Maximum scope DR CHRISTINE SIDDOWAY

A distinctive group of researchers on the EarthScope Bighorn Project are using innovative approaches in seismology and structural geology to study the formation of the Laramide Rocky Mountains. Dr Christine Siddoway, one of four female investigators in the team, explains their work

Could you begin by introducing the Mountains? Have your studies provided any hypotheses that have been proposed to explain Earthscope Bighorn project and highlighting further insight into this phenomenon? the Bighorn Mountains intracratonic arch. the reasons for its establishment? The Laramide Orogeny occurred during plate How important has collaboration been to the The EarthScope Bighorn Project is an convergence between the Farallon and North project? integrated geological and geophysical American tectonic plates during the time investigation of the Bighorn Mountains in period 75-45 million years ago. Distinctive There is extraordinary harmony and cooperation Wyoming. The aim of the multi-institution aspects of the orogeny are: the involvement among the collaborative group, with strong interdisciplinary investigation, led by University of crystalline basement in the near-surface communication and collaboration between of Wyoming investigator Eric Erslev, is to faulting (‘thick-skinned’ deformation); the graduate and postdoctoral researchers identify the process of formation for some the migration of diffuse magmatism and at the various institutions. This makes for the of the ‘signature’ mountain ranges of the distributed deformation into the continental strongest possible interpretations of the 3D/4D North American West: the Laramide Rocky foreland, distant from the plate boundary architecture of the Bighorn arch that are derived Mountains. Unlike many active mountain zone; and the formation of large asymmetric from multiple independent methods belts (orogens) of the world such as the Andes, sedimentary basins between basement- Himalayas, New Zealand Alps or St Elias ranges involved uplifts. In the Rocky Mountains, Is it signifi cant that the project is led by – that developed upon plate tectonic boundaries, deformation produced an anastomosing and involves a high proportion of – women the vast mountains of the Laramide Rockies array of basement-cored arches separated participants? formed well within the continental portion of by lens-shaped foreland basins. Estimates of the North American tectonic plate, far from a the amount of shortening necessary to form I believe this is a really distinctive and unusual zone of plate convergence. the Bighorn arch ranges from 8-13 per cent. aspect of the Bighorn Project. Considering the Rather than fold-thrust style deformation, demographics of the Earth sciences community What is the signifi cance of using geology and the Laramide features large-scale anticlinal in the US, the rich collaboration among women geophysics to study the Rocky Mountains in structures that are either asymmetrical and researchers who have a leadership role as particular? bounded by thrust faults, or symmetrical, with Principle Investigators (PIs), and the number of smaller reverse faults on both limbs. women students who are contributing to the The Bighorn Mountains have a structural research is notable. Four of the six PIs and both architecture that is so well known to geologists Are there unanswered questions that the postdoctoral researchers on the project are worldwide that it serves as an archetype for Bighorn project seeks to address? women. In addition, the project has supported within-plate, intracratonic deformation. One two MSc and four undergraduate women thing that geologists perceive but that it is Paradoxically – in light of the scale of the researchers. For us participants, the experience perhaps diffi cult for the casual traveller to Bighorn range and the large number of geology has been exemplary as an affi rmation that appreciate is the phenomenal scale of the visitors – there is not a consensus about which capable and innovative female researchers are structure: there is a difference in elevation of of the multiple hypotheses proposed for the attaining leadership stature in Earth sciences, 10,000 m from the ‘basement’-cover contact formation of the prodigious arch best explains a fi eld that has historically been dominated by beneath the adjoining Bighorn Basin to its its form. The mechanism for shortening of strong men at the top levels. We feel a great optimism restored position over the top of the mountain Archean lithosphere has long been unresolved about the future prospects for infl uential range. due to lack of geophysical imaging. The Bighorn future contributions by female researchers who Project seeks to obtain a 3D seismic image of the received academic training and advanced their Are you able to provide a brief background integral structures that form the architecture of professional standing through participating in on the Laramide orogeny of the Bighorn the range, and thereby to test the four principal the Bighorn Project. WWW.RESEARCHMEDIA.EU 113 DR CHRISTINE SIDDOWAY

Big discoveries in the Bighorns A group of EarthScope researchers are making important discoveries about the formation of intracratonic mountains, in an integrated geoscience project that has wide-reaching impacts

