Fissure Ridge Research Articles

Potential field and bathymetric investigation of the Monowai volcanic centre, Kermadec Arc: implications for caldera formation and volcanic evolution

The dynamics of caldera formation are affected by internal factors, such as magma chamber structure and properties and regional tectonic stresses. Understanding their interrelationship is important for assessing volcanic hazard and reconstructing the evolution of a volcano. The Monowai volcanic centre, located at the Tonga–Kermadec Arc, is a basaltic–andesitic submarine stratovolcano and caldera. Recent surveys have shown that it is active, but little is known about its structure and evolution. Here we present a combined analysis of swath bathymetry and potential field data from Monowai, acquired during cruise SO215 on the R/V Sonne, in 2011 April–June. The Monowai caldera is associated with a 20–25 mGal Bouguer gravity anomaly high and a broad positive magnetic anomaly. Short-wavelength magnetic anomalies of up to +1400 and −800 nT are observed along the caldera rim and on the summit of Monowai cone. Inversion of the Bouguer gravity anomaly data shows that the caldera high is caused by a buried dense body with density contrast of up to +450 kg m−3 with respect to the surrounding arc crust. The body has a volume of ∼250 km3 and is divided into a main unit extending from 3 to 6 km depth and a shallower ring structure that underlies the rim of Monowai caldera. We estimate the density of the body to be 2650–2850 kg m−3, suggesting a mafic composition. The ring structure is interpreted as set of ring dykes and the main unit as a solidified or partly solidified magma chamber system (the Monowai pluton). The observed magnetic anomalies are consistent with pluton intrusion within the Bruhnes magnetic polarity. Analysis of the swath bathymetry data shows that Monowai is located in a 20-km-wide graben, part of an en-echelon, left-stepping horst and graben system that spans the length of the arc and backarc. The Monowai caldera is elongated perpendicular to the direction of rifting, suggesting that it is affected by the regional stress regime. Geomorphological analysis suggests that the caldera was created by repeated collapse of a single caldera structure and that its complex shape and multiple ring faults can be attributed to prolonged activity and multiple cycles of inflation and collapse. We propose a multiphase evolution of the Monowai volcanic centre comprising initial magma emplacement and crustal inflation; magma chamber evacuation and caldera collapse; caldera resurgence and mass waisting; southward migration of the eruptive vent; and formation of Monowai cone. It remains unclear whether the current eruption at the Monowai cone is fed directly from a small underlying magma chamber, or laterally from the Monowai pluton. Observation of radial fissure ridges on its flanks suggests that a shallow magma body may have been recently emplaced beneath the summit causing inflation and the formation of extensional cracks. The summit appears unstable and may be prone to lateral or vertical collapse. Therefore, the region remains a potential source of regional volcanic and tsunami hazard.

Comparison of the Quaternary travertine sites in the Denizli extensional basin based on their depositional and geochemical data

In the Denizli Basin (Turkey), located in the western Anatolian extensional province, travertine and tufa deposition has been an ongoing process for at least 600,000years. Travertine bodies, which are 30 to 75m thick and each covers areas of 1 to 34km2, are up to 1km3 in volume.Today, spring waters in this area have temperatures of 19 to 57°C, are of the Ca–Mg–HCO3–SO4 type in the Pamukkale, Kelkaya and Pınarbaşı areas and the Ca–Mg–SO4–HCO3 type at Çukurbağ. Thermal waters along the northern margin of the basin are generally hotter than those in the east–southeast and south. The δ18O and δD values of the spring waters indicate a meteoric origin. The average temperatures of the hydrothermal systems in the Denizli Basin appear to have decreased from Pleistocene to Holocene.Travertine, which formed from the hotter water, is more widespread than the tufa that formed in the cooler spring waters. Deposition of the travertine, which formed largely on slopes, in depressions, and along fissure ridges (mostly on northern basin margins), was controlled by the interplay between various intrinsic and extrinsic parameters. The travertines are formed largely of calcite with only minor amounts of aragonite in some of the vertically banded, crystalline crust, raft and pisoid travertines found in some of the northern sites. The aragonitic samples, rich in Sr, are typically found around the spring orifices and along the central axis of the fissure ridges.The stable isotope values of the travertine found in the northwest and southeast parts of the basin are different. The δ13C values of the northern travertine deposits are more positive (3.7 to 11.7‰ VPBD) than those found in the south–southeast areas (−4 to 5.8‰ VPDB). In contrast, the travertine and tufa in the southeastern areas have higher δ18O values (−15.2 to −7.8‰ VPDB) than those of the northern areas (−16.6 to −4.8‰ VPDB). Available evidence indicates that spring activity and associated travertine precipitation in the Denizli Basin were controlled largely by tectonic activity rather than by climatic conditions.

