Paul Kelso

Lithospheric and crustal reactivation of an ancient plate boundary: the assembly and disassembly of the Salmon River suture zone, Idaho, USA

Abstract The Salmon River suture zone, western Idaho, is a fundamental lithospheric boundary between the North American craton and the accreted terranes of the Cordilleran margin. The initial juxtaposition along this north-south-oriented structure occurred during Early Cretaceous time. This zone was potentially reactivated twice by subsequent tectonism, once during Cretaceous time and once during Miocene time. The Late Cretaceous western Idaho shear zone formed along the Salmon River suture zone, as denoted by a sharp gradient in the isotopic signature of the granitoids that intruded the lithospheric boundary zone. The reconstructed Late Cretaceous orientation of the western Idaho shear zone contains subvertical fabrics (lineation, foliation). The same boundary also acted as a locus for subsequent Miocene Basin and Range extensional deformation. Domino-style normal faulting and deep (2100 m) basin formation accommodated the motion between the extending accreted terranes to the west and the unextended Idaho batholith to the east. Whereas either the mantle boundary or a crustal-scale structuring controls the regional extent of the extensionally reactivated zone, locally crustal basement faults and lithological contacts control the orientation and precise location of faults that accommodate reactivation. The multiple reactivation of the Salmon River suture zone is critical for several reasons. The Early Cretaceous suture zone apparently created a fundamental lithospheric flaw, which was reactivated after terrane accretion. Whether this zone was a fracture or a shear zone, the fabric in the mantle lithosphere was apparently not ‘healed’ during orogenesis. Thus, juxtaposition of mantle lithosphere, which is inferred to occur by faulting in the uppermost mantle, acts as a weakness during later tectonism. Second, the paucity of strike-slip plate boundaries in the geological record makes sense in the context of reactivation. The vertical, lithospheric-scale nature of these structures makes them particularly susceptible to lithospheric-scale reactivation during both transcurrent and/or extensional deformation. These reactivations both overprint the earlier deformation and modify the original geometry. Steeply dipping fabrics, rather than vertical fabrics, may be the general signature of major, ancient strike-slip faults.

The role of material anisotropy in the neotectonic extension of the western Idaho shear zone, McCall, Idaho

Basin and Range normal faulting in west-central Idaho reactivates preexisting Cretaceous structures, resulting in a series of normal fault–bound basins. The most intense extensional deformation is concentrated in the Late Cretaceous western Idaho shear zone, resulting in the development of the Long Valley basin. The area affected by the western Idaho shear zone displays two orientations of steep faults: one set of normal faults strikes north-south and is parallel to fabrics within the western Idaho shear zone; the other set strikes east-west and accommodates components of both normal and strike-slip movement. Areas within the Idaho Batholith that do not have strong Cretaceous fabrics (i.e., outside of the western Idaho shear zone) are characterized by a strong preferred north-northeast orientation of faults. From this preferred orientation we infer that the maximum infinitesimal stretch is oriented at 110/290° in this part of the Idaho Batholith. Gravity inversion indicates that the north end of Long Valley is an asymmetric basin about one kilometer deep, with the largest basin-bounding normal fault on the west side of the Long Valley. Paleomagnetic analysis of the Columbia River basalts indicates that the north-south elongate fault blocks within the western Idaho shear zone have not rotated. One block, located just to the west of the western Idaho shear zone, may have rotated counterclockwise. The lack of rotation of north-south oriented fault blocks, in combination with the fault orientations of the Idaho Batholith, indicate that the regional neotectonic deformation within and east of the western Idaho shear zone is characterized by dextral transtension with a divergence vector oriented 130/310°. The extensional reactivation of the western Idaho shear zone demonstrates the effect of material anisotropy at local and regional scales. On a local scale, the mylonitic foliation of the western Idaho shear zone is reactivated as normal faults, even though the regional flow field is oblique to the foliation. On a regional scale, the possible counterclockwise fault block rotation recorded west of the western Idaho shear zone is inconsistent with dextral transtension, suggesting that extensional deformation has reactivated the western edge of the arc-craton boundary as a kinematic domain boundary. We conclude that the preservation of initial features in vertical shear zones and/or plate boundaries is unlikely, due the tendency for well-developed, subvertical fabrics to reactivate, particularly in extension.

