A Study Of Quaternary Episodic Block Faulting In Xinding Basin

Xinding Basin is situated in the northern part of Fenwei Graben system; it is a typical Cenozoic extensional fault-depression basin. The basin is bounded by the NE-trending North Piedmont Fault of Wutai Mountain, North Piedmont Fault of Xizhou Mountain, South Piedmont Fault of Hengshan Mountain, and the NNE-trending East Piedmont Fault of Yunzhong Mountain. The vertical displacements of Wutai and Xizhou Mountain North Piedmont Faults are relatively stronger, which control the primary geomorphic feature of the basin. The basin is surrounded by continuously uprising fault-block mountains. The Wutai Mountain is on the east side of the basin. It is the roof of North China region, its highest peak Beitai is 3058m above sea level, after which the famous Beitai-stage planation surface is named. On the basin side of the fault-block mountains 3 levels of planation surfaces and 7 steps of river terraces were developed below the Beitai-stage planation surface, indicating multi-stage uplifting. In the history three earthquakes above magnitude 7 took place in the Xinding Basin. A study on the responses of earthquakes to episodic tectonism helps to reveal the dynamic mechanism of earthquakes.This dissertation took the stepped landforms of the piedmont zone surrounding Xinding Basin as the main thread, the north piedmont of Wutai and Xizhou mountains as the study area, and divided the Quaternary tectonic movement into stages based on the paragenetic relation between river terraces and pediments. By dating the river terraces and quantitatively analyzing the longitudinal river profile the division and timing of tectonic stages since 1.2 Ma were carried out. On this basis the responses of earthquakes to tectonism in the Late Quaternary were further discussed. The major achievements and new understandings are described as follows.1. The paragenetic evolution model of pediments and fluvial morphology under the action of episodic fault movementIn the piedmont zone of fault-block mountains the pediments and tectonic terraces developed during the same tectonic movement are possessed of a certain paragenetic relation. According to the classic fault scarp evolution theory, a fault block was uplifted to form a fault scarp; afterwards the gully on the scarp eroded and cut down to form a V-shaped valley. Upon entering a quiescent period of tectonic activity, the riverbed was widened to form a U-shaped wide valley, meanwhile the slope of fault scarp decreased because of denudation. If the quiescent period was sufficiently long, a pediment was formed by fault scarp recession. Under the control of downthrown basin ground as the common base level of erosion, the pediment surface and the valley bed were approximately situated at the same altitude. Later-stage block uplift formed new fault scarp, the previous pediment surface and the wide valley were uplifted, and the gully started retrogressive erosion from the scarp to produce a terrace. In the piedmont zone the terrace and pediment surface jointly formed a stair of surface; they were contiguous in space with comparable elevation. Multiple tectonic cycles may produce a multi-stepped landforms consisting of a series of pediments and corresponding river terraces. Therefore the history of tectonic uplifting can be determined from the stepped landforms, and the starting time of rapid uplift, i.e., the commencing of active tectonic period, can be determined by dating the tectonic terraces (the time of initial river downcutting).If an active tectonic stage is considered as a period of intense earthquake activities, then a scarp of tens meters high is the result of cumulated multiple earthquake dislocations, and a quiet tectonic stage means obviously decreased strength and frequency of fault slips, then downstream from the end of terrace is a rapid-flow reach consisting of a series of knickpoints, where the river bed is steep and the valley is narrow. On the other hand, upstream from the end of terrace is a slow-flow reach, where the riverbed is broad and gentle. The rapid reach represents an active tectonic period, while the slow reach means a stable tectonic period. If the retreating speeds of the knickpoints between two neighboring terraces are uniform, then the durations of active and quiet period can be estimated from the length of rapid and slow reach respectively. The time span of slow reach is T_Si = (T_i - T_i-1) L_Si/(L_Si + L_Ri), the time span of rapid reach is T_Ri = (T_i - T_i-1) L_Ri/(L_Ri + L_Si), where i is terrace stair numbered from bottom upward, T_i is the age of i-th terrace stair, L_Ri and L_Si