The origin and growth of a recently-active fissure ridge travertine over a seismic fault, Tivoli, Italy

An enigmatic, c. 2km-long and 15m-high travertine ridge, the Colle Fiorito ridge, occurs in the northwestern sector of the Tivoli travertine plateau, central Italy. The main questions addressed in this paper concern the origin and mode of growth of this prominent ridge. The presence of active structures beneath the studied ridge is inferred by recent and past earthquakes located at shallow depths immediately beneath Colle Fiorito. To understand the surficial structure of the Colle Fiorito ridge and the travertine depositional environment, we constructed a 10m-resolution DEM, analyzed recent and past aerial photographs, and conducted field surveys and meso- to micro-scale sedimentological analyses. To understand the ridge subsurface structure, we studied a set of 32 stratigraphic well logs available from previous works and from the local decorative stone industry, and realized a 2D electrical resistivity tomography (ERT) across the ridge. Results show a gentle antiformal structure affected by subvertical zones of strata discontinuity. The Colle Fiorito structure is interpreted as a previously-unknown fissure ridge travertine grown at the edge of the Tivoli travertine plateau, perhaps when the volumetric deposition rate reached its climax in the plateau for the abundance of fluid discharge and the rise of the water table. Such a fluid pressure may have activated the faults and fractures beneath Colle Fiorito, thus opening new pathways for the ascension of geothermal fluids toward the surface.

Growth of fissure ridge travertines from geothermal springs of Denizli Basin, western Turkey

Fissure ridge travertines grown from geothermal springs of Denizli Basin, southwestern Turkey, are investigated through stratigraphic, structural, geochemical, and geochronological methods, with the aim of understanding the growth of these elongate mound-shaped structures. Two main types of travertine deposits are recognized: (1) bedded travertines, which grew as flowstone on sloping surfaces and form the bulk of fissure ridges, and (2) banded travertines, which grew as veins within the bedded travertine chiefly along its central feeding conduit. Stratigraphic and structural observations shed light on the bedded-banded travertine relationships, where the banded features grew through successive accretion phases, crosscutting the bedded travertine or forming sill-like structures. The bedded and banded travertines alternated their growth, as demonstrated by complicated crosscutting relationships and by the upward suture, in places, of banded travertine by bedded travertine that was, in turn, crosscut by younger banded travertine. The bedded travertine is often tilted away from the central axis of the fissure ridge, thus leaving more room for the central banded travertine to form. U-series ages confirm the bedded-banded travertine temporal relationships and show that the growth of the studied fissure ridges lasted up to several tens of thousands of years during Quaternary time. The banded travertine was deposited mainly during cold events, possibly in coincidence with seismic events that might have triggered the outflow of deep geothermal fluids. C and O stable isotope and rare earth element data indicate a shallow feeding circuit for the studied structures with a fluid component deriving from a deeper geothermal circuit. A crack-and-seal mechanism of fissure ridge growth is proposed, modulated by the interplay of local and regional influencing factors and mechanisms such as geothermal fluid discharge, paleoclimate, tectonics, and the progressive tilting of bedded travertine limbs over a soft substratum creating the necessary space for the central veins to grow.

Morphometric analysis of the submarine arc volcano Monowai (Tofua–Kermadec Arc) to decipher tectono-magmatic interactions

