Saskia Erdmann

Miocene structural reorganization of the South Tibetan detachment, eastern Himalaya: Implications for continental collision

In the eastern Himalaya (Bhutan), there are two distinct top-down-to-the-north segments of the South Tibetan detachment system. The outer segment is a diffuse ductile shear zone preserved as klippen in broad open synforms. New age constraints show that it was active until at least ca. 15.5 Ma and cooled by ca. 11.0 Ma, as constrained by sensitive high-resolution ion microprobe (SHRIMP) U-Pb geochronology of magmatic zircon and 40 Ar/ 39 Ar thermochronology of muscovite in ductilely deformed leucogranite sills. The inner segment is a ductile shear zone active at least until ca. 11.0 Ma (constrained by SHRIMP U-Pb geochronology of magmatic zircon) and overprinted by more recent brittle faulting. These age constraints indicate that ductile deformation continued on the South Tibetan detachment more recently in the eastern Himalaya than in central and western parts of the orogen. These improved constraints on timing of South Tibetan detachment segments allow for a more detailed reconstruction of continental collision in the eastern Himalaya in which the outer South Tibetan detachment segment was abandoned in the mid-Miocene and passively transported southward in the hanging wall of the Main Himalayan thrust (the basal detachment of the orogen), while top-to-the-north ductile to brittle shearing continued on the inner South Tibetan detachment segment. Hinterland stepping of the South Tibetan detachment to maintain an orogenic critical taper (frictional wedge model) is a possible mechanism for this tectonic reorganization of the South Tibetan detachment during the Miocene. However, our data combined with published geochronologic data for the eastern Himalaya demonstrate that foreland translation and exhumation of a midcrustal dome (viscous wedge model) is the more tenable mechanism.

Origin of chemically zoned and unzoned cordierites from the South Mountain and Musquodoboit Batholiths, Nova Scotia

Textural relations and chemical zoning of cordierites in granites act as sensitive recorders of the conditions of their crystallisation history and underlying magma chamber processes. In this contribution, we present new data on texturally distinct and variably zoned cordierites from the late-Devonian, granitic South Mountain and Musquodoboit Batholiths, and infer the conditions of their formation. Using a combined textural (grain size, grain shape and inclusion relationships) and chemical (major element composition and compositional zoning) classification, we recognise the following six cordierite types: CG1/TT1, anhedral to subhedral macrocrysts with random inclusions and patchy normal zoning; CG2a/TT2, euhedral to subhedral macrocrysts with random inclusions and normal zoning; CG2b/TT2, euhedral to subhedral macrocrysts with random or oriented inclusions, and oscillatory zoning; CG3a/TT3, subhedral to euhedral microcrysts with no inclusions and reverse zoning; CG3b/TT4, euhedral macrocrysts with no inclusions and no zoning; and CG4/TT5, anhedral macrocrysts with random inclusions and normal zoning. The textural criteria suggest that these cordierites formed as a product of cotectic crystallisation from a melt, or as the result of a peritectic reaction involving country-rock material. The combined chemical and textural criteria suggest that: (1) normal zoning results from cotectic crystallisation during cooling, cotectic overgrowths on grains formed in a peritectic reaction with country-rock material, or cation exchange with a fluid; (2) oscillatory zoning results from cotectic crystallisation during variations in X Mg of the silicate melt following magma replenishment; (3) reverse zoning results from crystallisation during pressure quenching; and (4) the unzoned cordierite results from cotectic crystallisation under fluid-rich conditions.

Is stoping a volumetrically significant pluton emplacement process?: Comment

[Glazner and Bartley (2006)][1] have challenged the “large-magma-body-emplaced-by-stoping” model for granite batholiths. Instead, primarily as a consequence of their geochronological data on the Tuolumne Batholith, [Glazner and Bartley (2006)][1], building on [Glazner et al. (2004)][2] and [

Formation and transfer of stoped blocks into magma chambers: The high-temperature interplay between focused porous flow, cracking, channel flow, host-rock anisotropy, and regional deformation

Magmatic stoping, i.e., the formation, transfer into, and movement through magma of older plutonic and metamorphic host-rock xenoliths, was widespread in the Mesozoic Sierra Nevada batholith (California, United States). However, the prevailing view that stoped blocks form by rapid thermal shattering and collapse into chambers may not be the dominant process of block formation and displacement into chambers in the Sierra Nevada. In detailed studies in and around the Tuolumne Batholith and Jackass Lakes pluton, we found evidence for the following history of block formation in slightly older, fairly isotropic plutonic host rocks: (1) low stress sites developed, leading to planar zones of increased porosity; (2) focused porous flow of first felsic melts followed by intermediate melts led to growth of magma fingers, which in turn led to increased porosity and loss of host-rock cohesion; and (3) connection of magmatic fingers resulted in the formation of dike-like channels in which flow facilitated removal of all host-rock material in these planar zones. Once formed, blocks were initially displaced by repeated magma injections along these channels, often resulting in unidirectional growth in these zones creating local magmatic sheeted complexes along block margins. Free block rotation occurred when sufficient nonlayered magma surrounded the host block; in some cases, segments of former sheeted zones remain attached to rotated blocks. In anisotropic metamorphic host rocks, focused porous flow may have locally played a role, but the dominant processes during initial block formation were cracking, parallel and at high angles to anisotropy, and intrusion of magma by channel flow. Subsequent initial block displacement and eventual rotation are identical to those in the nearly isotropic host rock. The driving forces for the development of low-stress sites, cracking, dilation, and magma flow remain uncertain, but likely reflect the interplay between regional stress, magma buoyancy stresses, thermal gradients, and host-rock properties, and not simply rapid heating and thermal expansion cracking. Thus a number of processes may drive block formation, some of which are rapid (thermal shattering, roof collapse) whereas others occur over longer durations (incremental magma pulsing and formation of sheeted complexes, regional deformation).