Uwe Ring

This paper provides a regional-scale background for understanding gold-mineralising processes in the Otago Schist during the Cretaceous. At this time the schist belt was in the latter stages of formation as an accretionary complex with 2000 km strike length on the Pacific margin of Gondwana. The Otago Schist is interpreted as an exhumed accretionary wedge of structurally stacked clastic metasedimentary rocks with minor metabasic rocks. Metamorphic grade reached upper greenschist facies. Gold and other related elements were mobilised from the metasedimentary rocks during metamorphism, and these elements contributed to the high levels of orogenic gold endowment (>18 million ounces in the schist belt. Mesozoic Gold deposits were emplaced in two distinct pulses, one at the beginning of the Early Cretaceous (~140-135 Ma) and the other at the end of the Early Cretaceous (~112-100 Ma). These mineralising pulses were driven by regional tectonic events that may have involved episodic unde...

The East African Rift System (EARS) is one of the most prominent rift systems on Earth and transects the high-elevation East African Plateau. The EARS is famous for its tectonics and geology and has also been suggested to be the ‘cradle of mankind’ making it a natural laboratory for interdisciplinary research straddling the Earth and life sciences. Rifting commenced as a result of mantle plume activity under East Africa. Two distinct rift branches are observed: an older, volcanically active Eastern Branch and a younger, much less volcanic Western Branch. The Eastern Branch is generally characterized by high elevation, whereas the Western Branch comprises a number of deep rift lakes (e.g. Lake Tanganyika, Lake Malaŵi). The onset of topographic uplift in the EARS is poorly dated but has preceded graben development, which commenced at ~24 Ma in the Ethiopian Rift, at ~12 Ma in Kenya, and at ~10 Ma in the Western Branch. Pronounced uplift of the East African Plateau since ~10 Ma might b...

Structure and exhumation history of the Hellenide-Anatolide Orogen in the Aegean Sea region and the adjacent Anatolian peninsula is controlled by along-strike variations of pre-Alpine palaeogeography. In the Hellenides, Mesozoic extension created ribbon-like continental fragments of thinned and dense lithosphere that pinch out eastwards. In the east, the relatively large Anatolide microcontinent mostly escaped Mesozoic extension and lithospheric thinning, presumably because it had a distinctly different, thicker and more depleted lithosphere. In the Aegean transect these alongstrike differences in lithosphere structure ultimately resulted in sustained highpressure metamorphism followed by progressive slab retreat since about 60 Ma. Further east, collision of the Anatolide microcontinent at about 42 Ma formed a south verging greenschist-facies thrust-and-fold belt. Pronounced slab retreat in the Aegean forced differential extension resulting in a broad sinistral wrench corridor that ...

The exhumation of regionally coherent LT/HP-rocks in accretionary wedges is still not fully understood. The Otago Schist belt of New Zealand represents the former fore-arc high of the Mesozoic Torlesse accretionary wedge and is one example for exhumed high-P rocks that reached depths up to 35 km. To investigate exhumation processes and flow paths within the Torlesse wedge, we calculated

