Neotectonic deformation in the transition zone between the Dead Sea Transform and the East Anatolian Fault Zone, Southern Turkey: a palaeomagnetic study of the Karasu Rift Volcanism

In southern Turkey ongoing differential impingement of Arabia into the weak Anatolian collisional collage resulting from subduction of the Neotethyan Ocean has produced one of the most complex crustal interactions along the Alpine–Himalayan Orogen. Several major transforms with disputed motions, including the northward extension of the Dead Sea Fault Zone (DSFZ), meet in this region. To evaluate neotectonic motion on the Amanos and East Hatay fault zones considered to be northward extensions of the DSFZ, the palaeomagnetism of volcanic fields in the Karasu Rift between these faults has been studied. Remanence carriers are low-Ti magnetites and all except 5 of 51 basalt lavas have normal polarity. Morphological, polarity and K–Ar evidence show that rift formation occurred largely during the Brunhes chron with volcanism concentrated at 0.66–0.35 Ma and a subsidiary episode at 0.25–0.05. Forty-four units of normal polarity yield a mean of D/I=8.8°/54.7° with inclination identical to the present-day field and declination rotated clockwise by 8.8±4.0°. Within the 15-km-wide Hassa sector of the Karasu Rift, the volcanic activity is concentrated between the Amanos and East Hatay faults, both with left lateral motions, which have rotated blocks bounded by NW–SE cross faults in a clockwise sense as the Arabian Block has moved northwestwards. An average lava age of 0.5 Ma yields a minimum cumulative slip rate on the system bounding faults of 0.46 cm/year according with the rate deduced from the Africa–Arabia Euler vector and reduced rates of slip on the southern extension of the DSFZ during Plio-Quaternary times. Estimates deduced from offsets of dated lavas flows and morphological features on the Amanos Fault Zone [Tectonophysics 344 (2002) 207] are lower (0.09–0.18 cm/year) probably because they are limited to surface fault breaks and do not embrace the seismogenic crust.Results of this study suggest that most strike slip on the DSFZ is taken up by the Amanos–East Hatay–Afrin fault array in southern Turkey. Comparable estimates of Quaternary slip rate are identified on other faults meeting at an unstable FFF junction (DSFZ, East Anatolian Fault Zone, Karatas Fault Zone). A deceleration in slip rate across the DSFZ and its northward continuation during Plio-Quaternary times correlates with reorganization of the tectonic regime during the last 1–3 Ma including tectonic escape within Anatolia, establishment of the North and East Anatolian Fault Zones bounding the Anatolian collage in mid–late Pliocene times, a contemporaneous transition from transpression to transtension and concentration of all basaltic magmatism in this region within the last 1 Ma.     

Neotectonic and volcanic characteristics of the Karasu fault zone (Anatolia,Turkey): The transition zone between the Dead Sea transform and the East Anatolian fault zone

The Karasu Rift (Antakya province, SE Turkey) has developed between east-dipping, NNE-striking faults of the Karasu fault zone, which define the western margin of the rift and west-dipping, N–S to N20°–30°E-striking faults of Dead Sea Transform fault zone (DST) in the central part and eastern margin of the rift. The strand of the Karasu fault zone that bounds the basin from west forms a linkage zone between the DST and the East Anatolian fault zone (EAFZ). The greater vertical offset on the western margin faults relative to the eastern ones indicates asymmetrical evolution of the rift as implied by the higher escarpments and accumulation of extensive, thick alluvial fans on the western margins of the rift. The thickness of the Quaternary sedimentary fill is more than 465 m, with clastic sediments intercalated with basaltic lavas. The Quaternary alkali basaltic volcanism accompanied fluvial to lacustrine sedimentation between 1.57 ± 0.08 and 0.05 ± 0.03 Ma. The faults are left-lateral oblique-slip faults as indicated by left-stepping faulting patterns, slip-lineation data and left-laterally offset lava flows and stream channels along the Karasu fault zone. At Hacılar village, an offset lava flow, dated to 0.08 ± 0.06 Ma, indicates a rate of left-lateral oblique slip of approximately 4.1 mm·year^–1. Overall, the Karasu Rift is an asymmetrical transtensional basin, which has developed between seismically active splays of the left-lateral DST and the left-lateral oblique-slip Karasu fault zone during the neotectonic period.     