Morphometric analysis of multibeam bathymetry and backscatter data is applied to Monowai, a submarine volcano of the active Tofua–Kermadec Arc to map and document the structure and evolution of the volcanic centre. Low rates of erosion and sedimentation, and pervasive tectonic and magmatic processes, allow quantification through detailed structural analysis and measurement of deformation. The Slope, Aspect, Curvature, Rugosity, and Hydrology (flow) tools of ArcGIS provide a robust structural interpretation and the development of a model of Monowai evolution.A nested caldera structure with a volume of ~31km3 and a stratovolcano of ~18km3 dominate the magmatic constructs. The outer caldera is elongate along 125°, and the inner caldera along 135°. Numerous parasitic cones and fissure ridges are also observed, oriented at 039° and 041°, respectively. Northeast trending faults (with a regional average strike of 031°) are widespread within this part of the backarc, forming a nascent rift graben to the west of the Monowai caldera complex. The distribution of throw varies spatially, reaching a maximum total along-rift of 320m and across rift of 120m, with greater throw values measured in the west.Elongation directions of the two nested calderas are near-perpendicular to the trends of faults and fissure ridges. The inner caldera is more orthogonal to the magmatic constructs (fissure ridges and aligned vent cones) and the outer caldera is approximately orthogonal to the regional fault fabric, suggesting a strong interaction between magmatic and tectonic processes, and the directions of the horizontal principal stress. We present a detailed morphometric analysis of these relationships and the data are used to interpret the spatial and temporal evolution of the tectono-magmatic system at Monowai, and classify the type of rifting as transtensional. Similar analysis is possible elsewhere in the Kermadec backarc and within other regions of submarine volcanism.

Morphotectonics of fissure ridge travertines from geothermal areas of Mammoth Hot Springs (Wyoming) and Bridgeport (California)

Eleven Quaternary fissure ridge travertines from Mammoth Hot Springs (Wyoming) and seventeen ones from Bridgeport (California) were mapped and studied with a morphotectonic approach to understand possible relationships between travertines and active versus passive tectonics. Results are compared with other known geothermal fissure ridges on the Earth. The studied fissure ridges are all located in the hangingwall of normal faults, but the fissure ridges appear as non-dislocated by faults, rather by axial fissures. Both in the two principal study areas and elsewhere, azimuthal analyses of faults and fissure ridges show that the distribution of fissure ridge long axis is rather dispersed around the strike of the local normal faults. No correlation occurs between the fissure ridge length and the angle between the strike of the normal fault and the strike of fissure ridges. The studied fissure ridges are 2 to 360m long (mean length: 72.1m), 1 to 15m wide (mean width: 6.7m), and 0.5 to 8m high (mean height: 3.9m). Fissure ridge aspect ratios show a moderate correlation between the length and both the width and the height of fissure ridges, whereas the correlation between width and height is less marked. The growth in height and width of ridges appears as much more inhibited than in length. A model is proposed in which fissure ridge travertines grow with enhanced elongation along one sub-horizontal direction, which seems moderately controlled by the associated normal fault and the regional extension. Other factors, such as the inherited fracture network and the geothermal and artesian pressure of fluids (fluid discharge) may be important in the development of the studied fissure ridges. Results from this study may contribute to the knowledge of factors that control the long-term geothermal circulation and also the long-term hermetic durability of CO2 subsurface repositories.

U/Th dating of the travertine deposited at transfer zone between two normal faults and their neotectonic significance: Cambazli fissure ridge travertines (the Gediz Graben-Turkey)

The neotectonic characteristics of the travertines that outcrop near Cambazli Village to the west of the Gediz Graben in the Western Anatolia and the age determination of the travertine were carried out using the 230Th/234,238U disequilibrium method. The Cambazli fissure ridge travertines represent the travertine depositions that develop at a transfer zone. The extent of these travertines is at NW-SE and NE-SW orientations and the ridge crest-trend of these travertines range between approximately 55° and 82° and they are located at an intersecting position. The evaluation of the fissure ridge travertine directions indicated that the compression stress that was responsible for the deposition of the Cambazli travertine was determined to be in the N-S orientation and the extensional stress was determined to be in the E-W orientation. The orientation of the dominant extension in Western Anatolia during the neotectonic period was N-S and this orientation is not in accordance with the directions of stress for the travertines. This situation indicates that the travertines were deposited along a transfer zone in N10W orientation between two normal faults. The travertines were determined to be active since the Upper Pleistocene as indicated by the age determination conducted using the 230Th/234,238U disequilibrium method. The dilation rate of the travertines during dilation and the post-dilation period and the average dilation rate of the Cambazli travertines to the north of the Gediz Graben were calculated as 0.01–0.02 mm yr−1 during deposition and as 0.05 mm yr−1 during the post-dilation period. These dilation rates indicate lower tectonic activity along the northern ridge of the Gediz Graben than along the southern ridge. © 2011 Lavoisier SAS. All rights reserved

