MARGINS Education and Planning Workshop:
Rupturing of Continental Lithosphere in the Red Sea/Gulf of Suez
to be held March 17-23, 2001, Sharm-el-Sheikh, Sinai, Egypt
Abstracts; download as a PDF file or read on-line:
Mohamed G. Abdelsalam and Robert J. Stern
Geology of the Neoproterozoic Basement around the Red Sea
The Red Sea is flanked to the north, east and west by Precambrian (mostly Neoproterozoic) exposures collectively known as the Arabian-Nubian Shield. In NE Africa, the Nubian Shield extends from the Gulf of Suez in Egypt to the Gulf of Zula in Eritrea, forming mountainous terrain with moderate elevation known as the Red Sea Hills. In Asia, the Arabian Shield extends from the Gulf of Aqaba in Israel, Jordan and Saudi Arabia to the Bab al Mandab strait in Yemen. The exposure of the Precambrian Arabian-Nubian Shield around the Red Sea is related to early stages of doming before the opening of the Red Sea. The Arabian-Nubian Shield is dominantly underlain by Neoproterozoic juvenile material constituting intra-oceanic island arc terranes welded together along ophiolite-decorated arc-arc sutures at about 800-700 Ma. These are bounded to the east and west by arc-continental sutures that separate the Arabian-Nubian Shield from older continental fragments of east and west Gondwana. Arc-arc sutures in the central part of the Arabian-Nubian Shield trend E-W or NE-SW and can be traced across the Red Sea. The two prominent arc-arc sutures in this part of the Arabian-Nubian Shield are the Allaqi-Heiani-Onib-Sol Hamid-Yanbu and the Nakasib-Bir Umq sutures. These accretionary structures are defined by N- and/or S-verging fold and thrust belts. North of the Allai-Heiani-Onib-Sol Hamed-Yanbu suture, in the Eastern Desert of Egypt and northwestern Saudi Arabia, structural trends are different from the central and southern part of the Arabian-Nubian Shield, due to superimposition of the post-accretionary Najd fault system. The southern part of the Arabian-Nubian Shield is dominated by steep, N- to NNW-trending structures, the most prominent of which is the Barka suture in Eritrea. The Barka suture projects across the Red Sea in Saudi Arabia into a wide zone of N-trending structures such as the Bidah shear zone. Northerly-trending structures of the southern Arabian-Nubian Shield manifest continent-continent collision along the Mozambique belt further south. Early N-trending shortening zones and late NW-trending sinistral strike-slip faults deform the arc-arc sutures in the Arabian-Nubian Shield. The N-trending shortening zones formed at about 630-600 Ma as a result of convergence between east and west Gondwana. The most prominent of these structures is the Hamisana shear zone exposed in northern Sudan and southern Egypt. The NW-trending sinistral strike-slip faults of the Najd fault system formed at about 640-530 Ma and are exposed in the central and northern part of the Arabian Shield in Saudi Arabia and in the northern part of the Nubian Shield in Egypt. The evolution of the Najd fault system might be related to escape tectonics, indentation of west Gondwana with the Arabian Shield, or represent a zone of high shear strain following shortening of the Arabian-Nubian Shield between east and west Gondwana. Observations from the Precambrian geology of the Arabian-Nubian Shield suggest the following constraints on the evolution of the Red Sea: 1) Precambrian structures in the Arabian-Nubian Shield played relatively minor roles in controlling the evolution of the northern Red Sea. Neoproterozoic accretionary structures in the Arabian-Nubian Shield are oriented at high angles to the NNW-trend of the Red Sea and can be traced across it. These were not exploited by the developing rift, although the fundamental change between the northern and southern Red Sea happens at about the position of the Nakasib-Bir Umq suture; 2) N-S trending structures in the south were exploited by the Red Sea Rift, but NNW-trending Najd structures in the north were not; 3) Matching structures and formations across the Red Sea suggests that the evolution of the latter was accompanied by little crustal attenuation and that most of the Red Sea main trough is underlain by igneous rocks of mid-Cenozoic age; and 4) Matching Precambrian structures of the Arabian-Nubian Shield across the Red Sea indicate that the evolution of the Red Sea involved negligible strike-slip deformation.
Growth of Normal Fault Systems
Long rift systems such as the Red Sea/east African rift are the result of the linking together of numerous normal faults with isolated nucleation points. As individual normal faults grow they exhibit self-similar relationships between the length of the fault and its displacement. The self-similar growth relationship develops in accordance with the strength characteristics of the rock in which the normal fault is propagating. Growth continues in a self-similar manner until the leading down-dip edge of the fault intersects the ductile part of the crust or laterally propagating fault tips intersect one another or, as is commonly the case, parallel faults grow into the stress fields of one another. At the moment of physical linkage the two faults now share the same fault surface and the length of the fault increases to the sum of the two faults. Yet the maximum displacement is by definition the displacement on the larger of the two faults. Therefore, the displacement is significantly underrepresented for the new length of the combined faults. My students and I have suggested, on the basis of work on the east African rift, the Triassic rift basins of the eastern US, and on smaller normal fault systems in the Basin and Range Province of the western US, that the region of intersection experiences accelerated total displacement until the two faults achieve a self-similar relationship consistent with individual faults in the same region. However, when one fault grows into another's stress field, or region of stress drop, the rate of lateral propagation for both faults is greatly reduced resulting in an asymmetry to the displacement profile and footwall uplift pattern. As the normal faults grow toward each other, the summed displacement in the extension direction begins to increase until self-similarity is achieved. In many cases, the summed displacement, as measured in the direction of extension, is complemented by the growth of smaller faults. As the summed displacement in the region of overlap approaches that necessary to achieve a self-similar profile, the rate of fault propagation of individual faults is markedly reduced. This process accounts for the maintenance of fault scaling relationships in many extensional regions. Furthermore, this process results in a situation in which the strain is distributed roughly evenly over large regions during continental break-up. The process of fault growth described above results in a distribution of fault displacements that obviates the geometric necessity of substantial extension-parallel strike-slip faults. In no area that we investigated did we find a series of these extension-parallel transfer faults similar to those proposed to accommodate extension during the development of a passive margin (e.g., Lister et al., 1986). The self-similar growth patterns observed in a wide range of environments also yield results that are inconsistent with the extreme displacements thought to be associated with upper crustal low-angle normal faults (e.g., Wernicke 1995).