Neotectonic and volcanic characteristics of the Karasu fault zone (Anatolia,Turkey): The transition zone between the Dead Sea transform and the East Anatolian fault zone

Abstract

The Karasu Rift (Antakya province, SE Turkey) has developed between east-dipping, NNE-striking faults of the Karasu fault zone, which define the western margin of the rift and westdipping, N-S to N20°-30°E-striking faults of Dead Sea Transform fault zone (DST) in the central part and eastern margin of the rift. The strand of the Karasu fault zone that bounds the basin from west forms a linkage zone between the DST and the East Anatolian fault zone (EAFZ). The greater vertical offset on the western margin faults relative to the eastern ones indicates asymmetrical evolution of the rift as implied by the higher escarpments and accumulation of extensive, thick alluvial fans on the western margins of the rift. The thickness of the Quaternary sedimentary fill is more than 465 m, with clastic sediments intercalated with basaltic lavas. The Quaternary alkali basaltic volcanism accompanied fluvial to lacustrine sedimentation between 1.57 ± 0.08 and 0.05 ± 0.03 Ma. The faults are left-lateral oblique-slip faults as indicated by left-stepping faulting patterns, slip-lineation data and left-laterally offset lava flows and stream channels along the Karasu fault zone. At Hacilar village, an offset lava flow, dated to 0.08 ± 0.06 Ma, indicates a rate of leftlateral oblique slip of approximately 4.1 mm?year^?1. Overall, the Karasu Rift is an asymmetrical transtensional basin, which has developed between seismically active splays of the left-lateral DST and the left-lateral oblique-slip Karasu fault zone during the neotectonic period. © 2001 Éditions scientifiques et médicales Elsevier SAS     

Field evidences from northern Dead Sea Fault Zone (South Turkey): New findings for the initiation age and slip rate

The left-lateral strike–slip Dead Sea Fault Zone (DSFZ) extends from the Red Sea in the south to the East Anatolian Fault Zone (EAFZ) in the north. This study examines the northern part of the DSFZ around Amik Basin and presents surface and subsurface geological evidence for the Quaternary activity and initiation age of the northernmost DSFZ. The DSFZ extends N–S in the south of the Amik Basin where clear geological and morphological evidence exists for faulting. Geological observations around Amik Basin, analyses of borehole data and electrical resistivity profiles within the Amik Basin indicate that the activity of the northern DSFZ started after Pliocene in the Amik Basin. Subsurface data in the basin suggest that the DSFZ offsets a pre-Quaternary basin sinistrally by about 7.9 km. The offset pre-Quaternary basin suggests at least 4.94 ± 0.13 mm/year slip rate for the northern part of the DSFZ. The Karasu Fault Zone (KFZ) extends in an en-echelon pattern along the western margin of the Karasu Valley and it transfers the significant amount of slip from DSFZ to the EAFZ.     

Quaternary Tholeiitic to Alkaline Volcanism in the Karasu Valley,Dead Sea Rift Zone,Southeast Turkey: Sr-Nd-Pb-O Isotopic and Trace-Element Approaches to Crust-Mantle Interaction

Rare-earth-element, radiogenic and oxygen isotope, and mineral chemical data are presented for tholeiitic and alkaline Quaternary volcanism from Karasu Valley (Hatay, southeastern Turkey). Karasu Valley is the northern segment of the Dead Sea transform fault and is filled with flood-basalt type volcanics of Quaternary age. This valley is an active fault zone that is known as “Karasu fault,” extending in a NE-SW direction. The Karasu Valley basaltic volcanics (KVBV) are subaphyric to porphyritic, with variable amounts of olivine, clinopyroxene, and plagioclase phenocrysts. Alkali basalts are generally characterized by high contents of olivine, clinopyroxene, and plagioclase phenocrysts. Their groundmass contains olivine, clinopyroxene, plagioclase, and Fe-Ti oxides. Tholeiitic basalts are subaphyric to porphyritic (high contents of olivine, clinopyroxene, and plagioclase). Their groundmass is similar to that of alkali basalts. The range of olivine phenocryst and microlite compositions for all analyzed samples is Fo₈₁ to Fo₄₃. Plagioclase compositions in both tholeiitic and alkali basalts range from andesine, An₃₈ to bytownite, An₇₂. Clinopyroxene compositions range from diopside to calcic augite. Most of the olivine, plagioclase, and clinopyroxene phenocrysts are normally zoned and/or unzoned. Fe-Ti oxides in both series are titanomagnetite and ilmenite.

