Holocene slip rate for the central Altyn Tagh Fault:
Preliminary results from the Yue Ma Ke Qi site using displaced fluvial risers and ¹⁴C geochronology.

Previous studies of the active, left-slip Altyn Tagh Fault have produced two apparently contradictory Holocene slip rates for this first-order structure within the Indo-Asian collision zone.  Whereas GPS and paleoseismic studies indicate a rate of ~10 mm/yr over the last 0.01 ka and 0.5 to ~3 ka, respectively, reconstructions of displaced and dated landforms yield a significantly higher rate of ~30 mm/yr between 6 and 113 ka.  Importantly, the slip rate has not been measured for the 2 to 6 ka time interval—the time period over which the reported slip rates differ.  To address this data gap, we investigated the Yue Ma Ke Qi site (38.19° N, 88.52° E) along the central Altyn Tagh Fault, between the Cherchen He site in the west and the Aksay area in the east.  Our mapping and survey data reveal two fluvial terrace risers, T4/T3 and T3/T2, which have been left-laterally displaced by the Altyn Tagh Fault 51 ± 5 m and 42 m, respectively.  We have bracketed the abandonment ages of the surfaces bounding these displaced risers using 36 new radiocarbon dates on organic fragments collected from within the terrace treads (= maximum abandonment age) and the loess deposits capping the terrace surfaces (= minimum abandonment age).  These age data constrain the abandonment of the T2 and T3 surfaces to have occurred at ~2.3 ka and ~3.9 ka, respectively.  The data yield a minimum abandonment age for the T4 surface of ~5.8 ka.  Deducing the slip history for the fault using these data requires that the surface abandonment ages be paired with riser displacements.  We consider two end-member models.  In the lower-terrace reconstruction a fluvial riser accumulates no displacement until the lower-terrace surface is abandoned, thus providing a maximum constraint on the slip rate.   In contrast, the upper-terrace reconstruction presumes that a riser starts recording displacement as soon as the upper-terrace is abandoned, thus providing a minimum constraint on the slip rate.  At the Yue Ma Ke Qi site, lower-terrace reconstructions yield maximum slip rates of ~18 mm/yr from 2.3 ka to present (i.e., the T3/T2 riser offset combined with the T2 surface age) and ~6 mm/yr from 3.9 to 2.3 ka (i.e., the difference between the T4/T3 and T3/T2 offsets divided by the difference between the T3 and T2 ages).  In contrast, upper-terrace reconstructions yield minimum slip rates of ~11 mm/yr from 3.9 ka present (i.e., the T3/T2 offset combined with the T3 age) and ~5 mm/yr from 5.8 to 3.9 ka (i.e., the difference between T4/T3 and T3/T2 offsets divided by the difference between the T4 and T3 ages).  Although the preliminary rates derived from the T3/T2 riser offset span the range of previously published rates, our data do indicate that the Altyn Tagh Fault slipped no faster than ~10 mm/yr between formation of the T4/T3 and T3/T2 risers.

LaTEX formatting:

Previous studies of the active, left-slip Altyn Tagh Fault have produced two apparently contradictory Holocene slip rates for this first-order structure within the Indo-Asian collision zone.  Whereas GPS and paleoseismic studies indicate a rate of $\sim$10 mm/yr over the last 0.01 ka and 0.5 to $\sim$3 ka, respectively, reconstructions of displaced and dated landforms yield a significantly higher rate of $\sim$30 mm/yr between 6 and 113 ka.  Importantly, the slip rate has not been measured for the 2 to 6 ka time interval, the time period over which the reported slip rates differ.  To address this data gap, we investigated the Yue Ma Ke Qi site (38.19$\deg$N, 88.52$\deg$E) along the central Altyn Tagh Fault, between the Cherchen He site in the west and the Aksay area in the east.  Our mapping and survey data reveal two fluvial terrace risers, T4/T3 and T3/T2, which have been left-laterally displaced by the Altyn Tagh Fault 51 $\pm$ 5 m and 42$^{+9}_{-5}$ m, respectively.  We have bracketed the abandonment ages of the surfaces bounding these displaced risers using 36 new radiocarbon dates on organic fragments collected from within the terrace treads (= maximum abandonment age) and the loess deposits capping the terrace surfaces (= minimum abandonment age).  These age data constrain the abandonment of the T2 and T3 surfaces to have occurred at $\sim$2.3 ka and $\sim$3.9 ka, respectively.  The data yield a minimum abandonment age for the T4 surface of $\sim$5.8 ka.  Deducing the slip history for the fault using these data requires that the surface abandonment ages be paired with riser displacements.  We consider two end-member models.  In the lower-terrace reconstruction a fluvial riser accumulates no displacement until the lower-terrace surface is abandoned, thus providing a maximum constraint on the slip rate.   In contrast, the upper-terrace reconstruction presumes that a riser starts recording displacement as soon as the upper-terrace is abandoned, thus providing a minimum constraint on the slip rate.  At the Yue Ma Ke Qi site, lower-terrace reconstructions yield maximum slip rates of $\sim$18 mm/yr from 2.3 ka to present (i.e., the T3/T2 riser offset combined with the T2 surface age) and$\sim$6 mm/yr from 3.9 to 2.3 ka (i.e., the difference between the T4/T3 and T3/T2 offsets divided by the difference between the T3 and T2 ages).  In contrast, upper-terrace reconstructions yield minimum slip rates of $\sim$11 mm/yr from 3.9 ka present (i.e., the T3/T2 offset combined with the T3 age) and $\sim$5 mm/yr from 5.8 to 3.9 ka (i.e., the difference between T4/T3 and T3/T2 offsets divided by the difference between the T4 and T3 ages).  Although the preliminary rates derived from the T3/T2 riser offset span the range of previously published rates, our data do indicate that the Altyn Tagh Fault slipped no faster than $\sim$10 mm/yr between formation of the T4/T3 and T3/T2 risers.