Major Normal Faults Research Articles

First Installation of an Optical Ocean-Bottom Seismometer, Cabled Offshore Les Saintes, Lesser Antilles

Abstract The detection and analysis of offshore seismic processes worldwide often require the use of ocean-bottom seismometers (OBSs). However, most OBS deployments are done with stand-alone stations, with data recovery delayed by months. On the other hand, electrically cabled OBS, which allows for real-time monitoring, remains exceptional due to the high cost of manufacturing, installation, and maintenance. Here, we present a new perspective for cabled array of OBSs, using purely optical seismometers, plugged at the end of long fiber-optic cables, aimed at reducing their cost for observatories requesting real-time data. The optical seismometer was developed in the last decade by the École Supérieure d’Électronique de l’Ouest, based on the Fabry–Perot interferometer, tracking at high resolution the displacement of the mobile mass of a mechanical geophone (no electronics nor feedback). A prototype was successfully installed at the top of La Soufrière volcano of Guadeloupe in 2019. We replicated this sensor and installed it 5 km offshore Les Saintes islands, at 43 m depth (Guadeloupe, Lesser Antilles) to characterize the swarm-type activity persistent after the 2004 M 6.3 earthquake (Interreg Caraïbe PREST project). The installation cruise, FIBROSAINTES, was supported by the Flotte Océanographique Française. A plow designed by GEOAZUR carried the cable and was pulled on the seafloor by the vessel ANTEA. The landing cable was connected to the interrogator, with a real-time telemetry to the Institut du Physique du Globe de Paris/Observatoire Volcanologique et Sismologique de Guadeloupe. The OBS has been qualified with local land-based velocity broad band stations. The analysis of local earthquake swarms suggests transient creep on the major normal faults. This successful installation opens promising perspectives for real-time monitoring in on-land or offshore sites, presenting harsh environmental conditions, in which commercial, electrical seismic sensors are difficult and/or costly to install and maintain.

Structural assessment and petrophysical evaluation of the pre-Cenomanian Nubian sandstone in the October Oil Field, central Gulf of Suez, Egypt

The low quality of the seismic resolution of the studied Cambrian Pre-Cenomanian formations, which rest directly on the basement in the October Field in the Gulf of Suez, requires study, and the structure pattern controlling these formations is uncertain. The poor quality of the seismic data resolution in the Gulf of Suez is due to the presence of thick sequences of evaporites represented by the South Gharib and Zeit formations. Therefore, the current study used seismic reflection data integrated with well-log data to define the structural-stratigraphic setting of the pre-Cretaceous Nubian reservoir sequence. The study aims at locating and delineating zones having excellent reservoir potential within the Nubian sequence horizons (Nubian Transition, main Nubian, Nubian marker-1, and Nubia marker-2, referred to as NT, MN, M1 and M2, respectively) through the integration of the 3D static modelling and the petrophysical parameters deduced from the well-log data processing. A detailed petrophysical analysis, based on the wireline logs of some wells, was performed to determine the lithological units using two types of cross plots (Neutron-Density and M-N plots), and the different petrophysical parameters that characterize the various Nubian horizons were also computed. 3D modelling was used to visualise the lateral and vertical changes of the reservoir characteristics. The seismic mapping reveals that the Nubian structure is controlled by an NW-SE trending (Clysmic trend) of an NE-plunging asymmetrical anticlinal feature unconformably resting on a basement horst. The main structure is affected by two major normal faults, which trend NW, andis cut by two smaller sub-parallel faults that disturb the core of the structure, which is unaffected by the easterly oriented faults of the Aqaba trend. The petrophysical analyses indicate that the Nubian sandstones have intervals of good reservoir quality, with a total net-pay thickness of about 500 m, net sand ratios (60–90 %), effective porosity (10–25 %), and hydrocarbon saturation (85%), which may encourage further drilling, especially in the southeast portion. The result of this study can be extended to the other Nubia sandstones resting on the basement complex in the Gulf of Suez.

Petroleum system assessment of the Beni Suef Basin, Western Desert, Egypt

Within the northwestern Desert of Egypt, the Beni Suef Basin is one of the most significant hydrocarbon basins. After over 50 years of exploration and production, the main challenge today is to locate new areas that are prolific to oil and gas production. The study aims to identify potential drilling locations by integrating geochemical methods, 1D and 2D basin modelling, well log analysis, seismic interpretation, and hydrocarbon migration pathways. Results show that the Lower Kharita shale contain both oil and gas generating kerogen with generating potential that ranges from fair to very good. The source rocks within the Lower Kharita are mature (main oil window) and have a transformation ratio of up to 60% based on 1D basin modeling. The modelled 2D vitrinite reflectance map showed that hydrocarbon generation occurred at depths greater than 10850 ft (3306 m) with vitrinite reflectance values greater than 0.65% in both the northern and southern parts of the Beni Suef Field. Two main reservoirs were studied. The lower Bahariya and upper Kharita reservoirs are composed of well to moderately sorted sandstone. The subsurface structural maps, reservoirs, source, and charge modeling maps have been constructed to show the migration pathways and accumulation areas. There are three accumulation areas identified. The first accumulation area is located in the high structure up-dip section of the major normal fault, and migration direction is from northwest. The second and third accumulation areas are located in the low structure and down-dip from the normal fault, with migration of hydrocarbons from southeast. The latter two areas should be tested by drilling in the future.