represents respectively the length of rapid and slow reach between the i-th and i-1-th terrace. In this way we can obtain the starting and ending times of the active and stable tectonic periods.2. The step division of pedimentsOn the north piedmont of Wutai Mountain nearby Nanyukou of Fanshi County, as well as on the north piedmont of Xizhou Mountain nearby Hu Village of Dingxiang County, 6 pediments (P1 to P6) were developed between the fault scarp foot and the Tangxian-stage planation surface. They rise step by step, extend as belts along the piedmont, and are cut by transverse gullies. On the interfluves, the pediment surfaces are relatively flat with widths from meters to hundreds meters and slopes between 5°and 25°. Each step of pediment is connected to the next step via a frontal bank whose slope is between 30°and 60°. The frontal riser of the first pediment surface (P1) is the most recent fault scarp or diluvial platform; at the foot of pediment P1 outcrops the fault. The higher pediments resulted from recession and uplift of antecedent fault scarps, their frontal risers are retreating scarps. Above the P6 pediment there is a slope that transits into the flat Tangxian-stage planation surface (Pt). The six steps of pediments indicate six stages of rapid tectonic uplift during the Quaternary.The spatial pattern of pediments is relatively stable. Near the mouth of some large gullies (antecedent gullies), pediments are often united with the terraces or the bottoms of ancient wide valley. The altitude of the same stair pediment is basically identical on both sides of a gully. It changes little along the piedmont; if it does the undulation is continuous unless the pediment is offset by transverse faults. These features make it possible to distinguish pediments from local landform steps caused by differential erosion.The differentiation of activity along fault may result in different altitudes of pediments of the same stair in different regions. To the east and west of Yangyan River the north piedmont of Wutai Mountain is respectively controlled by two left-step en echelon faults. To the east of Yangyan River the altitudes of pediment P6 to P1 are successively 1510～1560m, 1450～1500m, 1350～1410m, 1290～1310m, 1250～1270m, and 1215～1230m above sea level. To the west they are 1488～1493m, about 1430m, 1385～1390m, 1320m, 1270～1290m, and 1225～1250m. Pediments P6 to P4 are higher on the east, while P3 to P1 are higher on the west, indicating that the western fault is more active in Late Quaternary. This is also seen in the relative heights of pediments. On the east piedmont P6 to P1 are successively 260～310m, 200～230m, 130～185m, 95～120m, 30～65m, and 10～25m above the alluvial plain on the basin side; while on the west piedmont they are successively 283～313m, 225～250m, 185～205m, 115～140m, 70～85m, and 40m. Starting from P4 the west is obviously higher than the east. P2 and P1 in west are nearly twice as high as that in the east. Therefore, a regional correlation of pediments needs to trace continuously , and the corresponding fluvial geomorphology should be referred to.3. Division and dating of river terracesOn the north piedmont zone of Wutai and Xizhou Mountains 7 steps of Quaternary terraces were developed in large transverse gullies. Along the valley of Hutuo River in east Xizhou Mountain 5 steps of terraces are rather completely preserved. Each terrace is covered by a loess-paleosol sequence of different thickness, which facilitates dating the geomorphic surfaces and making a regional correlation.Yangyan River is a large transverse gully on the north piedmont of Wutai Mountain. It originates from northeast Beitai peak of 3058m above sea level and is 35km long. It flows northwards out of the mountain then into the upper reach of Hutuo River. Below the Tangxian-stage planation surface of altitude 1600m there are 7 steps of Quaternary terraces along the Yangyan River(T1 to T7, T1 is the youngest). Among them T1 and T2 are aggradational terraces, T3 to T5 are strath terraces, T6 and T7 are degradational terraces. Below T5 the terraces are overlain by relatively complete loess-paleosol sequences. By means of paleomagnetic polarity and susceptibility tests the paleomagnetic stratigraphy of terraces T5 and T4 was established and correlated to the standard loess profile and abyssal oxygen isotope curve. It is determined that the oldest aeolian deposit on terrace T5 is paleosol S15, corresponding to oxygen isotope stage MIS 37, and the age of the terrace is 1.2 Ma.The oldest aeolian deposit on terrace T4 is paleosol S5, corresponding to oxygen isotope stage MIS 15, and the age is 0.6 Ma. The oldest aeolian deposit on