ABSTRACT The orogenic wedge model (Davis et al. 1983; Platt 1986) marks a conceptual breakthrough in understanding the growth and long-term evolution of accretionary wedges. The characteristic rheology of subduction-related accretionary wedges is thought to change from Coulomb to viscous when the wedge becomes thicker than ca. 15 km, a transition that may influence the stability and dynamics of these wedges. Platt (1986) proposed that viscous flow may trigger extensional faulting in the upper rear part of the wedge and Wallis et al. (1993) argued that viscous flow may cause vertical ductile thinning of the rear part of the wedge. Material fluxes control the geometric shape of an accretionary wedge (Brandon et al. 1998; Platt 1986). Frontal accretion and erosion both tend to drive the wedge into a subcritical condition as the taper angle of the wedge is progressively reduced. This leads to horizontal shortening across the wedge. If underplating is dominantly controlling the flow field in the wedge and frontal accretion or erosion at the rear of the wedge are small, the wedge is supercritically tapered and leading horizontal extension. Horizontal extension leads to a subhorizontal foliation and may eventually lead to normal faulting in the rear-part of the wedge. Despite the importance of these issues, there remains a paucity of detailed information about ductile deformation and how viscous flow influences the stability of subduction-related accretionary wedges. Strain measurements are an instrument to address whether viscous flow strongly influences the deformation in accretionary wedges. They provide direct information about the kinematics of ancient orogenic belts. Additionally, they allow understanding important tectonic processes in subduction wedges such as the pattern of flow within the wedge. We focus on deformation analysis on a suite of samples from the Otago wedge exposed in the South Island of New Zealand. The Otago accretionary wedge offers a unique opportunity to study the tectonic evolution of a typical subduction-related accretionary complex. Its across-strike length of ca. 600 km makes it one of the largest exposed ancient accretionary wedges on Earth. Pressure and temperature estimates indicate that our samples are representative of deformation conditions to depths as great as ca. 35 km. This is similar to maximum depths observed for subducting slabs beneath modern forearc highs. The deformation measurements show that the strain magnitude is generally small in the Otago wedge. The oct values, a measure of the distortion a sample experienced (independent from the strain geometry), range from 0.34– 3.87 for the Rf /? strains, 1.01–4.28 for XTG strains across the whole suite of the Otago rock pile, and 0.08–0.70 for the absolute strains obtained from low metamorphic grade rocks. The Otago samples are characterized by considerable volume strain that increases from the lower textural zones towards the high-grade interior of the wedge. Our strain results are inconsistent with the models which advocate supercritically tapering of accretionary wedges and that supercritical tapering eventually triggers normal faulting. Taking averages of our strain measurements, a residence time in the wedge of 35 Myr, burial depths of 30 km, coaxial deformation and a depth-dependent rate for ductile deformation, we calculate vertically-averaged strain rates. Because the principal strain axes of the tensor average are all inclined, the vertical averaging changes the principal stretches. The horizontal principal stretch parallel to the 160°-striking Otago wedge becomes 0.79, that for across strike 0.88 and for vertical strain 0.44. Averaged strain rates are −1.44−16 s−1 for parallel-strike horizontal strain, −6.2−17 s−1 for across-strike horizontal strain, and −8.02−16 s−1 for vertical strain. The strain rates are related to volume loss and to the efficiency with which dissolved chemicals are advected away. The rates are similar to the ones calculated by Bolhar & Ring (2001) and Ring & Richter (2004) for the Franciscan wedge. These strain rates are orders of magnitude smaller than the 1−14 s−1 strain rates assumed by Platt (1986). Thus, our data imply that the Otago wedge could not shorten horizontally fast, and hence could not have steepened up its surface slope. The fact that shortening was accompanied by volume loss has another important and interesting consequence. Even if a case was envisioned in which horizontal shortening was fast enough to steepen up the surface slope of the wedge, the volume loss would not necessarily change the wedge geometry into a supercritical configuration triggering normal faulting. As a consequence of the slow strain rates and the high volume loss, viscous flow probably was not fast enough to significantly influence the stability of the wedge and to form a supercritically tapered wedge.

... of Unit 3). Small to medium-scale lake-level changes, which may have occurred as a response to climatic changes comparable to lake-level changes in the younger history of Lake Malawi (Finney & Johnson, 1991), are documented by abundant palaeosols within these units. ...