Based on normative and geochemical data, the Karasu Valley basaltic volcanics are mostly olivine and quartz-tholeiites, and relatively lesser amount of alkali olivine-basalts. KVBV have low K₂O/Na₂O ratios, typically between 0.25 and 0.45. Olivine- and quartz-tholeiites are older than alkali olivine-basalts. Olivine tholeiites have Zr/Nb and Y/Nb ratios similar to alkaline rocks, but their Ba/Nb, Ba/La, and La/Nb ratios are slightly higher than alkali olivine-basalts. In contrast, quartz-tholeiites have the highest Ba/Nb, Ba/La, Zr/Nb, and Y/Nb and the lowest Nb/La ratios among the KVBV. Alkali basalts have ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios ranging from 0.703353 to 0.704410 and 0.512860 to 0.512910, respectively. In contrast, quartz-tholeiites have higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd ratios, which vary from 0.704410 to 0.705490 and 0.512628 to 0.512640, respectively. Olivine tholeiites have intermediate isotopic compositions ranging from 0.703490 to 0.704780 and 0.512699 to 0.512780, respectively. ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb isotopic ratios of KVBV range from 18.817 to 19.325, 15.640 to 15.718, and 39.054 to 39.223, respectively. The range of O isotope values is between +5.84 and +7.97‰. The higher O and Sr isotopes in olivine- and quartz-tholeiites relative to alkali olivine-basalts can be explained by contamination of magmas by crustal materials.

The KVBV have intraplate chemistry similar to that of other tholeiitic and alkaline basalts in other within-plate environments, and isotopes range from isotopically depleted mantle to enriched isotope compositions similar to some enriched ocean islands. Trace-element and isotope data indicate that the KVBV are derived from a common OIB-like asthenospheric mantle source, but they have experienced different degrees of crustal contamination during their ascent to the surface, contemporaneous with little fractional crystallization. Although quartz-tholeiites display significant effects of crustal contamination, alkali olivine-basalts appear to have negligible or no crustal contamination in their geesis.     

利用拉分盆地和错断河流阶地定量评估康西瓦断裂晚第四纪平均左旋走滑速率

康西瓦断裂为青藏高原西北缘的一条大型左旋走滑断裂。目前,不同学者对康西瓦断裂晚第四纪的平均走滑速率仍存在较大争议。文章以青藏高原西北缘喀拉喀什河谷段一处冲洪积扇上发育的一个小型拉分盆地以及该冲洪积扇上发育的一个错断河流阶地为研究对象,基于拉分盆地演化的两种简单模式,分别利用拉分盆地的长边和斜边限定对应冲洪积扇的水平位错位量和错位量的上限值,而通过光释光定年技术约束该冲洪积扇的形成年代,结合相关数据,分别估算出康西瓦断裂晚第四纪以来的平均左旋走滑速率为小于或等于8.6±1.0 mm/a和小于约12.4 mm/a。与此同时,利用该冲洪积扇上发育的错断河流阶地的水平位错和对应阶地的放弃年龄,估算出康西瓦断裂晚第四纪以来的平均左旋走滑速率为8.4±1.0 mm/a。

     

天景山断裂带晚第四纪左旋走滑运动速率确定及其空间分布特征

天景山断裂带是青藏高原东北缘一条重要的左旋走滑边界断裂,但是对该断裂左旋走滑运动速率的空间分布特征缺少足够的研究和认识.通过卫星影像解译和野外地质调查,我们在天景山断裂带中段滑动速率限定范围较宽泛的段落选择两个典型的断错地貌观察点,以期获得更为可靠的断裂晚第四纪滑动速率.首先,利用差分GPS和无人机摄影测量技术分别对孟家湾和崾岘沟两点的冲沟或河流阶地的左旋走滑位移量进行精确测量和恢复.然后,利用光释光测年技术对被断错地貌面的年龄进行限定.最后,在综合分析地貌标志物位移和地貌面年龄之间关系以及其形成演化过程,计算和讨论孟家湾和崾岘沟两个点的滑动速率,分别为(1.2±0.3)mm/a和(0.9±0.3)~(1.1±0.2)mm/a.通过与前人研究结果进行对比和分析,认为天景山断裂带晚第四纪左旋走滑运动速率在空间上较为稳定,约为1.1 mm/a.     

