

“…Indeed, k g / k s of our nonparametric fold test applied to Members I–IV is 1.22 [ Huang et al , ], which is just above F [294, 294] = 1.21 [ McElhinny , ]. Yi et al [] further suggest that these negative fold tests may be caused by northward latitude shift of the Tibetan Himalaya during the deposition of Member I and Members II–IV carbonate rocks. First and foremost, this argument is based on the assumption that the NRM would be primary.…”

“…Based on our comprehensive rock magnetic and petrographic results [ Huang et al , ], Yi et al [] agree that the magnetic carrier of the Zongpu carbonate rocks is authigenic magnetite. However, they argue that these authigenic magnetite grains were formed at the very early diagenetic stage with no significant lapse of time between of the deposition and remanence acquisition.…”

“…As we have previously, we argue that the remanence in the Zongpu and Zongshan carbonate rocks is thus of secondary origin. To support that the remanence is at least of a prefolding age, Yi et al [] question the validity of the fold tests in our reanalyses of the published paleomagnetic results from the Zongpu carbonate rocks in the Gamba area. Their fold tests applied to carbonate rocks from Member I, Members II–IV, and Members I–IV of the Zongpu Formation, no matter on individual specimen level or site level, are also negative, which are similar to our results.…”

“…Their fold tests applied to carbonate rocks from Member I, Members II–IV, and Members I–IV of the Zongpu Formation, no matter on individual specimen level or site level, are also negative, which are similar to our results. However, Yi et al [] argue that these negative fold tests are inconclusive because they fail the F test [ McElhinny , ]. This argument is not persuasive, however, because failure of the F test could well have been caused by the limited bedding variation within Member I and Members II–IV carbonate rocks.…”

“…To explain why the mean dip of Members II–IV is ~10° shallower than that of Member I, Yi et al [] argue that Member II–IV carbonate rocks were deposited on a forebulge, which has slightly tilted Member I carbonate rocks before the deposition of Members II–IV carbonate rocks. This argument is, however, against the geological observation that an unconformity exists between Member III and Member IV carbonate rocks, whereas Members I–III were deposited continually on a carbonate ramp [ Li et al , ], which could not have been deposited at a 10° primary dip, as this would lead to massive slope failure.…”