Complex Segment Linkage Along the Sevier Normal Fault, Southwestern Utah

Abstract Major normal fault systems are composed of segments that link as displacement accumulates, with linkage zone characteristics that reveal fault zone evolution. The steeply west-dipping Sevier fault zone in southwestern Utah, displays a complex fault network that developed between two long (>10 km), en echelon segments near the town of Orderville. Geologic map data and cross-sections of the transfer zone between the Mt. Carmel segment in the south and the Spencer Bench segment in the north reveal more than ten normal faults and four relay ramps displaying a range of geometries, including two relay ramps that display ramp-parallel folds. We suggest that transfer zone deformation was initially dominated by faults subparallel to the primary segments with later cross-faults that hard-linked these faults across most of the transfer zone. When the transfer zone was a soft-linked system, a displacement deficit likely existed relative to fault segments to the north and south. This early fault configuration would have reduced the efficiency of slip propagation associated with major earthquakes (>M7.0). In contrast, the present-day transfer zone, with a complex but hard-linked fault network, shows displacements that transition smoothly from the higher displacement (~800 m) southern segment to the lower displacement (~400 m) northern segment. That transition, combined with extensional strain within the zone, suggests that the Orderville fault network would be unlikely to impede propagation associated with future major earthquakes. The kinematic model of fault evolution presented here has implications for those investigating geothermal energy potential, groundwater flow, natural gas and oil reservoirs, mineral deposit formation, or seismic hazards.

Structural architecture of the Mapimí Biosphere Reserve and its surroundings, north-central Mexico: New insights from U–Pb geochronology and interpreted structural data

The Mapimí Biosphere Reserve is in northeastern México, near the boundary between the Mesozoic Coahuila platform and the Mexican central basin. A large portion of the Reserve is a broad bolson where isolated mountains are surrounded by flat terrain. Two slightly elevated areas (benches) are subtle topographic features in the Reserve. There, in rolling hills occur folded, Upper Cretaceous – Paleogene, poorly-lithified and -exposed transitional and continental sediments and tilted Paleogene volcanic and subvolcanic rocks. Each bench occupies a band that is ∼75 × 20 km. A land strip where playa lakes and salt-flats occur separates the benches. Deformation style within the benches differ from those in regions where platform carbonates or Mexican central basin rocks dominate. Structures within the benches are gentle, open folds and medium sized (15 - 3 km in diameter) structural domes and basins. Furthermore, orientation of the fold axes shows a wide scatter. There is at least one folded structure in the Reserve that displays an interference pattern that indicates refolding. Post-Oligocene normal faults bound the benches and overprinted on the folded structures. However, the most important normal fault, which cuts in half two domes and one structural basin, lacks a clear topographic scarp. The occurrence of broad exposures of the Upper Cretaceous Indidura Formation in flat areas, interpreted as the footwall block of a major normal fault, is consistent with the interpretation that the benches are rocky pediments partially covered by alluvium and/or eolian sediments. Thus, it appears that Cenozoic extension has been inactive for a long period of time in this portion of the Mexican Basin and Range province. The best evidence of NNW, down to the SW normal faults within the Reserve is in the Sierra de Mohóvano. There, an array of domino-style normal faults cut Oligocene volcanic rocks and gravel deposits. The master fault of the domino array is adjacent and parallel to a fold in Mesozoic marine rocks.We attribute the unusual characteristic of the folds in the Reserve to 1) the region is near the boundary among the large calcareous platform and the basin, and the boundary had a significant curvature, concave toward the W. This geometry caused complex deformation during crustal shortening, as tectonic transport associated with the Mexican orogen is toward the NE, 2) a significant difference in competence between the poorly consolidated transitional and continental deposits and the more competent rocks in the carbonate platform also played a role in the formation of the structures, as folds in the benches formed during the waning stage of the Mexican orogen, as suggested by the detrital zircon U–Pb ages, which are as young as ∼30 Ma, 3) Oligocene magmatic activity caused the emplacement of laccoliths, which locally modified older folds forming domes, 4) finally, Basin and Range extension (<27 Ma) cut across the folded sediments and overlying volcanic rocks. Evidence of extension in the Reserve is subtle as normal faults have remained inactive for a long time, while isostatic uplift and erosion developed the pediment, effectively masked most fault scarps.

