Uplifted Block Research Articles

Shaping the Rwenzoris: balancing uplift, erosion, and glaciation

The Rwenzori Mountains in Africa represent an extremely uplifted basement fault block at the eastern edge of the western branch of the East African Rift system, a large-scale rift system controlled by extensional stresses. The rugged alpine topography reaches an altitude of up to 5109 m, and the highest parts are ice-covered. Glacial landforms and moraines proof repeated more extensive glaciations during the last glacial cycles. In order to elucidate magnitudes and the varying role of erosional processes in shaping the relief of the Rwenzori Mountains over the past 2 mill. years, we performed numerical simulations with the landscape evolution programme ULTIMA THULE. It is controlled by a climate driver with temperature as a master variable as well as changing precipitation and evapotranspiration over time. The morphological processes considered are fluvial erosion, hillslope diffusion, and glacial abrasion, and the latter controlled by the simulated glaciation of the landscape. We provide three sets of model runs: the first one starting from the present-day topography and running for approx. 800 ka, the second one extending the modelling period to 2 Ma, and the third one starting from a peneplain and evolving for 2 Ma. Our results provide constraints on the temperature history of the Rwenzori Mountains, the interplay of morphological degradation and tectonic uplift, and a time frame for the formation of the mountain chain from a peneplain to the present relief. The modelled landscape evolves from a peneplain 2 Ma ago to a Rwenzori-type mountain range, when the fairly strong average rock uplift of 1–2 mm year−1 is compensated by a strong fluvial erosion component. The rock uplift rate is needed to obtain elevations above the equilibrium line altitude around 500 ka BP and results in surface uplift over time. Around that time, a periodic ice cap appears in the models, and glacial abrasion then limits the height of the Rwenzori Mountains to its present elevation.

Ore-forming timing of polymetallic-fluorite low temperature veins from Central Pyrenees: A Pb, Nd and Sr isotope perspective

Vein fluorite deposits (Tebarray, Lanuza, Bielsa-Parzán, Bizielle and Yenefrito) as well as one MVT-style fluorite mineralization (Portalet) in the Central Pyrenees are the focus in this contribution. These deposits are made up of fluorite, barite, base metal sulphides, calcite, and quartz and are hosted in sedimentary rocks and granites of Palaeozoic age. Generally, these mineral occurrences, typically associated with Late Palaeozoic steeply dipping faults are similar with respect to geologic setting, mineralogy and geochemical trends to other fluorite and base metal veins located in the Central Pyrenees. Veins occurring along such faults most likely represent channelways used by mineralizing solutions that were expelled from the basement. Previous work argued for genetic processes involving circulation of mineralising fluids during the Triassic–Lower Cretaceous period, which is often considered to represent a period of heat, fluid, and mass transfers related to rifting events in the western European basins, which is related to the opening of the Atlantic.A major goal of this study was to decipher the timing of fluid flow and ore formation on the basis of Nd–Sm dating of fluorite sampled from a number of deposits sharing a similar geological framework. No precise age(s) could be obtained due to a scatter in data, but results from the Portalet MVT-style deposit point to a mid-Triassic age (around 220Ma) for this mineralization. The model that best explains the diagenetic stratabound mineralization at Portalet is gravity-driven fluid flow involving basinal brines during a rifting stage. Indeed, the formation of horst and graben structures during Early Alpine extensional tectonics favoured the infiltration of meteoric water into uplifted blocks, followed by fluid migration through the deeper parts of the basins whereby heat and dissolved components were acquired. This model also explains diagenetic changes recorded in the host limestone at Portalet. Also, overall Pb, Sr and Nd isotopic ratios measured in galena and fluorite suggest that differences in host rock and in the lithology of the basement seem to have exerted control on the chemistry of mineralizing fluids providing each deposit with distinctive characteristics.

