Drs. Eric H. Christiansen and William Keach, Department of Geological Sciences

Introduction

The origin of Earth’s continents, many of our most important ore deposits and the largest and most damaging volcanic eruptions are all related to the emplacement of silicic plutons–large masses of once molten magma. Understanding the details of the mechanism by which these plutons are inserted into the crust is therefore a fundamental problem in the geological sciences. A recent report by the National Academy of Sciences has identified the origin of Earth’s continents and the triggers for volcanic eruptions as two of the ten “grand research questions” we need to answer in coming decades (National Research Council, 2008).

We sought to help understand these igneous processes by examining a unique set of data from western New Zealand. Here, an igneous intrusion has been imaged by a high quality three- dimensional seismic survey licensed to BYU by Plains Petroleum.

Evaluation of How Well the Academic Objectives of the Proposal Were Met

We sought to answer four fundamental questions.

1. What is the large scale geometry of intrusions?
2. How are the wall rocks (roof and margin) of a pluton deformed?
3. What is the age of the pluton? When was the magma emplaced?
4. What is the contemporary response of sedimentary systems at the surface to intrusions below the surface?

We were able to make significant progress toward answering all of these questions. For
example, we have concluded that the pluton is steep sided and have a domed and faulted upper contact (questions 1 and 2). The pluton is Neogene in age–ranging to perhaps as young as three million years old (Question 3). The seafloor was domed by the intrusion and levee-channel systems were diverted around it as it grew. Moreover a few small turbidites were shed from the rising dome (Question 4). The results are reported below. However, we still have significant work to completely answer some of the questions. In particular, we need to work on question 2 in more depth. Jason Luke has taken this on for his Master’s thesis to be completed here at BYU.

Evaluation of the Mentoring Environment

We conclude that The Primary Principles of Mentoring at BYU were met. Our project allowed two undergraduate students: to develop professional relationships with faculty and other group members, to be involved in significant scholarly activity, to enhance their technical skills, and to prepare for future experiences as professional geologists. These facets greatly enhanced our students’ potential for a career in the petroleum industry and at the same time elucidated an important problem.

Partly as a result of working on this project, Ryan Harbor has gone on to graduate school at the University of Texas at Austin where he is working on an MS degree in geology. He was able to work last summer as an intern for a major oil company. Jason Luke decided to stay at BYU to work on this project for his MS thesis. Jason, with the help of his faculty advisors, has submitted three proposals to private companies and professional societies to continue this work. Jason got an internship with Haliburton this past summer–partly as a result of his understanding of their software which he used in his research. He has lined up another internship this summer with Chevron and looks to have a promising career in the petroleum industry.

This project was a large investment of time and resources, but it provided a truly exceptional experience for two students training in a highly interactive environment with faculty and students. The research for this project was conducted in an environment that simulated as much as possible the conditions found in the professional workplace. The students involved in this research, Ryan Harbor and Jason Luke, became part of our research group. Interactions between the professors (Christiansen and Keach) researchers and students were both formal (lab work, field work, training exercises, regular seminars) and spontaneous (“I just figured out how to…”). The students were trained to use state-of-the-art analytical tools. Jason also attended a field trip to examine the structure of an igneous intrusion in the field led by an international group of experts from France, Italy, Germany, Switzerland, and the Unite Kingdom.

Additionally, both students wrote research abstracts that were published and Jason prepared a formal poster presentation to the international Conference in Moab. As he prepared the poster he received critiques from other members of our research group and then revised and rewrote the text on the poster. These are the critical elements of peer-reviewed scientific research.

The mentoring environment we created included five components.

1. Collaboration. Ryan and Jason worked together with two faculty members, a visiting geologist whose with additional expertise, and one another.
2. Training in seismic data interpretation. Experience with modern computer visualization techniques. Ryan and Jason developed skills in careful measurement, recording important details, becoming patient with computer data reduction and the uncertainties of inherent in real data. Jason also learned how to organize and analyze the geological data collected in the region by others.
3. A weekly visit one on one with Bill Keach or with Eric Christiansen. We also read and critiqued the important publications within our fields of study.
4. Extensive interaction with faculty and peers to learn skills of critical analysis. Throughout the year-long project, small group interaction ensured plenty of time for informal “comment” and analysis of the new data.
5. Writing formal reports for presentation at a scientific conference. We reviewed and required writing of a scientific abstract in a process that mimics the collaboration between coauthors of a scientific paper. Another type of review will come from other scientists when Jason Luke presented his research in a professional meeting.

