Active Thrusting And Folding Along The Pamir Frontal Thrust System

The Pamir is located in the northwestern corner of the Indo-Asian collision zone. During late-Cenozoic times,the northern margin of the Pamir has indented northward~300km, accommodated by south-dipping intracontinentalsubduction along the Main Pamir Thrust (MPT) and coupled the strike-slip on the western and eastern margins.Along the western margin, deformation has been persistently concentrated on the Pamir-Tadjik boundary faultDarvaz-Karakul Fault (DKF). In the Kyrgyz Alai Valley along the northern margin, the Pamir Frontal Thrust (PFT)has become inactive and deformation has stepped hindwards to the MPT. Along the estern margin, deformation hadbeen concentrated on the Pamir-Tarim boundary fault-Kashgar-Yecheng Transfer System (KYTS) since acitivtyinitiation (~25-20Ma); by late Miocene-Pliocene, strike-slip along the KYTS decreased dramatically from~11-15mm/a to~1.7-5.3mm/a, indicating the Pamir jointing together with the Tarim and subsequent northward motiontoward the Tian Shan. In response to tectonic setting transformation, regional straining was re-distributed. A series ofissues need to be addressed, such as the location, deformation characteristics, shortening rate accommodated of thedeformation frontier, the recent straining sptial distribution along eastern and northestern margin of the Pamir, thedifferences of straining distribution among the eastern, westen and northern margin, as well as the differences andsimilarities between the Tian Shan and the Pamir orogenic process. Keeping this motivation, this work investigatedactive deformation along northeastern margin of the Pamir. From this study, the following insights are attained:1) Spatial distribution of active structures along the NE margin of the Pamir.Along the NE margin of the Pamir, late-Cenozoic geologic deformation is concentrated on the MPT along thePamir’s mountain front, on the PFT along the deformation leading edge, and on the piggyback basin between them.The MPT, separating domains with contrasting geology and geomorphology, comprises several high-angle reversefaults involving the pre-Cenozoic basement, with slip initiating sometime between25-20Ma. Geologic mapping, rareearthquakes, geodetic GPS data and elastic half-space modeling indicate slip cessation along the MPT anddeformation has been transferred to the PFT along a detachment surface localized within Paleogene gypsum. ThePFT is a zone of active thrust faulting and folding. Active thrusts include the Biertuokuoyi Frontal Thrust, theMayikake Thrust and the Jilegeyoute Thrust in the Mayikake basin and the Tuomuluoan Thrust to the east of thebasin. Active folds include the Wulagen anticline, the Mingyaole anticline and the Mushi anticline.2) Deformation characteristics and rates of active thrusts in the PFTThe Biertuokuoyi Frontal Thrust, located along southwestern of the Mayikake basin, is a high-angle (~70°)north-vergent fault. According to displacement of the terrace surface, which was abandoned in the Last GlacialMaximum stage (LGM terrace), dip-slip rate and shortening rate of the fault are2.1+1.0/-0.5mm/a and~2.0mm/a,respectively. The Mayikake thrust is located at northern part of the basin, and is a low-angle (~15°) south-vergentthrust. The Mayikake thrust is interpreted as a bending-moment thrust of the Wulagen anticline based on flexural-slipthrusting, flexural-slip normal faulting and terrace tilt on the hanging wall of the fault. According to the LGM terracedisplacement, dip-slip and shortening rate of the fault are estimated to be3.1+3.0/-1.6mm/a and~3.0mm/a,respectively. The Jilegeyoute Thrust on the northern margin of the basin is a south-vergent fault dipping~45°, withdip-slip rate and shortening rate of~0.25mm/a and~0.2mm/a, respectively. The Tuomuluoan Thrust, to the east ofthe Mayikake basin, is a low-angle north-vergent thrust. The Paleogene gypsum on the hanging wall thrusts over the Plio-Pleistocene conglomerate in the south limb of the Mingyaole anticline along a~16°fault plane, with a minimumshortening of~2.1km. Paleomagnitostratigraphy suggests that top boundary age of the Xiyu formation is~0.35Ma,therefore the dip-slip rate and shortening rate of the PFT are>~6.3mm/a.3) Deformation characteristics and rates of active folds in the PFTThe Mingyaole anticline, initiating since~1.6Ma, is a detachement fold with average shortening rate of~1mm/a. A series of fold scarps are observed at the Kapaka water gap, Caijinchang of in the south limb andsouthwestern part of the anticline. Fold scarp is one type of geomorphic scarp resulting from kink band migrationfollowing underlying fault bending. For the detachment fold, although no fault bending is occurred, the kink bandmigration and limb rotation can also produce fold scarps, however there is no relevant report. Our study indicates thefold scarps at the Kapaka water gap and Caijinchang are