First posted May 30, 2024 For additional information, contact: Alaska Science Center staffU.S. Geological Survey4210 University Dr.Anchorage, AK 99508Alaska Mineral ResourcesAlaska Science Center In this paper, we provide new information on the stratigraphy and paleoflora of the Sheep Creek volcanic field in the Alaska Range that bolsters our understanding of a key interval in the tectonic, paleoclimate, and paleoenvironmental history of the northern Cordillera. Although the distribution and basic stratigraphy of these rocks have been previously reported, here we document the stratigraphic context of recently dated igneous rocks and paleosols ranging from the Paleocene–Eocene boundary to the early middle Eocene, describe a more complete fossil leaf flora from the succession, and place the Sheep Creek volcanic field in its regional tectonic context of ridge subduction and slab window migration in central Alaska.
First posted December 29, 2023 For additional information, contact: Volcano Science Center - Menlo ParkU.S. Geological Survey345 Middlefield Road, MS 910Menlo Park, CA 94025Contact Pubs Warehouse Ground-motion prediction equations (GMPEs) for the western Kingdom of Saudi Arabia are developed by employing a mixed-effects regression model to modify the Boore and others (2014) Next Generation Attenuation-West2 (NGA-West2) project GMPEs. NGA-West2 addressed several key issues concerning GMPEs for shallow crustal earthquakes in active tectonic regions. However, the NGA-West2 input data do not include many earthquakes in extensional regimes, such as those occurring in Saudi Arabia. This deficiency is redressed by calculating a magnitude scaling of the new Saudi Arabia-specific GMPEs compared to those of Boore and others (2014). Furthermore, there is a clear difference in distance scaling for the Saudi Arabian GMPEs in comparison with the NGA-West2 GMPEs. This difference is especially significant at large distances and is mainly due to lower anelastic attenuation in the crystalline crust of western Saudi Arabia. Our empirical data demonstrate that the GMPEs presented here are in good agreement with observed earthquake ground motions in western Saudi Arabia.
First posted December 29, 2023 For additional information, contact: Volcano Science Center - Menlo ParkU.S. Geological Survey345 Middlefield Road, MS 910Menlo Park, CA 94025Contact Pubs Warehouse Harrat Rahat is a Cenozoic volcanic field in the west-central part of the Kingdom of Saudi Arabia, 150 kilometers east of the Red Sea, and is the site of the most recent eruption in the country (1256 C.E.; 654 in the year of the Hijra). The city of Al Madīnah lies at the north end of Harrat Rahat, and its volcanic and seismic risks are frequently reassessed. In 2009 C.E. an earthquake swarm at Harrat Lunayyir, 200 km west-northwest of Al Madīnah, produced significant shaking and some building damage in nearby communities, which prompted a revision of seismic hazard models for the region. A network of seismic stations on this volcanically active western side of the Arabia Plate was installed, and stations were also added in the tectonically active northern part of the country. Although regional earthquakes may be used to determine the crustal structure of the western Arabia Plate, such crustal models are hindered by insufficient numbers of earthquakes in the stable plate interior. Tomography studies can be used to infer material properties of the subsurface, such as presence of partial melt, and are beneficial for volcanic hazard assessments. We use ambient seismic noise to compute Rayleigh and Love surface-wave dispersion maps between 5 and 12 second periods for a subset of seismic stations within and near northern Harrat Rahat. The surface-wave maps are inverted to produce shear-wave velocities using a neighborhood algorithm and interpolated into a pseudo three-dimensional model. The distributions of surface-wave and shear-wave velocities are heterogenous, varying from ±3–8 percent. However, low velocities are not restricted to Harrat Rahat. We observed a difference between Rayleigh- and Love-wave velocities that extends north of the site of the 1256 C.E. eruption and coincides with a low gravity anomaly. We obtain a shear-wave velocity increase of 10–15 percent between 15- and 25-kilometers depth, which is consistent with the presence of a transition between the felsic upper crust and the mafic lower crust of the Arabian Shield. The average shear-wave velocities of the upper and lower crust are estimated to be 3.64 and 3.95 kilometers per second using Rayleigh waves and 3.53 and 4.16 kilometers per second using Love waves, which are in good agreement with the results of other geophysical surveys in this area. The modest magnitude of the low-velocity anomalies within the crust and their locations extending well beyond the limits of Harrat Rahat indicate that they are not caused by a crustal magma chamber. If magma chambers exist, they are smaller than can be imaged with our seismological method (resolution corresponds to a 15-kilometer wavelength), deeper than 30 kilometers, or shallower than 5 kilometers with a small velocity contrast. The last possibility is discounted by weak-to-absent surface geothermal activity and by a lack of shallow seismicity that characterizes other areas with known shallow magmas, such as Hawaiʻi. We then expanded our work to the entire Arabian Shield with principally the same methodology to look at large-scale regional patterns. We found modest shear-wave velocity deviations on the order of only ±3 percent, which are within expected ranges for lithological variation.