柴达木盆地北缘断裂(阿木尼克山段)构造地貌及晚第四纪活动速率研究

柴达木盆地北缘断裂是控制盆地东北部的重要边界活动断裂,其晚第四纪活动特性的研究,对理解南祁连山应变分配模式及断裂活动向柴达木盆地内部挤压扩展过程具有重要意义。柴达木盆地北缘断裂在阿木尼克山段遥感影像线性特征明显,发育一系列断层陡坎、断层三角面、水系扭错等地貌现象。本研究通过遥感解译、地质调查、探槽开挖、GPS地形剖面测量、OSL地质测年等研究表明:在阿木尼克山山前存在长约30 km的地震地质遗迹,连续性好,发育断层陡坎、断层凹槽、地震鼓梁(包)、水系扭错等地貌现象。通过对典型地貌面GPS测量及OSL测年,得到全新世平均垂直速率为0.43±0.02 mm/a。通过DEM图解译得到Fan2洪积扇地貌面最大水平走滑量达48 m,得到13 ka以来走滑速率为3.38～4.21 mm/a; Fan1洪积扇地貌面最大水平走滑量达14 m,计算得到5.1 ka以来走滑速率2.50～2.75 mm/a。     

Late Cenozoic stress evolution along the Karasu Valley, SE Turkey

This study defines the Mio-Pliocene to present-day stress regime acting at the northeastern corner of the eastern Mediterranean region along the Karasu Valley (i.e., the Amanos Range), taking in the Antakya, Osmaniye and Kahramanmaras provinces. The inversion slip vectors measured on fault planes and chronologies between striations indicate that the stress regime varied from transpressional initially to transtensional, having consistent NW- and NE-trending σ_Hmax (σ₁) and σ_Hmin (σ₃) axes, respectively; there are significantly different mean stress-ratio (R_m) values however. The older mean stress state is characterized by N151±11°E-trending σ₁ and N59±12°E-trending σ₃ axes, and by a mean arithmetic R_m value of 0.76, indicating that the regional stress regime is transpressional. The younger stress regime is characterized by N154±8°E-trending σ₁ and N243±8°E-trending σ₃ axes, and by a mean arithmetic R_m value of 0.17, indicating a transtensional character for this regional stress regime. The low R values of the stress deviators related to the recent stress state reflect normal-component slips. The earthquake focal mechanism inversions confirm that the younger stress regime continues into the Recent. The inversion identifies a transtensional stress regime representing strike-slip and an extensional stress state with a consistent NE-trending σ_Hmin (σ₃) axis. These stress states are characterized by N66°E and N249°E-trending σ₃ axes, respectively. Both significant regional stress regimes induce left-lateral displacement along the southern part of the East Anatolian Fault (EAF, or Amanos Fault). The temporal change, probably in Quaternary time, within the regional stress regime—from transpression to transtension—resulted from the coeval influences of subduction processes in the west–southwest (i.e., along the Cyprus arc), continental collision in the east, and westward escape of the Anatolian block.     

Newly found Tunglo Active Fault System in the fold and thrust belt in northwestern Taiwan deduced from deformed terraces and its tectonic significance

We found active faults in the fold and thrust belt between Tunglo town and the Tachia River in northwestern Taiwan. The surface rupture occurred in 1999 and 1935 nearby the study area, but no historical surface rupture is recorded in this area, suggesting that the seismic energy has been accumulated during the recent time. Deformed fluvial terraces aid in understanding late Quaternary tectonics in this tectonically active area. This area contains newly identified faults that we group as the Tunglo Fault System, which formed after the area's oldest fluvial terrace and appears at least 16 km long in roughly N–S orientation. Its progressive deformations are all recorded in associated terraces developed during the middle to late Quaternary. In the north, the system consists of two subparallel active faults, the Tunglo Fault and Tunglo East Fault, striking N–S and facing each other from opposite sides of the northward flowing Hsihu River, whose course may be controlled by interactions of above-mentioned two active faults. The northern part of the Tunglo Fault, to the west of the river, is a reverse fault with upthrown side on the west; conversely the Tunglo East Fault, to the east, is also a reverse fault, but with upthrown side on the east. Both faults are marked by a flexural scarp or eastward tilting of fluvial terraces. Considering a Quaternary syncline lies subparallel to the east of this fault system, the Tunglo Fault might be originated as a bending moment fault and the Tunglo East Fault as a flexural slip fault. However, they have developed as obvious reverse faults, which have progressive deformation under E–W compressive stress field of Taiwan. Farther south, a west-facing high scarp, the Tunglo South Fault, strikes NNE–SSW, oblique to the region's E–W direction of compression. Probably due to the strain partitioning, the Tunglo South Fault generates en echelon, elongated ridges and swales to accommodate right-lateral strike–slip displacement. Other structures in the area include eastward-striking portion of the Sanyi Fault, which has no evidence for late Quaternary surface rupture on this fault; perhaps slip on this part of Sanyi Fault ceased when the Tunglo Fault System became active.     