U/Th dating of fissure ridge travertines from the Kirsehir region (Central Anatolia Turkey): structural relations and implications for the Neotectonic development of the Anatolian block

Travertines exposed in several locations in Central Anatolia are the important lithological product for the interpretation of local neotectonics. The fissure-type travertines provide significant information about stress orientation during deposition. Two travertine masses cropping out in the Kirsehir region have been studied and dated by the U-series method to obtain new chronological constraints, determine dilation rates and contribute to studies on the recent tectonic evolution of the area. The Kusdili and Kayabasi travertine masses are located on the hanging wall of the Kirsehir Fault, similar to numerous fissure ridge banded travertine deposits which are inactive today in the region. While individual fissures of the Kusdili travertine mass (Late Pleistocene-Holocene) have been dilated at rates of between 0.303 and 0.386 mm yr–1 during deposition, the Kayabasi travertine mass (Late Pleistocene) produced measured dilation rates of between 0.136 and 0.187 mm yr–1. The central fissures, filled by banded travertine, roughly follow the ridge crests. While the ridge crest has a NNE-SSW trend in the Kayabasi travertine mass, the ridge crest of the Kusdili travertine mass shows a NE-SW trend. This difference may be related to the clockwise rotation of the stress tensors from Late Pleistocene to Late Pleistocene-Holocene in the region.

Structure and petrology of newly discovered volcanic centers in the northern Kermadec–southern Tofua arc, South Pacific Ocean

The NZAPLUME III expedition of September–October 2004 to the northern Kermadec–southern Tofua (NKST) arc, between 28°52′S and 25°07′S, resulted in the discovery of at least seven new submarine volcanic centers and a substantial caldera complex adjacent to the previously known Monowai Seamount. The volcanic centers form a sublinear chain that coincides with the Kermadec Ridge crest in the south (Hinetapeka) and diverges ∼45 km westward of the ridge crest in the north (“V”) just to the south of where the Louisville Ridge intersects with the arc. All of the centers contain calderas or caldera‐like structures, as well as multiple cones, domes, fissure ridges, and vent fields. All show signs of recent eruptive and current hydrothermal activity. There are strong structural controls on edifice location, with cones and fissure ridges typically associated with faulting parallel to the regional ∼12° strike of the arc front. Several of the calderas are ellipsoidal, orientated northwest–southeast in the general direction of least compressive stress. Sampled volcanic rocks, representing the most recently erupted lavas, are all low‐K tholeiites. Two of the centers, Gamble and Rakahore, yielded only high‐silica dacite to rhyolite (69–74 wt% silica), whereas two others, Monowai and “V,” yielded only basalt to andesite (48–63 wt% silica). Mineral assemblages are plagioclase‐pyroxene dominated, with accessory Fe‐Ti oxides, apatite, olivine, and quartz/tridymite/cristobalite, typical of dry volcanic arc systems. Hornblende occurs only in a felsitic rhyolite from Hinepuia volcanic center, and zircon is absent. Glass contents range to 57% in basalts–andesites (mean 20%), and 97% in andesites–rhyolites (mean 59%) and other quench textures, including swallow‐tailed, plumose, or dendritic crystal forms and crystallites, are common. Most lavas are highly vesicular (≤63%; mean 28%) and have low volatile contents (mostly <2 wt%) which, together with the occurrence of tridymite or cristobalite, indicates explosive eruption and rapid cooling. Exceptions are rocks from “U” volcanic center, which have low vesicularity and low glass contents across a wide compositional range, indicating effusive eruption. Disequilibrium mineral textures, the frequent occurrence of xenoliths and xenocrysts, and macroscopic evidence for magma mingling indicate that many of the lavas are hybrids, having resided only a short time in upper crustal reservoirs prior to eruption. Silicic magmas are major components of NKST arc volcanism and caldera formation is the dominant eruptive style. The scale of silicic magmatism is in marked contrast to the dominant basaltic–andesitic magmatism in the southern Kermadec arc. With evidence from other arcs, silicic magmatism is now recognized as a major feature of intraoceanic arcs globally.