The lithospheric density structure below the western Himalayan syntaxis: tectonic implications

Abstract The differences in the tectonic regimes of the western part of the India–Asia collision zone (western Himalayan syntaxis), consisting of the Ladakh–Karakoram, Hindu Kush and Pamir terrains, are examined through the analyses and modelling of gravity anomalies, computed from global gravity model and terrestrial data. Long-wavelength (>450 km) anomalies are related to isostatic compensation and correspond to topographic variations. The short-wavelength gravity highs are primarily due to upthrust blocks and are mostly linked to crustal seismicity. The intermediate depth-focused earthquake region of Hindu Kush–Pamir is characterized by a gravity low probably caused by crustal underthrusting. The deep density structure of the crust and upper mantle constructed from the modelling of gravity anomalies with constraints from seismological information suggests crustal underthrusting of the Indian and the Asian plates in the Hindu Kush–Pamir section unlike that in the Ladakh–Karakoram region, where underthrust Indian lithosphere underplates beneath the Asian plate. The density model supports a hypothesis of slab break-off of Indian and Eurasian plates in the Ladakh–Karakoram segment, and of the Indian plate in the Hindu Kush–Pamir region, whereas the Eurasian plate drastically underthrusts deeper ( c. 200 km), causing deep seismicity in the Hindu Kush–Pamir section.

Aptian-Albian transition in central Tunisia: Tectonosedimentary and paleogeographic records

The Aptian-Albian series are characterized, in Tunisia, by several variations in subsidence rates and records discrete unconformity between Albian and Aptian in central and southern Tunisia. In central Tunisia Upper Aptian regressions were sustained until the beginning of Albian with a low sea level stand. In this paper, wells data gathered from central Tunisia exposes well expressed Aptian-Albian hiatus associated to several partial sedimentary gaps, erosion surface and unconformities and correlated with the “Aptian Crisis” highlighted at the Tethyan scale. This latter is coeval with an extensional regime in concurrence with the African rifting responsible for halokinesis movements and magmatism and leaded to the establishment of paleoreliefs as much as tilted blocks and half grabens in central Tunisia. Deposition shows regional hiatus and lateral transition from platform facies to south to, open marine facies to the north. The structural configuration of this domain within normal faulting, horsts and grabens leaded to the establishment of several distinct basins with different subsidence rates. Discordances occur specially around and upward uplifted blocks and emerged land and are especially associated with Aptian reef, karsts and shallow marine facies. They are unconformably overlaid by the transgressive open marine facies of Albian-Cenomanian series acknowledged as source rock in Central Tunisia. In addition, the reefal Aptian facies is considered a potential reservoir rocks in central Tunisia and it is frequently associated to mineralization and Hydrocarbure reservoirs.

Thermochronologic constraints on Late Cretaceous to Cenozoic exhumation of the Bendeleben Mountains, Seward Peninsula, Alaska

In the Bendeleben Mountains, Seward Peninsula, mid-Cretaceous granites are exposed in an uplifted block bounded on its south side by an E-W striking normal fault. The Bendeleben fault has well-preserved scarps 4–7 m in height that offset Holocene moraines. Seismic activity, young normal faulting, and Quaternary basaltic volcanism are all evidence of active extension. South of the Bendeleben fault, there is a 3–4 km deep basin. Fifteen apatite (U-Th)/He ages from granitic samples of the footwall yield an Eocene weighted mean age of 41.3±4.8 Ma. Biotite 40Ar/39Ar ages from the country rock of the Bendeleben pluton are 81–83 Ma. In spite of the young fault scarps, HeFTy and Pecube thermal modeling results illustrate that rapid exhumation of the Bendeleben Mountains occurred in the Late Cretaceous-Eocene and slowed since the Oligocene. A weak age-elevation relationship of apatite He ages and a lack of correlation between age and distance from the fault indicate that exhumation was accomplished with minimal block rotation on a steeply dipping, long-lived normal fault. Timing of extension in the Seward Peninsula can be correlated with deformation in the offshore Hope Basin where seismic reflection lines document Early Tertiary large-magnitude normal faulting followed by minor post-Miocene reactivation. The faulting observed in the Bendeleben Mountains is part of an extensional system that spans a large portion of the Bering Strait region. The tectonic model proposed in previous studies suggests that clockwise rotation of the Bering block relative to North America is the cause of extensional deformation in western Alaska.

