Table of Content

08 June 2006, Volume 28 Issue 2

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Brief Report

THE PRELIMINARY STUDY ON SEISMOTECTONICS OF THE 2003 AD BACHU-JIASHI EARTHQUAKE (M_S 6.8), SOUTHERN TIAN SHAN

XU Xi-wei, ZHANG Xian-kang, RAN Yong-kang, CUI Xiao-feng, MA Wen-tao, SHEN Jun, YANG Xiao-ping, HAN Zhu-jun, SONG Fang-min, ZHANG Lan-feng

2006, 28(2):  161-178. 

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Owing to strong and permanent Cenozoic re-orogenic processing, a lot of EW-striking active thrusts and folds have been developed in Tian Shan, resulting in crustal shortening in NS direction. There also exist NW-striking transform-like strike-slip faults that cut the Tian Shan and accommodate uneven crustal shortening larger in the west and smaller in the east. The seismogenic structures in and around the Tian Shan mainly include EW-striking thrust ramps or blind thrusts and NW-striking transform-like strike-slip faults. The 2003 AD Bachu-Jiashi earthquake is located at south of the Kalpintag nappe. A NE-trending deep seismic reflection profile about 50km long across the epicenter has been conducted after the earthquake. From this reflection profile four blind faults are identified. Together with earthquake relocation, these identified blind faults are used in the paper to interpret the seismogenic structures of the 1997 AD Jiashi strong earthquake swarm and 2003 AD Bachu-Jiashi earthquake. The 1997 AD Jiashi strong earthquakes were generated mainly by a NW-striking buried transform-like strike-slip fault, while the 2003 AD Bachu-jiashi earthquake by blind thrusts in front of the Kalpintag nappe.

DEFORMATION PATTERN AND SHORTENING RATES IN THE EAST PART OF KALPIN THRUST SYSTEM IN SOUTHWEST TIANSHAN DURING LATE QUATERNARY

RAN Yong-kang, YANG Xiao-ping, XU Xi-wei, CHENG Jian-wu, CHEN Li-chun

2006, 28(2):  179-193. 

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Less attention has been paid to the deformation pattern, shortening amount and shortening rate in late Quaternary across Kalpin thrust system. This paper tries to discuss them, mainly in the eastern part of the thrust system by doing field investigation along the faults and folds, measuring geomorphic deformation, excavating trenches in some important sites where young alluvial fans were displaced obviously and dating young deposits of alluvial terrace. There are two types of deformation in surface and near surface for the Kalpin thrust system in late Quaternary i.e. the movement of thrust faults at lower angles and bending of young folds. Both kinds of deformation are shown by shortening and uplifting of young geomorphic surfaces. The surface ages of 3 stages are concluded by dating 20 examples using TL method in the studying area and comparing the results of previous people on deposition and incision times of alluvial terraces in Tianshan mountain area, and they are 100ka, 33～18ka and 6.6～8.2ka, respectively, for large-scale deformed alluvial surfaces: T₃, T₂, and T₁ in kalpin region. Then, the 19 shortening amounts and rates are obtained in the 13 sites along 4 rows of anticlines in front of Kalpin thrust system and Piqian fold. The shortening amounts and rates show that there are two sections where deformation is stronger than others. The two sections consist of two arcs that is toward the south. Shortening rates near top of arcs are 1.32mm/a in the west and 1.39mm/a in the east across the thrust system, respectively. Besides, deformation on front rows is stronger than that behind for bifurcate folds.

THE ANALYSIS FOR CRUST SHORTENING OF KALPIN THRUST TECTONIC ZONE, SOUTH-WESTERN TIANSHAN, XINJIANG, CHINA

YANG Xiao-ping, RAN Yong-kang, SONG Fang-min, XU Xi-wei, CHENG Jian-wu, MIN Wei, HAN Zhu-jun, CHEN Li-chun

2006, 28(2):  194-204. 