terrace T3 is paleosol S1, corresponding to oxygen isotope stage MIS 5, and the age is 0.13 Ma. The oldest aeolian deposit on terrace T2 is loess L1, corresponding to oxygen isotope time MIS2. The ages of terrace T1, T2 and T3 given by ¹⁴C and luminescence dating are 6ka, 20ka and 134ka BP respectively. The gullies on the north piedmont of Xizhou Mountain are of smaller scale, and the higher terraces are rather severely damaged by erosion. Nearby Hu Village 6 stairs of terraces were developed in the transverse gully, among which T1 and T2 are aggradational terraces, T3 is strath terrace, T4 to T6 are degradational terraces. The age for terrace T1, T2, and T3 from luminescence dating is respectively 6ka, 20ka, and 140ka BP. The oldest aeolian deposit on T2, T3, and T4 is respectively L1, S1, and S5.The Hutuo River flows through the entire Xinding Basin, then crosses the east segment of Xizhou Mountain on the southeast side of the basin to continue eastward. In the gorge area along Rongjiazhuang, Zhaojiazhuang, to Lingzidi in the eastern Xizhou Mountain, 5 stairs of river terraces are well preserved along the river. Among them T1 and T2 are aggradational terraces, T3 to T5 are strath terraces. Luminescence dating resulted the ages of T1 to T3 terraces as 7.6ka, 20～26ka, and 130ka BP. The oldest aeolian deposit on T2, T3, and T4 is L1, S1, and S5 respectively. The loess stratum on T5 was eroded.In summary the river terraces of the same stair on the north piedmont of Wutai Mountain and on Xizhou Mountain piedmont are overlain by identical loess-paleosol sequences, the ages of terraces are generally consistent, indicating that the river terraces in this region were formed by the same mechanism.Geomagnetic stratigraphic study of the Quaternary loess in Xinding Basin was carried out for the first time, age determination and regional correlation of the Quaternary river terraces were done systematically. This is a major progress achieved by this dissertation.4. The paragenetic relation between pediments and river terraces and its tectonic implicationsNearby Nanyukou on the northern piedmont of Wutai Mountain and Hu Village on the north piedmont of Xizhou Mountain, the pediments (P1 to P6) and the terraces of lateral gullies (T2 to T7) are united at the gully mouth step by step, constructing six levels of stepped surfaces. These unified geomorphic surfaces extend as belts along the piedmont, with two ends extending toward the upper reach of gullies. In a plan view they are "U"-shaped and rise and encircle step by step. This step-by-step correspondence of pediments and terraces is consistent in the region.The episodic uplift of fault block played the leading role in the formation of the unified surfaces, however, the climatic change and drainage difference of gullies resulted in the undulation of the surfaces which is accordant with the landform of the alluvial plain on the basin side. The Yangyan River is the largest river in the study area of north Wutai piedmont, abundant material source and relatively large discharge produced the huge alluvial fan with the valley mouth as its vertex, and made the united surface to maintain a descending trend from near the vertex outward in its development. The last glacial stage caused aggradation of the Yangyan River and the reduction of discharge made a large quantity of material stagnant nearby the fan vertex, thus increased the height difference between the fan vertex and the edge. By contrast, the gullies R1 to R6 on the east side of Yangyan River are of small size, insufficient material sources and steep riverbed, aggradation in glacial stage was not significant, and alluvial fans were not well developed. Glacial and local factors jointly enhanced the altitude difference of alluvial fans on the basin side, resulted in the large undulation of the first-step unified surface.The first-step unified surface (consisting of P1 and T2) appears merely as a slope break of the fault scarp on the bedrock piedmont, while the unified surfaces of second and above steps have obvious pediment surfaces, indicating that a sufficiently long quiet tectonic period is necessary to form a unified geomorphic surface. Fault recession and river evolution are two different kinds of geomorphologic processes. According to field investigation, the headward erosion of the river was generally much quicker than the recession of fault scarp. If the quiescent period was short, even though the gully reached an equilibrium state and the riverbed was widened by lateral erosion, but the fault scarp did not obviously retreat or become less steep, the following tectonic uplift might cause river incision