Quaternary faulting along the Caribbean-North American plate boundary in Central America

Recent detailed mapping along the Motagua fault zone and reconnaissance along the Chixoy—Polochic and Jocotán—Chamelecón fault zones provide new information regarding the nature of Quaternary deformation along the Caribbean—North American plate boundary in Central America.The southern boundary of the Motagua fault zone is defined by a major active left-slip fault that ruptured during the February 4, 1976 Guatemala earthquake. The recurrent nature of slip along the fault is dramatically demonstrated where stream terraces of the Río El Tambor show progressive left-slip and vertical (up-to-the-north) slip. Left-slip increases from 23.7 m (youngest mappable terrace) to 58.3 m (oldest mappable terrace) and vertical slip increases from 0.6 m to 2.5 m. The oldest mappable terrace crossed by the fault appears to be younger than 40,000 years and older than 10,000 years.Reconnaissance along the Chixoy—Polochic fault zone between Chiantla and Lago de Izabal has located the traces of a previously unmapped major active left-slip fault. Geomorphic features along this fault are similar to those observed along the active trace of the Motagua fault zone. Consistent and significant features suggestive of left-slip have so far not been observed along the Guatemala section of the Jocotán—Chamelecón fault zone.In Central America, the active Caribbean—North American plate boundary is comprised of the Motagua, Chixoy—Polochic, and probably the Jocotán—Chamelecón fault zones, with each accommodating part of the slip produced at the mid-Cayman spreading center. Similarities in geomorphic expression, apparent amount of left-slip, and frequency and magnitude of historical and instrumentally recorded earthquakes between the active traces of the Motagua and Chixoy—Polochic fault zones suggest a comparable degree of activity during Quaternary time; the sense and amount of Quaternary slip on the Jocotán—Chamelecón fault zone remain uncertain, although it appears to be an active earthquake source. Uplift of major mountain ranges on the north side of each fault zone reflects the small but consistent up-to-the-north vertical component (up to 5% of the lateral component) of slip along the plate boundary. Preliminary findings, based on offset stream terraces, indicate a late Quaternary slip rate along the Caribbean—North American plate boundary of between 0.45 and 1.8 cm/yr. Age dating of offset Quaternary terraces in Guatemala will allow refinement of this rate.     

Late Quaternary activity and dextral strike-slip movement on the Karakax Fault Zone, northwest Tibet

Field observations and interpretations of satellite images reveal that the westernmost segment of the Altyn Tagh Fault (called Karakax Fault Zone) striking WNW located in the northwestern margin of the Tibetan Plateau has distinctive geomorphic and tectonic features indicative of right-lateral strike-slip fault in the Late Quaternary. South-flowing gullies and N–S-trending ridges are systematically deflected and offset by up to ~ 1250 m, and Late Pleistocene–Holocene alluvial fans and small gullies that incise south-sloping fans record dextral offset up to ~ 150 m along the fault zone. Fault scarps developed on alluvial fans vary in height from 1 to 24 m. Riedel composite fabrics of foliated cataclastic rocks including cataclasite and fault gouge developed in the shear zone indicate a principal right-lateral shear sense with a thrust component. Based on offset Late Quaternary alluvial fans, ¹⁴C ages and composite fabrics of cataclastic fault rocks, it is inferred that the average right-lateral strike-slip rate along the Karakax Fault Zone is ~ 9 mm/a in the Late Quaternary, with a vertical component of ~ 2 mm/a, and that a M 7.5 morphogenic earthquake occurred along this fault in 1902. We suggest that right-lateral slip in the Late Quaternary along the WNW-trending Karakax Fault Zone is caused by escape tectonics that accommodate north–south shortening of the western Tibetan Plateau due to ongoing northward penetration of the Indian plate into the Eurasian plate.     