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Kalpin thrust tectonic is an active reverse fault fold zone at the southwestern Tianshan front piedmont, it consists of five to six rows of arc fold zones which are formed by Cambrian-Quaternary sedimentary rocks. The major morphology of the anticline is multiple box-shaped or asymmetry inclined, mostly, and it is similar to that of fault-bend fold and fault-propagation fold. Depending on the seismic reflection surveys data, the reverse faults on the front of the nappe in Kalpin thrust tectonic zone form an integrate detachment surface in the deep along the gypsum stratum in Cambrian. The depth of the detachment is shallower in the southeast and deeper in the northwest. The depth of the detachment fault is deeper in the west part (about 9km deep) and shallower in the east part (about 5km deep) of the Piqiang fault. In the middle part of the Kalpin reverse fault-fold zone, we have made two balanced cross-sections at the two sides of Piqiang fault. On the two geological cross-sections, we construct the structure mode at depth using fault-bend fold or fault-propagation fold model. The length of the two sections is 73km and 78km, respectively. The restored sections yield a crustal shortening of 40km to 45km, the shortening rate is 33% and 37%, respectively. Calculating the long-term shortening rate from these two across-sections is difficult, because the time of initiation of deformation is poorly known. Geological evidence suggests that most of the shortening began in the beginning of the deposition of the thick conglomerate unit in lower Quaternary. If the initiation time is about 2.5Ma, the shortening rate of Kalpin thrust tectonic zone is 15.4～17.3mm/a.

THE SEISMOGENIC TECTONICS OF THE M_S 6.8 BACHU-JIASHI,XINJIANG EARTHQUAKE IN FEB.24, 2003

SHEN Jun, CHEN Jian-bo, WANG Cui, WU Chuan-yong, SONG Zheng-na

2006, 28(2):  205-212. 

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The M_S 6.8 Bachu-Jiashi earthquake of Feb.24, 2003 occurred in the western Tarim Basin and is possibly the continuation of the Jiashi strong earthquake swarms in 1997-1998. However, its focal mechanism and rupture process are different from that of the Jiashi strong earthquake swarms, according to our preliminary study on its seismic tectonics with geomorphologic information from satellite images, the deep structures from the petroleum seismic exploration, the macro damage and isoseismic features from field investigation, the relocation of the epicenters of the aftershocks, and the regional seismic tectonics from both deep and surface tectonics. The occurrence of the M_S 6.8 Bachu-Jiashi earthquake is closely related with the revealed reverse fault on the Maigaiti slope belt between the Bachu uplift and Kashi depression in western Tarim Basin. The sites of the ground fissures found in the field fit with the revealed reverse fault. Isoseismal features are also corresponding to the rupture direction of the fault. These evidences indicate that the M_S 6.8 Bachu-Jiashi earthquake is the result of the southward rupturing from deep to shallow along a north-dipping reverse fault in Tarim Basin. This reverse fault is possibly the result of the propagation of the thrust fault-fold system named Kalpintag thrust belt in the front of Tianshan.

SEISMOTECTONIC STUDY ON WEST PART OF THE INTERACTION ZONE BETWEEN SOUTHERN TIANSHAN AND NORTHERN TARIM

TIAN Qin-jian, DING Guo-yu, HAO Ping

2006, 28(2):  213-223. 

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The interaction zone between southern Tianshan and northern Tarim locates at northeast side of Pamir. It is a region with high seismicity. We built a seismotectonic model for the west part of this zone from data of geological profiles, deep crust seismic detection and earthquake focal mechanisms in this paper. Geological profiles show different structure styles between the north of Artux anticline and the south of it. There are southward thrust blocks with basement rocks and strongly deformed Mesozoic and Cenozoic folds at the north, but northward reverse faults and flat Neogene - Quaternary shallow folds at the south. A fault named Qiligaike at the north side of the Artux anticline can be identified from its linear feature on satellite image. It cuts many folds at the north, and controls the twisting deformation of Artux anticline at the south. These deformation features on two sides of the fault imply the fault is a high angle strike-slip fault. Deep crust seismic detection also shows different velocity structures between Tianshan and Tarim Basin. Depth of the crystalline basement is 4km in Tianshan and 10km in Tarim; Depth of crust in Tianshan is 55km and 50km in Tarim. Steep slope of the crust exists at the northwest of Artux. These indicate the existence of high angle faults. Based on the synthesized geological features, deep crust structure, and earthquake focal mechanisms, we think that the main regional tectonic is featured as that the Tianshan tecto-lithostratigraphic unit overthrusts on the Tarim block. The Tianshan tectonic system includes the Maidan fault and thrust sheets in front of the fault; The Tarim tectonic system includes the underground northern Tarim margin fault, conjugate fractures in basement and overthrust faults in shallow. The northern Tarim margin fault is a high angle fault existing in deep of the Tarim crust, adjusting different trending deformation between Tianshan and Tarim. It is the major active fault that can generate large earthquakes. The other faults as the Tianshan overthrust system and the Tarim basement faults in this area may generate moderately strong earthquakes with different styles.