to form terrace, but there would not be corresponding pediment or slope break of the fault scarp. Terrace T1 did not develop a corresponding pediment at the valley mouth, but formed a scarplet several meters high across the fault, its synchronous sediments on the downthrown wall are buried below the contemporary alluvial fan. Its formation was a manifestation of fault activity, and was also accordant with the optimal climate period in Holocene. What roles the tectonic and climatic factors played is worthy of further investigation.According to the sedimentary texture of alluvium and the oldest aeolian deposit overlying, terraces T3 to T5 were formed when the river was in an erosion or equilibrium state, so fault-block uplift caused synchronous and rapid river incision, the age of terrace can be considered as the starting time of speedy tectonic uplift. Terrace T2 was formed when the river was in an aggradation state (in large gullies) or in an erosion and equilibrium state (in medium and small gullies), but the arid climate of the last glacial maximum did not prevent river incision. It can be inferred that climatic factors did not significantly affect the incision caused by tectonic uplift. Even if the incision was postponed slightly, the time lag should not be very long. Therefore, the age of T2 can be approximately considered as the starting time of the last speedy tectonic uplift.Based on the study of paragenetic relation of pediments and river terraces, it is deduced that Wutai Fault-block Mountain experienced 6 relatively strong tectonic uplifting events in the Quaternary. The last 4 events started at 1.2Ma, 0.6Ma, 0.13Ma, and 0.02Ma B.P. respectively. This result is consistent with previous studies in southern Fenwei Graben system and in the northeastern part of Tibetan Plateau.Paragenetic relation of pediments and terraces of transverse rivers was put forward for the first time, then uplifting stages of fault block was rebuilt by the paragenesis, thus effectively reducing the uncertainty of uplifting-event identification with single landform type. This is another major progress achieved by this dissertation. 5. Longitudinal profile of gully and the rhythm of mountain upliftingBased on the surveyings of longitudinal profile along 3 gullies on the north piedmont of Wutai Mountain and 4 gullies on the north piedmont of Xizhou Mountain, it is found that the riverbed consists of serial rapid reaches with steep slope and slow reaches with gentle slope. The rapid reach is composed of a series of dense knickpoints, while on the slow reach the knickppoints have small height and large interval. The extinction point of terrace corresponds to the slope-break of riverbed, from which downstream there is narrow rapid reach and upstream there is broad slow reach. Because the episodic uplifting of fault-block plays a leading role in the incision of transverse river, the above features in the river profile are difficult to explain by differential erosion due to lithological differences. Instead they should be the result of episodic fault slippings. The rapid reachs were formed in active tectonic periods, the knickpoints resulted from many times of large-displacement surface faultings at short recurrence intervals; while the slow reachs were formed in stable tectonic periods when the displacement and frequency of faulting were low.If the retreating speeds of the knickpoints between two neighboring terraces were uniform, then the duration time of active and quiet tectonic periods can be estimated from the ages of terraces and the lengths of rapid and slow reachs. In this way the uplifting rhythm of Wutai fault-block mountain in the recent 120Ma can be described as follows. The rapid uplifting periods are 1200 to 1056ka, 600 to 522ka, 130 to 99ka, and 20 to Oka; the rest times are quiet periods. The uplifting rhythm of Xizhou fault-block mountain in the recent 120Ma can be described as follows. The rapid uplifting periods are 1200 to 1063ka, 600 to 486ka, 130 to 106ka, and 20 to 0ka; the rest times are quiet periods. The results from two regions are comparable but with some differences, which are considered due to insufficient statistic samples and local lithological and drainage differences. With the increase of surveying gullies and statistic samples, the local factors can be further suppressed. The statistics indicates that the duration of an active period is much shorter than the consecutive stable period, being about l/2.4～1/18.8 of the previous stable period and 1/2.0～1/6.7 of the following stable period. Moreover, the longer the stable period lasted, the longer the following active period was.Nearby the