Lithospheric structure of the western United States and the Tibetan Plateau: Implications on their mechanism of uplift

Copyright 1991, Mohammed Noureddine Beghoul. See also: http://atlas.geo.cornell.edu/dissertations/Beghoul_1991.htmThis dissertation seeks to determine the upper mantle structure and the mechanisms responsible for the Cenozoic uplift of the high terranes located near plate boundaries: the Tibetan Plateau and western North America. The upper mantle structure is determined using the first P arrivals obtained from the International Seismological Centre (ISC) at regional distances (2 - 22 degrees). In the first chapter a methodology is presented for computing mantle lid Pn velocities using ISC data together with a detailed error analysis. Application of this algorithm to Colorado Plateau yields an average Pn velocity of 8.12 +/- 0.09 km/s. This value is higher than the one reported in the literature but similar to that beneath stable midcontinent regions. We use this Pn value and the Cenozoic history of the plateau to constrain the mode of uplift. In chapter two, using the same techniques, we confirm the lower Pn velocity beneath the Basin and Range Province and show the presence of about 4% intrinsic azimuthal Pn velocity anisotropy in the mantle lid beneath the Basin and Range. The direction of high velocity coincides with the direction of present-day extension in the Basin and Range Province (i.e., NW - SE). We show that this anisotropy is the result of Cenozoic extension rather than a cumulative signature of older tectonic events. In chapter three, a modified version of the algorithm and detailed mapping of Sn attenuation allow the determination of mantle lid thickness beneath the western United States and Tibet. We show that the mantle lid thickness beneath the southern 2/3 of Tibet ranges from 135-165 km thick. This value is similar to the one we find for the Great Plains. The deep structure, Cenozoic uplift, and various other geophysical and geological data of these two high terranes are consistent with the subduction of flat slabs beneath them. The continental Indian Plate is still beneath the southern 2/3 of Tibet, but the oceanic Farallon Plate has already been delaminated from the overriding North American Plate

Active megadetachment beneath the western United States

Geodetic data, interpreted in light of seismic imaging, seismicity, xenolith studies, and the late Quaternary geologic history of the northern Great Basin, suggest that a subcontinental-scale extensional detachment is localized near the Moho. To first order, seismic yielding in the upper crust at any given latitude in this region occurs via an M7 earthquake every 100 years. Here we develop the hypothesis that since 1996, the region has undergone a cycle of strain accumulation and release similar to “slow slip events” observed on subduction megathrusts, but yielding occurred on a subhorizontal surface 5–10 times larger in the slip direction, and at temperatures >800°C. Net slip was variable, ranging from 5 to 10 mm over most of the region. Strain energy with moment magnitude equivalent to an M7 earthquake was released along this “megadetachment,” primarily between 2000.0 and 2005.5. Slip initiated in late 1998 to mid-1999 in northeastern Nevada and is best expressed in late 2003 during a magma injection event at Moho depth beneath the Sierra Nevada, accompanied by more rapid eastward relative displacement across the entire region. The event ended in the east at 2004.0 and in the remainder of the network at about 2005.5. Strain energy thus appears to have been transmitted from the Cordilleran interior toward the plate boundary, from high gravitational potential to low, via yielding on the megadetachment. The size and kinematic function of the proposed structure, in light of various proxies for lithospheric thickness, imply that the subcrustal lithosphere beneath Nevada is a strong, thin plate, even though it resides in a high heat flow tectonic regime. A strong lowermost crust and upper mantle is consistent with patterns of postseismic relaxation in the southern Great Basin, deformation microstructures and low water content in dunite xenoliths in young lavas in central Nevada, and high-temperature microstructures in analog surface exposures of deformed lower crust. Large-scale decoupling between crust and upper mantle is consistent with the broad distribution of strain in the upper crust versus the more localized distribution in the subcrustal lithosphere, as inferred by such proxies as low P wave velocity and mafic magmatism

About the lithospheric structure of central Tibet based on seismic data from the INDEPTH III profile