CENOZOIC DEFORMATION AND PROPAGATION OF THE KALPINTAG FOLD NAPPE

SONG Fang-min, MIN Wei, HAN Zhu-jun, XU Xi-wei

2006, 28(2):  224-233. 

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The Kalpintag fold nappe is located at the northwestern foot of the Tianshan Mountains. Since Cenozoic, owing to the Indian-Eurasian collision, the Mesozoic fold structures of Tianshan have been rejuvenated, uplifted and pushed northward and westward. As a result, several rows of fold-reverse fault zones have been progressively formed within the foreland basins. This paper describes in detail the Cenozoic deformation features and propagation of the fold-reverse fault zone on the west of the nearly south-north-trending Piqiang fault zone. The results show that the Cenozoic deformation of the nappe was characterized by wavy differential uplift, and this has caused the successive formation of the fold-reverse fault zone from the southern side of the Tianshan Mountains to the Tarim Basin. Among them, the early-formed folds are close to the Tianshan Mountains, while the latter-formed folds are close to the Tarim Basin, indicating the general tendency of northward propagation of the fold-reverse fault zone during their formation process. The distance of propagation may reach up to 76km. Moreover, the front of individual fold-reverse fault zone consists of several fault strands, which are associated with folds and have different ages of formation and time of recent activity. The early-formed faults are close to the Mountain side and the latter-formed close to the basin, indicating the northward propagation of the frontal faults of the individual fold. The distance of propagation is about 100～500m. The mechanism of the propagation of the fold-reverse fault zone is discussed in this paper as well.

THE PRELIMINARY STUDY ON PALEOEARTHQUAKES ALONG THE WESTERN SEGMENT OF KALPINTAG FAULT

MIN Wei, SONG Fang-min, HAN Zhu-jun, XU Xi-wei

2006, 28(2):  234-244. 

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The Kalpintag Fault locates at the most forefront of Kalpintag nappe tectonics, and can be separated into eastern and western segments by Piqiang Fault. Six large trenches are excavated along the western segment and four paleoearthquakes can be distinguished in the three trenches of them. The first paleoevent occurred about 12ka BP, the second event occurred about 8.6ka BP, the third event occurred about 5ka BP,and the last event occurred after (1.73±0.15) ka BP, which probably is the Xike'er M6.8 earthquake in 1961 AD. The four paleoevents are characterized by 3～4ka quasi-periodic recurrence interval. The Kalpintag nappe structure are composed of 5～6 rows of fold-reverse fault zones. The faults with the latest activity are located at the forefront of the fold-reverse fault zones with 10km spacing between each fault. The north-dipping and listric style fault surfaces merged into the detachment surface in the deep along the bottom of Cambrian at 6～10km depth. The field investigation discovered that earthquake ruptures and paleoearthquake traces can be found not only along Kalpintag Fault but also along other faults, but the rupture length and seismic slip are smaller than that formed by an M≥7 earthquake. Although five paleoearthquakes since 14ka BP are obtained along western segment of Kalpintag Fault, some events are probably missed because of less trenches and dating samples. Many problems such as magnitude of these events, seismogenic fault and their rupture zones formed by one or several events await study in the future.

PALEOEARTHQUAKES ALONG THE EAST SECTION OF THE RANGE FRONT FAULT OF KALPINTAG DURING LATE QUATERNARY IN KALPIN STRUCTURE SYSTEM, THE SOUTHWEST TIANSHAN MOUNTAINS

RAN Yong-kang, YANG Xiao-ping, CHENG Jian-wu, XU Xi-wei, CHEN Li-chun

2006, 28(2):  245-257. 

