Residents of the Prince Rupert area have become increasinglyinterested in their physical surroundings as a result of following theprogress of the multidisciplinary science project called ACCRETE, and theACCRETE project has benefited in innumerable ways from help given by theresidents. I hope this article will answer some of the most frequentlyasked questions about ACCRETE and that it will provoke additionaldiscussions that lead to a deeper understanding of the surrounding region.
What is ACCRETE?
ACCRETE is a project that involves scientists of many disciplinesfrom 11 universities to study how continents grow by the accretion of newearth materials. The disciplines include geology, geochemistry, nuclearphysics, paleomagnetism, and seismology. The project has a web site: http://geoweb.princeton.edu/research/ACCRETE/accrete.html.
Why Prince Rupert?
Prince Rupert is in a dramatic geologic location.
The bold ice carved rock faces along the Skeena River bear witnessto the region's recent glacial history. The glaciers retreated rapidlysome 8000 years ago; this and the consequent sea level rise probably causedsome dispersal of the aboriginal groups that were here before, during, andafter this glacial episode. The glaciers made a sculpture of thecountryside, and the line made on the sculpture by the level of the seacreated the intricate, scenic shoreline.
Most people know about the glacial periods, but what is rarelythought about is the fact that the combination of geologic forces liftingup the continental crust with the erosion by the glaciers and other naturalforces over the past 50 million years has exposed rocks that were formedsome 20 km below the ancient surface. Thus, we are able to see with thenaked eye what happened long ago, deep in the crust below us.
In the Prince Rupert area, the plates are now sliding past eachother along the Queen Charlotte fault, Canada's most important activeearthquake fault. This is the way the plates have moved relative to eachother at the latitude of Prince Rupert for about the last 85 million years.The fault line was not always where the Queen Charlotte fault now is.Between 85 and 65 million years ago, some scientists think, it was locatedalong a line from about Juneau, Alaska, to just east of Prince Rupert, moreor less along Work Channel to Telegraph Point, and south from there.
If the NNW-SSE line along Work Channel was the site of an ancientgreat strike slip fault like the Queen Charlotte fault, that means the landunder Prince Rupert was once further south relative to that under Terrace.If we assume a conservative rate of plate motion of 5 cm per year for 20million years, there would be about 1000 km of movement. This rate is aboutthat of the present day Queen Charlotte fault, or the San Andreas fault. Afaster rate, for a longer period of time, both of which are reasonable,could easily increase the distance Prince Rupert traveled to over 3000 km.
Imagine, rainy Rupert could have been at the latitude of balmy BajaCalifornia 85 million years ago! However, not everyone agrees with thishypothesis. Some scientists argue that the geologic history between 140 Maand the present, from Digby Island to Terrace, occurred more or less at thesame latitude. Thus, a scientific question is posed: did Prince Rupertmove 1000 km north relative to Terrace since 85 Ma ago, or has it been atthe present latitude since then?
What's the problem?
If the Work Channel line is indeed the site of an exposed ancientstrike slip fault, then we can study how one of these faults works in thelower part of the continental crust. For active faults, like the QueenCharlotte or San Andreas, we cannot with our naked eyes see what is nowhappening 20 km below the surface.
On the other hand, if there was no relative strike-slipdisplacement between Prince Rupert and Terrace, the alternate answer would lead to a prediction of what to expect in the Himalayas and Tibetan Plateau over the next 20 million years or so. I will not go into the details here of the consequences of this answer, because the first task is to tellwhether or not the strike-slip motion occurred.
How can geologists tell if 1000-2000 km of strike-slip faultingoccurred across any fault line? The best way is to match rocks across thefault line that formed at the same time. The older the rocks, the more therelative movement. However, this technique doesn't work if one or bothsides were destroyed by later geologic events, like melting andmetamorphism. Most of the rocks between Telegraph Point and Shames Riverwere partially molten between 85 and 50 million years ago: intense heatingcreated melt (liquid rock) in many places and led to chemical reactionsthat transformed the rocks in others (metamorphism), and they werecompressed as they were transforming (deformation). Any distinctivefeatures that existed before the melting, metamorphism, and deformationwere obliterated.