First posted December 29, 2023 For additional information, contact: Volcano Science Center - Menlo ParkU.S. Geological Survey345 Middlefield Road, MS 910Menlo Park, CA 94025Contact Pubs Warehouse Active volcanic systems pose serious hazards to people and property including inundation and incineration by lava, blanketing by tephra (volcanic ash), exposure to noxious volcanic gases, and damage from shallow earthquakes triggered by ascending molten material (magma). To improve understanding of volcanism and associated seismicity on the western Arabia Plate, the Saudi Geological Survey and the U.S. Geological Survey conducted a multi-year investigation of the northern Harrat Rahat volcanic field adjacent to the city of Al Madīnah al Munawwarah, Kingdom of Saudi Arabia. Project components included creation of a high-resolution digital topographic base; interpretation of eruptive history supported by detailed geologic mapping, paleomagnetism, and abundant high-precision geochronology of volcanic deposits; assessments of eruptive styles and volcanic hazards by physical volcanology; investigation of the origins of magmas in the mantle and of their differentiation in the crust revealed by chemical and isotopic petrology; gravity and magnetotelluric surveys to reveal crustal structures and to search for magma reservoirs; and regional and local seismic tomography and analyses of seismic hazards. Project results are presented in this Professional Paper as chapters written for technical scientific audiences. This initial chapter introduces the project and briefly summarizes results in plain language for readers who have more general backgrounds.
Cover.Photograph showing Proterozoic metamorphic and plutonic rocks of the Arabian Shield exposed in the rugged topography of low hills and lower parts of flat-topped hills south of Al Madīnah and west of Harrat Rahat.Subdued topography of upper Jabal Jimmah (left) and Jabal 'Ayr (center with telecommunication towers) consist of Miocene, pre-Harrat Rahat lava flows that unconformably overlie the Arabian Shield rocks.Hills were either uplifted by now-obscured faults, or were incised relative to the basin that hosts Harrat Rahat, after deposition of the Miocene lavas.Harrat Rahat lava domes are visible on the skyline between Jabal 'Ayr and Jabal Jimmah.View is to the southsoutheast from the As Salam district in the western suburb of Al Madīnah.U.S.
First posted December 29, 2023 For additional information, contact: Volcano Science Center - Menlo ParkU.S. Geological Survey345 Middlefield Road, MS 910Menlo Park, CA 94025Contact Pubs Warehouse We present a probabilistic seismic-hazard analysis (PSHA) for the west-central part of the Arabian Peninsula. Our study area includes the northern Harrat Rahat volcanic field and the nearby city of Al Madīnah, Kingdom of Saudi Arabia. This young, active volcanic field experienced one historical eruption in 1256 C.E. (654 in the year of the Hijra) that vented 20 to 22 kilometers (km) southeast of the center of Al Madīnah, which has a present population of about 1.4 million. The field also erupted numerous times in the late Pleistocene and possibly in the early Holocene. We used recently developed regional ground-motion prediction equations for Saudi Arabia to calculate the severity of ground shaking as a function of distance from the earthquake source. This information was combined with two key volcanic parameters for this area: (1) the return period of volcanic eruptions and (2) the spatial probability of the next vent opening. The calculated ground-motion levels of peak ground acceleration and peak ground velocity with a 2-percent probability of exceedance in 50 years are expected to be about 0.14 the acceleration due to gravity (g) and 10 centimeters per second (cm/s), respectively, at the most probable vent opening location, about 25 km southeast of Al Madīnah’s center, and about 0.07 g and 3 cm/s, respectively, in the city interior. These ground motions are higher than previous estimates that did not consider the nearby earthquakes associated with volcanic activity.