Travertine deposition and faulting: the fault-related travertine fissure-ridge at Terme S. Giovanni, Rapolano Terme (Italy)

In this paper we describe an example of travertine fissure-ridge development along the trace of a normal fault with metre displacement, located in the eastern margin of the Neogene–Quaternary Siena Basin, in the Terme S. Giovanni area (Rapolano Terme, Italy). This morphotectonic feature, 250 m long, 30 m wide and 10 m high, formed from supersaturated hot waters (39.9°C) flowing from thermal springs aligned along the trace of the normal fault dissecting travertines not older than Late Pleistocene (24 ± 3 ka). A straight, continuous fissure with a maximum width of 20 cm occurs at the top of the ridge, along its crest. Hot fluids flow from cones mainly located at the extremities of the ridge, where travertine is depositing. The travertine fissure-ridge shows an asymmetrical profile since it buries the fault scarp. The difference in height of slopes corresponds to the vertical displacement of the normal fault. Fissuring of the recent travertine deposits along the strike of the crestal fissure, as well as recent hydrothermal circulation, lead us to believe that the Terme S. Giovanni normal fault may be currently active. On the whole, the Terme S. Giovanni fissure ridge can be defined as a travertine fault trace fissure-ridge, adding a helpful example for studying the relationship between faulting and travertine deposition.

Travertine

The term ‘travertine’ encompasses a wide variety of continental carbonate rocks deposited from groundwaters and surface waters. These include, but are not limited to, mineral spring deposits, stream and lake deposits, and cave formations and are recognized by interesting morphologies such as spring mounds, fissure ridges, cascades, step pools, and speleothems. These beautiful deposits are often popular tourist destinations, such as Havasu Falls, Ariz., and Yellowstone National Park hot springs. Larger economic deposits are mined for building stone such as that from Bagni di Tivoli, Italy, which was used for the construction of many famous buildings in ancient Rome. Humankind has used travertine systems literally for millennia for art, building stone, and water sources.

Palaeomagnetic and rock magnetic properties of travertine: Its potential as a recorder of geomagnetic palaeosecular variation, environmental change and earthquake activity in the Sıcak Çermik geothermal field, Turkey

Travertine, the product of incremental growth of inorganic carbonate, is potentially a high-resolution recorder of geomagnetic palaeosecular variation (PSV) when it incorporates small amounts of ferromagnetic material. It grows most regularly in regions of neotectonic activity where geothermal waters feed into extensional fissures and deposit successive layers of carbonate as fissure travertine. The same waters spill out onto the surface to deposit bedded travertine which may incorporate wind blown dust including ferromagnetic particles. Tectonic travertine deposits are linked to earthquake activity because the geothermal reservoirs are reset and activated by earthquake fracturing but tend to become sealed up by carbonate deposition between events. This study investigates whether sequential deposition can identify cycles of PSV and provide a means of estimating rates of travertine growth and earthquake frequency. The palaeomagnetic record in three travertine fissures from the Sıcak Çermik geothermal field in Central Anatolia and nearby bedded travertines dated up to 360,000 years in age (U–Th) are investigated to evaluate magnetic properties and relate the geomagnetic signature to earthquake-induced layering. Sequential sampling of bedded travertine from the margins (earliest deposition) to centres of fissures (last deposition) identifies directional migrations reminiscent of PSV. Thermal demagnetisation shows that goethite pigment is not a significant remanence carrier; instead hematite, and more rarely magnetite, is the carrier. Magnetic susceptibility of fissure travertine is proportional to the calcite:aragonite ratio. Two-frequency susceptibility analysis identifies a ferromagnetic content in bedded travertine dominated by fine superparamagnetic grain sizes whereas the fissure travertine has mostly single and multidomain grain sizes, a difference interpreted to reflect contrasting energies of the two environments plus atmospheric input in the bedded travertine. Fissure travertine possesses strong lineated anisotropy of magnetic susceptibility (AMS), with horizontal k max axes oriented along the fissure axes and k int and k min distributed within the orthogonal plane; this is explained by rolling of ferromagnetic grains up the side of the fissure during repeated water ejection until fixed by the host carbonate precipitation. In contrast bedded travertine has low magnitude AMS with near neutral ellipsoid shapes controlled by settling of grains during weak outflow from the axis of the fissure ridge. The source of the magnetic minerals in the fissure travertine is probably in material washed down by meteoric waters from the local terra rossa soil and concentrations of these minerals (and hence magnetic susceptibility) could be a signature of pluvial environments. Fissure travertine is a reliable recorder of the ambient field when layered although bedded travertine is found to exhibit inclination shallowing. On the assumption that PSV cycles record periods of 1–2 ky, layering in the travertine identifies resetting of the geothermal system by earthquakes every 50–100 years in this region. Travertine precipitation occurs at rates of 0.1–0.3 mm/year on each side of the extensional fissures and possibly at a rate an order higher as bedded travertine on the surface. Earthquakes of magnitude M ≤ 4 occur much too frequently to have any apparent influence on travertine deposition but earthquakes with M = 4.5–5.5 occur with a frequency compatible with the travertine layering and appear to be the events recorded by the layering. Two signatures of much larger earthquakes on a 1–10 ky timescale are also recorded by travertine deposition. These are (i) incidental emplacement of massive travertine or fracturing of earlier travertine without destruction of the fissure as a venue of travertine emplacement and (ii) termination of the fissure as a site of deposition with transfer of geothermal activity to a new fracture. Palaeomagnetic estimates of fissure duration and the presence of some 25 fractures in the ∼300,000 year old Sıcak Çermik field growing at rates of 0.1–0.6 mm/year suggests that the type (ii) signature is achieved by an M ∼ 7.5 event approximately every 10,000 years.