Mahmoud A. Atta
Deep Marine Anhydrite in Gebel El Zeit - Esh El Mellaha Basin Gulf of Suez, Egypt
Gebel El Zeit - Esh El Mellaha Basin is generally asymmetric half graben basin, and having the trend of northeast-southwest. It is considered as one of the prolific inland basin on the southwestern coastal area of the Gulf of Suez. The sedimentary section is very thick and consists of Pre-Rift, Syn-Rift and Post-Rift sediments with markable unconformities in-between. They are ranging in age from Cretaceous to Recent and have been deposited under various environmental conditions. Kareem Formation is one of the Syn-rift Miocene aged sediments and composed mainly of foraminifera-rich calcareous shale with minor interbeds of limestone, sandstone and anhydrite. These anhydrite beds generally mark the base of the Kareem formation and differentiating it from the underlying organic rich unit known as Rudies Formation. It is always believed that these anhydrite beds were deposited in very shallow water or Sabkha. This is contradicting with the environment under which both Kareem and Rudies formations were deposited which is open marine with no evidence of the sea level drop during the deposition of these two formations. Due to sedimentological, paleontological and petrological evidences from the outcrops and the subsurface core studies in the subject basin, the deep marine origin is suggested for the deposition of these anhydrite beds.
Strength evolution in the brittle crust during low-angle normal faulting
Low-angle normal faults (LANFs) are increasingly documented in the geologic, seismologic, and paleoseismologic records. The latter two are relatively recent additions that seriously erode the remaining ground upon which theories that deny low-angle normal faulting stand. Combined with the apparent strength paradox of the world's most well studied fault (San Andreas), the Earth science community is faced with inadequate mechanical theories of faulting and, therefore, of our understanding of crustal and lithospheric strength and how it evolves. Resolution of these issues is hindered by a shortage of appropriate data with which to test and improve the many existing models. Field data from LANFs and their surroundings from different levels in the brittle crust provide tests and bases for modifications to these theories. Orientation data from outcrop-scale tensile and tensile-shear fractures in the footwall of the Brenner LANF (Alps) show that the maximum principal stress (sigma-1) was at a high angle (~70) to the LANF as it evolved from ductile to brittle at the base of the brittle crust (Axen et al., in press, JGR). Conjugate tensile-shear fractures from the footwall of the Whipple LANF (California) show that sigma-1 was at a high angle (~55-80) to the LANF while it was active between ~5 and 10 km depth (Axen and Selverstone, 1994, Geology). In addition, paleoseismic studies (e.g., Caskey et al., 1996, BSSA; Axen et al., 1999, Geology) document surficial faulting related to shallow (<100m) LANF slip where sigma-1 was very likely subvertical. Hence, significant rotation of sigma-1 away from vertical during LANF slip is not required. The Brenner and Whipple LANFs were surrounded or underlain by rock with significant cohesive strength. The Whipple LANF was surrounded by continuously healing or recently healed "chlorite breccia" relict from the preceding history of cataclastic flow and synchronous mineralization. The Brenner LANF was underlain by previously formed mylonites. In both cases, the outcrop-scale, brittle, mixed-mode structures that evolved in footwall rocks subjacent to the LANFs cut across pre-existing fabrics, so involved failure of cohesive rock that must have been stronger than the LANF. Integration of these orientation data with relevant experimentally derived failure envelopes on Mohr's diagrams provides a basis for comparison with theories of fault mechanics. In particular, it appears that LANF slip does not require unusually low friction if the LANF surroundings are cohesive. Thus, although experimental mechanical results are robust, their integration into mechanical models has been over simplified, in particular through dismissal of the cohesive strength of intact or healed rock surrounding major faults. In turn, local strength (and differential stress?) gradients are probably important near major faults and reflect an intermediate-scale fault-parallel mechanical layering that needs to be better understood. This layering may act in concert with the broader surroundings of major faults to control overall strength of the brittle crust. The existence of such cohesive layers implies that the broader surroundings may also have strength (and stress?) levels above that of Byerlee's law, which is commonly taken as the strength limit of brittle crust. This may be an appropriate assumption only where and when optimally oriented faults are actually present. For example, many LANFs probably evolve from ductile to brittle with initial low dips controlled by the orientation of the immediately preceding mylonitic foliation. Their upper plates may be cohesive initially, lose strength suddenly and drop to a Byerlee-compatible level due to formation of upper-plate faults, then strengthen gradually as these faults rotate out of optimal orientation. Ultimately, the original upper plate strength may be regained and the cycle may begin again. The frictional strength maximum on such faults is probably at their bases where normal stress is highest, but rapid healing there may increase the long-term strength to higher (cohesive) levels, promoting continued slip on the underlying LANFs. Such a history implies interactions of processes over several orders of magnitude of length (10-3 to 105 m) and time (1 to 1012 s), implying important strength and stress evolution on those same scales. Fault mechanics will be better understood when processes operating over these scales are integrated.
William Bosworth* and Ken McClay**
Structural and stratigraphic evolution of the Gulf of Suez rift, Egypt: A synthesis
The structural and stratigraphic development of the Gulf of Suez rift reflects the interplay of five principal factors: 1) the presence of pre-existing fault systems, penetrative fabrics and basement terrane boundaries, 2) eustatic sea-level changes, 3) changes in basin connectivity to the Mediterranean Sea and Indian Ocean, 4) rapid changes in African intra-plate stress fields, and 5) activation of the Levant-Aqaba transform plate boundary. The Gulf of Suez-Red Sea rift initiated in the Late Oligocene, probably propagating northwards over a time span of no more than about 6 Ma. Rifting intersected a major east-west structural boundary of Late Eocene age at the latitude of Suez City. North of Suez, extension was more diffuse but mostly focused on the Manzala rift that is presently buried beneath the Nile Delta. Earliest syn-rift, mainly continental sediments (Chattian-Aquitanian) consisted of red beds containing minor basalts. In the Gulf, marine Oligocene strata are presently only suspected in the south, at the juncture with the northern Red Sea. By the Aquitanian, a shallow to marginal marine environment prevailed in most of the rift. The prolonged Burdigalian sea-level rise enabled marine waters to flow freely between the Mediterranean Sea and the Gulf of Suez, resulting in deposition of thick Globigerina shales and deep-water carbonates. During the Langhian and early Serravallian, rapid eustatic sea-level changes resulted in pronounced facies changes within the rift. During the late Serravallian significant fall in sea-levels, the Mediterranean water connection was either completely or intermittently blocked, leading to deposition of evaporites in the central and southern Gulf sub-basins. Thick halite sections accumulated in the Late Miocene, and later loading resulted in the formation of salt diapirs and salt walls. Normal marine conditions were re-established during the Pliocene, but waters were then provided by the Red Sea-Gulf of Aden connection to the Indian Ocean, and a permanent land-barrier separated the Gulf of Suez from the Mediterranean. Analysis of fault geometries, fault kinematics and sedimentation patterns indicates that rift-normal extension predominated throughout the Oligocene to early Middle Miocene evolution of the rift. In the Middle Miocene, the Levant-Gulf of Aqaba transform boundary was established, linking the Red Sea rift plate boundary to the convergent Bitlis-Zagros plate boundary. This resulted in a dramatic decrease in extension rates across the Gulf of Suez and a clockwise rotation of stress fields in Sinai. During the Late Pleistocene, the intra-Gulf of Suez extension direction rotated counter-clockwise to N15E.