The 2020 Mw 6.5 Stanley, Idaho, Earthquake and Aftershock Sequence: Complex Faulting at the Northern End of the Basin and Range Province

ABSTRACT The magnitude 6.5 Stanley, Idaho, earthquake occurred on 31 March 2020 in a sparsely populated region north of the Sawtooth normal fault. We used seismic data from temporary and permanent stations to derive a 1D velocity model and relocate 1401 M ≥ 2.4 earthquakes with hypoDD, including a foreshock, the mainshock, and 3 yr of aftershocks. We used broadband data to determine seismic moment tensors for 173 Mw≥3.1 earthquakes. Combining locations and mechanisms shows the mainshock ruptured an unmapped north-trending, steeply west-dipping, left-lateral strike-slip fault. Rupture initiated near the bottom of the seismogenic zone and propagated upward and bilaterally for ∼20 km north and ∼3–5 km south, where the fault likely ends and deformation changes to extension. There, the rupture may have jumped west to another unmapped blind fault accommodating oblique extension. Support for a late mainshock rupture on a northwest-trending, likely east-dipping fault comes from normal faulting aftershocks. The total rupture length is ∼25–30 km, because oblique fault activity ends at the latitude of the northern terminus of the Sawtooth fault, but its trace, if it reached the surface, would be ∼6 km to the west. The Sawtooth fault was not active, even though aftershock clusters indicate that short strike-slip and normal faults are abundant in its footwall and hanging wall. Extension in the northern Basin and Range seems to terminate where major normal faults reach the inactive Eocene Trans-Challis fault system (TCFS), suggesting the TCFS exerts structural control. The deformation north of the TCFS is low, and the strike-slip character was unknown before the Stanley earthquake. Faults rupturing in the Stanley earthquake lack surface expression and are immature with low cumulative displacement. Complex transitions between tectonic regimes are common and may result in blind ruptures on unknown, immature faults, posing an underrated hazard.

Integrating 3D subsurface imaging, seismic attributes, and wireline logging analyses: Implications for a high resolution detection of deep-rooted gas escape features, eastern offshore Nile Delta, Egypt

The eastern part of the offshore Nile Delta is one of Egypt's most productive gas provinces. This study utilizes well logs dataset and 2D seismic from Port Fouad marine (PFM) Field to investigate reservoir lithofacies, rock-types as well as the associated gas chimneys, pockmarks, bright anomalies of escaping fluids in the seafloor, and high-faulted zones caused by the chimney's developments. First, structural smoothing and median filters are used to improve the continuity of the stratigraphic horizons while preserving various geological structures, in order to pick all formation tops and structural faults. Afterwards, variance and energy attributes were computed and extracted for amplitude enhancement of various geobodies such as faults, gas chimneys, and bright reflectors. After that, petrophysical analyses revealed that the Upper Miocene Wakar Formation hosts scattered, thin gas pay zones (<10m) with low hydrocarbon saturation values (average Shc = 27.9%), high water saturation (average Sw > 70%), and high shale volume (average Vsh > 30%). These poor reservoir characteristics prevented accumulation of thick gas zones, thereby several gas escape features (e. vertical chimneys) are observed in the region. Finally, the estimated petrophysical parameter logs and the structure maps derived from seismic data are crucial components of the built 3D structural model. This model reveals the presence of four major normal faults (F1, F2, F3, and F4) oriented NW-SE and WNW-ESE, creating grabens and half grabens. These major faults (F1, F2, F3, and F4) are directly related to the inverted tectonics during the Cretaceous-Paleogene compression (Syrian Arc phase). A link between these structural patterns and gas escape zones is proposed, and the absence of good reservoir facies likely promoted an active phase of gas seepage in the study region. In addition, such gas escape chimneys can create catastrophic events on the seafloor and are considered direct evidence for the presence of gas generation and failure in accumulation.

Architecture and Evolution of the Southern Chotts‐Jeffara Basin, Tunisia

AbstractSouthern Tunisia is known to be less deformed and simpler than its neighboring Atlassic domain to the north. This area is complex and basin evolution in the Southern Chotts‐Jeffara (SCJ) basin is debated. In this paper we combined surface and subsurface data with low temperature thermochronology (LTT) to reinvestigate the tectono‐sedimentary evolution of the SCJ basin from Permian to Jurassic. We reconstruct the present‐day architecture of the SCJ basin along two regional sections. In these sections, we focused mainly on regional thickness variations and on internal reflections interpreted from seismic data. We observe three structural elements: (a) A Paleozoic culmination, oriented E‐W, capped by Mid‐Upper Triassic deposits; (b) the Tebaga of Medenine (ToM), a culmination also oriented E‐W but located ∼50 km north of the Paleozoic culmination; and (c) A Triassic culmination in the eastern part of the area, oriented NW‐SE. We note the absence of major normal faults along the sections. The LTT data we present are the first published in this area and allow to reconstruct the timing and magnitude of vertical movements. These data prove: (a) exhumation at ∼230 Ma of the Permian and Lower Triassic units associated with the onset of the ToM removing locally about 900 m of pre‐Cretaceous sediments; and (b) the development of the Triassic culmination ∼180 Ma removing 2000 m of pre‐Cretaceous sediments in the Jebel Rehach. This study demonstrates that vertical movements in the SCJ basin are controlled by long‐wavelength processes developed essentially in shortening regimes.