ABSTRACT The bulk of the exhumation of (ultra)high-pressure rocks usually occurs soon after these rocks were metamorphosed in the course of lithospheric convergence and deep underthrusting during early orogenic stages. A number of studies have demonstrated great exhumation rates during these early exhumation stages. It is poorly understood, however, how this early exhumation is being kinematically achieved. One possibility are extrusion wedges, which are characterized by a thrust-type shear zone at their base and a normal-sense shear zone at their top. The normal-sense shearing at the top of the wedge is a geometric effect only and must not be mistaken as an effect of net horizontal extension of the region. This is a very important characteristic of extrusion wedges. Therefore lithospheric extension is not required for very rapid early exhumation of (ultra)high-pressure rocks. For plausibly arguing for the existence of an extrusion wedge, it is critical to demonstrate that thrust-related mylonitic deformation in the footwall shear zone occurred contemporaneously with normal shearing in the hangingwall shear zone. We present three examples of fossil extrusion wedges from the European Alps and the Cycladic Blueschist Unit in the Aegean. The Eclogite Zone in the Tauern Window of the Eastern Alps was metamorphosed at 25-27 kbar and ca. 650°C at 31.5 Ma. Exhumation occurred very rapidly at rates exceeding 40 km Myr-1. Rb-Sr multimineral geochronology shows that the thrust at the base of the Eclogite zone operated at the same time as the oblique normal fault at the top of the Eclogite Zone. Early exhumation occurred in less than 2 Myr and accomplished about 50-70 km of exhumation of the Eclogite Zone. The two examples from the Cycladic Blueschist Unit in the Aegean are from Evia Island at the western side of the Aegean Sea, and Samos Island and the adjacent western Turkish mainland at the eastern limit of the Aegean Sea. In Evia, the extrusion wedge was active from 33 to 21 Ma and accomplished 15-30 km of blueschist exhumation. In Samos and western Turkey an extrusion wedge was active from 42 to 32 Ma and caused 30-40 km of blueschist exhumation. In both cases structural analysis reveals complementary senses of shear for the wedge-delimiting faults, with thrust-type shear zones at the base and normal-sense shear zones at the top of the extrusion wedges. In all three cases there is no evidence for regional-scale lithospheric extension or the development of contemporaneous extensional sedimentary basins. Instead, wedge extrusion is concurrent with the development of thrust belts in the footwall of the basal thrust of the extrusion wedges. We therefore conclude that all three extrusion wedges formed during overall lithospheric convergence.

... into four main stages: (1) Eocene and earliest Oligocene ∼ESE–WNW-oriented nappe stacking (D 1 ... Nonetheless, the late-stage D 3 emplacement of the Kallithea nappe between 9 and 10 ... Menderes Massif is indicated by the fact that the Aegean, especially the Cretan Sea, is ...

ABSTRACT The Sisters Shear Zone (SSZ) on Stewart Island, New Zealand, is a greenschist-facies extensional shear zone active prior to and possibly during the development of the Pacific-Antarctica spreading ridge at ∼76 Ma. We report quantitative kinematic and rotation data as well as apatite fission-track (AFT) ages from the SSZ. Early kinematic indicators associated with the NNE-trending stretching lineation formed under upper greenschist-facies metamorphism and show alternating top-to-the-NNW and top-to-the-SSE senses of shear. During progressive exhumation lowermost greenschist-facies and brittle-ductile kinematic indicators depict a more uniform top-to-the-SSE sense of shear in the topmost SSZ just below the detachment plane. Deformed metagranites in the SSZ allow the reconstruction of deformation and flow parameters. The mean kinematic vorticity number (Wm) ranges from 0.10 to 0.89; smaller numbers prevail in the deeper parts of the shear zone with a higher degree of simple shear deformation in the upper parts of the shear zone (deeper and upper parts relate to present geometry). High finite strain intensity correlates with low Wm and high Wm numbers near the detachment correlate with relatively weak strain intensity. Finite strain shows oblate geometries. Overall, our data indicate vertical and possibly temporal variations in deformation of the SSZ. Most AFT ages cluster around 85-75 Ma. We interpret the AFT ages to reflect the final stages of continental break-up just before and possibly during the initiation of sea-floor spreading between New Zealand and Antarctica.

Abstract We constrain the timing and kinematics of the Serifos detachment in the southwestern Cyclades, Greece, using low-temperature thermochronometry. Fission-track dating shows that the Serifos detachment was active between *13 and 6 Ma and that the Serifos ...

