Research

The Northern Highlands terrane (NHT), Scotland preserves a complex Neoproterozoic–Paleozoic tectono–metamorphic evolution. Work over the past three decades has helped constrain the timing and conditions of metamorphism in the northern NHT, yet parts of the southern NHT remain less well-constrained. New petrochronologic analyses (accessory phase geochronology, Mnz-Xtm and Grt-Bt thermometry, microstructural analysis) of the high-grade Sgurr Beag Thrust sheet in the southern NHT provide significant constraints on the thermal and deformational history. Monazite inclusions in garnet cores and inner rims yield two age populations; a ~750–720 Ma population correlated with the Knoydartian (840–720 Ma) orogeny and a ~680–600 Ma population provisionally linked to a regional hydrothermal event. A third event ranging from ~465–435 Ma, peaking at ~450 Ma, is recorded by monazite and xenotime in garnet inner rims and the matrix. Paragenetic fabric relationships between garnet, monazite, and xenotime indicate at least two distinct phases of post-450 Ma garnet growth and associated Mnz-Xtm and Grt-Bt thermometry yields 640–680°C conditions. These data suggest a continuous and prolonged high temperature orogenic event that contrasts with traditional interpretations of distinct Grampian and Scandian events or simple Scandian re-working of older Grampian and/or Knoydartian metamorphic assemblages.

The Wet Mountains of Colorado are underlain by exhumed Proterozoic metamorphic rock that lies north of the recently identified 1.46 to 1.40 Ga Picuris orogen in northern New Mexico.  The Picuris orogen is inferred to have formed in response to a Mesoproterozoic (ca. 1.49-1.40 Ga) contractional to transpressional event extrapolated to be regional in extent.  It has been proposed that the Picuris orogen composes the westernmost tectonic segment of the Pinware-Baraboo-Picuris orogen that formed along an evolving convergent margin that becomes younger from northeast to southwest.  This study is focused on the record of high temperature deformation, metamorphism, and plutonism in the southern Wet Mountains interpreted as a consequence of Mesoproterozoic tectonism related to the Picuris orogeny.  Geochronology results yielded two populations:  ~ 1.45 Ga age of crystallization for granitic intrusions and ~ 1.7 Ga date for a metasedimentary rock.  P-T thermobarometric results for a Grt + Sil + Crd + Bt + Kfs + Pl + Qtz migmatite yielded inferred peak conditions of > 0.6 GPa and > 700oC.  These results suggest that the Picuris Orogeny was the deformative and metamorphic event that displaced sediments to > 0.6 GPa and caused a regional overprinting of foliation throughout the Wet Mountains. 

The Sierra Nevada Batholith is an amalgamation of hundreds of granitic plutons that are well exposed throughout the Sierra Nevada Mountains, displaying a complex network of joints spanning kilometers to centimeters in length.  The field study area encompasses approximately 72 mi2 and is located approximately 20 miles southwest of Lake Tahoe in Mokelumne Wilderness.  The study area was selected in Google Earth, and primary domains were determined on the basis of mega-lineaments that can be traced for a kilometer or more and are evident as linear bands of vegetation in topographic depressions, often with associated drainage.  There are three distinct areas that possess a high density of these mega-lineaments, and individual sites for more detailed study were chosen to investigate the congruency between mega-lineaments and fractures within the intervening areas.  Outcrop images from 21 sites [~1,000 m across], were taken in Google Earth and traced in Adobe Illustrator, and traced joint orientations were computed and analyzed using histograms.  The two dominant joint sets trend NE-SW and NW-SE, forming a consistent orthogonal set with locally clear truncations in Google Earth.  When analyzing truncation relationships on the domain scale, there was inconsistency in which direction was truncating versus truncated.  Subsequent field work indicated this was in part due to exfoliation slab break-up that reactivated earlier joints or fabrics.  Field work consisted of 2 weeks of joint orientation data collection from 6 accessible sites.  While the same preferred orientations seen in Google Earth were also observed in the field, field data displayed more variable joint orientations.  Valley floors likely exhibit pre-exfoliation joint patterns that formed at depth and ridges exhibit more joints formed by near surface processes involving reactivation.  Field joint truncation data yielded consistent results that represent a clear age relationship between ENE-WSW and NNW-SSE, with the former displaying an older joint set.  Georectified drone image analysis was conducted at two sites, representing a third scale of analysis.  Results indicate the same truncation relationships as the field data.  We believe that there may be a Google Earth Pro resolution ‘filter’ that favors the earlier formed and overall longer joints that developed at depth.  These joints possess shadowing, vegetation, and weathering features that delineate the individual fractures, at the expense of shorter and more irregular joints related to more modern reactivation and exfoliation slab break-up.  The mega-lineaments that define the three designated domains bound and influence joint patterns visible on the outcrop scale and are correlated with smaller scale joints at individual sites.