SYNCHRONOUS TRANSPRESSIONAL UPLIFT ALONG WIDELY-SPACED INTRACONTINENTAL STRIKE-SLIP FAULTS, GOBI AND MONGOLIAN ALTAI, CENTRAL ASIA

We combine geologic, thermochronologic, terrestrial cosmogenic nuclide (TCN) geochronology, and inverse modeling of bedrock channel longitudinal profiles to reconstruct the rate of rock uplift for block-like ranges in the transpressional bends of unconnected, widely-spaced intracontinental strike slip faults bounding the Gobi and Mongolian Altai ranges in central and western Mongolia. Among the world’s largest, active intraplate structures with four historic ruptures > Mw 8, geologic, basin growth strata, and thermochronologic evidence suggest that these faults became active ~30 Ma, perhaps related to changes in far-field stresses originating in the Himalayan orogen to the south, or in the Pacific plate boundary to the east. We test this idea by modeling the base level fall at the mouths of the north Altanteel basin located on the eastern flank of the Baatar Hayrhan range of the Mongolian Altai, and the Bitut watershed located on the northern flank of Ikh Bogd in the Gobi Altai, both of which are underlain by relatively uniform crystalline rocks. We employ a linear inversion of basin channels using the detachment-limited stream power model where the power dependency on slope is assumed to be 1, and rock-erodibility is determined from channel steepness and erosion rate observables. Published TCN erosion rates for the Bitut drainage range from ~36 to 77 m/Myr for interglacial and glacial climates respectively. Similarly, a new TCN erosion rate of 47±3.9 m/Myr for the Holocene Altanteel drainage that is not building a piedmont fan contrasts with a TCN exposure age of 20±1.6 ka on an extensive granite-boulder fan surface built by the same drainage indicating that the late glacial sediment flux was probably higher. Channel inversion results show synchronous rock uplift commencing ~30-35 Ma at rates of ~10-30 m/Myr, increasing to 65-80 m/Myr at 15 Ma, and decreasing rapidly to the current rate of ~40 m/Myr. These results are consistent with and extend the temporal range of an age-elevation apatite fission track and apatite helium profile constructed for Ikh Bogd and a pending, corresponding thermal profile for Baatar Hayrhan. They also provide an important new constraint on geodynamic models considering the timing and steadiness of stress transfer to central Asia from distal plate boundaries.