ABSTRACT In extending orogens like the Aegean Sea of Greece and the Basin-and-Range province of the western United States, knowledge of rates of tectonic processes are important for understanding which process is primarily extending the crust. Platt et al. (1998) proposed that homogeneous stretching of the lithosphere (i.e. vertical ductile thinning associated with a subhorizontal foliation) at rates of 4-5 km Myr-1 is the dominant process that formed the Alboran Sea in the western Mediterranean. The Aegean Sea in the eastern Mediterranean is well-known for its low-angle normal faults (detachments) (Lister et al., 1984; Lister &Forster, 1996) suggesting that detachment faulting may have been the primary agent achieving ~>250 km (McKenzie, 1978) of extension since the Miocene. Ring et al. (2003) provided evidence for a very fast-slipping detachment on the islands of Syros and Tinos in the western Cyclades, which suggests that normal faulting was the dominant tectonic process that formed the Aegean Sea. However, most extensional detachments in the Aegean do not allow to quantify the amount of vertical ductile thinning associated with extension and therefore a full evaluation of the significance of vertical ductile thinning is not possible. On the Island of Ikaria in the eastern Aegean Sea, a subhorizontal extensional ductile shear zone is well exposed. We studied this shear zone in detail to quantify the amount of vertical ductile thinning associated with extension. Numerous studies have shown that natural shear zones usually deviate significantly from progressive simple shear and are characterized by pronounced shortening perpendicular to the shear zone. Numerous deformed pegmatitic veins in this shear zone on Ikaria allow the reconstruction of deformation and flow parameters (Passchier, 1990), which are necessary for quantifying the amount of vertical ductile thinning in the shear zone. Furthermore, a flow-path and finite-strain study in a syn-tectonic granite, which intruded into the shear zone, was carried out. Consistent results show that the mean kinematic vorticity number in the shear zone was close to 1, indicating that the bulk deformation path was close to simple shear. This in turn indicates that vertical ductile thinning was not important during extensional faulting. We conclude that detachment faulting was the primary agent that extended the Aegean crust.

ABSTRACT The orogenic wedge model (Davis et al. 1983; Platt 1986) marks a conceptual breakthrough in understanding the growth and long-term evolution of accretionary wedges. The characteristic rheology of subduction-related accretionary wedges is thought to change from Coulomb to viscous when the wedge becomes thicker than ca. 15 km, a transition that may influence the stability and dynamics of these wedges. Platt (1986) proposed that viscous flow may trigger extensional faulting in the upper rear part of the wedge and Wallis et al. (1993) argued that viscous flow may cause vertical ductile thinning of the rear part of the wedge. Material fluxes control the geometric shape of an accretionary wedge (Brandon et al. 1998; Platt 1986). Frontal accretion and erosion both tend to drive the wedge into a subcritical condition as the taper angle of the wedge is progressively reduced. This leads to horizontal shortening across the wedge. If underplating is dominantly controlling the flow field in the wedge and frontal accretion or erosion at the rear of the wedge are small, the wedge is supercritically tapered and leading horizontal extension. Horizontal extension leads to a subhorizontal foliation and may eventually lead to normal faulting in the rear-part of the wedge. Despite the importance of these issues, there remains a paucity of detailed information about ductile deformation and how viscous flow influences the stability of subduction-related accretionary wedges. Strain measurements are an instrument to address whether viscous flow strongly influences the deformation in accretionary wedges. They provide direct information about the kinematics of ancient orogenic belts. Additionally, they allow understanding important tectonic processes in subduction wedges such as the pattern of flow within the wedge. We focus on deformation analysis on a suite of samples from the Otago wedge exposed in the South Island of New Zealand. The Otago accretionary wedge offers a unique opportunity to study the tectonic evolution of a typical subduction-related accretionary complex. Its across-strike length of ca. 600 km makes it one of the largest exposed ancient accretionary wedges on Earth. Pressure and temperature estimates indicate that our samples are representative of deformation conditions to depths as great as ca. 35 km. This is similar to maximum depths observed for subducting slabs beneath modern forearc highs. The deformation measurements show that the strain magnitude is generally small in the Otago wedge. The oct values, a measure of the distortion a sample experienced (independent from the strain geometry), range from 0.34– 3.87 for the Rf /? strains, 1.01–4.28 for XTG strains across the whole suite of the Otago rock pile, and 0.08–0.70 for the absolute strains obtained from low metamorphic grade rocks. The Otago samples are characterized by considerable volume strain that increases from the lower textural zones towards the high-grade interior of the wedge. Our strain results are inconsistent with the models which advocate supercritically tapering of accretionary wedges and that supercritical tapering eventually triggers normal faulting. Taking averages of our strain measurements, a residence time in the wedge of 35 Myr, burial depths of 30 km, coaxial deformation and a depth-dependent rate for ductile deformation, we calculate vertically-averaged strain rates. Because the principal strain axes of the tensor average are all inclined, the vertical averaging changes the principal stretches. The horizontal principal stretch parallel to the 160°-striking Otago wedge becomes 0.79, that for across strike 0.88 and for vertical strain 0.44. Averaged strain rates are −1.44−16 s−1 for parallel-strike horizontal strain, −6.2−17 s−1 for across-strike horizontal strain, and −8.02−16 s−1 for vertical strain. The strain rates are related to volume loss and to the efficiency with which dissolved chemicals are advected away. The rates are similar to the ones calculated by Bolhar & Ring (2001) and Ring & Richter (2004) for the Franciscan wedge. These strain rates are orders of magnitude smaller than the 1−14 s−1 strain rates assumed by Platt (1986). Thus, our data imply that the Otago wedge could not shorten horizontally fast, and hence could not have steepened up its surface slope. The fact that shortening was accompanied by volume loss has another important and interesting consequence. Even if a case was envisioned in which horizontal shortening was fast enough to steepen up the surface slope of the wedge, the volume loss would not necessarily change the wedge geometry into a supercritical configuration triggering normal faulting. As a consequence of the slow strain rates and the high volume loss, viscous flow probably was not fast enough to significantly influence the stability of the wedge and to form a supercritically tapered wedge.