Displacement and timing of left-lateral faulting in the Kunlun Fault Zone,northern Tibet,inferred from geologic and geomorphic features

The E-W to WNW-ESE striking Kunlun Fault Zone, extending about 1600 km, is one of the large strike-slip faults in the northern Tibet, China. As a major strike-slip fault, it plays an important role on the extrusion of Tibet Plateau in accommodating northeastward shortening caused by the India-Asia convergence. However, the time of initiation left-lateral faulting of the Kunlun Fault Zone is still largely debated, ranging from the Middle to Late Triassic (240–200 Ma) to early Quaternary (2 Ma). We document displaced basement rocks and geomorphic features along the Kunlun Fault Zone, based on tectono-geomorphic interpretation of satellite remote sensing images and field geologic and geomorphic observations. Our results show that the largest cumulative offset of basement rocks is likely to be 100 ± 20 km. Meanwhile, a series of pull-apart basins (Kusai, Xiugou and Tuosu lake basins) and pressure ridges (East Deshuiwai and Maji Snow Mountains), each 45–70 km long and ∼8–12 km wide, are developed along the Kunlun Fault Zone, which resulted from long-term tectono-geomorphic growth since the Late Miocene or Early Pliocene. Geologic evidence indicates that the Kunlun Fault Zone had a long-term slip rate of ca.10 mm/yr during the late Quaternary. This slip rate is similar to that shown by present-day GPS measurements. Thus, we estimate that the Kunlun Fault Zone probably began left-lateral faulting at 10 ± 2 Ma based on a total displacement of 100 ± 20 km, and assuming a constant long-term slip rate of ca.10 mm/yr for several millions of years. And this timing constraint on initiation of left-lateral faulting of the Kunlun Fault Zone is consistent with widespread tectonic deformation which occurred in the Tibetan Plateau.     

内蒙古乌兰哈达-高勿素断裂的新活动证据及构造意义初探

近期在鄂尔多斯地块东北缘、蒙古高原南缘的乌兰哈达火山群附近发现一条长约100余公里的NW向断裂——乌兰哈达-高勿素断裂,并基于高分辨率卫星影像解译和野外地质调查对该断裂的新活动特征进行了初步研究。断裂活动的地貌证据包括线性展布的断层陡坎、断塞塘、断层槽谷以及位错冲沟、断头沟等。跨断裂冲沟的同步性左旋位错及广泛发育的反向陡坎(倾向NE)等指示断裂应为左旋走滑为主兼具由SW向NE逆冲的运动性质。乌兰哈达-高勿素断裂的构造位置及几何学、运动学特征指示其应归属于NW向左旋走滑的张家口-渤海断裂带,该断裂的左旋走滑运动应在调节其南、北两侧块体向E的差异运动中起着重要的作用。另一方面,鄂尔多斯地块东北缘的乌兰哈达-高勿素断裂及张家口断裂、洗马林断裂等NW向断裂所表现出的逆冲运动特征指示鄂尔多斯地块东北缘可能持续受到青藏高原东北缘对鄂尔多斯地块西南缘自晚中新世以来推挤作用远程效应的影响,这些伴有逆冲运动的NW向断裂应是鄂尔多斯地块东北缘地区响应青藏高原东北缘NE向生长和扩展的一种具体表现。乌兰哈达-高勿素断裂新活动证据的发现不仅完善了张家口-渤海断裂带的几何图像,也为认识和理解鄂尔多斯地块东北缘的构造变形和评价地震危险性提供了新约束。

     

Geochemistry and geochronology of the mafic lavas from the southeastern Ethiopian rift (the East African Rift System): assessment of models on magma sources,plume–lithosphere interaction and plume evolution

Major and trace element and isotopic ratios (Sr, Nd and Pb) are presented for mafic lavas (MgO > 4 wt%) from the southwestern Yabello region (southern Ethiopia) in the vicinity of the East African Rift System (EARS). New K/Ar dating results confirm three magmatic periods of activity in the region: (1) Miocene (12.3–10.5 Ma) alkali basalts and hawaiites, (2) Pliocene (4.7–3.6 Ma) tholeiitic basalts, and (3) Recent (1.9–0.3 Ma) basanite-dominant alkaline lavas. Trace element and isotopic characteristics of the Miocene and Quaternary lavas bear a close similarity to ocean island basalts that derived from HIMU-type sublithospheric source. The Pliocene basalts have higher Ba/Nb, La/Nb, Zr/Nb and ⁸⁷Sr/⁸⁶Sr (0.70395–0.70417) and less radiogenic Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb = 18.12–18.27) relative to the Miocene and Quaternary lavas, indicative of significant contribution from enriched subcontinental lithospheric mantle in their sources. Intermittent upwelling of hot mantle plume in at least two cycles can explain the magmatic evolution in the southern Ethiopian region. Although plumes have been originated from a common and deeper superplume extending from the core–mantle boundary, the diversity of plume components during the Miocene and Quaternary reflects heterogeneity of secondary plumes at shallower levels connected to the African superplume, which have evolved to more homogeneous source.     