Late Paleozoic-Jurassic tectonic evolution of the eastern Deseado Massif in central-southern Patagonia

Previous tectonic studies have indicated that the peri-cratonic lithosphere, located away from continental margins, is sensitive to far-field stresses propagating from active plate margins, which induce variable deformation. In order to gain a better understanding of potential intraplate tectonic events associated with the geodynamic evolution of the active margin of southwestern Gondwana, we conducted a tectono-sedimentary study of the Permian-Jurassic volcano-sedimentary record in the Deseado Massif, located in southern Patagonia. Our multidisciplinary analysis includes detailed geological mapping of an area of approximately 150 km2, structural analysis, geoelectric tomography, 2D seismic data, new geochronological dating, petrographic studies, and stratigraphic loggings of the volcano-sedimentary basin record. This comprehensive data set has allowed us to establish the tectonic, sedimentary, and magmatic evolution of the eastern Deseado Massif. Specifically, we have identified major normal faults associated with the syn-extensional deposition of late Permian and Jurassic sedimentary and volcanic rocks, as well as the Late Triassic emplacement of intermediate and felsic intrusive bodies. Additionally, interspersed large-scale shortening events were recognized, which induced positive tectonic inversion events in the region, recording contrasting stress fields during the analyzed lapse. Based on this, six major intraplate tectonomagmatic events were defined: (i) a potential post-Devonian pre-late Permian exhumation of the Neoproterozoic-early Paleozoic igneous-metamorphic basement, which we tentatively link to the Gondwanide orogeny; (ii) intraplate extension in the Late Permian (255 ± 4 Ma) related to the deposition of the Dos Hermanos Member of the La Golondrina Formation; (iii) Late Triassic (231 ± 3 Ma) intrusion of andesitic bodies, tentatively linked to the inland migration of arc magmatism associated with the South Gondwana flat slab; (iv) subsequent Late Triassic positive tectonic inversion of Permian extensional faults caused by a large-scale contractional event linked to the South Gondwana flat slab; (v) the extension-related emplacement and deposition of Early-Middle Jurassic (176 ± 3 Ma; 172 ± 4 Ma) sedimentary (lacustrine and fan deltas-related deposits), pyroclastic rocks (ignimbrites and ash tuffs), and lavas (lava domes and dykes) related to the Chon Aike silicic large igneous province; and (vi) poorly-constrained post-Middle Jurassic positive tectonic inversion of Jurassic faults. Therefore, we suggest that the geological events preserved in the Deseado Massif provide a key deformational record of the distal effects associated with ancient geodynamic processes that occurred along the southwestern active margin of Gondwana.

Geophysical characteristics of a fault system in the northern Central Range of Taiwan and its applications for geothermal energy exploration

The northern Central Range of Taiwan is a high-potential geothermal region. Since the formations are mainly tight metasandstone and slate, permeable structures associated with faults are commonly considered as conduits of geothermal fluids. This study determines the characteristics and orientations of the permeable fault zones by analyzing the geophysical logs and microresistivity formation image log (FMI) of the JT-4 well in Jentse, an important geothermal area in the northern Central Range. Between 720 and 1480 m measured depth (MD), the effective porosity of the intact host rock is mostly below 3% calculated by the geophysical log. Zones with porosity greater than 5% are only clustered within a few thin intervals. The FMI interpretations show these porous zones are in the interior of the fractured and faulted intervals. These porous fault zones comprise fault damage zones with a high density of open fracture planes and fault cores with porous fault breccias. There is a highly brecciated fault core in 1334–1339 m MD, which would be the most permeable interval of the well. Additionally, some healed fault zones with sealed fractures are observed. The picked drilling-induced tensile fractures signify that the direction of the present-day maximum horizontal principal stress is N40–50°E, and most of the open fractures also strike parallel to the NE–SW direction. The study results show that the open fractures are concentrated in the four fault zones belonging to one major normal fault system. After integrating the orientations and locations of the fault zones, we propose that the permeable normal fault system is about 200 m wide, trends N50–70°E, and dips 70–80° to the NW. The development of the open fractures and the permeable fault system in the northern Central Range may be controlled by the current rifting of the Okinawa Trough offshore northeastern Taiwan. The study exhibits the characteristics of fractured fluid conduits of the regional geothermal system, which will benefit future geothermal exploration in northeastern Taiwan.