The Denizli graben-horst system and the eastern limit of western Anatolian continental extension: basin fill, structure, deformational mode, throw amount and episodic evolutionary history, SW Turkey

The Denizli graben-horst system (DGHS) is located at the eastern-southeastern converging tips of three well-identified major grabens, the Gediz, the Küçük Menderes and the Büyük Menderes grabens, in the west Anatolian extensional province. It forms a structural link between these grabens and the other three NE-NW-trending grabens—the Çivril, the Acıgöl and the Burdur grabens—comprising the western limb of the Isparta Angle. Therefore, the DGHS has a critical role in the evolutionary history of continental extension and its eastward continuation in southwestern Turkey, including western Anatolia, west-central Anatolia, and the Isparta Angle. The DGHS is a 7-28-km wide, 62-km long, actively growing and very young rift developed upon metamorphic rocks of both the Menderes Massif and the Lycian nappes, and their Oligocene-Lower Miocene cover sequence. It consists of one incipient major graben, one modern major graben, two sub-grabens and two intervening sub-horsts evolved on the four palaeotectonic blocks. Therefore, the DGHS displays different trends along its length, namely, NW, E-W, NE and again E-W. The DGHS has evolved episodically rather than continuously. This is indicated by a series of evidence: (1) it contains two graben infills, the ancient graben infill and the modern graben infill, separated by an intervening angular unconformity; (2) the ancient graben infill consists of two Middle Miocene-Middle Pliocene sequences of 660 m thickness accumulated in a fluvio-lacustrine depositional setting under the control of first NNW-SSE- and later NNE-SSW-directed extension (first-stage extension), and deformed (folded and strike-slip faulted) by a NNE-SSW- to ENE-WSW-directed phase of compression in the latest Middle Pliocene, whereas the modern graben infill consists of 350-m thick, undeformed (except for local areas against the margin-bounding active faults), nearly flat-lying fanapron deposits and travertines of Plio-Quaternary age; (3) the ancient graben infill is confined not only to the interior of the graben but is also exposed well outside and farther away from the graben, whereas the modern graben infill is restricted to only the interior of the graben. These lines of evidence imply an episodic, two-stage extensional evolutionary history interrupted by an intervening compressional episode for the DGHS. Both the southern and northern margin-bounding faults of the DGHS are oblique-slip normal faults with minor right- and/or left-lateral strike-slip components. They are mapped and classified into six categories, and named the Babadağ, Honaz, Aşağıdağdere, Küçükmalıdağ, Pamukkale and Kaleköy fault zones, and composed of 0.5-36-km long fault segments linked by a number of relay ramps. Total throw amounts accumulated on both the northern and southern margin-bounding faults are 1,050 m and 2,080 m, respectively. In addition, the maximum width of the DGHS and the thickness of the crust beneath it are more or less same (~ 28 km). The total of these values indicate a vertical slip rate of 0.15-0.14 mm/year and averaging 7% extension for the asymmetrical DGHS. The master faults of the Babadağ, Honaz, Küçükmalıdağ, Pamukkale and Kaleköy fault zones are still active and have a potential seismicity with magnitudes 6 or higher. This is indicated by both the historical (1703 and 1717 seismic events) to recent (1965, 1976, 2000 seismic events) earthquakes sourced from margin-bounding faults and some diagnostic morphotectonic features, such as deflected drainage system, degraded alluvial fans with apices adjacent to fault traces, back-tilting of fault-bounded blocks, and actively growing travertine occurrences. The kinematic analyses of main fault-slip-plane data, Upper Quaternary fissure ridges and focal-mechanism solutions of some destructive earthquakes clearly indicate that the current continental extension (second-stage extension) by normal faulting in the DGHS continues in a (mean) 026° to 034° (NNE-SSW) direction. Detailed and recent field geological mapping, stratigraphy of the Miocene-Quaternary basins, palaeostress analysis of fault populations and main margin-bounding faults of these basins, extensional gashes to fissures, and focal-mechanism solutions of destructive earth-quakes that have occurred in last century strongly indicate that extension is not unidirectional and confined only to western Anatolia, but also continues farther east across the Isparta Angle and west-central Anatolia, up to the Salt Lake fault zone in the east and the inönü-Eskişehir fault zone in the north-northeast. Therefore, the term “southwest Turkey extensional province” is proposed in lieu of the term “west Anatolian extensional province”.