extinction point of terrace the river may sometimes have a transition zone of gradient, i.e., the rapid reach and slow reach form an S-shaped slope break zone, which means that the change of tectonic mode may have a transition period. But not all cases are like this, further verification is needed.Comparing to the result of paleo-earthquake studies for the recent 20ka, the number of surface-rupture earthquakes is not consistent with the number of knickpoints below terrace T2, the latter is generally less than the former. The reason should be further investigated.On the premise that fault-block uplifting plays a leading role in the formation of river terraces, the durations of tectonic stages can be estimated by dating the terraces and analyzing the longitudinal profile of the river, thus improving the accuracy of age of tectonic stages. This is the third major progress achieved by the dissertation. 6. Mechanism discussion of episodic block upliftingTwo possible mechanisms are responsible for the episodic block uplifting around Xinding basin—regional tectonism and climate changes. Regional crust pulling-apart induce sinking of basin, multiperiodic activity of boundary faults and uplifting of mountain. Cooling and greater variability of Quaternary climate cause intense periodical erosion and isostasy rebound of the mountain, which is a factor which cannot be neglected either in the episodic uplifting of block mountains.The heights of pediment series indicate that the long term average uplifting rate of the block mountains around Xinding basin changed little since late Pliocene, but the uplifting frequency increased distinctly since the last 1.2Ma, and the uplifting has a tendency of shorter duration, shorter recurrent time and bigger uplifting rate, which cannot be ascribed to pure regional tectonism. On the other hand, quick uplifting stages did not accord completely with the fluctuation of climate, which favor that fluctuation of climate is not the dominant agent. However three of four uplifting stage started from early interglacial, and two are completely in interglacial stage, which suggest some temporal relation between uplifts and climate changes.The greater variability of climate since 1.2Ma ago enhance the interglacial exhumation of mountain and the accumulation of basin, intensifying the isostasy rebound and favoring the episodic uplift of mountains, which maybe the reason why the mode of mountain uplifting changed. The erosion of mountain in the switch from glacial to interglacial is the strongest, so the isostasy is the most prominent, which may be the reason why mountain uplifts mostly start at early interglacial.In a word, some combination of regional tectonism and climate changes has been responsible for episodic uplifting of block mountain in Quaternary. Crust isostasy rebound induced by fluctuation of climate has been intensifying since Quaternary, and periodically acting on crustal dynamic process, which may be the key mechanism for more frequent uplift of mountain, shorter duration and higher rate of uplift since Quaternary.Rules of regional tectonism and climate changes in episodic uplift of block mountains since Quaternary has been discussed preliminarily. This is the fourth major progress achieved by the dissertation.7. Responses of earthquakes to tectonic episodes in late QuaternaryThe Xizhou Mountain north piedmont fault was not active between 100ka and 22ka BP, no surface-faulting event was found; on the other hand, the vertical displacement since 22ka amounts to 16～18m, 6～7 faulting events have taken place. The active period of earthquakes accords to the most recent active tectonic stage, while the time of seismic quiescence consists with the last stable tectonic stage.Since the last 22ka, the earthquakes were clustering in the forepart and the terminal. but relative sparse in the middle stage. The magnitudes of earthquakes waned in the early stage, then largened in the late stage. The Wutai north piedmont fault underwent a quiet period 6 to 7 ka before, then entered a active period of earthquakes until now. The longitudinal profiles of transverse gullies indicates that since the last 20ka the two fault have experienced three inferior tectonic stages near equally long which are in turn active, quiet and active. Paleoearthquakes show that switch from the last stable to the recent active tectonic stage was abrupt and intense.The responses of earthquakes to tectonic episodes in late Quaternary have been explored initially, which helps to unveil the mechanism of earthquake clustering of fault segment in long time span. This is the fifth major progress achieved by the dissertation.