Kachchh Mesozoic Domes, Western India: Study of Morphotectono Character and Evolution

Kachchh domes are recognized by the oval to elliptical- shaped outcrop patterns marked by outlines of bedding surfaces which invariably dip in the outward directions. The occurrence of domes in rows without having corresponding basin-like features implies that these are not superposed folds resulting due to constriction- type tectonic forces. Further, in spite of the close time-space relationship, the occurrence of domes is exclusively in the Mesozoic rocks on the uplifted block of the fault. The absence of any such rock formation on the other side of the fault rules out the possibility that these are 'drape folds' developed during the adjustment of the sedimentary blanket over the faulted-up edges of the basement blocks. Hinging on the evidence of intrusive plutonic (mafic) masses in the core of some of the domes, we suggest that the structures evolved through diapiric rise of magma bodies causing dome-shaped up-warping (bending) of the pre-existing (Mesozoic) flat-lying sedimentary formations. Linear disposition of domes is explained as due to channellization of magma along the fractures that developed around large-scale crustal doming during the early phase of the Reunion Plume impingement under the Indian Lithosphere.

Tectonics from fluvial topography using formal linear inversion: Theory and applications to the Inyo Mountains, California

AbstractTectonic activity generates topography, and the variability of tectonic forcing is responsible for topographic patterns and variability of relief in fluvial landscapes. Despite this basic relation, the inverse problem, by which features of the topography are used for inferring tectonic uplift rates, has proven challenging. Here we develop formal linear inversion schemes to infer a record of the rate of relative uplift as a function of space and time from the long profiles of rivers. The relative uplift rate is the difference between the rates of rock uplift and of the base level change. The inversion schemes are based on a closed‐form analytic solution to the transient linear stream power model, and to increase model resolution they make use of the multiplicity of information made available by multiple rivers and their tributaries. The distribution of the fluvial response time to tectonic perturbations is a key component of the inversion scheme, as this determines which tectonic events are preserved in the topography. We develop two inversion parameterizations that differ in their assumptions about the tectonic forcing: space‐invariant and time‐space variability with an assumed spatial distribution. The inversion schemes are applied to the Inyo Mountains, an uplifted block along the western boundary of the Basin and Range Province in California. Inversion results indicate that the range has been experiencing an acceleration of the relative uplift in the past ∼2‐3 Ma. We use the inversion results to constrain the paleotopography and paleo‐erosion rate along the range and to recover the throw rate history along the fault that bounds the Inyo range.

Juruá Orogeny: Brazil and Andean Countries

Seismic data from Petrobras (Brazilian state-owned petroleum company) show wide deformation and many reverse faults throughout the Solimões and Acre basins of northern Brazil. This deformation was observed for the first time in the Juruá River in 1976 in the Solimões Basin, and it increases toward Acre and Subandean basins from Argentine and Chile to Colombia and Venezuela. Structural inversions, block uplifts, and asymmetrical folds are attributed to compression and shearing stresses along this wide area. The severe diastrophism seen in the Juruá River area is of Late Jurassic (Kimmeridgian) age. It probably coincided with the final separation between Laurasia and Gondwana continents and the initial opening of the Central Atlantic Ocean. In Peru and neighboring countries, the same Late Jurassic orogeny is also present. It occurs in the form of widespread regional uplifts, structural inversions, tilting, faults and asymmetrical folds beneath a pronounced regional parallel or angular unconformity of latest Jurassic age, marking a first-order sequence boundary above the Upper Jurassic Sarayaquillo Formation and equivalent formations, as well as above older formations. The depositional hiatus at the upper part of the Jurassic System is attributed to reorganization of stress fields that resulted in basin inversions, followed by widespread peneplanation. The uppermost Jurassic or lower Cretaceous beds, deposited above this regional unconformity, were not affected by this tectonism in Brazilian and Subandean basins. The stratigraphy of Peruvian Subandean sedimentary basins is similar to that of the Acre Basin.