Accretionary wedges are major sites of continental growth at subduction zones. Their growth and dynamic evolution is still a matter of debate, especially how sediments are accreted during progressive deformation and how tectonic erosion influences its stability with respect to the critical taper theory. At the frontal part of the Torlesse Accretionary wedge in New Zealand, the so-called Esk-Head melange

ABSTRACT The bulk of the exhumation of (ultra)high-pressure rocks usually occurs soon after these rocks were metamorphosed in the course of lithospheric convergence and deep underthrusting during early orogenic stages. A number of studies have demonstrated great exhumation rates during these early exhumation stages. It is poorly understood, however, how this early exhumation is being kinematically achieved. One possibility are extrusion wedges, which are characterized by a thrust-type shear zone at their base and a normal-sense shear zone at their top. The normal-sense shearing at the top of the wedge is a geometric effect only and must not be mistaken as an effect of net horizontal extension of the region. This is a very important characteristic of extrusion wedges. Therefore lithospheric extension is not required for very rapid early exhumation of (ultra)high-pressure rocks. For plausibly arguing for the existence of an extrusion wedge, it is critical to demonstrate that thrust-related mylonitic deformation in the footwall shear zone occurred contemporaneously with normal shearing in the hangingwall shear zone. We present three examples of fossil extrusion wedges from the European Alps and the Cycladic Blueschist Unit in the Aegean. The Eclogite Zone in the Tauern Window of the Eastern Alps was metamorphosed at 25-27 kbar and ca. 650°C at 31.5 Ma. Exhumation occurred very rapidly at rates exceeding 40 km Myr-1. Rb-Sr multimineral geochronology shows that the thrust at the base of the Eclogite zone operated at the same time as the oblique normal fault at the top of the Eclogite Zone. Early exhumation occurred in less than 2 Myr and accomplished about 50-70 km of exhumation of the Eclogite Zone. The two examples from the Cycladic Blueschist Unit in the Aegean are from Evia Island at the western side of the Aegean Sea, and Samos Island and the adjacent western Turkish mainland at the eastern limit of the Aegean Sea. In Evia, the extrusion wedge was active from 33 to 21 Ma and accomplished 15-30 km of blueschist exhumation. In Samos and western Turkey an extrusion wedge was active from 42 to 32 Ma and caused 30-40 km of blueschist exhumation. In both cases structural analysis reveals complementary senses of shear for the wedge-delimiting faults, with thrust-type shear zones at the base and normal-sense shear zones at the top of the extrusion wedges. In all three cases there is no evidence for regional-scale lithospheric extension or the development of contemporaneous extensional sedimentary basins. Instead, wedge extrusion is concurrent with the development of thrust belts in the footwall of the basal thrust of the extrusion wedges. We therefore conclude that all three extrusion wedges formed during overall lithospheric convergence.