First posted December 29, 2023 For additional information, contact: Volcano Science Center - Menlo ParkU.S. Geological Survey345 Middlefield Road, MS 910Menlo Park, CA 94025 Harrat Rahat is an alkali basalt, continental, intraplate volcanic field located within the central-western part of the Kingdom of Saudi Arabia. The northern quarter of Harrat Rahat contains evolved volcanic products that achieve trachyte compositions (>60 weight percent SiO2). Within the Al Efairia volcanic center, pyroclastic-flow and -surge deposits that reflect explosive trachyte volcanism (and minor exposed lava domes that reflect effusive trachyte eruptions) sit at the surface as the youngest expression of volcanic activity within this part of the Harrat Rahat volcanic field. Five trachyte deposits emplaced explosively have been identified within the Al Efairia volcanic center based on geologic mapping, petrography, geochemistry, and paleomagnetism. These units are the trachytes of Um Rgaibah, Gura 5, Gura 4, Al Efairia, and Al Qayf, in descending stratigraphic order. Here, we present 14 40Ar/39Ar analyses from four of these units, which yield eruption ages of 4.2±5.2 thousand years (ka) for the trachyte of Um Rgaibah, 79.7±1.6 ka for the trachyte of Gura 5, 84.3±1.6 ka for the trachyte of Gura 4, and 88.0±1.8 ka for the trachyte of Al Efairia. The eruption age of the trachyte of Al Qayf has been constrained to between 410.3±3.4 and 418.8±1.9 ka using paleomagnetic correlations and 40Ar/39Ar ages from overlying and underlying intermediate composition lava flows. Most of these trachytes have distinct geochemical compositions, petrographic characteristics, and directions of remanent magnetization. The exceptions are for the trachytes of Gura 4 and Gura 5, which overlap in their geochemical, petrographic, paleomagnetic, and geochronologic affinities. Based on these similarities, we interpret the trachytes of Gura 4 and Gura 5 to have erupted during a closely spaced (a few decades) time interval from the same magma batch but from craters that are >2 kilometers (km) apart. The eruption of the trachyte of Al Efairia at 88.0±1.8 ka is the result of a different magma batch that erupted a few thousand years prior to the trachytes of Gura 4 and Gura 5. The Al Efairia volcanic center is remarkably different from the Matan volcanic center located ~10 km to the north, which has also erupted young (<150 ka) trachytes. The Matan volcanic center has been shown to produce trachyte compositions only after eruption of basalt followed by intermediate lava flows, whereas only trachyte compositions have erupted within the Al Efairia volcanic center over this same time interval.
First posted December 29, 2023 For additional information, contact: Volcano Science Center - Menlo ParkU.S. Geological Survey345 Middlefield Road, MS 910Menlo Park, CA 94025Contact Pubs Warehouse Mafic volcanic fields are widespread, but few have erupted in historical times, providing limited observations of the magnitudes, dynamics, and timescales of lava flow emplacement in these settings. The Harrat Rahat volcanic field in western Saudi Arabia offers a good opportunity to study eruptions in such a setting, with a historical eruption in 1256 C.E. (654 in the year of the Hijra) and numerous well-preserved late Pleistocene lava flows. We combine historical observations and rheological and morphological analyses of the youngest flows with analytical models to reconstruct eruptive histories and lava flow emplacement conditions in Harrat Rahat. Petrologic analysis of samples for emplacement temperatures and crystallinities show cooling trends from vent to toe of ~1,140 to ~1,090 degrees Celsius (°C) at rates of 2 to 7 °C per kilometer, crystallinities increasing from 0.5 to 60 volume percent, and apparent viscosities increasing from 102 to 109 pascal seconds. High-resolution topographic data facilitate quantitative analysis of morphology and interpolation of preeruptive surfaces to measure flow thicknesses, channels, and levees, and enable calculation of eruptive volumes. Analytical models relating flow morphology to emplacement conditions are applied to estimate effusion rates. Within the suite of studied flows, minimum volume estimates range from 0.07 to 0.42 cubic kilometers dense rock equivalent, with effusion rates on the order of tens to hundreds of cubic meters per second and durations from 1 to 15 weeks. These integrated analyses quantify past lava flow emplacement conditions and dynamics in Harrat Rahat, improving our understanding and observations of fundamental parameters and controls of effusive eruptions in Harrat Rahat and other mafic volcanic fields.