Model of formation of gold-hematite-barite mineralization of the Prognoznoe deposit, Northeastern Russia

The results of geological-structural, petrographical, and mineralogical-geochemical studies allowed the authors to offer an original model of formation of the Prognoznoe deposit. The relict shallow basin accumulated clasts of the Archean Au-bearing iron formations and metamafic rocks from the uplifted block of the Khivach inlier of the Precambrian basement of the Omolon craton terrane. As a result, the rounded and semirounded material composes a gentle volcanic sequence of the Middle-Upper Devonian Kedon tuff sandstones and gravelites (red-colored member of ferrous volcanosedimentary rocks) with dispersed clastic and supergene gold. Secondary enrichment and redistribution of gold and iron in zones of the quartz-hematite-barite-carbonate veinlets are related to the intrusion of thick Cretaceous dikes and sills. The areas without dikes and sills are free of economic gold-bearing bodies. An extremely low amount of sulfides is typical of this deposit and of the epithermal deposits of the Kedon volcanic belt in the southern part of the Omolon craton terrane.

U-Pb dating and provenance of detrital zircons from the Upper Precambrian deposits of North Timan

Timan comprises the southwest edge of the Pechora Plate. The plate basement is composed of variably metamorphosed sedimentary, mainly terrigenous, and igneous rocks of the Late Precambrian age that are generally overlain by Ordovician-Cenozoic platform cover. Poor exposition and discontinuous dis� tribution of the Upper Precambrian outcrops of dominantly fossilfree sedimentary rocks cause considerable disagreements in stratigraphic correlation. This applies equally to North Timan, which represents an uplifted block of basement, in which sedimentary-metamorphic rocks form the Barminskaya Group (~5000 m thick), previously dated as Early Riphean to Vendian. Earlier Rb-Sr and Sm-Nd isotope dating of schist and crosscutting gabbro-dolerite and dolerite establish ed the timing of greenschist facies metamorphism at 700 Ma. Thus, Late Riphean age of the Barminskaya Group has been suggested. Results of local U-Pb dating of detrital zircon from silty sandstones of the Malochernoretskaya Formation, which constitutes the middle part of the outcropping section of the Barminskaya Group, confirm this conclusion. Age data for 95 zircon grains cover the range of 1035-2883 Ma with age peaks at 1150, 1350, 1550, 1780, and 1885 Ma. The mini� mum age of zircons, considered as the lower age constraint on sediment deposition, provides grounds to date the Barminskaya Group as Late Riphean and indicates eroded rock complexes of the Fennoscandian Shield as the possible provenance areas.

Quaternary tectonics along oblique deformation zones in the Central Andean retro-wedge between 31°30′S and 35°S

Abstract The distribution of the Quaternary deformation in the outer retro-wedge of the Andes (31°30′–35°S) is controlled by the subduction geometry, the position of the structural front, and the location of oblique pre-Cenozoic mechanical anisotropies. In the Southern Precordillera, Quaternary structures tend to group along the Barreal–Las Peñas deformation zone with a NW to NNW trend. This Cenozoic belt (31°30′–32°40′S) developed on the northern segment of the Triassic Cuyo Basin and broadened laterally during the Quaternary. New radiocarbon ages on dam deposits confirm Holocene tectonic activity on the northwestern edge. The oldest ages of dam deposits are 5810±90 a BP (Cabeceras Creek) and 810±50 a BP (Dolores Creek). Palaeoseismological and seismic data suggest active tectonic growth on the NW and SE extremes. Quaternary tectonics has contributed to modifying the relief along this oblique belt. This contribution is evident from the tectonic uplift of blocks (minimum 90–120 m in the Barreal block), the initial development of intermontane basins (Pampa de los Burros Basin), the Quaternary rejuvenation of tectonic depressions (at least 32–37 m of tectonic subsidence in the Vizcacheras half-graben) and the incipient development of low-relief morphotectonic units by soft-linkage of Quaternary structures.

Mechanics and response of a surface rock block subjected to pressure fluctuations: A plucking model and its application

Plucking (removal of rock blocks) is often the dominant mechanism for producing a scour hole on riverbeds comprised of heavily jointed rock masses being subjected to pressure fluctuations from a jet flow. This paper explores the mechanics and response of a surface block subjected to pressure fluctuations. First a particle-flow simulation was conducted to demonstrate how repeated pressure fluctuations are able to gradually remove rock bridges in discontinuities surrounding a rock block, if a pressure fluctuation's intensity is substantial. As a consequence, these weak planes may become fully persistent. The block's uplift speed then depends on the pressure differences on the opposite (horizontal) faces, and the frictional resistance of the lateral discontinuities. This paper proposes a theoretical framework to model the mechanics and response of a rock block subjected to a sinusoidal pressure fluctuation. This model can be applied to estimate the development of a scour hole, through plucking, during a specific flood event. An example demonstrates the applicability of the proposed approach in predicting the potential depth of the scour hole.