A second way to recognize displacement is by paleomagnetic studies.We know a compass needle points to the magnetic north pole. The needlealso lies parallel to the magnetic lines of force. These lines areparallel to the surface at the equator but gradually steepen with latitudeuntil they are vertical to the surface at the magnetic poles. If we were atthe magnetic north pole our compass needles would try to stand on theirtips, which would make it difficult to tell directions.
Many rocks, when they form, preserve the direction of the magneticforce lines at the time that they formed. If we can reconstruct horizontalwhen the rocks formed, and this is easy with lava beds and layers ofsediments, and if we can measure the direction of the ancient lines ofmagnetic force, which we can do in the laboratory, then we can determinethe latitude at which the rocks formed. Comparing that latitude with thepresent latitude gives the northerly component of the distance traveled bythe rock sample. (Note that, just as for navigation on the seas beforedevelopment of the chronometer in the 18th century, latitude is easy todetermine, but longitude is extremely difficult).
Work on determining paleolatitude for the rocks around PrinceRupert shows that indeed the rocks could have formed far to the south.However, for the rocks around Prince Rupert, there is no independentdetermination for paleohorizontal for when the magnetic field was imprinted on them. The rocks which provide the paleomagnetic information are plutonic igneous, which means they cooled slowly from a liquid that wasemplaced deep below the earth's surface. This permits an alternativeexplanation: the apparent anomalous direction of the magnetic pole couldhave been produced by rotation of the rocks after the record of themagnetic field was locked in. Some scientists argue very convincingly thatsuch a rotation did occur, involving east side up around a north trendingaxis of rotation.
Thus, the problem is to put together enough evidence to be able toargue that either no strike slip motion took place between Prince Rupertand Terrace, or that a lot of such relative motion could have happened.The search for the answer is a challenge for earth scientists.
Who are we?
Some of the ACCRETE scientists are the geologists who have beenworking for many years, in a couple cases over 30 years, on unraveling thegeologic history of the Coast Mountains. For perspective, 30 years agowas before helicopters could easily fly to high elevations and before theplate tectonics paradigm gave us a framework to place many seeminglyunconnected bits of information. Both the science and our ability to dothe science have evolved tremendously over the last 30 years. In fact, theproblem that consumes our interest now wasn't even defined until the lastdecade or so.
To explore the depths of the crust beyond where the eye can see, wecalled upon seismologists who, with man-made sound sources and portableseismometers to measure the sound after it passed through the earth'scrust, are able to image rock features deep in the continental crust. Thistakes our knowledge to at least 30 km depth. The technique works much likeusing ultrasound to image a baby in a womb. Although the scientificprinciples are very similar, the scales are different: in wavelength ofenergy, in density contrasts being imaged, and in size of features imaged.Abrupt changes in the physical properties of rocks in the lower crust mightshow where blocks of rock had slid past each other. The seismic experimentwas done during 10 days in 1994, when a 250 foot ship towed an array ofacoustic pulse generators and sensors up and down Portland Canal, aroundDundas Island, up Clarence Strait in southeast Alaska, and in and out ofDixon Entrance.
Age of the rocks is determined by measuring how much decay ofradiogenic elements (parent elements) has occurred. Radiogenic elementsdecay to stable isotopes of the elements (daughter elements) at knownrates. By measuring the quantities of parent and daughter elements inminerals (rocks are made of different minerals) it can be determined howold the mineral is. With advances in techniques over the past decade, wecan now date rocks in the Coast Mountains that were impossible to date tenyears ago. Thus, we will be able to tell exactly when a particular rockmoved or crystallized and compare such pieces of information from acrossthe Coast Mountains. From this comparison, we will understand how therocks were responding to the pushing of plates against each other, and whatthe directions of this push were, and when.
The geochemists of ACCRETE measure trace amounts of most of theelements in the periodic table. A rock on the surface that hascrystallized from a melt that was generated some 20 km below the surfacecontains a fingerprint-like distribution of the elements that reflects thechemical make-up of the region where the rock originated. If one source ofa rock came from 1000 km away from another, it might have a differentchemical signature. We shall try to map these subsurface patterns bymeasuring the chemical make-up of the many igneous rocks that occur in theCoast Mountains. We shall try to match the physical distinctionsdetermined by the seismologists with the chemical signatures determined by the geochemists.