Probing Understanding in Physical Geology Using Concept Maps and Clinical Interviews

Using concept maps and clinical interviews, we assessed the extent to which undergraduate students restructure their conceptual knowledge at progressively more sophisticated levels over the course of a two-semester lecture-based physical geology sequence. Students completed concept mapping exercises and clinical interviews at regular intervals throughout the two semesters and data indicated that the course did not address integration of concepts into student knowledge domains. Concept maps and clinical interviews both illustrated acquisition of geologic concepts longitudinally with a disproportionately small increase in integration of those concepts into frameworks of understanding. However, concept maps and clinical interviews appear to be viable and complementary approaches in determining the degree to which student accomplishment objectives are being met.

Intrusive and depositional constraints on the Cretaceous tectonic history of the southern Blue Mountains, eastern Oregon

We present an integrated study of the postcollisional (post–Late Jurassic) history of the Blue Mountains province (Oregon and Idaho, USA) using constraints from Cretaceous igneous and sedimentary rocks. The Blue Mountains province consists of the Wallowa and Olds Ferry arcs, separated by forearc accretionary material of the Baker terrane. Four plutons (Lookout Mountain, Pedro Mountain, Amelia, Tureman Ranch) intrude along or near the Connor Creek fault, which separates the Izee and Baker terranes. High-precision U-Pb zircon ages indicate 129.4–123.8 Ma crystallization ages and exhibit a north-northeast–younging trend of the magmatism. The 40Ar/39Ar analyses on biotite and hornblende indicate very rapid (<1 m.y.) cooling below biotite closure temperature (∼350 °C) for the plutons. The (U-Th)/He zircon analyses were done on a series of regional plutons, including the Lookout Mountain and Tureman Ranch plutons, and indicate a middle Cretaceous age of cooling through ∼200 °C. Sr, Nd, and Pb isotope geochemistry on the four studied plutons confirms that the Izee terrane is on Olds Ferry terrane basement. We also present data from detrital zircons from Late Cretaceous sedimentary rocks at Dixie Butte, Oregon. These detrital zircons record only Paleozoic–Mesozoic ages with only juvenile Hf isotopic compositions, indicating derivation from juvenile accreted terrane lithosphere. Although the Blue Mountains province is juxtaposed against cratonic North America along the western Idaho shear zone, it shows trends in magmatism, cooling, and sediment deposition that differ from the adjacent part of North America and are consistent with a more southern position for terranes of this province at the time of their accretion. We therefore propose a tectonic history involving moderate northward translation of the Blue Mountains province along the western Idaho shear zone in the middle Cretaceous.

Introductory geology for elementary education majors utilizing a constructivist approach

Field Excursions in Earth Science. is designed as a non-prerequisite field-based course for elementary education majors. Classic Canadian Shield and Michigan Basin outcrops and Quaternary features are used to teach those Earth science objectives considered most important for K-8 teachers by the Michigan State Board of Education and by others. We integrated these objectives into five conceptual pathways rather than presenting them as discrete pieces of information. A variety of teaching techniques based on constructivist educational theory are employed, so that pre-service teachers experience active-learning strategies in the context of how science is practiced. Our learning strategies address the cognitive and affective domains and utilize personal experiences in conjunction with pre- and post-experience organizers to allow students to develop individual meanings. We place emphasis on observations and concepts and we encourage students to explain their understanding of concepts verbally and in a variety of written formats. Activities address spatial concepts and map reading; mineral, rock, and fossil identification; formation of rocks; surficial processes and landform development; structural deformation and plate tectonics; and environmental issues. Students keep field notes and have daily projects. They address the pedagogical structure of the course in a daily diary.

A geoscience curriculum for the 21st century

Geology faculty at Lake Superior Sate University (LSSU), a comprehensive state-funded university in Michigans eastern Upper Peninsula, have designed and implemented a new undergraduate geoscience curriculum that utilizes innovative instructional strategies. The purpose of our major undertaking was to remediate the learning problems that we associated with traditional geoscience curricula and pedagogy, problems such as lack of student attentiveness and retention of concepts, inability of students to integrate core concepts after a series of discrete courses, lack of student communication skills, and poor student motivation. We have all witnessed unengaged students during traditional lecture sessions; we as instructors wanted to provide excitement in the geosciences, stimulate interest, and improve student problem-solving skills. Thus, after years of tweaking our traditional courses in the usual manner by creating active and open-ended laboratory exercises, providing increasing amounts of field work, and i...