Paleoproterozoic metamorphic and deformation history of the Thompson Nickel Belt, Superior Boundary Zone, Canada, from in situ U–Pb analysis of monazite

The Thompson Nickel Belt is a 35km×200km northeast-trending segment of the northwest margin of the Archean Superior craton in Manitoba, bounded to the west by the Paleoproterozoic Reindeer Zone. The belt was metamorphosed and deformed during the Trans-Hudson orogeny, resulting in an elongate, nested metamorphic zonal pattern, parallel to the overall trend of the belt. The metamorphic pattern consists of an inner middle amphibolite-facies zone, flanked on both sides by upper amphibolite-facies zones, flanked in turn by granulite-facies zones. U–Pb data were collected by laser ablation–multiple collector–inductively coupled plasma–mass spectrometry from metamorphic monazite in polished thin sections from each of the metamorphic zones. In the middle amphibolite-facies and upper amphibolite-facies zones, monazite in metapelite is interpreted to have developed during the dominant, foliation-forming deformation event and yields ages of ca. 1776 and 1779Ma, respectively. Metapelite from the eastern granulite-facies zone contains ca. 1834Ma monazite grains intergrown with sillimanite that is pseudomorphous after andalusite and grew during the dominant foliation-forming event. Metawacke from the eastern granulite-facies zone contains ca. 1826Ma and 1792Ma monazite grains that are interpreted to be contemporaneous with the main foliation-forming event and were unstable in the presence of melt, and a third population of ca. 1748Ma monazite grains. In the western granulite-facies zone, metapelite contains ca. 1803Ma monazite grains intergrown with sillimanite that grew during the dominant foliation-forming event, and meta-iron formation contains ca. 1752Ma monazite grains characterized by oscillatory zoning and associated with melt pseudomorphs. A tectonic model is presented in which the initiation of collision between the Reindeer Zone and the Superior craton begins around ca. 1850–1840Ma. With continued convergence, rocks now present in the western and eastern granulite-facies zones were progressively buried and heated to middle amphibolite-facies conditions by ca. 1803 and 1792Ma, respectively. As the collision progressed, rocks now present in the upper amphibolite-facies and middle amphibolite-facies zones were buried and subjected to lower amphibolite-facies metamorphic conditions by ca. 1779 and 1776Ma, respectively. Progressive regional metamorphism may have been punctuated by local thermal events related to magmatism, such as at ca. 1830Ma in the eastern granulite-facies zone. Post-peak metamorphic cooling and differential uplift of crustal blocks related to transpressional deformation is recorded by ca. 1752 and 1748Ma monazite grains from the western and eastern granulite-facies zones, respectively.

“Arc‐continent collision” of the Aleutian‐Komandorsky arc into Kamchatka: Insight into Quaternary tectonic segmentation through Pleistocene marine terraces and morphometric analysis of fluvial drainage

At the NW corner of the Pacific region, just south of the Kamchatsky Peninsula, the northern tip of the Pacific plate subduction and associated volcanic arc interacts with the western end of the Aleutian‐Komandorsky dextral transform plate boundary and associated arc. Study of both Holocene and Pleistocene sequences of uplifted marine terraces and also of fluvial drainage patterns on the Kamchatsky Peninsula allows us to highlight active tectonics produced by complex plate interaction. Our results show that the central eastern coast of the peninsula is currently divided into four different zones consisting in uplifted blocks associated with various uplift rates in front of a fold‐and‐thrust zone to the west. Our main tectonic benchmark—the altitude of the shoreline correlated to the Last Interglacial Maximum (Marine Isotopic Stage 5e)—yields late Pleistocene uplift rates ranging from 0.2 to 2.74 mm/yr. One of the main active faults bounding the coastal blocks is dextral and is interpreted as a prolongation of an offshore fault of the Aleutian‐Komandorsky dextral transform plate boundary. We suggest that structures on the Kamchatsky Peninsula accommodate a part of the transform motion, but that mainly, the arc‐continent collision of the Aleutian arc against Kamchatka produces a “bulldozer” effect on the Kamchatsky Peninsula.