Rapid facies changes in Holocene fissure ridge hot spring travertines, Rapolano Terme, Italy

Holocene hot water travertine continues to form at Terme San Giovanni, near Rapolano Terme, central Italy, although artificial diversion of the water has reduced deposition. Mesothermal water (≈38–39 °C) emerging from fault‐controlled vents located on a hilltop has created a linear fissure ridge 240 m long and up to 10 m high. Active parts of the ridge crest are covered by small cones; inactive parts are locally neotectonically fissured and have small pools. Ridge deposits include crystalline crust, paper‐thin raft and shrub lithotypes. The ridge has both smooth and terraced marginal slopes, dominated by crystalline crusts with small shrubs in terrace pools. At the base of the ridge, there is a rapid transition to lateral flats and depressions, where water from the ridge collects and deposits shrub, irregular pisoid, reed, paper‐thin raft and fine‐grained and organic‐rich travertines. Water channelled to nearby valley sides deposits thick crystalline crusts on valley slopes and waterfall overhangs, locally with small pools filled by smooth spherical pisoids. On the valley floor, mixing of waters forms varied stream‐fill deposits that include micritic reed, paper‐thin raft and coated bubble travertines. The diversity of travertine facies observed results from the location of the Terme San Giovanni hot springs on a hill crest, thus providing a wide array of downslope locations for further deposition. The abrupt facies transitions observed are characteristic of hot spring carbonates and result from a combination of rapid decrease in precipitation away from vents, variations in local surface topography and the feedback effect of travertine deposition itself, which dams and diverts water flow.

Back‐arc rifting in the Izu‐Bonin Island Arc: Structural evolution of Hachijo and Aoga Shima Rifts