Student Participants

Ryan Harbor and Jason Luke both worked on this project as undergraduate students. Jason is now pursuing the project as his graduate degree. Both of them wrote abstracts of their research. Harbor’s appeared in the most important annual geophysical meeting in the world–the fall meeting of the American Geophysical Union. Luke presented a poster at an international meeting on the how magma is emplaced in the crust. Fortunately, the meeting was held in Moab, Utah, where classic exposures could be visited at the same time.

Harbor, R.L., Christiansen, E.H., and Keach, R.W., 2008, 3D Seismic Studies of Igneous Intrusions, Taranaki Basin, off-shore west New Zealand: American Geophysical Union Eos Transactions, v. 89, no. 53, Fall Meeting Supplement, Abstract V54a-08.

Luke, J., Christiansen, E.H., and Keach, R.W., 2010, Three-D-Seismic images reveal the external structure of igneous intrusions, Taranaki basin, off-shore western New Zealand: Hints for emplacement mechanisms: Laccoliths and Sills 4 Conference (LASI 4), Moab, Utah.

Description of the Results

Below we have copied the abstract that Jason Luke wrote in conjunction with his faculty mentors. We have also attached a pdf file of the poster presenting our initial results from the LASI 4 meeting.

Several off-shore volcano-plutonic complexes are imaged in a detailed 3D seismic survey acquired by Pogo New Zealand/Plains Exploration. The new data provide insight into the sizes, shapes, and wall rock deformation associated with the emplacement of plutons. The seismic survey, conducted in 2005, covers 1700 km2 and was processed with modern techniques used in hydrocarbon exploration. The images and structures have to be interpreted with care because of distortions caused by “velocity pull ups” created by the large seismic wave velocity contrast between sediment and igneous rock.

The magmatic rocks may be part of the Mohakatino Volcanic Centre (15 to 1.6 Ma) that intrudes and partially fills the Taranaki graben, which began to form in the Cretaceous. Imaged plutons range from less than 1 to as much as 12 km across. The intrusions are steep-sided and do not resemble sills, but their bases are poorly resolved. The top of the largest complex is sharply delineated and marked by multiple apophyses as much as 2 km across and hundreds of meters high. Deformation along the sides of the intrusion is dominated by highly attenuated dipping strata with apparent dips of 45° or higher. Dips decrease rapidly away from the intrusion but doming extends several hundred meters from the margins.

A series of high-angle faults fan out from the margin of the pluton and cut the folded strata along the margin. These faults terminate against the margins of the intrusion, extend as much as 1 pluton diameter away from the margin, and then merge with “regional” faults that are part of the Taranaki graben. Offset along these radiating faults is on the order of a few hundred meters.

Strata on the top of the complex are thinned but are deformed into a faulted dome with an amplitude of about 1 km. Steep, dip-slip faults form a semi-radial pattern in the roof rocks but are strongly controlled by the regional stress field as many of the faults are sub-parallel to those that form the graben. The longest roof faults are about the same length as the diameter of the pluton and cut through approximately 1 km of overlying strata, but offset gradually diminishes vertically away from the top of the intrusion.

The pluton appears to be composite and formed from multiple, steep-sided intrusions as evidenced by the complex margins and multiple apophyses. Small sills are apparent several hundreds of meters above the top of the main complex. Multiple episodes of deformation are also indicated by a series of unconformities in the sedimentary strata around the complex. In fact, doming appears to have generated a series of channeled turbidite deposits that fan out around the intrusion.

The intrusion lies in a relay zone between two NE-trending en echelon normal faults. Little oblique-slip has occurred and space for the intrusion may have been created by doming and floor subsidence; stoping may have occurred, but stoped blocks are not apparent in the seismic images.

Budget: How the funds were used

The budget for this project was used to: (1) pay a salary to Ryan Harbor and Jason Luke, (2) to pay for travel and registration at the LASI 4 conference and field trip (described above) where Jason presented his paper on the New Zealand intrusions, (3) support the visit of Scot Paterson from the University of Southern California, and (4) to purchase new computer equipment so that the large amounts of seismic data could be processed and visualized as an “X-ray” of the Earth’s interior.