Signals from 11 shots and 8 earthquakes, and numerous teleseismic events were recorded along the 400-km seismic line INDEPTH III in central Tibet and interpreted together with previous seismic and tectonic data. The abnormal behavior of various mantle phases reveals a complex Moho-transition zone, especially in the northern part of the line, in the Changtang Block, where the lower crust and the mantle show unusually low velocities, a shingled appearance of Pn and no low-velocity layer in the upper crust. The strong east-west anisotropy in the Changtang Block is related to an easterly escape movement of the whole lithosphere, facilitated by the warm and weak layers in the lower crust and the upper mantle, bounded apparently by two prominent west-east running fault zones

He, Ne and Ar 'snapshot' of the subcontinental lithospheric mantle from CO2 well gases

The subcontinental lithospheric mantle (SCLM) constitutes a significant portion of the upper mantle sourcing magmatic volatiles to the continents above, yet its geochemical signature and evolution remain poorly constrained. Here we present new interpretation of noble gas datasets from two magmatic CO2 fields in the SW US, namely Bravo Dome and Sheep Mountain, which provide a unique insight into the volatile character of the SCLM sourcing the Cenozoic volcanism in the region. We identify that reduction of 3He/4Hemantle ratio within the Sheep Mountain CO2 field can be attributed to radiogenic production within the SCLM. Using a Reduced Chi-Squared minimisation on the variation of derived 4He/21Necrust ratios within samples from the Sheep Mountain field, combined with a radiogenically raised 21Ne/22Nemantle end member, we resolve 3He/4Hemantle ratios of 2.59 ± 0.15 to 3.00 ± 0.18 Ra. These values correspond with a 21Ne/22Nemantle value of 0.136. Using these 3He/4Hemantle end member values with 21Nemantle resolved from Ne three component analysis, we derive the elemental 3He/22Nemantle of 2.80 ± 0.16 and radiogenic 4He/21Ne*mantle range of 1.11 ± 0.11 to 1.30 ± 0.14. A second Reduced Chi-Squared minimisation performed on the variation of 21Ne/40Arcrust ratios has allowed us to also determine both the 4He/40Armantle range of 0.78 to 1.21 and 21Ne/40Armantle of 7.66 ± 1.62 to 7.70 ± 1.54 within the field. Combining these ratios with the known mantle production ranges for 4He/21Ne and 4He/40Ar allows resolution of the radiogenic He/Ne and He/Ar ratios corresponding to the radiogenically lowered 3He/4Hemantle ratios. Comparing these values with those resolved from the Bravo Dome field allows identification of a clear and coherent depletion of He to Ne and He to Ar in both datasets. This depletion can only be explained by partial degassing of small melt fractions of asthenospheric melts that have been emplaced into the SCLM. This is the first time that it has been possible to resolve and account for both the mantle He/Ne and He/Ar ratios within a SCLM source. The data additionally rule out the involvement of a plume component in the mantle source of the two gas fields and hence any plume influence on the Colorado Plateau Uplift event

