Gateways into the tectonics of mid-oceanic ridges: Tethyan ophiolites
The spreading axes of mid-oceanic ridges, where two plates separate and magma wells up to form new ocean crust, are the most voluminous volcanic system on Earth. However, direct investigation of this enormously important tectono-magmatic system is hampered by 2 to 4 km deep column of seawater. Geophysical survey of the seafloor and its igneous basement is thus mostly done from heavily equipped research ships.
In 1996 scientific expeditions exploring the mid-Atlantic ridge discovered remarkable seafloor edifices termed "oceanic core complexes". These features have two distinctive characteristics: first, a turtle back shaped topographic high exposing deep rocks of the ocean lithosphere on the seafloor, and second, conspicuous corrugations that formed as part of a faulting process. The recently discovered oceanic core complexes were interpreted as footwalls of low angle 'detachment faults' that cut deep through the ocean crust. If extension occurs for a long enough period on a fault, then a significant portion of the oceanic crust and even upper mantle rocks could be exhumed on the seafloor.
We look for evidence for oceanic long-lived faulting not on the seafloor under kilometers of water, but in 'ophiolites', thick slices of fossil ocean lithosphere emplaced on-land by tectonic convergence. Particularly we look at the Troodos ophiolite exposing on Cyprus an in-tact section of a Cretaceous Tethyan oceanic basin. If deep oceanic faulting occurred in the Troodos then seawater should have penetrated deep into the crust along major fault planes. Silicate minerals forming deep crust and mantle rocks interacted with seawater to form serpentine, a highly hydrated Mg-silicate mineral. Serpentine, in turn, is a mechanically weak substance that would turn the serpentinized initial fault plane into a preferable site for long-lived faulting.
Fig. 3. Serpentinite exposure in the Limassol Forest, located in the southern part of Troodos ophiolite, Cyprus. |
In his recently completed MSc thesis Bar Elisha explored a serpentinite shear zone in the Limassol Forest complex of the Troodos ophiolite, Cyprus, suspected as an on-land exposed oceanic detachment fault. Detailed field and petrographic work tracked progressive stages in the water-rock interaction, while oxygen and boron isotope analysis constrained the physical conditions of serpentinization.
The origin of a super-continents: Tracking the consolidation of Gondwana
The solid substrate on which humanity thrives did not exist when Earth formed 4.5 billion years ago. The continents formed by protracted accumulation of low-density granitic magma that crystallized as a thin crust on top of the denser earth mantle. The assembly of the Gondwana supercontinent during the waning stages of the Proterozoic (900-600 Ma) provided the tectonic background for profound biological, climatological and geochemical changes on Earth that lead, eventually, to the Cambrian radiation.
The amalgamation of Gondwana resulted in an extensive mountain chain, comparable and even exceeding in scale the Cenozoic Alpine-Himalayan chain. Our knowledge of the sequence of tectonic events that comprised the assembly of Gondwana is not as nearly detailed as that of the closure of Tethys Ocean.
Fig. 1. Gondwana super-continent and Pan African sutures. |
The lack of a reliable and adequate chronometer for precisely dating collision-related greenschist- to amphibolite-facies metamorphism in complicated poly-metamorphosed terrains is a crucial obstacle in the reconstruction of the Gondwanan puzzle. Monazite [(LREE)PO4], an accessory phosphate mineral that is common in metapelitic rocks and partitions U and Th into its structure, has the potential to bridge this gap providing a reliable U-Th-Pb geochronometer of collision-related metamorphism. U-Th-Pb dating of monazite potentially times aluminosilicate growth and formation of metamorphic isograd. The development of in situ dating techniques (MC-LA-ICP-MS) makes it possible to obtain U-Th-Pb ages on individual spots in a grain of monazite. This is significant of two reasons. First, in situ dating permits correlation of the age of the mineral and its textural setting. For example, it is possible to measure the age difference between monazites that are included within garnet and those in a rock’s matrix. Second, chemical zoning in monazite requires a technique that permits age determination on individual chemical zones within a crystal, otherwise mixed ages are inevitable.
In his PhD research Bar Elisha aims to determine the age of the major tectonic events that led to the amalgamation of Eastern Gondwana along the northern segment of the East African orogen: the Arabian Nubian Shield. Bar set upon the painstaking task of discovering and documenting tiny monazite crystals in the high grade metamorphic rocks of southern Israel.
Fig.2. Panoramic backscattered scan of garnet-andalusite schists from the Elat association, southern Israel and WDS thorium maps of monazite grains from within the schist. |
Preliminary U-Pb dating of monazite from the Elat schist performed in the LASS (Laser Ablation Split Stream) lab in University of California, Santa Barbara are thrilling. One of the most interesting samples analyzed during this campaign consists of a schist xenolith entrapped by a granitic orthogneiss (Fig. 3). The major events in the evolution of Eastern Gondwana in our area are recorded in this tiny sample by crystals of zircon, xenotime and monazite – all of which are accessory minerals and geological watches, beginning to tick at variable temperatures and circumstances of crystal growth.
Fig. 3. Inverse Concordia and ages of zircon, monazite and xenotime from Elat schist xenoliths within the Elat granitic gneiss. |
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