Silke Schmidt

A new occurrence of microdiamond-bearing metamorphic rocks, SW Rhodopes, Greece

We describe a new locality with microdiamond-bearing, ultrahigh-pressure metamorphic rocks near the village Sidironero in the Rhodope Metamorphic Province in northern Greece, about 70 km west of the nearest known location at Xanthi. High- and ultrahigh-pressure metamorphic conditions are preserved in an intensely strained melange zone which is sandwiched between upper-greenschist to lower-amphibolite-facies rocks in the footwall (Pangaion-Pirin Complex) and upper-amphibolite-facies rocks in the hanging wall (Rhodope Terrane). The melange zone consists of various paragneisses, orthogneisses and metamafics. A strong overprint at upper-amphibolite-facies conditions associated with migmatisation in the orthogneisses and subsequent intense mylonitisation at lower-amphibolite-facies conditions almost obliterates peak-pressure assemblages. Relics of high-pressure conditions are preserved in mafic boudins and in garnet-kyanite-mica schists. Garnet in garnet-kyanite-mica schists displays inclusions of microdiamonds and swarms of non-oriented rods of rutile and quartz. The lithological and structural appearance of the melange zone resembles the exposure of ultrahigh-pressure metamorphic rocks further east at Xanthi, whereas the location at Kimi may occupy a higher structural level.

Optical dating of alluvial deposits at the orogenic front of the andean precordillera (Mendoza, Argentina)

Well constrained numerical ages of alluvial fan sediments are key to understanding the chronology of alluvial episodes and tectonic activity at the front of the Andean Precordillera. We tested the application of radiocarbon and optically stimulated luminescence (OSL) dating in the distal part of an alluvial fan five kilometers north of Mendoza. For OSL dating a large number of aliquots (n > 70) — each composed of ∼50 quartz grains — were measured in order to obtain reliable burial ages despite scattered dose distributions. Owing to a feldspar contamination in all samples, an infrared stimulation was inserted before each OSL measurement, which reduced the feldspar OSL signal successfully. By using the minimum age model we obtained stratigraphically consistent burial ages of alluvial deposits in a depth profile. The uppermost ∼1 m of sediment is composed of debris flow deposits buried 770±76 years ago. Three plant remnants used for radiocarbon dating from the same layer, however, yielded ages younger than 350 years, which are interpreted to underestimate the depositional age. Underneath the debris flow, a major unconformity cuts a series of distal alluvial fan sediments with interstratified floodplain deposits, which are composed of sandy and calciterich silt layers, respectively. Three samples from this unit which were distributed over one meter of sediment thickness yielded statistically concordant OSL ages of 12.3±1.2 ka, 12.3±1.2 ka, and 11.7±1.1 ka. The deposition of these sediments during the latest Pleistocene coincides with a phase of cool and humid climate, which occurred before the alluvial fan propagated farther into the foreland. The overlying debris flow sediments are associated with alluvial fan incision during the arid Late Holocene.

Repeated Folding during Late Holocene Earthquakes on the La Cal Thrust Fault near Mendoza City (Argentina)

In 1861, one of the most destructive earthquakes in the history of Argentina destroyed the city of Mendoza (currently 1 million inhabitants). The magnitude M S∼7.0 earthquake is inferred to have occurred on the 31‐km‐long La Cal thrust fault, which extends from Mendoza to the north, where it offsets an alluvial fan and small inset terraces along a well‐preserved fault scarp. A trench excavated on a terrace that is vertically offset by ∼2.5  m exposes two main stratigraphic units separated by an erosional unconformity. The coarse‐grained upper unit is deformed by three east‐vergent folds ( F 1– F 3). Retrodeformation of these folds yields total displacements of ∼2.0  m, ∼2.4  m, and ∼0.5  m on the underlying fault splays, respectively. The displacement of ∼2.0  m recorded by fold F 1 is interpreted as the result of the fault rupture that caused the 1861 earthquake. F 2 and F 3 were presumably generated during the penultimate event with an inferred magnitude of M w∼7.0, although formation during two distinct ruptures cannot be excluded. Finite‐element modeling shows that coseismic folding above the tip of a blind thrust fault is a physically plausible mechanism to generate these folds. A published luminescence age of 770±76 years, which is interpreted to date the formation of the deformed terrace, indicates that the two (or possibly three) scarp‐forming events occurred during the last ∼800 years. The fine‐grained sediments below the erosional unconformity—that contain evidence for at least one older earthquake—are dated at ∼12  kyr. Our results indicate that elastic strain energy, which is accumulating at the front of the Precordillera today as shown by Global Positioning System (GPS) data, was repeatedly released during earthquakes on the La Cal fault in the past. Hence, the La Cal thrust fault poses a serious threat to the city of Mendoza.