THE STRIKING APPEARANCE of the Bighorn lithosphere-scale linkage of foreland arches to academic theses that are part of the Bighorn Mountains has long fascinated geologists and plate tectonics and regional fracture patterns Project tourists alike. The crystalline rocks and the in basement-involved orogens that commonly layered sedimentary rocks that cover them control oil and gas production”. The team of SEISMIC SOLUTIONS form an elongated, doubly-plunging arch, which Bighorn Project investigators hope to determine visitors can actually see when they observe the mechanisms that drive the formation of The fi rst of these encompasses the Bighorns that the sedimentary layers on the east side of basement-involved arches, like the Bighorns, Arch Seismic Experiment (BASE), which in the range slope down to the east, and those on which will greatly advance knowledge of intra- 2010 was successful in imaging the crust and the west dip to the west. On the outer fl anks continental deformation worldwide. In doing mantle below the Bighorn Arch by carrying of the range, geological faults cut through the so, this research deepens the understanding of out an active-source wide-angle refl ection rock creating abrupt changes in the angle of continental lithospheric rheology, one of the and refraction survey led by Kate Miller and the layers, as Dr Christine Siddoway, a Principal fundamental problems in plate tectonics. Steve Harder. This allowed project researchers Investigator for the EarthScope Bighorn Project, to measure crustal velocity and thickness, and explains: “The high, interior parts of the Bighorn identify large-scale structures: “Extraordinarily AN INTEGRATED PROJECT Mountains are underlain by crystalline-textured high-resolution images were obtained through granite and gneiss that have properties of great As an integrated Geoscience initiative, the use of 21 seismic shots recorded on 1,800 ‘Texan’ strength and resistance to erosion. The granites EarthScope Bighorn Project brings together an dataloggers, with 4.5 Hz vertical component and gneisses – often referred to as ‘basement’ innovative combination of approaches to its geophones,” Siddoway explains. The visionary rocks – have these characteristics partly because study of the range. Notably, these are: experiment produced 15,000 total travel times they are of great age, but also because they for inversion, obtaining the fi rst high-resolution formed at depth in the Earth’s crust during the • The use of a hybrid active/passive high- P-wave velocity models of the crust and upper Archean Eon, more than 2.5 billion years ago”. resolution seismic studies mantle of the Bighorn region.

• The use of iterative retrodeformation The project employs passive seismic data, MECHANISMS OF FORMATION techniques that balance surface areas and 3D recorded principally from broadband The process of formation of the basement arch volumes to develop GIS-based geometric/ is the focus of the EarthScope Bighorn Project, kinematic models of the upper crust over which addresses not solely the Bighorns but also time-integrated steps seeks to increase understanding of the structures responsible for within-continent deformation, • The involvement of student participants as Siddoway outlines: “Our fi ndings apply who undertake individual research, to an array of global problems, ranging from through internship training and as

FIGURE 1. Simplifi ed cross section of the Bighorns Arch, showing four contrasting Moho geometries for the four lithospheric models being tested. Scenarios one and four have been ruled out.