Basement structures exert a crucial influence on the structural configuration of later rift development: Insights from the Shanan and Chengbei sag, Bohai Bay Basin

The Early Cretaceous and Paleogene rift structures of the Shanan and Chengbei sags were inherited from basement tectonic deformation of Indosinian and Yanshanian tectonism. However, the architecture of the basement structures and its influence on the structural configuration of later rift development are poorly understood, which impeded hydrocarbon exploration efforts in the study area. Based on 2D high-resolution seismic reflection data in the study area and onshore field data in the Luxi area, we recognize the Indosinian NW-striking low-angle fold-thrust belts and Yanshanian NE-striking high-angle thrusts in basement of the study area. The pre-rift NW- and NE-striking basement structures exert a crucial influence on the two-phase rift architecture and structural evolution of the study area: (1) Low-angle NW-striking Indosinian thrusts are dominantly reactivated as dip-slip major normal faults during Early Cretaceous, such as the Shanan fault and Chengbei faults. The high-angle Yanshanian NE-striking thrusts are primarily reactivated as left-lateral strike-slip faults during this stage, such as the Shaxi fault. (2) During the Paleogene, oblique extension took place across the pre-existing NW-striking reactivated normal faults. A set of half-grabens formed along the major NW-striking oblique-slip faults and large amount of ENE-striking secondary normal faults developed in the NW-striking sags. The pre-existing left-lateral NNE-striking faults shifted to become right-lateral and extensional faults. The Early Cretaceous rift was primarily controlled by NE-SW-directed extension, which was derived from the large-scale left-lateral strike-slip movement of the Tan-Lu Fault Zone. The Paleogene oblique rift was induced by NNW-SSE-directed extension related to the upwelling water-rich mantle wedge of the Pacific subduction zone.

Investigation and mapping of geological construction materials in parts of chemoga river sub basin, debre markos, Ethiopia

Currently the demands of geomaterials are intensively increasing all over the country regarding to the expansions of towns for the consumption of engineering structures and buildings. Therefore, searching workable, serviceable and accessible geomaterials nearby areas is a vital subject to stabilize the inflation of geomaterials. The present research was carried out in the Chemoga river sub basin in the Southern direction of Debre Markos town, North Western Ethiopia. To meet out the objectives, detailed surface explorations were carried out through systematically selected travel lines (across the strike and along streams) to detect the different lithologic formations and the contacts; A number of representative rock samples were collected for visual interpretations; GPS reading and metallic meter were the materials used in the field to quantify stratigraphic units, bedding thickness and lamination the lithologic formations; The Brunton compass was used in the field to measure the orientation (dip direction, dip amount and strikes) of geological structures. Besides to this, related literatures were reviewed to have a general background on the subject matter.Basalt and Sandstone are the dominant lithogical units in the study area and have existed with about an average thickness of 500 m and 145 m respectively. Fresh to slightly weathered basalt and columnar basalt that used as a masonry stones are ease to access, workable and the most dominant lithologic units in the study area. Fine to coarse grain reddish to light color sandstone interbedded with siltstone and mudstone is well exposed. Based on the detail field exploration and judgments, Sandstone unit has been well exposed in a desirable potential and quality. And based on the detail field investigation and researcher judgment, the sandstone resource could answer the demands of construction materials for the surrounding areas of Debre Markos town for several thousand of years. Geometrically, mostly massive and steep rock faces are well developed which are difficult for quarrying process and required high explosive materials. Generally, a sandstone rock unit is surrounded by three major normal faults and inaccessible as compared to basalt unit. Regarding to environmental influences/effects, the area is sparsely farmed and it is not common that an extractive activity does not show any environmental and social impacts. Chert is exposed on geographically limited area and rarely observed following stream cuts. Chert and basalt units are slightly to highly weathered and showed alterations on the surface. Finally, the potential and the spatial distribution of geomaterials are investigated and revealed on the map using ArcGIS software and on the basis of the findings of the present research, recommendations are forwarded.

