Accelerated extension of Tibet linked to the northward underthrusting of Indian crust

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1 Accelerated extension of Tibet linked to the northward underthrusting of Indian crust Richard Styron*, Michael Taylor, and Kurt Sundell DOI: 1.138/NGEO2336 Methods summary The modeling strategy uses a two-stage inversion scheme following Styron 11, incorporating both geologic estimates of rift geometry and extension magnitude, as well as thermal histories obtained from the ahe and zhe data. The thermokinematic modeling is conducted using Pecube 3 and accounts for radiogenic heating, heat advection due to rock uplift, heat flux through both the Moho and the earth s surface, and structural complexities such as fault interaction, down-dip changes in fault geometry, and isostatic flexure of the rift flanks. This work relies on structural and neotectonic mapping to obtain the location and geometry of modeled faults, as well as to yield acceptable ranges for cumulative extension across the Lunggar rift. Mapping was performed in four field campaigns between 26 and 21. Bedrock samples for (U-Th)/He analysis were primarily taken from transects oriented in the direction of fault slip, to best capture the cooling signal produced by tectonic exhumation 31. For each sample, zircons and apatites were separated and 3-6 grains for each were hand-picked. Geological Constraints The maximum extension value used in the modeling is determined by the width of the exposure of exhumed rocks in the footwall block; this is from fault trace to fault trace where the rift has a central horst block bound on both sides by normal faults, or from a fault trace on the structurally lowest side of the footwall to the point where no significant rift flank uplift has occurred (ideally the footwall cutoff), where the rift is a half-graben system with a single master fault. Additionally, a small correction (2 km, which is 1-2% of the footwall width) has been added to account for the effects of a thin (~1 km) synextensional hanging wall basin, as these basins would place the hanging wall cutoffs farther into the basin from the fault trace 11. The minimum extension value is set significantly lower than the approximate amount of uplift necessary to exhume rocks from beneath the zircon He partial retention zone. In all cases, this value is much lower than the lowest net extension derived from the modeling results. Modeling methods Each Pecube thermokinematic model represents a 3D section through the Tibetan crust. The base of the model is at 8 km below sea level, and the top of the model is the modern-day topography. The width of the model is slightly larger than the sampled width (distance N-S) for NATURE GEOSCIENCE Macmillan Publishers Limited. All rights reserved

2 DOI: 1.138/NGEO2336 each modeled E-W transect. The fault geometry at depth is assumed to be planar (for moderatelydipping normal faults) or anti-listric (for detachment faults), consistent with the structural maps and cross sections, and current models for metamorphic core complex formation 32,33 ; however, sensitivity tests 11 show that the modeled cooling ages of the samples are relatively insensitive to the deep fault geometry. Additionally, topography is assumed to be steady-state throughout the model; this does not affect the modeled cooling ages because the faults dip at a low-enough angle that the samples cool below the rift basin (relatively low-elevation and flat throughout the deformation history) rather than below the footwall mountain range, where topography does change through time. Thermal parameters for the crust are taken from an earlier study on the South Lunggar rift 11 which has published sensitivity tests for these parameters. Our modeling of the extensional history of the Lunggar rift parameterizes the extension history into a set of variables: when slip began on each fault, what its initial slip rate was, when the slip rate changed on each fault, and its more recent slip rate. Note that the first and second slip rates are allowed to be equal, meaning that we do not enforce a slip rate change on the faults. Each variable is given a set of possible values. Then, we make a set of all of the possible unique combinations of variables for each transect (typically tens to hundreds of thousands of combinations for each transect; transects with two faults have many more combinations, as they have more variables), and then we arithmetically calculate the net horizontal extension for each combination of variables, based on the slip history and fault geometry, and filter the list so that only combinations of variables that produce net extension within the predefined boundaries are considered. This reduces the total number of possible variable combinations that need to be modeled in Pecube by 1-2 orders of magnitude. At this point, 1,-18, models were run for each transect, depending on the number of faults and the extension ranges. Models were run in parallel on identical Ubuntu Linux 11.4 virtual environments on Amazon s EC2 cloud computing platform, using PiCloud, a Python-based commercial API to Amazon s servers. Finally, model results were filtered so that modeled runs were deemed successful if the modeled thermochronometer ages are within 2σ of the mean of the measured aliquots for the sample. Note that σ here is the larger of either the standard deviation of the measurements, or of a standard laboratory uncertainty for each thermochronometer (5% of the mean age for zircon and 6% for apatite, respectively) based on hundreds of repeated measurements of laboratory standards. For some models, one sample outlier was allowed per transect, as otherwise no model results could fit the data. All data used in the paper is published2,21 and all code is publicly available online at 2 NATURE GEOSCIENCE Macmillan Publishers Limited. All rights reserved

3 DOI: 1.138/NGEO2336 References 31: Stockli, D. F. Application of low-temperature thermochronometry to extensional tectonic settings. Reviews in Mineralogy and Geochemistry, 58, (25) 32: Buck, W. R. Flexural rotation of normal faults. Tectonics, 7, (1988). 33. Tirel, C., Brun, J. P., & Burov, E. Dynamics and structural development of metamorphic core complexes. J. Geophys. Res. B, 113, 1-25, (28). NATURE GEOSCIENCE Macmillan Publishers Limited. All rights reserved

4 DOI: 1.138/NGEO2336 W E W E mm/yr mm/yr 1 mm/yr Figure S NATURE GEOSCIENCE Macmillan Publishers Limited. All rights reserved

5 DOI: 1.138/NGEO2336 Figure S1: North-looking view of Pecube model cross-sections with topography (grey lines), faults (red lines) and example velocity fields (arrows) for all model transects (See Figure 1b for location). All cross-sections are taken through the center of each Pecube model, and are oriented E-W. Velocity fields shown are relative to the hanging-wall basin with the minimum vertical velocity. In models 4 and 6, positive velocities in the opposite hanging-wall basin indicate that the basin with zero velocity is subsiding relative to the basin with positive velocities; there is no material flux through the top of the model in the hanging-wall basins. Velocity fields are at Ma in all models. Note that the velocity fields shown are taken from the best-fitting set of results, but these velocity fields are different in each model run. Also note change in velocity scale for various models. NATURE GEOSCIENCE Macmillan Publishers Limited. All rights reserved