Research on the Mechanism of Tectonic Stress Field Evolution for Coal and Gas Outburst

The geological characteristics of Pingdingshan mining area in China is fault block uplift and the depression in all around. The faults in the region are dominated by NW trending faults. Our researches show that most of faults experience different mechanics process and movement character because of the evolution of tectonic stress field. The regional distribution of coal and gas outburst is consequent on the regional distribution of regional tectonic stress, tectonic coal and gas storage. At the action of SW-NE extrusion stress by the effect of Qinling orogeny belt, the NWW strike structure and the NNE composite tectonics are prone to occur coal and gas outburst because of concentrated stress, low penetrability and good gas storage condition. Also incorporated with in situ stress measurements and the numerical simulation of regional tectonic stress field by using self-developed Rock Mass Stress-state Analysis System software, the mechanism of the tectonic stress field on coal and gas outbursts is analyzed. The results indicate that the distribution of rock stress depends upon the faults and is formed again because of superposition of the action of faults. Tectonic stress field and its uneven distribution provide dynamic conditions for the occurrence of coal and gas outbursts.

Miocene block uplift and basin formation in the Patagonian foreland: The Gastre Basin, Argentina

Fil: Bilmes, Andres. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Centro de Investigaciones Geologicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geologicas; Argentina

Dynamic response of mortar-free interlocking structures

Economical earthquake-resistant housing is desirable in seismically active rural areas of developing countries. These regions often suffer a significant loss of life during strong ground motion because of lack of seismic-resistant housing. To enable an efficient and cost-effective solution, a new concept of construction was investigated utilising structures consisting of (i) interlocking blocks with relative movability at the block interface and (ii) coconut-fibre rope reinforcement. The interlocking blocks were prepared with coconut fibre reinforced concrete. A mortar-free construction can facilitate more energy dissipation during a seismic event, because of the relative movement at the block interfaces. Four structures were considered: two columns (with and without ropes) and two walls (with and without ropes). This paper reports on the investigation of the in-plane behaviour of the mortar-free structures under different loadings: push over, snap back, impact, harmonic and earthquake loadings. The influence of un-grouted rope reinforcement on the fundamental frequency, damping ratio, induced accelerations, top relative displacement, base shear, overturning moment and block uplift is investigated. The harmonic tests were more accurate in finding the fundamental frequencies of the structures compared to the push over and snap back tests. As expected, the structures with ropes were stiffer than the structures without ropes. The structures with ropes had smaller relative top displacements and block uplifts compared to the structures without ropes. The damping of the structures without ropes was higher than that of the structures with ropes. The damping increased with the amplitude of input motion. The same trend was observed for the maximum overturning moment, uplift and rope tension.

Kariba Dam Plunge Pool Scour: quasi-3D Numerical Predictions

Kariba Dam is a 128 m high double-curvature arch dam located on the Zambezi River at the border between Zambia and Zimbabwe. The dam is known worldwide for its impressive plunge pool scour hole of almost 90 m below the tailwater level, which is an unprecedented value in dam history.This paper presents a numerical study performed to assess both past scour and future scour potential at the dam site. Based on gate opening durations and bathymetric surveys since 1960, quasi-3D time-dependent numerical scour computations have been made to assess the scour potential of the gneiss rock for the next 50 years.The computations use the Comprehensive Scour Model (CSM) [Bollaert, 2004]. This model is physics based and allows predicting rock scour evolution with time for the main rock break-up mechanisms (rock mass fracturing, rock block uplift and peeling off of rock blocks along the wall). The model has been calibrated based on past flooding and the corresponding scour observed. Dynamic pressures at the pool bottom have been analytically determined and compared with values measured on a 1/65 scaled model of the dam and its plunge pool constructed at the Laboratory of Hydraulic Constructions of the Swiss Federal Institute of Technology (LCH-EPFL). The numerical model automatically adapts these values during subsequent scour formation by applying a 2D turbulent jet diffusion model, downstream of each of the gates separately. Main assumptions and outcomes of the numerical scour study are presented. It specifically focuses on the potential for future scour as well as on the potential beneficial influence of several proposed countermeasures against further scour formation.