The evolution of catchment‐depositional system relationships on the dip slopes of intra‐rift basement highs: An example from the Frøya High, Mid‐Norwegian rifted margin

AbstractBasement highs form one of many potential sediment source areas during the evolution of continental rifts and rifted margins and add to the topographic complexity typical of active rifts. Footwall basement highs acting as a source area to sedimentary systems in the hangingwall of major faults have been documented in many systems worldwide. However, the back‐tilted footwall dip slopes of such highs have received comparatively little attention. Here, we investigate a subsurface case study from the Norwegian continental shelf, where catchments and shallow marine syn‐rift sedimentary systems on a dip slope are preserved due to early transgression of an intra‐rift high. At the onset of Late Jurassic rifting, the Frøya High emerged as a prominent, N‐S trending, 25 km‐wide basement high tilted towards the east in response to several kilometres of displacement along the Klakk Fault Complex, a major normal fault zone at the Frøya High's western edge. Using well‐calibrated 3D seismic reflection data, we observe a series of conspicuous Upper Jurassic wedges along the eastern edge of the Frøya High along the margin of the Froan Basin. Internally, these wedges show sigmoidal geometries marking top and foresets of clinoform packages with a maximum thickness of ca. 200 m with foresets between 30 and 200 m high, dipping ca. 10° towards the east, southeast and northeast. We interpret these wedges to represent a series of eastward prograding deltas positioned along a constructional shoreline, connected to E‐W trending valleys and river catchments up‐dip. The deltas show strong progradation, interpreted to reflect the impact of the continued uplift of their catchments prior to the abrupt termination of sediment supply from drainage capture by footwall scarp drainages. The presence of a connected, largely constructional shoreline has implications for Late Jurassic sediment distribution around the Frøya High, providing primary sedimentary input for longshore‐driven sedimentary systems in the Draugen Ridge to the north. Comparisons with other syn‐rift dip slope systems highlight a broadly similar evolution but shows a distinct lack of the protracted backstepping observed in other dip slope systems. We postulate that different structural configurations of dip slope systems, being footwall uplift or hangingwall subsidence driven, may drive the strongly progradational character of the deltaic systems on the Frøya High. The Frøya High example highlights the need to constrain primary sediment input points to aid the interpretation of volumetrically significant, but short‐lived and subtle depositional systems, especially within complex, tectonically active settings.

Structural analysis of fracture networks controlling geothermal activity in the northern part of the Malawi Rifted Zone from aeromagnetic and remote sensing data

We investigated the relationship between the geothermal activity and fracture networks that control storage and fluid pathways of geothermal systems in the northern part of the Malawi Rifted Zone (MRZ). It is suggested that low to medium temperature geothermal systems in the MRZ are mainly non-magmatic and structurally controlled. However, structural controls and favorable settings of geothermal activity are poorly understood in this region. We used an approach of remote sensing and aeromagnetic data analysis to 1) identify and characterize fracture networks to better understand the structural controls of geothermal activity in the northern part of the rift, 2) quantify and identify permeable areas using topological analysis of fracture intensity and connectivity frequency proxies, 3) understand the role that inherited structures play in the geothermal systems of the northern part of the rift, and 4) build preliminary geothermal conceptual models of selected geothermal systems. Our findings show that fracture networks in the Karonga and Nkhata regions comprise a varying degree of complexity along strike. This structural complexity occurs mainly in favorable structural settings such as 1) two fault segments coalescing to form hard and soft-linked relays, 2) two different oriented fracture segments intersecting each other, and 3) on the tips of major normal faults. The hot springs are located where one or more of these favorable settings occur. The remote sensing analysis shows that Quaternary normal faults are the primary controlling structures of thermal waters at shallow depths in the Karonga (NW-SE to N-S strike) and Nkhata (N-S strike) regions. The aeromagnetic data revealed that permeable NW-striking foliation planes of the Precambrian Mugesse Shear Zone and WNW to ENE-striking foliation planes of the Mwembeshi Shear Zone are important structural controls of geothermal fluids at greater depths in the Karonga and Nkhata region, respectively. The Mwankeja-Mwesia and Chiweta geothermal systems in the Karonga region show favorable structural settings for geothermal fluids related to NNW and NW-striking normal faults segments that coalesce to form hard and soft-linked relay ramps and NW, and NNE -striking faults intersecting each other and NW-striking foliation planes. The Mtondolo geothermal area in the Nkhata region shows that the intersection of N-striking normal faults and ENE-striking foliation planes is the favorable structural setting that controls the emergence of hot springs through the surface. We conclude that high fracture intensity and connectivity are related to the location of the hot springs at the surface and can be used to determine permeable zones and hidden geothermal fluids together with other methodologies. The aeromagnetic data analysis suggested that buried faults and inherited structures (e.g., foliation planes) are controlling the geothermal fluids at depth. Additionally, our aeromagnetic analysis of magnetic basement depth estimation suggests that some of the geothermal reservoirs in the northern part of the MRZ could have an estimated depth of ~ 500 to 1000 m.b.g.l. Finally, the low-cost methodology applied in this study can reduce the risk of drilling non-productive wells in the early exploration stages and become an exploration strategy for similar geothermal systems in countries of the Western Branch of the East African Rift System.