Plagiarism: The Weakest Link in Teaching Research Skills

تتناول هذه المقالة ظاهرة استفحلت وأخذت بعدا عالميا وهي ظاهرة السرقة العلمية في عصر رقمي أصبحت فيه المعلومات والمعطيات العلمية افتراضية أكثر من ذي قبل وفي متناول الجميع. باستعمال استبيان وزِّع على عينة من الطلبة وعينة من الأساتذة يحلل هذا البحث هذه الظاهرة في سياق مذكرة يقدمها طلبة ل م د لنيل شهادة  الماستر في اللغات التطبيقية في قسم الآداب واللغة الإنجليزية، جامعة قسنطينة 1. ويفترَض في هذا البحث أن كثيرا من الطلبة لا يدركون تماما ما تعنيه السرقة العلمية حقا لأن هذا المفهوم لا يدرَّس إلا نادرا في مقياس تدريس مناهج ومهارات البحث.  هذا بالإضافة إلى أن الطلبة يقدمون أبحاثهم في وقت قصير ويشرف عليهم عدد قليل من الأساتذة. كل هذا وذاك عقد مشكل السرقة العلمية فأصبحت صعبة التشخيص ولا مفر منها عند كثير من الطلبة. وتوحي أجوبة الأساتذة والطلبة على السواء أيما إيحاء على ما هو مفهوم السرقة العلمية في واقعهم اليومي وما يترتب عليه من نتائج وخيمة على المدى القريب والمدى البعيد وكيفية كشف هذه السرقة من قبل المشرفين أثناء إشرافهم على أبحاث متعددة وفي أجل قصير وكذا كيفية اجتناب الطلبة لهذه السرقة من أجل سلوك أخلاقي والتزام بقوانين الملكية العلمية وهي أمور من شأنها ترقية الفكر الإبداعي والأصيل            This paper examines plagiarism which is a widespread phenomenon that has gained a universal dimension in a digital age where knowledge and data are growing more virtual and more accessible than ever.  Using a questionnaire administered to a sample of students and a sample of teachers, the author tries to analyse this phenomenon in the context of the research projects undertaken by LMD students in partial fulfilment for the requirements of a Master’s in applied languages at the Department of Arts and English Language, University 1 of Constantine. The hypothesis is that many students are not aware of what plagiarism really includes as they are seldom taught this appropriately in the module of research methods and skills. Furthermore, the great number of students, the short time for submissionand lack of supervisors have made the problem even more complex, far out of control and practically unavoidable. The responses of both teachers and students are quite insightful in matter of what plagiarism fully implies, what its consequences are in the short and long run, how to detect it by supervisors in short term mass research projects and how to avoid it by students in order to behave ethically, abide by the laws governing intellectual property and, by the same token, promote original and creative thought.Key Words: Digital, intellectual property, original thought, plagiarism, unethical 

Apport et contribution de l'analyse de diagraphies a la connaissance d'un bassin sedimentaire. Application au bassin de Timimoun (Algerie)

SIGLEINIST T 77572 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Slurry shield tunneling in soft ground. Comparison between field data and 3D numerical simulation

In urban areas, the control of ground surface settlement is an important issue during shield tunnel-boring machine (TBM) tunneling. These ground movements are affected by many machine control parameters. In this article, a finite difference (FD) model is developed using Itasca FLAC-3D to numerically simulate the whole process of shield TBM tunneling. The model simulates important components of the mechanized excavation process including slurry pressure on the excavation face, shield conicity, installation of segmental lining, grout injection in the annular void, and grout consolidation. The analysis results from the proposed method are compared and discussed in terms of ground movements (both vertical and horizontal) with field measurements data. The results reveal that the proposed 3D simulation is sufficient and can reasonably reproduce all the operations achieved by the TBM. In fact, the results show that the TBM parameters can be controlled to have acceptable levels of surface settlement. In particular, it seems that moderate face pressure can reduce ground movement significantly and, most importantly, can prevent the occurrence of face-expected instability when the shield crosses very weak soil layers. The shield conicity has also an important effect on ground surface settlement, which can be partly compensated by the grout pressure during tail grouting. Finally, the injection pressure at the rear of the shield significantly reduces the vertical displacements at the crown of the tunnel and, therefore, reduces the settlement at the ground surface

In situ test of a borehole extensometer

cited By 2Stresses in the Earth's crust are known to represent a key element of geodynamical processes at various spatial and temporal scales. Static stresses within lithospheric plates can be estimated on the basis of direct in situ measurements in boreholes or near the surface. Earthquake focal mechanisms yield the same type of information by sampling at greater depths. The quantification of geological deformations happens to be the principal tool to determine possible changes in time of the tectonic stress pattern. These changes can however be recorded only qualitatively and on the geological time scale. In seismic regions it is clear that the regional stress field varies on a time scale defined by the recurrence of earthquakes, say several tens of years. Our goal was to design a robust and cheap instrument which could continuously monitor stress changes at depths large enough to reduce meteorological influences. © 1989 Springer-Verlag