Morphology and magnetic survey of the Rivera-Cocos plate boundary of Colima, Mexico

The propagation of the Pacific-Cocos Segment of the East Pacific Rise (EPR-PCS) has significantly altered the plate configuration at the north end of the Middle America Trench. This ridge propagation, the collision of the EPR-PCS with the Middle America Trench, the separation of the Rivera and Cocos plates and the formation of the Rivera Transform have produced a complex arrangement of morphotectonic elements in the area of Rivera-Cocos plate boundary, atypical of an oceanic transform boundary. Existing marine magnetic and bathymetric data has proved inadequate to unravel this complexity, thus, a dense grid of total field magnetic data were collected during campaigns MARTIC-04 and MARTIC-05 of the B/O EL PUMA in 2004 and 2006. These data have greatly clarified the magnetic lineation pattern adjacent to the Middle America trench, and have revealed an interesting en echelon, NE-SW oriented magnetic high offshore of the Manzanillo Graben. We interpret these new data to indicate that the EPR-PCS ridge segment reached the latitude (~18.3°N) of the present day Rivera Transform at about Chron 2A3 (~3.5Ma) and propagated further northward, intersecting the Middle America Trench at about 1.7 Ma (Chron 2). At 1.5 Ma spreading ceased along the EPR north of 18.3°N and the EPR-PCS has since retreated southward in association with a southward propagation of the Moctezuma Spreading Segment. North of 18.3°N the seafloor near the trench has been broken into small, uplifted blocks, perhaps due to the subduction of the young lithosphere generated by the EPR-PCS.

Influence of fault trend, bends, and convergence on shallow structure and geomorphology of the Hosgri strike-slip fault, offshore central California

We mapped an ∼94-km-long portion of the right-lateral Hosgri fault zone in offshore central California using a dense network of high-resolution seismic reflection profiles, marine magnetic data, and multibeam bathymetry. These data document the location, length, and continuity of multiple fault strands, highlight fault-zone heterogeneity, and demonstrate the importance of fault trend, fault bends, and fault convergence in the development of shallow structure and tectonic geomorphology along strike-slip faults. Eight sections (A through H) of the Hosgri fault are mapped. The fault trends ∼335° to 341° in the southern ∼40 km of the study area (sections A through C) where shallow deformation is primarily dilational. The absence of tectonic uplift in this area has contributed to localization of the Santa Maria River and delta and, as a result, Holocene sediments cover the fault zone. The Hosgri fault generally trends 329° to 337° in the central ∼24 km of the study area (sections D through F), which coincides with oblique convergence of the Hosgri and the more northwest-trending Los Osos and Shoreline faults. This convergence has resulted in local restraining and releasing fault bends, transpressive uplifts, and extensional basins of varying size and morphology. Notably, development of a paired fault bend is linked to indenting and bulging of the Hosgri fault by a strong crustal block translated to the northwest along the Shoreline fault. Two diverging Hosgri fault strands bounding a central uplifted block characterize the northern ∼30 km of the Hosgri fault (sections G and H) in this area. The eastern Hosgri passes through significant releasing (329° to 335°) and restraining (335° to 328°) bends before passing onland at San Simeon; the releasing bend is the primary control on development of an elongate, asymmetric, 15-km-long × 300- to 2400-m-wide, “Lazy Z” sedimentary basin. The western strand of the Hosgri fault passes through a significant restraining bend (329° to 316°) and continues northward until slip is transferred to faults underlying the Piedras Blancas fold belt. Earthquake hazard assessments should incorporate a minimum rupture length of 110 km based on continuity of the Hosgri fault zone through this area. Lateral slip rates may vary along the fault (both to the north and south) as different structures converge and diverge but are probably in the geodetically estimated range of 2–4 mm/yr.