W. Roger Buck
Magma Assisted Rifting and Faulting of Thick Lithosphere
The mechanism that allows rifting of cold, strong lithosphere is poorly understood. The tectonic force available to drive rifting (from rift push and slab pull) is estimated to be as much as an order of magnitude below the yield strength of intact lithosphere, according to standard models. Moreover, data from a number of rifts and passive margins seem to require less tectonic subsidence, relative to the longer-term thermal subsidence, than is predicted by lithospheric stretching models. Examining the role of dikes, a ubiquitous feature in many rift zones, may help resolve some of these dilemmas. Although substantial extensional tectonic force is required to accommodate emplacement of lithosphere-scale dikes, that force may be as little as one-tenth the force required for amagmatic rifting. If regional tectonic force in plume areas is insufficient to allow diking, the magma will instead be extruded, as in the case of ocean-island basalts. Simple, two-dimensional thermal and stress modeling suggests that diking may also contribute to the subsidence pattern seen in many rift zones. The injection of hot, low-density material through the lithosphere reduces the amount of tectonic subsidence, while having less effect on thermal subsidence. I will report more a new set of high-resolution numerical models of rifting that include dike intrusion and faulting in a self-consistent manner. The are used to explore the time-evolution of lithospheric strength, subsidence, and surface deformation in rift zones, under different assumptions of magma supply and rifting rates. These patterns are compared with real-world examples, including two well-studied rift areas in which magmatism may have played an important role: North Atlantic rifted margins and the Red Sea rift zone.
Richard T. Buffler*, Robert C. Walter**
Rift basins and rift processes in the southern Red Sea - northern Danakil region, Eritrea
Sedimentary basins in continental rifts contain long and detailed records of tectonism, volcanism and sedimentation that document the origin and evolution of the rift itself. Here we report the progress of an ongoing long-term cooperative field-oriented research project being conducted in Eritrea by an international team of investigators from Eritrea, the United States, Mexico, The Netherlands and France. A primary goal of the project is to study active geological rift processes (sedimentation, tectonics and volcanism) along an accessible active plate boundary centered in the southern Red Sea - northern Danakil region. Another major goal is to establish a geological framework for the study of early hominid evolution, adaptation, and migration. The study area extends for over 100km along the coastal lowlands from the village to Foro south to the village of Bada, and from the mountain front to the Red Sea. This and adjacent areas of the Afar have long been recognized as a natural laboratory to study active geological processes. The work so far has concentrated in four main areas within the study area, as follows: 1) The Foro Area, located between the Eritrean Escarpment and the Gulf of Zula, is characterized by a large Pleistocene alluvial fan/flood basin sequence, modified by recent volcanism and graben formation along the escarpment front, which initiated a new cycle of alluvial fan deposition. On geomorphic grounds, fluvial terraces likely correspond to sea level highstands of the middle and upper Pleistocene. Some have been tectonically elevated several hundred meters (on the flanks of Mt. Ghedhem) indicating that significant uplift has occurred in this zone in the past 500 ka. 2) The Buri Peninusula - The Abdur Archeological Site, located across the Gulf of Zula, is where the discovery of Middle Stone Age artifacts (hand axes and blades) within an uplifted fringing reef system dated to 125,000 (the last interglacial maximum) has shed important new information about early human adaptative strategies and migration paths out of Africa (Walter et al., 1999, Nature, v. 405, 4 May, p.65). Here active faulting, folding and doming have deformed the young reef system as well as the underlying Pleistocene? Buri Sequence, a widespread fluvial to shallow marine siliciclastic and limestone sequence. 3) The Alid-Dandero-Mahable area, located along the mountain front in the southern part of the study area, where there are extensive outcrops of Pleistocene fluvial/lacustrine/flood basin to shallow marine sediments containing rich paleontological and archeological sites. This area also has been modifed by active faulting, folding and volcanism, including the prominent Alid Volcanoe, a large lacolithic and extrusive complex with pumice beds dated by the USGS as young as 10,000 years and even younger basalt flows on its flanks. 4) The Bada area, located further to the south in the northern Danakil, contains a widespread older (Miocene?-Pliocene?) fluvial/flood basin red bed sequence, overlain unconformably by a Pleistocene shallow marine sequence. This younger sequence contains fan-delta conglomerates, shallow marine fine-grained sandstones, and extensive evaporites and carbonates, all deposited in a former arm of the Red Sea that was located in the northern Danakil during previous high stands of sea level. The sea has subsequently been cut off from the northern Danakil during this most recent high stand by recent volcanism and tectonism along the Red Sea coast. An active graben system occurs along the mountain front at Bada, and recent basalt flows extrude from the bounding fault zones. This active graben system extends north through Alid all the way to Abdur.
Barry I. Cameron
Magmatism Associated with the Rupturing of the Red Sea: A Modern Analogue for the Sverdrup Basin in the Canadian Arctic
Several potential, though inherently different, sources of data constrain models that describe stretching and rupturing of the continental lithosphere. From a sedimentological viewpoint, sedimentary deposits that fill the rift basin are surface phenomena of a process occurring at greater depth. Geophysical data show the present crustal expression of the rifting process in the mantle, and therefore, penetrates closer to the true source. Both the sedimentological and geophysical lines of evidence suffer from their indirect nature, i.e. both record ensuing geological consequences of extension. Magmatic rocks have the advantage of forming at depth in the mantle during the incipient rifting event. The nature of the volcanic rocks divulges direct information on the depth and degree of partial melting of the mantle upwelling beneath the extended crust. Changes through time of these key igneous parameters would constrain a model for rupturing of the continental lithosphere. Stretching of the continental crust causes decompression melting in the mantle. Mantle upwelling underneath the thinned crust produces a partial melt as temperature remains nearly constant but pressure decreases sharply. General disagreement amongst igneous petrologists has centered on whether the melting that produces the basaltic rocks erupted at a continental rift originates in the deeper, plastic lower mantle called the asthenosphere or the rigid upper mantle (+crust) called the lithosphere. The chemical composition of the basaltic rock contains a fingerprint of the melt region, and therefore, offers a resolution to this contentious issue. In the high Canadian Arctic, Permian basaltic rocks of the Esayoo Formation were generated by a weak initial stretching episode that formed the Sverdrup Basin. These alkaline basalts display identical trace element signatures to ocean island basalts that characterize magmas derived from the asthenospheric mantle with minimal subcontinental lithospheric contribution and crustal contamination. More voluminous Cretaceous basalts in the Sverdrup Basin were products of a much stronger stretching episode and they exhibit disparate trace element signatures more similar to continental flood basalts. These tholeiitic basalts possess a more significant lithospheric contribution to the primary mantle melts. The possibility exists that a small rate of continental extension (i.e., low b) yields alkaline melts from an asthenospheric mantle source, whereas higher rates of extension promote volcanic rocks with a greater lithospheric component. The Red Sea is a modern analogue to basin development in the Canadian Arctic. Magmatic rocks in the northern and central Red Sea/Gulf of Suez region contain important information on the tectonic events that led to rupturing of the continental lithosphere. In addition to further investigations of newly discovered volcanic rocks in the Sverdrup Basin of Canada, geochemical studies utilizing modern analytical techniques are proposed for volcanic rocks associated with the opening of the Red Sea. Field studies will include the measuring and sampling of numerous sections of sedimentary-volcanic sequences. Major and trace elements and the radiogenic isotopic chemistry of the volcanic rocks will be analyzed by x-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), and thermal ionization mass spectrometry (TIMS), respectively. Mafic tephric deposits associated with volcanic edifices will be sampled so that the volatile content of melt inclusions in olivine phenocrysts can be determined by Fourier transform infrared spectroscopy. Water content in the melt inclusions may provide information on the fluid fluxes that facilitate extension.
