Julien Charreau

Magnetostratigraphy of the Yaha section, Tarim Basin (China) : 11 Ma acceleration in erosion and uplift of the Tian Shan mountains

We report a magnetostratigraphic and rock magnetic study of the Yaha section, located on the southern flank of the central Tian Shan mountains, Asia. Our results show a two-fold increase in sedimentation rate as well as marked changes in rock magnetic characteristics ca. 11 Ma. After 11 Ma, sedimentation rate remained remarkably constant until at least 5.2 Ma. These findings are consistent with sedimentary records from other sections surrounding the Tian Shan. We conclude that uplift and erosion of the Tian Shan accelerated ca. 11 Ma, long after the onset of the collision between India and Asia, and that the range rapidly evolved toward a steady-state geometry via a balance between tectonic and erosion processes.

Neogene uplift of the Tian Shan Mountains observed in the magnetic record of the Jingou River section (northwest China)

The Tian Shan Mountains constitute central Asias longest and highest mountain range. Understanding their Cenozoic uplift history thus bears on mountain building processes in general, and on how deformation has occurred under the influence of the India-Asia collision in particular. In order to help decipher the uplift history of the Tian Shan, we collected 970 samples for magnetostratigraphic analysis along a 4571-m-thick section at the Jingou River (Xinjiang Province, China). Stepwise alternating field and thermal demagnetization isolate a linear magnetization component that is interpreted as primary. From this component, a magnetostratigraphic column composed of 67 polarity chrons are correlated with the reference geomagnetic polarity timescale between ∼1 Ma and ∼23.6 Ma, with some uncertainty below ∼21 Ma. This correlation places precise temporal control on the Neogene stratigraphy of the southern Junggar Basin and provides evidence for two significant stepwise increases in sediment accumulation rate at ∼16–15 Ma and ∼11–10 Ma. Rock magnetic parameters also undergo important changes at ∼16–15 Ma and ∼11–10 Ma that correlate with changes in sedimentary depositional environments. Together with previous work, we conclude that growth history of the modern Tian Shan Mountains includes two pulses of uplift and erosion at ∼16–15 Ma and ∼11–10 Ma. Middle to upper Tertiary rocks around the Tian Shan record very young (<∼5 Ma) counterclockwise paleomagnetic rotations, on the order of 15° to 20°, which are interpreted as because of strain partitioning with a component of sinistral shear that localized rotations in the piedmont.

Miocene to present kinematics of fault-bend folding across the Huerguosi anticline, northern Tianshan (China), derived from structural, seismic, and magnetostratigraphic data

We combine surface structural measurements, subsurface seismic imaging, and magnetostratigraphic dating to retrieve, through geometric modeling, the detailed history of fold growth and sedimentation across the Huerguosi anticline, on the northern Tianshan piedmont, taking advantage of a beautifully exposed section of growth strata. The model assumes a fault-bend folding mechanism, consistent with subsurface fold geometry. The shortening history is obtained by least-squares fi tting of the measured dip angles of the growth strata. The shortening rate across the anticline is shown to have been remarkably constant: it increased only slightly from 0.84 ± 0.04 mm/yr between 10 and 4 Ma to 1.14 ± 0.02 mm/yr over the past 4 m.y. This approach also allows correcting syntectonic sedimentation rates for the effect of the fold growth and shows that the sedimentation rates in the piggyback basin increased abruptly from ~0.4 to ~0.7 mm/yr ca. 4 Ma.

Modeling the shortening history of a fault tip fold using structural and geomorphic records of deformation

[1] We present a methodology to derive the growth history of a fault tip fold above a basal detachment. Our approach is based on modeling the stratigraphic and geomorphic records of deformation, as well as the finite structure of the fold constrained from seismic profiles. We parameterize the spatial deformation pattern using a simple formulation of the displacement field derived from sandbox experiments. Assuming a stationary spatial pattern of deformation, we simulate the gradual warping and uplift of stratigraphic and geomorphic markers, which provides an estimate of the cumulative amounts of shortening they have recorded. This approach allows modeling of isolated terraces or growth strata. We apply this method to the study of two fault tip folds in the Tien Shan, the Yakeng and Anjihai anticlines, documenting their deformation history over the past 6–7 Myr. We show that the modern shortening rates can be estimated from the width of the fold topography provided that the sedimentation rate is known, yielding respective rates of 2.15 and 1.12 mm/yr across Yakeng and Anjihai, consistent with the deformation recorded by fluvial and alluvial terraces. This study demonstrates that the shortening rates across both folds accelerated significantly since the onset of folding. It also illustrates the usefulness of a simple geometric folding model and highlights the importance of considering local interactions between tectonic deformation, sedimentation, and erosion.

Addendum to “Late Cenozoic magnetochronology and paleoenvironmental changes in the northern foreland basin of the Tian Shan Mountains” by Jimin Sun, Qinghai Xu, and Baochun Huang

[1] Sun et al. (2007) constructed a magnetostratigraphic record of the Kuitun River section (Xinjiang Province, China) and reinterpreted a magnetostratigraphic record from the same section previously published by Charreau et al. (2005). In this paper, we show that Sun et al. miscorrelated their column with respect to that of Charreau et al. A compatible correlation, recognized by both sides, is presented.

Lake Tauca highstand (Heinrich Stadial 1a) driven by a southward shift of the Bolivian High

Our paleoprecipitation map of the Altiplano reveals the atmospheric processes underlying a major hydroclimatic change 16 ka ago. Heinrich events are characterized by worldwide climate modifications. Over the Altiplano endorheic basin (high tropical Andes), the second half of Heinrich Stadial 1 (HS1a) was coeval with the highstand of the giant paleolake Tauca. However, the atmospheric mechanisms underlying this wet event are still unknown at the regional to global scale. We use cosmic-ray exposure ages of glacial landforms to reconstruct the spatial variability in the equilibrium line altitude of the HS1a Altiplano glaciers. By combining glacier and lake modeling, we reconstruct a precipitation map for the HS1a period. Our results show that paleoprecipitation mainly increased along the Eastern Cordillera, whereas the southwestern region of the basin remained relatively dry. This pattern indicates a southward expansion of the easterlies, which is interpreted as being a consequence of a southward shift of the Bolivian High. The results provide a new understanding of atmospheric teleconnections during HS1 and of rainfall redistribution in a changing climate.