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Kalpin thrust tectonic system is an important part of foreland thrust tectonic of the south Tianshan. This paper tries to discuss paleoearthuakes along the east section of the range front fault of Kalpintag during late Quaternary by doing field investigation along the fault, measuring geomorphic deformation and excavating trenches in some important sites where young alluvial fans were displaced obviously. There are 6 rows of fold-thrust faults from Puchang fault to Shanchakou and 4 rows from Shanchakou to Kalpin. Furthermore, the offset alluvial fans of Holocene have been found along two sections of the fault. One is Sanjianfang-Wudaoban section to the west of Sanchakou, another is Dakouzidaoban section to the east of Sanchakou. Both sections are located near arcuate top of thrust-sheet. We can find more than two periods of Holocene fans were displaced by thrust fault except the modern fan in both sections, and only Pleistocene fans were offset in other parts along the fault. It suggests that there are two segments to the east of Puchang fault along the range front fault of Kalpintag. 3 trenches are excavated along the Sanjianfang-Wudaoban segment and 4 events are discovered. Ages of the events are obtained by controlling deposit dating of TL samples. Referenced ages of events are 22, 14, 6.5 and 4.4ka BP and recurrence intervals are 8, 7 and 2ka, respectively. The vertical displacement of events is 1～1.2m and shortening slip 1.3～1.4m for events of long return-time, and they are 0.2～0.3m and 0.6～0.7m for events of short return-time, respectively. And one trench is excavated and 2 events since the end of late Pleistocene are exposed along Dashankoudaoban segment. Ages of the two events in this segment are a bit older than 13 and younger than 6ka BP, respectively. The recurrence interval is about 7ka. The coseismic displacement of each event is about 0.5m and the shortening slip 1.3m, respectively. It is necessary to tell that there is an uncertainty for the event age given in this paper because it is difficult to get good samples for dating. However, we can learn more on the large earthquake activity during late Quaternary along the range front fault of Kalpintag from these events.

CHARACTER OF LATE QUATERNARY ACTIVITY OF THE EAST SEGMENT OF THE FRONT-EDGE FAULTS OF KALPINTAG

CHENG Jian-wu, RAN Yong-kang, YANG Xiao-ping, XU Xi-wei

2006, 28(2):  258-268. 

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Based on a lot of outcrop geological surveys, we discussed the thrust fault belt in the front of the east section of Kalpintag in Kalpin area and found many evidences, including the fault incising the surface of diluvium and fluvial channel sediments and terraces, and also the fracture and subsection character of the faults. The evidences indicate that the frontal fault of the Kalpintag is an active thrust-fault of low angle. According to the interpretation and analysis of the satellite image and field investigation, we analyzed the geometry structure of the fault. Many samples in several trenches are used to date the diluvium and fluvial channel sediments and terraces and the sections of the fault scarps at 7 observation points in the field were measured too. Using these data we estimated the slip rate of the fault. The research results implicate that the front edge fault of the east section of kalpintag is divided into two arc segments: from west to east, the first segment is from Bachulinkuang to Sanchakou, and the other segment is from Sanchakou to east of Qionggemandaoban. The Wudaoban, Sanjianfang and Dashankoudaoban are at the crest of the arc of the fault which incises the surface of T₁ and T₂ diluvium and terraces, the horizontal averaged shortening rate of the fault is 0.35～0.44mm/a since Holocene and 0.16～0.30mm/a in the late Pleistocene to Holocene. The neotectonic movement at these sites is strong. Whereas the fault only incises T₃ diluvium at such sites as Bachulinkuang, the north of Sanchakou and Dashankou which are not at the crest of the arc of the fault. The horizontal averaged shortening rate in these areas is 0.05～0.07mm/a since late Pleistocene, being smaller than that of other sites.

THE BASIC FEATURES OF THE ACTIVE TECTONICS IN THE KUQA DEPRESSION OF THE SOUTHERN TIANSHAN

SHEN Jun, WU Chuan-yong, LI Jun, XIANG Zhi-yong, CHEN Jian-bo, XIE Tian, SONG Zheng-na, WANG Cui

2006, 28(2):  269-278. 

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The distribution, tectonic style and new displacement and other features of the main active tectonics in Kuqa depression in the front of southern Tianshan were introduced in this paper. This depression is an “eye-shaped” tectonics in plane. It is composed of two fold zones in the south and north respectively. The northern one close to the main southern Tianshan Range is a southward thrusting fault-folding system. The most recent active fold in this system is the Kasangtuokai fold belt. The southern one close to the Tarim Basin is a northward thrust fault-fold system. The recent active folds in this system are the Qiulitage fault-fold belt and other young folds in its south, such as the Yaken fold. These two folding systems embrace the Baicheng Basin which likes an eyeball in the eyelids. The Kasangtuokai Fault with a length of 60km in the north and the Qiulitag Fault with the length over 200km in the south are the most important active faults in Kuqa depression. The younger and smaller folds in the south of Qiulitag anticline belt indicate the southward propagation of the thrust fault in Kuqa depression. The petroleum seismic profiles show that the folding and faulting processes are controlled by the detachment fault between the sediment cover and the basement of the basin. The depth of the detachment fault is around 10km and possibly defines the main seismogenic zone in the depression area.