Evolution of a single incised valley related to inherited geology, sea level rise and climate changes during the Holocene (Tirso river, Sardinia, western Mediterranean Sea)

We performed a morpho-stratigraphic study of the Tirso River incised valley (Sardinia Island, western Mediterranean Sea), an erosional feature crossing the Sinis fault, a major normal fault bordering the Campidano basin between the Gulf of Oristano and the western Sardinia shelf. High-resolution seismic reflection profiles and multibeam echosounder data, integrated by age-constrained stratigraphic logs derived from 9 sediment cores enabled us to reconstruct the valley evolution during the Holocene. We found that the Tirso valley is the result of a single event of incision and infill during the last eustatic cycle, strongly controlled by the presence of the Sinis fault. In fact, this structure represents a geological threshold that marks an abrupt change in substrate lithology and seabed slope, which controlled the valley morphology, narrow when downcutting early Pliocene formations along the steeper open shelf, and wider inside the Gulf, in the Pleistocene alluvial deposits of the flatter Gulf of Oristano. The sedimentary record starts with alluvial sediments filling the valley along the shelf during the initial phase of sea level rise, i.e., over 10 ka. During the last ~9.0 ka, a bay head delta developed, with the formation of barriers at the gulf entrance. In the mid-late Holocene, the progressive sea-level rise led to rapid drowning of the barrier system, recorded by marine and estuarine sediments filling the valley. Analysis of ecological associations in the cores, collected along a valley-normal transect, allowed for a detailed reconstruction of the paleo-environmental conditions during the latest phase of the incised valley filling controlled by global climatic variations in the Mediterranean region between ~9.0 and ~ 4.5 ka. Together with eustasy, our work reveals that the evolution and sedimentary infill of the Tirso incised valley was strongly controlled by inherited geological constraints, which influenced the morphology of the valley and the stratigraphic pattern.

The Memory of a Fault Gouge: An Example from the Simplon Fault Zone (Central Alps)

Faut gouge forms at the core of the fault as the result of a slip in the upper brittle crust. Therefore, the deformation mechanisms and conditions under which the fault gouge was formed can document the stages of fault movement in the crust. We carried out a microstructural analysis on a fault gouge from a hanging-wall branch fault of the Simplon Fault Zone, a major low-angle normal fault in the European Alps. We use thin-section analysis, together with backscattered electron imaging and X-ray diffractometry (XRD), to show that a multistage history from ductile to brittle deformation within the fault gouge. We argue that this multistage deformation history is the result of continuous exhumation history from high to low temperature, along the Simplon Fault Zone. Because of the predominance of pressure solution and veining, we associated a large part of the deformation in the fault gouge with viscous-frictional behaviour that occurred at the brittle-ductile transition. Phyllosilicates and graphite likely caused fault lubrication that we suggested played a role in localizing slip along this major low-angle normal fault.

Long-term morpho-structural development of major normal fault zones, Gran Sasso area, Central Apennines (Italy)

Although the Central Apennines of Italy are frequently struck by earthquakes related to normal faulting, so far, no detailed regional study into long-term development and mode of normal faulting exists. In this study, we focus on the Assergi fault zone (AFZ) and the Campo Imperatore fault zone (CIFZ) in the Gran Sasso area.Both fault zones are S-dipping and ~ 20 km in length. Range front faults with ~1 km of footwall topography are associated with basins in the hanging wall. To create footwall relief, long-term throw rates of >0.6 mm/a are required. We compared geologic throw with morphologic markers, such as range and triangular facet height, drainage basin size, spacing between drainage basin outlets, and surface offsets. The morphologic analysis allows to distinguish fault sectors that share morphological characteristics but typically include several mapped fault segments. The history of faulting of the CIFZ indicates that normal fault segments first acquire offset at a constant length until reaching a threshold, and then link with neighbouring fault segments into longer fault zones.Our study of the fault zones suggests the following general concept for long-term development of range-front fault morphology and associated slopes: (1) Surface offset by active faulting is followed, or accompanied, by incision of numerous small alluvial-fan catchments with narrow outlet spacing into the fault footwall. (2) Upon further fault growth, large triangular facets separated by gullies develop. By headward and lateral erosion the small fan catchments merge into fewer and larger catchments with larger outlet spacing. (3) During fault dormance, fault scarps degrade to lower-dipping slopes typically veneered by scree. On these slopes, scarps excavated by differential erosion expose Riedel shears of the main faults.