Rupturing of the Continental Lithosphere (Red Sea/Gulf of Suez)
Sediments provide the best available record of vertical crustal motion, and of fault-induced tilting, fault propagation and the growth of folds. Reading this record is not necessarily simple because it reflects a composite picture of deformation in three dimensions, and at a range of length scales and timescales; sedimentation above as well as below sea level, and not necessarily initially horizontal; episodic localized erosion; temporal and spatial variations in paleobathymetry or paleoelevation (some large and/or poorly constrained); differential compaction of sedimentary wedges; and uncertainties in timing. Our perception of this record is influenced also by the resolution of the tools that may be brought to bear in the subsurface, and by the vagaries of partial exhumation beneath modern topography. Moreover, since at least the mid-Eocene, sedimentation in and adjacent to marine embayments has been modulated by glacial-eustasy as well as by tectonic phenomena, and sediment fluxes have undoubtedly been influenced by the changing climate as well as by the emerging physiography of tectonically active regions. Recent work (by others) in both the subsurface and available outcrop of the Red Sea/Gulf of Suez region indicates that this is a potentially fertile area in which to combine sedimentological/stratigraphic studies and structural analysis at selected sites and at varying scales. My personal interest is in finding places where such observations can be brought to bear on the evolution of fault systems ("growth structures") and on the episodic tilt patterns of extensional fault blocks. Underpinning any stratigraphically based research will be the lateral tracing of physical surfaces at high resolution, recognizing that one of the more interesting aspects of stratigraphy in tectonically active basins is that some of the standard concepts of sequence stratigraphy don't necessarily apply. Among these are the development of diachronous unconformities at growth folds and the potential for base-level changes of opposite sign, at the same time, in different parts of the same basin.
James R. Cochran
Nucleation of an oceanic spreading center in a continental rift: the northern Red Sea
The northern Red Sea is an amagmatic continental rift in which a mid-ocean ridge spreading center is beginning to develop. The rift consists of narrow continental shelves and a main trough, which forms a series of fault-bounded terraces 20-30 km wide stepping down to a 15-30 km-wide axial depression. Free-air gravity anomalies form a pattern of elongate high-amplitude (50 mGal) highs and lows oriented subparallel to the trend of the rift and extend for 50-70 km along strike with gravity highs located on the seaward edges of the bathymetric terraces Gravity contours are systematically terminated or offset across NE-SW trending zones at which bathymetric contours are also offset. The gravity anomalies display the basement relief under the main trough, interpreted as a series of tilted fault blocks 15-30 km across and roughly 60 km in length separated by accommodation zones which absorb the differential motion between adjacent sets of fault blocks. The axial depression differs from the oceanic "axial trough" of the southern Red Sea in that sedimentary sequences are continuous across it, lineated magnetic anomalies are not present and it is shallower. It is marked by a free-air gravity minimum with a relative amplitude of 30-60 mGal and very high heat flow (300-600 mW/m2). The axial depression often appears to be fault bounded. Deformation of the sediments in the axial depression is more intense and concentrated than in the marginal areas. The axial depression is not only the locus of recent deformation, but is also the location of a series of small axial deeps spaced at 50-75 km intervals along it. Deeps are consistently located almost exactly at the midpoint of segments, halfway between the accommodation zones and are associated with large normally magnetized, dipolar magnetic anomalies. The axial depression is not a continuous axis of deep water as suggested in published descriptions, but is systematically segmented. Both depth and the amplitude of the gravity lows decrease away from the deeps with minima at the accommodation zones. Accommodation zones are not simply saddle points, but also offset the axial depression. The bathymetric and gravity lows associated with deeps do not intersect, but rather overlap without joining. The axial depression thus appears to be divided into discrete, independent segments separated by the same accommodation zones that define the geometry of the continental basement fault blocks within the marginal areas. Within each segment, there is an axial deep located almost exactly half way between the accommodation zones and associated with high-amplitude normally-magnetized dipolar magnetic anomalies which appear to result from large recent intrusions. Lithospheric extension in the northern Red Sea has reached the point where magma is beginning to be produced. This melt is focused at the center of rift segments where it ascends along faults bounding the axial depression to create deeps. We hypothesize that with continued intrusion, the deeps develop into small seafloor spreading cells which propagate and grow together to form an oceanic spreading center. The segmentation of the newly formed mid-ocean ridge is thus inherited from the rift geometry established during continental rifting
Robert Dunn and Don Forsyth
Surface wave constraints on the shallow shear wave structure beneath the southern East Pacific Rise
Short period seismic surface waves yield information on the crust and shallow mantle, particularly the shear wave velocity which is sensitive to temperature and melt fraction. We use short period surface waves (4-20 sec) generated by regional earthquakes and recorded by the MELT seismic array to model lateral variations in the shear velocity structure of the uppermost mantle and crust beneath the southern East Pacific Rise. We present records of Love wave energy that show clear evidence for lateral focusing of seismic energy due to the anomalous velocity structure beneath the rise. For stations located near the rise axis, arrivals are delayed, although paradoxically amplitudes are relatively larger than for stations located away from the rise. These observations can be explained by a low shear velocity region beneath the rise, due to high temperatures and perhaps melt, that acts as a lateral wave-guide to surface wave energy and that focuses energy towards the rise. Because amplitudes and delays are sensitive to the velocity gradients in this wave-guide, inverse modeling of the waveforms of the data place constraints on the width and magnitude of the low velocity region. Knowledge of the shear velocity can thus be used to infer the nature of the shallow thermal structure and melt distribution beneath the East Pacific Rise.