PRELIMINARY STUDY OF LATE QUATERNARY CRUSTAL SHORTENING RATE ALONG KUQA DEPRESSION IN SOUTH TIANSHAN, XINJIANG

WU Chuan-yong, SHEN Jun, CHEN Jian-bo, LI Jun, XIANG Zhi-yong, XIE Tian, SONG Zheng-na, WANG Cui

2006, 28(2):  279-288. 

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The Kuqa depression is located in the middle segment of the southern Tianshan Mountains. There are four E-W extending rows of reverse fault and anticline zones in the depression. From the south Tianshan Mountains towards the Tarim basin, they are the mountain piedmont, the Kasangtuokai, the Qiulitag and the Yaken reverse fault and fold zones. After a month field working, we find the crustal shortening of the Kuqa depression in late Quaternary is almost caused by the Kasangtuokai, the Qiulitag and the Yaken reverse fault and fold zones. The reverse fault and anticline zones in the Kuqa depression are very different in tectonic feature. We accurately surveyed these tectonics with total station and differential GPS in order to get a new cognition of the deformation characteristic and the slip rate. Based on the deformation characteristics of conceptual fault-propagation fold and field investigation, we think the deformation of the fault-propagation fold in the Kuqa depression is caused by faulting rather than folding. The crustal shortening rate caused by the fault is approximately near to the actual rate. So we only surveyed the deformation near the fault. The Kasangtuokai anticline is a fault-propagation fold. From late Quaternary, the deformation of Kasangtuokai anticline is mainly caused by total-uplift of the hanging wall. The deformation rate is about 1.0～2.0mm/a. The deformation feature of the Dongqiulitag anticline is similar to that of the Kasangtuokai, while the crustal shortening rate is little more than that of Kasangtuokai, about 2.5mm/a. The Qiulitag anticline is a very complicated tectonic. It is a fault-bend fold. There are two reverse faults on the core and the north limb of the Qiulitag anticline. Its tectonic deformation includes two parts: the fold rise and the uplift of the hanging wall of the fault. By surveying and dating, we get the crustal shortening rate of the Qiulitag anticline limb of about 1.06～2.0mm/a. Considering the shortening of the core fault and southern limb, the total rate is possibly more than 3.0mm/a. The Yaken anticline is a blind thrust fault-anticline fold. Its shortening rate is 1.5～2.0mm/a. So the total crustal shortening rate of the Kuqa depression is more than 5.0～7.0mm/a from late Quaternary.

ETM IMAGE CHARACTERISTICS AND INTERPRETATION OF ACTIVE TECTONICS OF THE AREA AROUND THE KALPINTAG THRUST SYSTEM

CHEN Li-chun, CHEN Gui-hua, CHEN Li-ze, RAN Yong-kang, YANG Xiao-ping

2006, 28(2):  289-298. 

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In the area around Kalpintag thrust system, earthquakes occurred frequently and intensively. The fundamental data and detailed investigation about active tectonics and the law of earthquake recurrence in this region are desiderated. So, 12 scenes of Landsat-7 ETM digital images are chosen and processed exclusively for the active tectonics. Then, two color composite images of 30m-resolution over the region are portrayed with bands 3-2-1 and bands 7-4-3 as R-G-B respectively, while two color composite images of 15m-resolution over the field workaround are fused with bands 3-2-1-8 and bands 7-4-3-8, respectively. With these resulting images, we designed some geological interpretation criteria for active tectonics of Kalpintag thrust system. According to these criteria, active tectonics of the area around Kalpintag thrust system are interpreted with scales of 500,000, while those of three sites on the field workaround are interpreted in detail with scales of 50000, by means of contrastive interpretation with two kinds of different images at one time. At last, the field investigation confirmed that the used techniques of processing and interpretation in this research were resultful.