Eocene to Quaternary Deformation of the Southern Puna Plateau: Thermochronology, Geochronology, and Structural Geology of an Andean Hinterland Basin (NW Argentina)

AbstractCordilleran orogens are complex tectonic systems, strongly influenced by lithospheric deformation and exhumation. However, in the type region of the Central Andes, the timing and mechanisms of deformation and mountain building remain poorly constrained, especially for the high‐elevation Puna Plateau of NW Argentina. We use geologic mapping, structural cross‐sections, low‐temperature thermochronology, and U‐Pb dating of ash and detrital zircons to elucidate the history and style of orogenic deformation in the Antofalla region of the Puna Plateau. Cooling ages of major reverse fault hanging walls and dated growth strata indicate that crustal shortening activated major reverse faults during the Eocene and continued out‐of‐sequence through the Early Miocene. This protracted period of shortening, which resulted in the filling of the Antofalla Basin, was followed by a transient episode of upper‐crustal E‐W extension on a major normal fault system during the Late Miocene. Pliocene to Quaternary time saw E‐W shortening return to the region. Our structural cross‐section permits only a low magnitude of shortening (∼15–25%), which stands in contrast to the 60‐km thick crust. Triassic to Cretaceous cooling ages from footwall rocks and minor structures indicate that the entire region experienced little exhumation or sedimentation during the Mesozoic, perhaps retaining a crust thickened during multiple episodes of Paleozoic orogenesis. The crust may also reflect lower‐crustal thickening due to the growth of a lithospheric drip during the Late Miocene.

Assessing active and capable faulting as best practice for post-earthquake reconstruction activities: the Sant’Eutizio Abbey case study, in the epicentral area of the 2016 central Italy seismic sequence

Surface faulting is, together with strong ground shaking, a hazard associated with major earthquake faults. Assessing surface faulting potential of a given active tectonic structure is a fundamental prerequisite to adequately plan the use of territories and to perform new constructions, in order to act practices aimed to mitigate the associated risk. Assessing the surface faulting potential represents also ground for correctly performing re-construction and retrofitting of buildings and infrastructures during post-earthquake activities. We investigated a branch of a major seismogenic normal fault in the central Apennines of Italy, the Campi-Preci fault, along which the monumental Sant’Eutizio Abbey is located. The medieval Abbey is one of the most important cultural/religious edifices of the central Apennines, heavily damaged by the MW 6.5 October 30, 2016, earthquake, focused a few km to the south. Our study, based on field geological, geomorphological and structural survey and trenching investigations revealed that I) the trace of the Campi-Preci active fault branch is not actually located where presently reported in the available literature, II) the supposed morpho-tectonic features (basically, some km-long scarp carved on the Meso-Cenozoic carbonate bedrock), that suggested the presence of the fault segment in the area of the Sant’Eutizio Abbey, are not related to the active fault but are probably associated to a presently inactive reverse fault and III) the Sant’Eutizio Abbey is likely not potentially affected by primary surface faulting. Our work highlights that only a comprehensive multidisciplinary approach allows to correctly assess surface faulting potential in both seismotectonic and engineering perspectives.

Thermal Spring System Plumbing across a Major Normal Fault: Pah Tempe, Utah, USA

Abstract Faults, including their cores and associated fracture zones, can play an important role in controlling the flow of ground water. Pah Tempe Hot Springs at Timpoweap Canyon in southwestern Utah provides excellent exposures of the damage zone of a major normal fault and associated groundwater discharge. This includes numerous point discharges of ~40°C Na-Cl water into the Virgin River along a ~500 m stretch of the footwall damage zone of the Hurricane fault, part of the greater Wasatch-Hurricane fault system. An impermeable fault core and fine-grained rocks in the hanging wall impede cross-fault flow and force thermal waters to rise to the surface. The spectacular canyon-wall exposures permit an investigation of a fractured bedrock vadose zone as an analogue to subjacent aquifers. Photogrammetry (visual and infrared) enabled the mapping of discrete inflow locations, as well as permeable discontinuities in the canyon walls. Geochemistry, including reaction-path models, defined plausible chemical evolution pathways of thermal waters, whereas high-resolution geophysical surveys provided information on subsurface disruption and the depth to bedrock beneath Virgin River channel alluvium. The synthesis of these approaches indicates that permeable pathways in fractured carbonate rocks exhibit heterogeneous patterns. Some rock volumes contain abundant permeable fractures, bedding-plane partings, and solution cavities. Yet, within such volumes, one fracture may be permeable, whereas another only a few cm away may not. On a large scale (tens of meters), entire volumes of bedrock may be impermeable. Geophysical studies can shed light on gross fracture patterns in the bedrock aquifer but are incapable of showing which features accommodate flow. Patterns of permeability in the thick vadose zone may be the best predictor of subjacent flow paths in the aquifer. Thermal waters represent the leakage of deep brines or the mixing of Virgin River underflow with deep brines. Carbonate rocks exhibit self-organization where stochastic distribution of fracture apertures channel flow into wider features that channel additional flow. In turn, these features self-widen through solution weathering. By contrast, siliciclastic and basement rocks are not nearly as prone to dissolution. Thus, fluxes are partitioned into north-south fractures that will tend to have aperture in this east-west extending region. The results of this study help improve the conceptualization of flow systems near fault zones, and the methods employed may be a template applicable to the study of other systems.