Abstract Multi‐ and single‐channel seismic profiles are used to investigate the structural evolution of back‐arc rifting in the intra‐oceanic Izu‐Bonin Arc. Hachijo and Aoga Shima Rifts, located west of the Izu‐Bonin frontal arc, are bounded along‐strike by structural and volcanic highs west of Kurose Hole, North Aoga Shima Caldera and Myojin Sho arc volcanoes. Zig‐zag and curvilinear faults subdivide the rifts longitudinally into an arc margin (AM), inner rift, outer rift and proto‐remnant arc margin (PRA). Hachijo Rift is 65 km long and 20–40 km wide. Aoga Shima Rift is 70 km long and up to 45 km wide. Large‐offset border fault zones, with convex and concave dip slopes and uplifted rift flanks, occur along the east (AM) side of the Hachijo Rift and along the west (PRA) side of the Aoga Shima Rift. No cross‐rift structures are observed at the transfer zone between these two regions; differential strain may be accommodated by interdigitating rift‐parallel faults rather than by strike‐ or oblique‐slip faults. In the Aoga Shima Rift, a 12 km long flank uplift, facing the flank uplift of the PRA, extends northeast from beneath the Myojin Knoll Caldera. Fore‐arc sedimentary sequences onlap this uplift creating an unconformity that constrains rift onset to ∼1‐2Ma. Estimates of extension (∼3km) and inferred age suggest that these rifts are in the early syn‐rift stage of back‐arc formation. A two‐stage evolution of early back‐arc structural evolution is proposed: initially, half‐graben form with synthetically faulted, structural rollovers (ramping side of the half‐graben) dipping towards zig‐zagging large‐offset border fault zones. The half‐graben asymmetry alternates sides along‐strike. The present ‘full‐graben’ stage is dominated by rift‐parallel hanging wall collapse and by antithetic faulting that concentrates subsidence in an inner rift. Structurally controlled back‐arc magmatism occurs within the rift and PRA during both stages. Significant complications to this simple model occur in the Aoga Shima Rift where the east‐dipping half‐graben dips away from the flank uplift along the PRA. A linear zone of weakness caused by the greater temperatures and crustal thickness along the arc volcanic line controls the initial locus of rifting. Rifts are better developed between the arc edifices; intrusions may be accommodating extensional strain adjacent to the arc volcanoes. Pre‐existing structures have little influence on rift evolution; the rifts cut across large structural and volcanic highs west of the North Aoga Shima Caldera and Aoga Shima. Large, rift‐elongate volcanic ridges, usually extruded within the most extended inner rift between arc volcanoes, may be the precursors of sea floor spreading. As extension continues, the fissure ridges may become spreading cells and propagate toward the ends of the rifts (adjacent to the arc volcanoes), eventually coalescing with those in adjacent rift basins to form a continuous spreading centre. Analysis of the rift fault patterns suggests an extension direction of N80°E ± 10° that is orthogonal to the trend of the active volcanic arc (N10°W). The zig‐zag pattern of border faults may indicate orthorhombic fault formation in response to this extension. Elongation of arc volcanic constructs may also be developed along one set of the possible orthorhombic orientations. Border fault formation may modify the regional stress field locally within the rift basin resulting in the formation of rift‐parallel faults and emplacement of rift‐parallel volcanic ridges. The border faults dip 45–55° near the surface and the majority of the basin subsidence is accommodated by only a few of these faults. Distinct border fault reflections decreases dips to only 30° at 2.5 km below the sea floor (possibly flattening to near horizontal at 2.8 km although the overlying rollover geometry shows a deeper detachment) suggesting that these rifting structures may be detached at extremely shallow crustal levels.

Travertines: Depositional Morphology and the Bacterially Constructed Constituents

ABSTRACT Investigation of travertine accumulations throughout central Italy and the west-central U.S. has shown that the carbonate is precipitated in response to both inorganic and organic processes. Individual deposits range up to 85 m thick and hundreds of square kilometers in areal extent; all of the carbonate is low-magnesian calcite. Water chemistry, temperature, and morphology of the accumulation greatly influence the constituents comprising these deposits. Harsh environmental situations favor inorganic deposits, while increasingly more moderate conditions result in the formation of a greater abundance of organically precipitated material. Morphological variations of travertine deposits recognized include 1) waterfall or cascade, 2) lake-fill, 3) sloping mound, fan, or cone, 4) terraced ound, and 5) fissure ridge. Gross morphology, internal stratification, and constituents comprising these deposits vary systematically depending upon the type of accumulation and chemistry of the waters. Bacterially precipitated calcite forms a large percentage of the carbonate in many travertine accumulations, exceeding 90% of the framework grains comprising some of the lake-fill deposits. Bacteria are among the first taxa to inhabit and reproduce in harsh spring environments and produce a variety of fascinating constituents. The bacteria are primarily rods, generally 0.2 µm in diameter and less than 1.0 µm in length. The rods readily decay resulting in calcite crystals loaded with micropores. The basic building block of bacterially constructed travertine is a clump of bacteria averaging 20 µm in diameter enclosed in a single crystal of calcite. Aggregates of these crystals produce a variety of deposits including 1) crudely laminated carbonate mud, 2) finely laminat d layers of mud, 3) intraclasts, 4) foam rock, and 5) shrubs. The shrubs are most striking, commonly forming layers 1-3 cm thick but also producing bacterial pisoids. At some locales, bacterial stromatolites composed of layers of shrubs alternating with finely laminated layers of bacterial mud comprise essentially the entire deposit. The shrub layers are the result of flourishing summer growth of bacteria and, furthermore, show remarkable daily laminae 0.1-0.5 mm thick. The importance of bacteria in the formation of travertine and their universally recognized abundance in modern sediments provides an impetus for a reexamination of the role of bacteria in the origin of other types of ancient deposits.