Raafat E. Fat-Helbary
Abu Dabbab area is located in the Central Eastern Desert of Egypt north of Idfu-Mersa Alam asphaltic road, 35 km to 50 km northwest of Mersa Alam. It is adjacent to the coastal plain of the Red Sea. It bounded by longitudes 33.76° and 35.31° E and latitudes 24.55° and 25.65° N as in the figure. It is a part of the Red Sea mountain range. The area is characterized by rugged topography and comprises mainly upper metamorphic rocks intruded by igneous rocks. Abu Dabbab area is drained by three main wadis and their tributaries which are Wadi Abu Dabbab,Wadi Mubark, and Wadi Dabr. Abu Dabbab area is very important area on the Red Sea coast because there are two main shock earthquakes of magnitude 6.0 and 5.3 occurred in 1955 and 1984 respectively, both of these earthquakes were felt strongly in upper Egypt at Aswan, Qena and Quseir. In last few years, the Egyptian Government was planing to construct many projects in southern Egypt. One of these projects is to chang the southern part of Red Sea coast to an international attracted tourism area. The important infrastructions planned to construct in the area are Safaga airport ,Marsa Alam ariprt and many tourism hotels and villages. The main aspect of this study will take into consideration the study of seismic activity and seismoctectonics. As well as to determine the relationship between the identified spatial distribution of the earthquakes and structural geology and tectonics of the area and tectonics of the Red Sea. The results of this study can be use for seismic hazard analysis for puropose of planning and policy-making.
Andrew M. Goodliffe*, Fernando Martinez*, and Brian Taylor*, Trevor J.
Lewis**, Alan E. Taylor**, Elizabeth J. Screaton
The Geothermal Signature of Continental Breakup: Results from the Woodlark Basin
In the fall of 1999 a N-S geothermal transect of marine heat flow measurements was collected across the margins of the Woodlark Basin directly west of the westward propagating spreading center. The Pacific Geoscience Center heat flow probe was used, with 11 thermistors spaced over 3 m in a "violin string" configuration to determine in-situ temperature gradients and thermal conductivity. Existing seismic, sidescan and bathymetric data were used to locate sites. The breakup geothermal traverse shows low values (~30 mW/m2) in the northern margin, which agrees with downhole measurements from ODP Leg 180. Heat flow values increase rapidly starting about 10 km to the north of the rift-bounding fault system, and reach 100 mW/m2 within the active rift. The values peak at almost 350 mW/m2 further south, on strike with the center of spreading segment 1, within a region of elevated but lower heat flow (100-250 mW/m2) that is likely a result of hydrothermal circulation close to the spreading center. Heat flow decreases to the south through a province dominated by large-throw block faulting, dipping below 100 mW/m2 on strike with the southern continent-ocean boundary, and continuing with shorter wavelength peaks up to 250 mW/m2, leveling off at 40-50 mW/m2. Broadly, the thermal asymmetry correlates with the prominent structural asymmetry of the margins. In the north, minor small-offset faults and a general downflexing of the margin toward the center of the rift imply the majority of extension has been confined to the lower crust. Low heat flow values imply little extension in the northern margin and that it has been chilled by the southward subduction of the Solomon Sea Plate. To the south, large-throw block faulting dominates. A more than ten fold increase in heat flow across the margins demonstrates a clear thermal signal consistent with dramatic thinning of the lithosphere and transition to seafloor spreading. The 30-40 km wide heat flow high (where values exceed 100 mW/m2) implies a focusing of extension ahead of the spreading segment that is greater than would be suggested by the observed pattern of rifting. The measurements support a new model of asymmetric continental extension caused by an initial lateral thermal asymmetry.
Andrew M. Goodliffe, Fernando Martinez, and Brian Taylor
A Reconstruction of the Margins of the Woodlark Basin: Predictions of Past Continental Thickness
The Woodlark Basin, in the southwest Pacific, has been formed by continental rifting and the westward propagation of seafloor spreading since at least 6 Ma. Extension east of 151 40 E is primarily by seafloor spreading; the once contiguous Pocklington and Woodlark Rises are separated by oceanic lithosphere formed on the Woodlark spreading center. To the west, extension is by continental rifting, continuing into the Papuan Peninsula towards the current Euler pole at 12 S, 144 E (Taylor et al., 1999). The geology of the Rises, including results from commercial and ODP drilling, indicate that the Rises formerly comprised a substantial landmass that has since subsided and been split into two by seafloor spreading. The distribution of crustal strain within the continental margins has been characterized by bathymetric mapping and seismic reflection profiles. On a basinwide scale, extension has been confined to the region between the oceanic lithosphere of the Solomon Sea to the north and the Coral Sea to the south. In contrast to areas to the south, multi-channel seismic (MCS) surveys have imaged only small offset faults in the Trobriand forearc region. In addition, low heat flow (~30 mW/m2) implies that this region has experienced minimal extension during the opening of the Woodlark Basin. However, this conclusion is not consistent with the observed subsidence in the region. The dating of a rift onset unconformity in the west and the oldest magnetic anomaly in the basin at approximately 6 Ma indicates that extension has been occurring in the west for at least 6 Ma in association with a largely static Euler pole. Using well constrained opening rates derived from kinematic reconstructions of the seafloor spreading history, the amount of continental stretching prior to the initiation of seafloor spreading has been derived. We reconstruct key continental cross sections, using estimates of crustal thickness derived using gravity data and basic isostatic assumptions. We keep the cross-sectional area of the continental crust constant throughout. We discuss adjustments to the cross-sectional area due to uplift resulting from the replacement of lithosphere by asthenosphere, and the addition of material from arc volcanism and magmatism.