Active faulting in the western Pyrénées (France): Paleoseismic evidence for late Holocene ruptures

We have identified a 50-km-long active fault scarp, called herewith the Lourdes Fault, between the city of Lourdes and Arette village in the French Pyrénées. This region was affected by large and moderate earthquakes in 1660 (Io = VIII–IX, MSK 64,), in 1750 (Io = VIII, MSK 64) and in 1967 (M_d = 5.3, Io = VIII, MSK 64). Most earthquakes in this area are shallow and the few available focal mechanism solutions do not indicate a consistent pattern of active deformation. Field investigations in active tectonics indicate an East–West trending and up to 50-m-high fault scarp, in average, made of 3 contiguous linear fault sub-segments. To the north, the fault controls Quaternary basins and shows uplifted and tilted alluvial terraces. Deviated and abandoned stream channels of the southern block are likely due to the successive uplift of the northern block of the fault. Paleoseismic investigations coupled with geomorphic studies, georadar prospecting and trenching along the fault scarp illustrate the cumulative fault movements during the late Holocene. Trenches exhibit shear contacts with flexural slip faulting and thrust ruptures showing deformed alluvial units in buried channels. ¹⁴C dating of alluvial and colluvial units indicates a consistent age bracket from two different trenches and shows that the most recent fault movements occurred between 4221 BC and 2918 BC. Fault parameters and paleoseismic results imply that the Lourdes Fault and related sub-segments may produce a M_W 6.5 to 7.1 earthquake. Fault parameters imply that the Lourdes Fault segment corresponds to a major seismic source in the western Pyrénées that may generate earthquakes possibly larger than the 1660 historical event.     

长春市南湖-桑家窝堡断层特征及活动性研究

南湖-桑家窝堡断层是一条穿越长春市区的北西向隐伏活动断层,该断层对长春地区的地壳稳定性影响较大。利用高分辨率遥感信息解译、探地雷达探测、浅层人工地震勘探和钻孔联合剖面探测等资料,并结合第四纪沉积物的ESR 年龄测定资料,讨论了南湖-桑家窝堡断层的空间展布和运动特征。南湖-桑家窝堡断层沿逯家窝堡、后三家子、南湖公园、长春公园、桑家窝堡、大合隆一线展布,断层产状为45°∠65°,断层线较平直。在断层西北端大合隆一带,南湖-桑家窝堡断层错断了上白垩统嫩江组( K2n) 岩层,并在嫩江组岩层顶部沿断层形成了侵蚀沟谷,但没有错断早更新世和中更新世地层。在断层东南端的逯家窝堡附近,南湖-桑家窝堡断层错断了白垩纪泉头组( Kq) 岩层,但没有错断中更新世和晚更新世地层。南湖-桑家窝堡断层形成于中生代末期,第四纪早期曾经活动,活动方式以左旋走滑运动为主,第四纪中更新世以来断层没有明显的活动迹象。     

The 1984 Abruzzo earthquake (Italy): an example of seismogenic process controlled by interaction between differently oriented synkinematic faults