THE IDENTIFICATION AND APPLICATION OF GROWTH STRATA LINKED TO THE FORELAND FOLD-AND-THRUST BELT DURING MOUNTAIN BUILDING

ZHANG Guang-liang, ZHANG Pei-zhen, MIN Wei, CHEN Jie

2006, 28(2):  299-311. 

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Growth strata (i.e. progressive unconformities) are linked to a particular structure at depth and a record for different tectonic and sedimentation processes. they locate in foreland basins on the fronts of fold and thrust belts and exhibit extremely varied attitudes. The inherent synchroneity of growth strata and coupled fold or fault activity make growth strata crucial to interpret fold-and-thrust geometry and kinematics. On the balanced cross-section the sequences of growth strata have characteristic wedge-shaped sedimentation. In coeval depositional systems, fault-bend folding, fault-propagation folding and detachment folding are interpreted as the dominant mechanisms. The other modes of folding are recognized later in the 1980s,for example, Trishear folding and Chester and Chester folding, and so on. Though several types of theoretical behavior are expected, all growth strata can be grouped into two fundamental mechanisms: hinge migration and Limb rotation. Growth strata result from a simultaneous interference of several processes such as tectonics, sedimentation and erosion. The interplay between tectonic and surface processes has been shown to constrain the evolution of orogens through a feedback mechanism, the competition between tectonic uplift and shortening, syntectonic sedimentation rate and syntectonic erosion rate controls the final shape and the occurrence and geometries of fault breakthrough in thrust-related anticlines. According to variety of limb and hinge, synsedimentary wedge, variety of strata occurrence and thickness and regional geological setting, growth strata can be identified. In the future, the study of thrust-related folding processes within folds and thrusts belts will be developed by multi-models and ways. Synchroneity and continuity of growth strata and coupled fold or fault activity can be depicted accurately. Based on the present work and good examples of growth strata, paleomagnetic stratigraphy can provide some important information about chronology and tectonic process. Through reviewing briefly the importance, geometry and kinematics of growth strata, we conclude that the Sikouzi section should be a molasses basin occurring in the front of thrust-fold mountain belt where there exist growth strata and progressive unconformities. However,detailed investigation should be done on this in the future.

DISCUSSION ON SEISMOGENIC FAULT OF THE 1976 TANGSHAN EARTHQUAKE

JIANG Wa-li

2006, 28(2):  312-318. 

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The opinions of two papers from this publication are discussed in the paper. One is that the Tangshan Fault is a high-angle, west-dip and thrust with strike-slip fault. The other is that the Fuzhuang-Xihe Fault distributed at the east side of Tangshan city is the seismogenic fault of Tangshan earthquake. For the former opinion, it needs to explain the relationship between the active style of the thrust Tangshan Fault and the formation genesis of a Quaternary depression along the west side of Tangshan city. For the latter opinion, if the Fuzhuang-Xihe Fault is the seismogenic fault of the Tangshan earthquake, it needs to explain the genesis relationship between this surface rupture zone with west-dip and the surface rupture that extents through the Tangshan city with strike-slip. And it needs more evidence to get rid of the possibility, that is, the surface rupture belongs to secondary structure rupture. The paper suggests doing more work on the active fault that controls the Caobo Quaternary depression.

SINISTRAL STRIKE-SLIP FAULTS ALONG THE SOUTHERN ALASHAN MARGIN AND EASTWARDS EXTENDING OF THE ALTUN FAULT

CHEN Wen-bin, XU Xi-wei

2006, 28(2):  319-324. 

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The southern Alashan margin fault bundle is composed of 5 faults which strike EW, converge westward and splay eastward. Each fault of the fault bundle extends more than the 100km, controls the distribution of EW-trending Quaternary basins, and is featured with sinistral slip since the late Quaternary. The western part of the fault bundle-the Jintananshan Fault, tends to meet with the Kuantanshan Fault-the east part of the Altun Fault and has some relations to the Wenzhushan uplift in the transformation. According the spatial correlation, it is believed that the Jintananshan Fault as well as the whole fault bundle is the eastward-extending part of the Altun Fault. The special modality of the fault bundle, which spreads out eastwards, is in favor of decomposing and absorbing the slip movement, which is normal at a strike-slip fault's ends. The sinistral strike-slip movement of the fault bundle is the result of the Altun Fault extending further eastward, which might have begun between the end of the early Pleistocene and the beginning of the middle Pleistocene.