Dennis L. Harry and John Londono
Crustal Structure along Continent-Ocean Transects in Western Alabama and Mississippi, Southeast USA: Constraints from Gravity, Seismic, and Well Data
Gravity, seismic reflection data, and well data have been integrated to develop crustal-scale cross sections along two NNE-SSW trending transects in eastern Mississippi and western Alabama. The transects cross from eastern Tennessee into the Gulf of Mexico Abyssal Plain. Five tectonic provinces are distinguished on the basis of long wavelength (>200 km) Bouguer gravity anomalies. From north to south, these include 1) the Lower Paleozoic Mississippi Valley Graben system; 2) the Late Paleozoic Black Warrior foreland basin and Ouachita Fold and Thrust belt; 3) the Mesozoic Mississippi Interior Salt Basin, 4) the Wiggins Arch (a Mesozoic horst), and 5) the Mesozoic/Cenozoic Gulf of Mexico coastal basin. The thickness of the crust along these transects varies from 40 km to 15 km between their landward extent and the shelf edge. Anomalously thin crust is present beneath the Mississippi Valley Graben (< 35 km) and the Ouachita Fold and Thrust belt/Mississippi Interior Salt Basin region (ca. 26 km-thick). The crust between the Mississippi Valley Graben and Black Warrior foreland basin reaches a thickness of ca. 40 km. The Wiggins Arch, which separates the Mississippi Interior Salt Basin from the Gulf Basin, is underlain by ca. 35 km-thick crust. Crystalline crust becomes progressively thinner south of the Wiggins Arch as a result of Mesozoic continental extension associated with the opening of the Gulf of Mexico. A pronounced basement hinge zone is located just landward of the shelf edge, where the thickness of the crystalline crust decreases abruptly to less than 10 km. Basement seaward of the shelf edge is interpreted as continental crust that was highly extended during Mesozoic rifting. The Ouachita Fold and Thrust belt is interpreted to involve deformed sedimentary rocks extending to a depth of 16 km, and is located just landward of the suture between Laurentian (Grenville) basement and a continental fragment or island arc system that collided with North America during the Late Paleozoic. The Jackson Dome, which marks the southern edge of the Ouachita Fold and Thrust belt , is located above the suture in a region where the crust is locally thickened by ca. 8 km. Granites recovered from a deep well on the Wiggins Arch, south of the suture, provide a 270 Ma age for the formation of the allocthonous crust. Numerous short wavelength (< 75 km) Bouguer gravity anomalies suggest an irregular basement topography in both the allocthonous and autocthonous crust that is attributed to normal faulting during Late Precambrian and Mesozoic rifting, respectively, but Paleozoic normal faulting during subsidence of the Black Warrior basin can not be ruled out as a contributor to these features. Gravity modeling supports the presence of a high density lower crust beneath the Ouachita fold and thrust belt. This high density crust is best interpreted as relict oceanic crust that formed on the southern margin of Laurentia during Precambrian rifting, and became wedged into its present position during the Late Paleozoic Ouachita orogeny. The Paleozoic oceanic crust is estimated to be about 10 km thick, suggesting excessive volcanism along the Laurentian margin during rifting. These results are similar to those obtained by Mickus and Keller along a transect further west, in Arkansas and Louisiana, and strengthen the contention that the Precambrian Laurentian rifted continental margin be considered a ?volcanic? margin.
A 2-D seismic reflection data set was collected over the Iberia Abyssal Plain in 1997 in order to learn more about the evolution of rifting in this region. Nine dip lines image the peridotite ridge. My project will initially focus on improving the processing sequence for the 2-D lines. The goal is to obtain improved images for a more accurate structural interpretation, and a more detailed velocity model to constrain the geologic interpretation. The geophysical work will be used as a tool to understand the geology. While the existence of the peridotite ridge has been known for some time, its origin and structure remain largely unknown. One possibility is that theperidotite was brought to the surface along sub-horizontal detachment faults that were active during the rifting of Pangea. Enhanced seismic images would allow for a more accurate structural interpretation to test this theory or develop new ones. Velocity data would also help to identify the crust-mantle boundary in this area. Ultimately, we hope to determine the relation of the peridotite ridge to the ocean-continent transition and what role the peridotite ridge played in the evolution of rifting.
Magmatic activity in Jordan and adjacent areas took place after the end of the Pan African Orogeny only in a couple of phases related to rifting and hot spots or mantle upwelling: Late Triassic, Late Jurassic and Early Cretaceous and Oligocene to Recent times. During the last phase i.e. Oligocene to Recent, magmatic activity gave rise to large volcanic fields and was consanguineous with uplifting and continental breakup in the course of the formation of the Red Sea and Dead Sea Transform. This last phase is divided into two stages: Latest Oligocene Early Miocene, produced magmas mainly of transitional to tholeiitic character (e.g. the Red Sea Dike system) while the second stage produced many volcanic fields of mildly to strongly alkaline nature. The magmas of the second stage are more voluminous than those of the first stage. The Red Sea Dike System started just before the beginning of the significant subsidence and plate separation along the Red Sea and Suez Rift (e.g. Said, 1962; Garfunkel, 1989). K-Ar dating of this system in Sinai yielded ages between 25 and 19 Ma (e.g. Steinitz et al. 1978) and one of these dikes has been dated in southern Jordan (East of the Dead Sea Transform) at 21.3 Ma. A detailed investigation (geochemical, goechronological and emplacement style) of the Red Sea Dike System across the political borders would be of utmost importance in understanding the Early stages of the Rupturing of the Continental Lithosphere in the Red Sea/Gulf of Suez. Understanding of this system could also have some important implications on the tectonic environment during the last stage of the Pan-African Orogeny during which extensive diking of various compositions took place.
Stephen L. Karner
Healing and Lithification of Simulated Faults
In the stick-slip model for earthquakes, instabilities occur on a pseudo-regular basis, with seismic events separated by periods of quasi-stationary contact. Observations from seismic data show that coseismic stress drops are larger for faults that have longer earthquake recurrence times (coseismic stress drop c. 1-5 MPa per decade time). During the interseismic period (stick), fault strength may evolve as a function of the real area of contact between opposing fault walls or, if fault gouge is present, as a function of gouge consolidation. In this way, faults regain strength (or heal) between the discrete seismic events. However, laboratory studies indicate that several other key parameters have an effect on fault strength. Data from room temperature experiments show that friction increases with time of 'stationary' contact (healing), consistent with seismic estimates of fault healing. Fault healing rates are known to depend on physical parameters (e.g. stressing rate, fault stiffness, normal and/or shear stress variations) and the character of the shear zone (e.g. surface roughness, accumulated shear displacement, presence of gouge). Yet, these are not the only parameters that influence the frictional properties of fault zones. Laboratory data from tests at hydrothermal conditions indicate that fluid-rock interactions and the chemical character of the system also effect fault strength. The processes involved include mineral diagenesis and production of alteration products (e.g. clays), fault sealing due to solution transport and precipitation, gouge cementation via solution-assisted creep compaction, fluid-assisted deformation of contacting asperities, among others. Several empirically derived constitutive laws have been derived to help describe frictional sliding behavior. Of these, the rate- and state-dependent friction laws have received considerable attention and are increasingly being used for models of faulting and earthquake rupture dynamics. Numerical simulations using these laws are capable of matching many of the laboratory results. However, these laws fail to match some experimental observations - such as the frictional response to large perturbations in loading conditions, or the effects of complicated geochemical environments. In light of such laboratory results, it is important to develop constitutive laws that are capable of predicting friction for a wide range of conditions.