DOCTORAL STUDENT WILL YECK AND INTERN AUSTIN ANDRUS AT WORK INSTALLING A BROADBAND SEISMOMETER

114 INTERNATIONAL INNOVATION INTELLIGENCE instruments, such as the EarthScope USArray BIGHORN PROJECT and BASE’s EarthScope FlexArray. Investigator Anne Sheehan conceived the innovative use FORMATION OF BASEMENT-INVOLVED of 850 of the Texan instruments as recorders FORELAND ARCHES: AN INTEGRATED for naturally occurring distant (teleseismic) EARTHSCOPE EXPERIMENT earthquakes. Siddoway elaborates: “Traditionally, OBJECTIVES the ‘Texan’ instruments are used solely as active sensors, so it is an innovation in our experiment The EarthScope Bighorn Project is an to extend their use to the passive realm as a integrated geological and geophysical means of enhancing the continuity of the seismic investigation of the enigmatic basement- imaging”. Meanwhile, Dr Megan Anderson at involved foreland arches that formed the Colorado College uses the broadband seismic Bighorn Mountains of Wyoming. The project data to refi ne the team’s understanding of the employs innovative integration of geological structure of the deeper lithosphere, by examining and seismological research. the frequency dependence of splitting for KEY COLLABORATORS teleseismic shear waves (shear wave splitting analysis). Anderson’s work tests whether the Eric Erslev, University of Wyoming asthenospheric mantle ‘fabric’ that is delineated FIGURE 2. Shaded relief digital elevation model of by shear wave transmission behaviour is due to Kate Miller and Lindsay Worthington, the Bighorn Mountains, Wyoming. The white area shearing in response to current North American Texas A&M University corresponds to the elevated terrain that approximately absolute plate motion or whether the anisotropy Anne Sheehan, Ziaohui Yang, Colin corresponds to the crest of the elongate NW-SE- is a residual product of Archean tectonism. O’Rourke, and Will Yeck, University of orientated structural arch. Colorado – Boulder INTERPRETING AND CORRELATING Steve Harder, University of Texas – El Paso Dr Eric Erslev at the University of Wyoming and institutions, but the involvement of student Christine Siddoway and Megan Anderson, and Siddoway at Colorado College have been participants. The Bighorn Project included Colorado College correlating the seismic interpretations with contributions from studies undertaken by more structural geology datasets obtained from fault than 10 Masters, doctoral and postdoctoral FUNDING arrays in the fi eld by graduate and undergradute researchers, and helped to facilitate an industry National Science Foundation – students. “The faults provide geometrical, internship that allowed one student to gain Award Nos. 0844202, 0843889, 0843657, kinematic and temporal control on the profi ciency with the use of ELFEN software. and 0843835. lithospheric scale structures from seismology,” Individual research was conducted by more than details Siddoway. Through procedures designed 20 undergraduate students, through the Keck Air Force Research Laboratory 09NA29249 to test the viability of competing models for Geology Consortium programme, Integrated arch formation, a grand scale 4D model is being Research Institutions for Seismology (IRIS) CONTACT developed, tested and refi ned by Erslev, using internships, and thesis-research opportunities innovative methods of 3D structural balancing. from the project’s participating institutions. Dr Christine Smith Siddoway Proving its commitment and impact across all Professor of Geology levels of study and research, further education STRIKING DISCOVERIES Geology Department and training opportunities were provided to over Colorado College The project fi ndings indicate that the Bighorn 40 volunteers from schools around the country. Colorado Springs arch was formed upon a pre-existing (probably CO 80903 Precambrian) fault, which acted as a nucleation WIDE-RANGING OUTREACH USA point for a ramp that broke upwards from a sub- horizontal detachment at a depth of around 30 Dissemination is a key component of the T +1 719 389 6717 km. The researchers have discovered a prevalence EarthScope Bighorn Project’s objectives. It aims E [email protected] of strike-slip faulting that accompanied to give access to its 4D models to education and www.bighorns.org the ‘characteristic’ Laramide contractional training programmes and enable participation deformation: “We document distributed from geologists across academia and industry. www.coloradocollege.edu/dept/GY/ strike slip in wide zones of pervasive fracturing As well as this, the project’s PIs will continue faculty_christine_siddoway.asp and shear that correspond with pronounced to disseminate in peer-reviewed publications, topographic lineaments within the crystalline at conferences and through online resources: DR CHRISTINE SMITH SIDDOWAY rocks of the arch core, and within a zone along the “The valuable seismic datasets from passive and graduated with a BA from Carleton College northern limit of the Bighorn arch”. The project active source instruments will be available after in 1984, before studying for an MSc at the is testing whether relative displacements across the proprietary period via the IRIS PASSCAL University of Arizona in 1987 and attaining these zones are responsible for the development data repository and archive,” Siddoway outlines. her PhD from the University of California of the vast basement arch with its overarching Other data recorded during EarthScope’s USArray – Santa Barbara in 1995. She is currently anticline observed in the sedimentary cover rocks. Transportable Array (TA) fi eld campaign have Excitingly, the 4D Earth model of the Bighorns already been disseminated within the USArray Professor of Geology at Colorado College, is nearing completion. Based on key seismic community. Whilst the delimitation of the where her research interests include constraints provided by the 3D confi guration of the lithospheric-scale structures is notable, continued structural and metamorphic geology, Moho and mid-crustal interfaces imaged by BASE, work is needed to resolve the crustal-scale faults Gondwana tectonics and tectonic evolution it will enable the fuller visualisation of the temporal within the crystalline core of the Bighorn arch and of the Rocky Mountain foreland. dimension of formation process. their infl uence upon seismic wave transmission. This will not only impact on our knowledge of the Bighorns, but will help accomplish the aim of the COLLABORATION ACROSS THE BOARD national EarthScope programme to characterise Central to the goals of the Bighorn Project is not the lithosphere in the mid-continent and cratonic only the strong collaborative ethos between PIs regions of North America. WWW.RESEARCHMEDIA.EU 115