The evolution of the seismogenic process associated with the M_s 5.8 Sangro Valley earthquake of May 1984 (Abruzzo, central Italy) is closely controlled by the Quaternary extensional tectonic pattern of the area. This pattern is characterised by normal faults mainly NNW striking, whose length is controlled by pre-existing Mio–Pliocene N100±10° left-lateral strike-slip fault zones. These are partly re-activated as right-lateral normal-oblique faults under the Quaternary extensional regime and behave as transfer faults.Integration of re-located aftershocks, focal mechanisms and structural features are used to explain the divergence between the alignment of aftershocks (WSW–ENE) and the direction of seismogenic fault planes defined by the focal mechanisms (NNW–SSE) of the main shock and of the largest aftershock (M_s=5.3).The faults that appear to be involved in the seismogenic process are the NNW–SSE Barrea fault and the E–W M. Greco fault. There is field evidence of finite Quaternary deformation indicating that the normal Barrea fault re-activates the M. Greco fault as right-lateral transfer fault. No surface faulting was observed during the seismic sequence. The apparently incongruent divergence between aftershocks and nodal planes may be explained by interpreting the M. Greco fault as a barrier to the propagation of earthquake rupturing. The rupture would have nucleated on the Barrea fault, migrating along-strike towards NNW. The sharp variation in direction from the Barrea to the M. Greco fault segments would have represented a structural complexity sufficient to halt the rupture and subsequent concentration of post-seismic deformation as aftershocks around the line of intersection between the two fault planes.Fault complexities, similar to those observed in the Sangro Valley, are common features of the seismic zone of the Apennines. We suggest that the zones of interaction between NW–SE and NNW–SSE Plio-Quaternary faults and nearly E–W transfer faults, extending for several kilometres in the same way as M. Greco does, might act as barriers to the along-strike propagation of rupture processes during normal faulting earthquakes. This might have strong implications on seismic hazard, especially for the extent of the maximum magnitude expected on active faults during single rupture episodes.     

秦岭造山带内宁陕断裂带构造演化及其意义

宁陕断裂是秦岭造山带内部发育的一条近东西向区域性断裂。研究表明,宁陕断裂运动学性质为左行走滑,变形早期为韧性变形,晚期叠加脆性变形。早期变形形成的同变形变质矿物的⁴⁰Ar-³⁹Ar定年结果显示,变形时代为169~162Ma左右,属于秦岭造山带碰撞后陆内变形阶段产物。宁陕左行走滑断裂的存在暗示着在中晚侏罗世之前,现今南秦岭构造带很可能分属于两个不同的构造单元。宁陕断裂北西侧具有古老变质基底,并有大量早中生代花岗岩体侵入;南东侧只发育中上元古宙浅变质火山-沉积组合,发育晚元古宙-早古生代基性侵入岩脉及一些碱性岩脉。中晚侏罗世-早白垩世期间,围绕着扬子地块西缘和北缘,发生过左行走滑变形,这可能与扬子地块在这个时期的顺时针旋转相关。     

Reactivated strike–slip faults: examples from north Cornwall, UK

Several strike–slip faults at Crackington Haven, UK show evidence of right-lateral movement with tip cracks and dilatational jogs, which have been reactivated by left-lateral strike–slip movement. Evidence for reactivation includes two slickenside striae on a single fault surface, two groups of tip cracks with different orientations and very low displacement gradients or negative (left-lateral) displacements at fault tips.

Evidence for the relative age of the two strike–slip movements is (1) the first formed tip cracks associated with right-lateral slip are deformed, whereas the tip cracks formed during left-lateral slip show no deformation; (2) some of the tip cracks associated with right-lateral movement show left-lateral reactivation; and (3) left-lateral displacement is commonly recorded at the tips of dominantly right-lateral faults.

The orientation of the tip cracks to the main fault is 30–70° clockwise for right-lateral slip, and 20–40° counter-clockwise for left-lateral slip. The structure formed by this process of strike–slip reactivation is termed a “tree structure” because it is similar to a tree with branches. The angular difference between these two groups of tip cracks could be interpreted as due to different stress distribution (e.g., transtensional/transpressional, near-field or far-field stress), different fracture modes or fractures utilizing pre-existing planes of weakness.

Most of the d–x profiles have similar patterns, which show low or negative displacement at the segment fault tips. Although the d–x profiles are complicated by fault segments and reactivation, they provide clear evidence for reactivation. Profiles that experienced two opposite slip movements show various shapes depending on the amount of displacement and the slip sequence. For a larger slip followed by a smaller slip with opposite sense, the profile would be expected to record very low or reverse displacement at fault tips due to late-stage tip propagation. Whereas for a smaller slip followed by larger slip with opposite sense, the d–x profile would be flatter with no reverse displacement at the tips. Reactivation also decreases the ratio of d_max/L since for an original right-lateral fault, left lateral reactivation will reduce the net displacement (d_max) along a fault and increase the fault length (L).

Finally we compare Crackington Haven faults with these in the Atacama system of northern Chile. The Salar Grande Fault (SGF) formed as a left-lateral fault with large displacement in its central region. Later right-lateral reactivation is preserved at the fault tips and at the smaller sub-parallel Cerro Chuculay Fault. These faults resemble those seen at Crackington Haven.