Mesbah Khalil*, A. Rashed, S.A. Rahma. and T. Dahroug**
The control of the pre-Suez rift structures on the syn-rift clastics, their entry points and accumulations inside the Suez Rift
The eastern margin of the Suez Rift in Sinai is a unique outcrop model for the pre-existing structural fabric, controlling the shape of the eastern rift margin and the clastics supply during the rift phases. These clastics are the main hydrocarbon productive reservoir in the Suez rift system (Morgan, Badri, North October and many other fields). Results of this study indicate that the accumulations of the syn-rift clastics are controlled by three main elements. These elements are:1) well integrated drainage system with continuous clastics supply from the rising rift shoulders, 2) structurally controlled entry points inside the rift basin 3) active fault system associated with active subsidence in the sediments entry areas. This active subsidence allowed for less sediment re-cycling when reaching the basin (thicker accumulation in one spot). The active rising of the eastern rift shoulders during Middle and Late Miocene was extending to central Sinai massif and associated with active drag toward the rift side producing drainage water divide along NS line in central Sinai massif. Intersection of the main pre-rift fault trends (NS, WNW and EW) with the NW rift trend controlled the eastern rift boundary, the geometry of most of the blocks and consequently the clastics entry points inside the rift basin. Outcrops of the type section of the basal Miocene clastics (Nukhul Fm.) in Wadi Nukhul show a clear 4D model for the syn-rift clastics accumulation contolled by the pre-rift structural fabric. Two main northeasterly tilted normal fault blocks of pre-rift sequence control the shape of the depositional surface to fill the in-between low with clastics (down-dip of the western block and down-thrown of the eastern block). Drag in Eocene from the northern side along old EW fault closed this pocket to be filled by clastics from floods of Wadi Nukhul during Oligocene and Miocene from east. The Wadi Nukhul block is terminated from south at EW pre-rift fault that joins the coast with the eastern rift border fault at Abu Zenima area. The predominated pre-existing NS faults (Precambrian to Paleozoic) in Hadahid and Sidri-Baba area (eastern side of the Suez rift) are dissected by the Tertiary rift faults producing several sediments entry points in that part of the rift. Well-integrated drainage pattern extended from the southern foot-slope of the Central Sinai plateau feed these areas with clastics during Early and mid rifting phases. In sub-surface the Morgan field (The largest oil production from Miocene) is a good analog model for the structurally controlled sediments entry points. Huge clastics supply from Sinai massif feed the Morgan area from Wadi Feiran through El Qaa plain with entry point at El Tor area. The transfer zone between the central and southern dip provinces of the Suez rift controlled the sediments entry point of Feiran drainage at the offshore El Tor area. Shallower structures along the transfer zone south of El Tor with less subsidence prevented the clastics from dispersing toward south. They were funneled throgh the structurally low pass toward north (the transfer zone between the down-thrown of the Araba fault from the east and the down-thrown of the Morgan fault from the west). Active subsidence and block rotation in the Morgan area controlled the clastics to be funneled and trapped in El Morgan-Badri pocket.
Inward salt flowage toward the Red Sea axis, with extensional faulting of its sedimentary overburden: Near-bottom observations around Atlantis II Deep
Thick Miocene evaporites that accumulated on Red Sea "transitional" crust (inferred to be thinned and intruded continental crust) have flowed inward over parts of the axial zone of Plio-Pleistocene oceanic crust which accreted by slow seafloor spreading. Where spreading centre processes have created high-relief rift mountains of back-tilted oceanic crust, basement relief has dammed this flow, preventing evaporites from smothering the axis. At central latitudes (~20deg.N to ~23deg.N), obliquity of the Red Sea trough to Africa- Arabia motion has led to the development of a staircase of en echelon spreading axes bounded by right-stepping transforms. The fracture zones produced by these transforms have provided passages by which Miocene salt can penetrate to the plate boundary. Consequences include involvement of the salt in the axial hydrothermal ciculation (producing pools of dense hot brines), and topographic isolation of spreading segments into evaporite-enclosed "deeps". Near-bottom side-scan and photographic images help document the recency and pattern of evaporite flowage, because this slow flow causes tensional fracturing of the sedimentary overburden, just like the crevasses on glaciers or, indeed, the subaerial 'canyonland grabens' found where brittle sedimentary strata overlie a ductile evaporite layer. Our Deep Tow imagery around Atlantis II Deep defines both salt-bounded and basalt-bounded transforms, but is not complete enough to resolve whether the entire broad "inter-trough zones" that separate this deep from its en echelon neighbours are merely wide evaporite-filled transform valleys rather than (as alternatively hypothesised) residual belts of continental crust between the isolated spreading cells that might be characteristic of the temporal transition from rifting to spreading.
Arturo Martin Barajas and Lance Forsythe
Magmatic evolution of the Gulf of California rift system: comparisons tothe Red Sea
Volcanism in the Gulf of California rift system and the Extensional Province (GEP) records the transition from subduction to continental rifting (15-4 Ma), and to oceanic rifting in the last 4 Ma. This transition is accompanied by a change in the eruptive processes and magma composition from dominantly calc-alkaline dacitic-andesitic volcaniclastic aprons to alkaline and tholeiitic basalt and andesite flows. Coeval calc-alkaline rhyolite to dacite dome complexes, caldera-type ignimbrite deposits, and composite andesitic volcanoes occur along the eastern margin of the Baja Peninsula and several islands within the Gulf during the rifting. From ~12 Ma up to present alkali basalt and andesite erupted intermittently in Baja California to the west of the main Gulf escarpment, and to a lesser extent, on the opposite side in central Sonora. These alkalic lavas have distinctive geochemical patterns apparently inherited from subduction with Nb-Ta negative anomalies, and high LILE concentration, principally Ba, Sr, and light REE. In Quaternary times more typical intra-plate alkaline basalts have erupted in a few places on both rift sides. Low-K sub-alkaline basalt, and differentiates erupted from 10 to 5 Ma along both margins and over a broad region in southern Baja California. Later (<5 Ma), these sub-alkaline rocks erupted in a more restricted area within the Gulf, and evolved into MORB-like lavas in the nascent spreading centers. When compared to the magmatic evolution of the Red Sea some fundamental distinctions can be made. Rifting in the Gulf of California occurred along axis of the waning volcanic arc, which strongly influenced syn-rift volcanism, whereas voluminous plume-related flood basalt is precursor of rifting in the Red Sea. Evolved subalkaline volcanism with geochemical features of arc-lavas is a common feature during rifting in the Gulf of California. This type of volcanism is rare(?) along the margins of the Red Sea, which probably represents a more typical situation for magmatic evolution of rifting the continental lithosphere.