First posted December 29, 2023 For additional information, contact: Volcano Science Center - Menlo ParkU.S. Geological Survey345 Middlefield Road, MS 910Menlo Park, CA 94025 Pleistocene and Holocene basalts, hawaiites, mugearites, benmoreites, and trachytes from the northern part of the Harrat Rahat volcanic field, Kingdom of Saudi Arabia, were analyzed for Sr, Nd, Hf, and Pb isotopic compositions. Evolved trachytes with Mg number <0.1 (Mg# = Mg/[Mg+Fe2+], molar) have relatively radiogenic Sr isotopic compositions indicating that they were influenced by contamination probably in the upper crust. Volcanic rocks with Mg# >0.1, consisting chiefly of alkali basalts but encompassing hawaiites, mugearites, and benmoreites, show a limited range in Hf, Nd, Sr, and Pb isotopic compositions. Although the total Pb isotope variation is only 1 percent, the Pb isotope values correlate with Mg#, where the least radiogenic Pb is in samples with the lowest Mg#. The trend formed in Pb isotope space points toward an unradiogenic Pb composition that is similar to the Pb isotopic composition of lower crust of the Precambrian Arabian-Nubian Shields, as well as to feldspars and galena in the upper crust of the western Arabian Shield. This trend is interpreted as progressive but overall minor (no more than 5 weight percent) assimilation of shield rocks, or their partial melts, during fractional crystallization.Isotopic compositions of the least evolved northern Harrat Rahat magmas are most similar among analyzed Arabian harrats to depleted spreading-ridge basalts of the active Red Sea rift, but isotopic values are displaced toward those of spreading-ridge basalts of the Gulf of Aden that are proximal to the site of the Afar mantle plume. The Pb isotopic compositions very near the Northern Hemisphere Reference Line indicate no discernable lithospheric contribution to yield the parental basalts of northern Harrat Rahat, and their isotopic compositions are consistent with derivation predominantly from depleted Northern Hemisphere asthenosphere with a subordinate (20–30 weight percent) component from the Afar mantle plume.Trace-element variations show that appreciable portions of melting were in the garnet stability field, confirming the sub-lithospheric origin of the magmas, and that melting extents were low, accounting for the alkalic, trace-element-enriched character of the suite. The presence of possible Afar mantle beneath the western part of the Arabian Shield and its absence beneath the Red Sea rift may result from capture and channelized flow along high-relief structures incised into the base of the sub-continental lithosphere, as revealed by geophysical images.
First posted December 29, 2023 For additional information, contact: Volcano Science Center - Menlo ParkU.S. Geological Survey345 Middlefield Road, MS 910Menlo Park, CA 94025Contact Pubs Warehouse A fundamental goal of igneous petrology is to quantify the durations required to produce evolved magmas following influx of basalt into the crust. However, in many cases, complex field relations and (or) the presence of a long-lived magmatic system make it difficult to assess how basaltic inputs relate to more evolved magmas, therefore precluding calculation of meaningful timescales. Here we present field relations, geochemistry, 40Ar/39Ar ages, and 36Cl surface-exposure ages for volcanic rocks from the Matan volcanic center, located in the northern part of the Harrat Rahat volcanic field, in the Kingdom of Saudi Arabia. These data document a systematic and repeated temporal progression from alkali basalt to trachyte for the youngest eruptive products. From ~155–17 thousand years ago, the following eruptive sequence occurred four times: (1) alkali basalt, (2) hawaiite, mugearite, or benmoreite, and (3) trachyte. We interpret each eruptive sequence to result from injection of basalt into the crust, and its subsequent differentiation and eruption of progressively evolved magmas. We use the interval time between successive eruptions within a given sequence to calculate the timespans required to produce trachyte from alkali basalt. Differentiation from alkali basalt to intermediate compositions (hawaiite, mugearite, and benmoreite) took ≤3 thousand years (k.y.) on average. Differentiation from intermediate compositions to trachyte took a maximum of 6.7±3.6 to 22.9±1.7 k.y. Thus, the total duration of differentiation was ~10–25 k.y. Timescales presented here are independent of the processes evoked to drive differentiation because they are based solely on the ages and compositions of eruptive products from a system characterized by a simple, repeated differentiation sequence.
First posted December 29, 2023 For additional information, contact: Volcano Science Center - Menlo ParkU.S. Geological Survey345 Middlefield Road, MS 910Menlo Park, CA 94025Contact Pubs Warehouse Earthquake swarms caused by volcanic activity, tectonic stresses, or industrial operations (oil and gas production) can pose considerable risk for nearby settlements. As a rule, a probabilistic seismic hazard assessment (PSHA) that is based on time-independent earthquakes does not take into account earthquake swarms because of their statistically time-dependent nature. We describe the technique and application of a scenario-based method for the estimation of seismic hazard from earthquake swarms that may occur in areas of volcanic activity in western Saudi Arabia. The method consists of the generation of a large number of scenario seismic swarms followed by the calculation of ground motion in a site of interest for all earthquakes in all of the swarms. The set of calculated ground-motion values permits the construction and analysis of probability distribution functions for possible ground-motion levels during the hypothetical earthquake swarms. The level of ground motion representing the hazard is selected considering an appropriate value of the probability of exceedance or non-exceedance, depending on the goal of the study. This swarm-scenario-based seismic hazard assessment may provide a valuable supplement to an ordinary PSHA and (or) deterministic seismic hazard assessment that are commonly used for emergency response and post-earthquake recovery management.