resulted from kink band migration and the fold scarps atsouthwest part of the anticline are resulted from limb rotation, and both formed in detachment folding. Comparingthis new type of fold scarp with classical fold scarp shows lots of similarities and differences, and the fold scarpcategory needs to determine before using them to calculate incremental shorntening since the terrace abandonment.Folding deformation in three dimensions involves shortening, uplift and lateral propagation. Terrace surfaces,as a useful strain marker linking underlying structure to surface deformation, are increasingly applied to econvolveshortening and uplift of a fold. The syntectonic passively deformed terraces, however, can also reflect lateralpropagation, which is poorly investigated. The Mushi anticline, located at eastern part of the PFT, is a geometricallysimple fault tip fold, with total shortening of~700m and total uplift of~1300m. Flights of wide, continuous, andclearly deformed fluvial terraces are preserved over most of the northern half of the fold, providing a good chance toexploit how to use the deformed terrace to constrain the three-dimensional folding history. In analysis of terracesurface deformation, the Mushi anticline grows by progressive rotation of the limbs, with a late-Quaternaryshortening rate of1.5+1.4/-0.5mm/a and uplift rate of2.3+2.3/-0.9mm/a. Along strike of the fold, longitudinalprofiles of terraces also display tilting. A new model of lateral propagation of this work suggests that the eastwardlengthening of the Mushi anticline ceased since at least~134ka, and lateral propagation is dominated by rotation.4) Strain distribution on the NE marginRecent deformation on the NE margin of Chinese Pamir is concentrated on the PFT, a thrust activated in thelatest foreland-ward propagation sequence of the Pamir. In the Mayikake basin, shortening is accommodated by theBiertuokuoyi Frontal Thrust and the Mayikake Thrust, with a total rate of~5.0mm/a (~2.0mm/a and~3.0mm/a,respectively) since~18ka. To the east, the shortening rate of the Tuomuluoan Thrust is>6.3mm/a over the past~0.35Ma and the shortening rate of the Mingyaole anticline is~1mm/a since~1.6Ma, with a total rate of~7-8mm/a. The geologic rates of~5-8mm/a at two time scales are comparable to the geodetic rate of~6-9mm/a acrossthis same zone, indicating on the NE margin of the Pamir, deformation is concentrated on the PFT since~1-2Ma.This rate is comparable with the Holocene shortening rate of~5.3mm/a in the Kyrgyz Alai Valley and the averageshortening rate of~5.0mm/a since~4Ma in the Atushi-Kashi fold system in the foreland of the Southern Tian Shan.Therefore, the relative movement rate of the Pamir and the Tian Shan appear roughly uniform since~4Ma, in spite ofaccommodation by different deformation styles and an evolving spatial distribution.Different straining spatial distributions on different margins of the Pamir reflects different orogenic processes.Along E and NE margin, the slip cessation along the Pamir-Tarim boundary fault MPT-KYTS and subsequentnorthward migration of deformation frontier to the PFT-Kashi-Atushi fold system in the southern Tian Shan foreland indicate the Pamir jointing together with the Tarim and moving toward the Tian Shan as a whole. In contrast,deformation of the Pamir’s mirror image on the NW margin is persistently concentrated on the Pamir-Tadjikboundary fault DKF indicates strong relative motion between the Pamir and the Tadjik basin. In the Kyrgyz AlaiValley, the deformation frontier has stepped hindwards to the MPT, but persistently concentrated on the frontier of thePamir.5) Difference between the Pamir and the Tian ShanIn the Pamir, deformation is concentrated on the outer margins. In the Tian Shan, to the north of the Pamir,however, deformation is distributed to a series of fault relavely steeply dipping through the brittle crust, implyingdifferent orogenic processes. Both the Pamir and the Tian Shan migrated forelandward implusivley to produce aseries of fold belts. Deforamtion in the Pamir foreland is dominated by thrusting, each strand has short length andactive deformation is concentrated in a narrow zone. In the southern Tian Shan foreland, however, deformation isdominated by folding, each strand can extend very long distance, and active deformation is distributed in a muchwider zone.Highlights in the thesis:1) The active bending-moment thrust and the flexural-slip normal fault are firstly reported with field evidence inChina.2) Two new fold scarp models, which are quite different from the classical fold scarp formed in fault bending,have been eatablished.3) The method using deformed fluvial terraces to constrain three-dimensional folding was firtly suggested and anew relative model was built.4) In China, the Monte Carlo modeling (see appendix material) was firstly used to constrain deformation rateuncertainty of active thrusts and active folds.