Heat flow measurements in active rifts: indications of the pattern of crustal and lithospheric deformation
Heat flow measurements of active rift systems can provide significant information on the pattern of crustal and lithospheric extension and the transition from rifting to seafloor spreading. In the Fall of 1999 R/V Maurice Ewing conducted a heat flow survey in the sedimented marine continental rifts and margins of the Woodlark Basin. 247 thermal gradient and in situ conductivity measurements were made. These data are used to study the active rift-to-spreading evolution of the basin at three stages: (1) the early rift (Goodenough Basin transect), (2) the breakup thermal signal just ahead of the initiation of seafloor spreading (Moresby Seamount transect), and (3) the conjugate margins and continent-ocean transition (COT) following oceanic seafloor accretion (Pocklington and Woodlark Rises transects). The Goodenough Basin transect crosses a small rift basin ~100 km ahead of the westward propagating spreading center. Average measured and sedimentation corrected heat flow values in the basin interior are low (~56 and 74 mW/m2 respectively) relative to predictions from uniform lithospheric thinning models. Average observed and corrected heat flow increase abruptly, however, to ~147/190 mW/m2 at the northern end of the basin within 10 km of the D'Entrecasteaux Islands, metamorphic core complexes created by the rapid exhumation of lower crust. The heat flow pattern may be explained by a model involving horizontal flow of ductile lower crust from beneath the basin and its vertical extrusion at the D'Entrecasteaux Islands. The Moresby Seamount transect shows low values (~30 mW/m2) in the northern margin, in agreement with ODP Leg 180 drilling results. These values remain low in the northern margin where it is primarily downflexed and shows little faulting. About 10 km from the main faults which delimit the southern edge of the hanging wall the values progressively increase peaking at >300 mW/m2 in the area ahead of the main Brunhes age spreading tip-a ten fold increase displaying a clear thermal signal consistent with thinning of the lithosphere and transition to oceanic spreading. The values then decrease on the southern margin but level off at more than twice the values of the northern margin, suggesting that subduction of the Solomon Sea plate beneath the northern margin may be responsible for the low northern values typical of forearcs. This broad thermal asymmetry also correlates with the prominent structural asymmetry of the margins here. The Pocklington and Woodlark Rises transects show generally elevated values (~50-200 mW/m2) but are less than the peak breakup values, consistent with margin cooling following breakup. Systematic variations are noted in some of the traverses crossing the area of the COT. In several cases, higher values in the margins abruptly decrease and remain lower on crossing major faults which drop the seafloor to the level of the oceanic basin. The lower values toward the spreading center are opposite to predictions from thermal conduction models and suggest that the COT involves a change from a conductive heat flow regime in the margins to one dominated by hydrothermal circulation in the oceanic basin. Comparison of similarities and differences between Woodlark Basin and northern Red Sea will be made.
Strain Patterns in Southern Baja from 3 Ma to the Present
The modern rifted margin of Baja is the result of a long tectonic process which ultimately resulted in the formation of the Gulf of California through highly-oblique extension and sea-floor spreading. Seismicity suggests that modern strain is largely confined to faults submerged by the Gulf of California or along its major transforms. Preliminary results on the rate and nature of three dimensional strain partitioning along the extended gulf coast of southern Baja determined by geomorphology, mapping, coastal uplift studies, and DEM analysis, suggests that the ultimate phase of Gulf development was characterized by a rapidly shifting of the tectonic framework whereby continental faults ultimately play a very small part in the overall plate strain. In southern Baja, highly-active oblique-normal faults on the continent quickly transferred strain out to the Gulf leaving only a relatively low level of faulting on the continent by the late Pleistocene. This pattern, characterized by active faulting in the Pliocene and then rapidly dying out Late Pleistocene, is in strong contrast to northernmost Baja which exhibits strong seismicity in the region north of the Aqua Blanca fault, presumably under the influence of the San Andreas fault. SAR interferometry that captures six years of strain record in the region around the Agua Blanca fault itself, by Lu and Mayer, documents a low level of strain accumulation.
Techniques for reconstructing the Quaternary history of extensional terrains, based on lessons learned in the Basin and Range, and Rio Grande Rift Provinces
The Quaternary history of extensional terranes can be reconstructed at several spatial and temporal scales. At the smallest scale, geomorphology of fault blocks and their intervening drainage systems gives clues to long-term evolution covering the past several My. Long-term uplift histories are represented by range front tectonic geomorphology, i.e. faceted spur structure, and these can be correlated with the more traditional fission-track studies. The early descriptive studies of faceted spurs (e.g. Wallace, 1978) and correlation of facet heights (Hamblin, 1976; McCalpin, 1994) have given rise to more recent, quantitative techniques. For example, dePolo (2000) has used basal facet height in the Basin and Range to classify ranges into categories of vertical slip rate. Sophisticated numerical models of mountain-block erosion, have reproduced horst mega-geomorphology by specifying rates of uplift and transport-limited erosion (diffusion-dominated versus wash-dominated); some of this simulated topography greatly resembles actual mountain ridges. On a somewhat larger scale, Quaternary geologic mapping results in identification and dating of geomorphic surfaces (pediments, fans, fluvial and marine terraces) that act as datums to record Quaternary deformation. In many cases the oldest of these datums only exist as isolated fragments of the original, areally-extensive surface. However, it is these oldest datums that are important because they record the greatest cumulative deformation, and any spatial migration of deformation. The old datums may also record drainage reversals and capture that result from neotectonic movements. Because most of these datums result from basin-wide or regional climate changes, their counterparts can often be observed in the adjacent offshore stratigraphic record, making it possible to correlate the subaerial and submarine records. At the largest scale, the movement history of individual faults in the late Pleistocene and Holocene is assessed by large-scale geologic strip mapping and by fault trenching (McCalpin, 1996). Although fault trenching is typically done for seismic hazard studies, the resulting seismic source parameters are also relevant to neotectonic questions such as: 1) are there persistent seismogenic fault segments?, 2) if so, what determines the segment boundaries?, 3) are surface ruptures "characteristic"?, 4) what is the recurrence time between characteristic earthquakes? We answer such questions by retrodeforming the trench logs using computer graphics software, in a manner similar to the preparation of palinspastic maps. The chronology of faulting is based on the sequence of events inferred from the retrodeformation, added to the numerical-age geochronology of trench samples. Finally, the displacement chronology from multiple trench sites is correlated to produce the 4-dimensional pattern of late Quaternary deformation. All of the above techniques have been field tested in the arid to semi-arid terrain of the Basin and Range and Rio Grande rift provinces (USA) over the past 2 decades. I believe they could also be applied successfully to the Red Sea area.
Simon McClusky and Robert Reilinger
GPS constraints on Arabia-Africa motion: Results, on-going activities, and future needs
Since 1988, our group in collaboration with L-DEO, Cornell University, European colleagues, and host country partners, has been using the Global Positioning System (GPS) to monitor plate motions and inter- and intra-plate deformation in the eastern Mediterranean region (McClusky et al., 2000 JGR, 105, 5695-5719). The GPS observation network was extended into the western Caucasus in 1991, Egypt in 1994, the eastern Caucasus in 1998, and Syria in 2000. During 2001 we are scheduled to establish two continuous GPS stations along the eastern margin of the Red Sea in Saudi Arabia; our Saudi partners (KACST) are simultaneously installing three additional stations, one of which will also be located along the eastern margin of the Red Sea. Also in 2001, we will begin a focused effort to monitor motion of the Sinai "block" in collaboration with our Egyptian partners (NRIAG), including establishing a continuous station in the Sinai, and 12 GPS-survey. Overall, the GPS network will include a substantial part of the zone of interaction of the African, Arabian, and Eurasian plates. Results to date provide constraints on the present-day northward motion of the African plate (at least the NE part of the plate in Egypt) and the northern part of the Arabian plate in Turkey. Specifically, sites on the northern Arabian plate move 18 2 mm/yr. at N25