We are undertaking a systematic study of the paleomagnetic polesacross the Coast Mountains. This will establish exactly what thediscrepancies are between present latitude and apparent latitude ofcrystallization, and what the systematics are between these discrepancieswith respect to space and age of the rocks.
What have we learned since the 1994 seismic experiment?
In order to reach a conclusion that will be accepted by our peers,we must, in scientific articles available to everybody, examinealternatives closely and present abundant supporting data. We have not yethad the time to reach supportable conclusions. Nevertheless, throughdebate with each other and with other colleagues at professional meetings,we are focusing on what we can do to distinguish the alternatives. This isan application of the scientific method.
So far the major new conclusions are:
1). From a coordinated effort to determine the history of relativemovement of rock between Telegraph point and Shames River, we havedetermined that a very large amount of strike-slip motion could have occurredwithin this region. This is a major breakthrough from the earlierprevailing conclusion that no strike slip motion could have occurredthrough this region. The breakthrough could not have happened without thedevelopment over the last 5 years of a new understanding of how rocks flowwhen they are very hot.
2). The seismic results suggest that an ancient fault through the villageof Kitkatla may be an important feature for understanding the assemblingof the continental crust west of Prince Rupert.
3). We can indeed find discontinuities in physical features in the lowercrust below Work Channel. And geochemical data also suggest a fundamentaldiscontinuity at about the same place.
4). We have discovered major faults that go through most of thecontinental crust and have gentle slopes towards the southwest. They stopsome 10-15 km below the surface. We think these represent the tracks leftby a giant stretching of the crust some 5-10 million years ago. One ofthese "tracks" stops under the Nass River lava beds. We think there may bea relation between it and where these lavas reached the surface because thefault would provide a convenient weak zone in the crust for the lavas topush through.
The most exciting factor driving interest in the ACCRETE project isthat the roots of an ancient strike-slip fault may be exposed in themountains between Prince Rupert and Terrace. If we find this to be thecase, then we can study what happens in the lower crust when a strike-slipfault is active. We cannot go 20 km below the surface now to study the SanAndreas fault. We must find where the roots of ancient strike-slip faultswere exposed to the surface by the combined processes of uplift of the landand erosion of the uplifted land by water and ice. Thus, everybody who isconcerned about understanding earthquakes is interested in the results ofACCRETE.
What we do has a very large educational component. Mankind ingeneral has a curiosity about how and why things are the way they are. Thesearch for answers has occupied us during all recorded history, and,undoubtedly, from well before. Questioning is a characteristic of ourspecies, Homo sapiens. The better we understand our surroundings, thebetter we are able to cope with them: to understand violent changes such asearthquakes and catastrophic floods; to understand the consequences of ouractions, such as road building on slope stability; to understand thenatural limits of agricultural productivity; to understand the availabilityand limits of natural resources, such as coal or copper. Geologists areuniquely able to help the general population to understand the environment.
An important part of the education process is the training offuture scientists. We identify motivated students in college and interestthem in the study of the earth. Some of these go on for postgraduateeducation to learn more on their way to a master's degree; some go furtherby learning to do original research, thus earning a doctorate degree.
There is great effort by geologists around the world to understandwhy there are mountains. Most people take mountains for granted until theyare confronted with the question of why are there mountains. Ice and waterare commonly credited with making the mountains. However, the elevationabove sea level must be there before ice and water can flow downhill toproduce the forms we now see. The Himalayas are an excellent laboratoryfor finding a solution to the problem of how land is uplifted, because itis rising now. But we cannot see there, now, what is happening 20 km belowthe surface. It is part of the science of geology to look at the historyof old mountain belts to determine whether something like the Himalayasonce existed somewhere else and since been eroded down to where the former roots are now exposed at the surface. Such a place is the Coast Mountains of British Columbia and southeast Alaska.
How will the results be communicated?
It is the nature of science that the results must be published andavailable to everyone. This must happen before any new idea is accepted.In order to be published, the manuscript must pass through the hands ofreferees and editors to be sure that all relevant other work has been takeninto account in the new study, to assure that no undetected mistakes weremade by the scientists, and to be sure the results are easilyunderstandable by the widest possible audience. In addition, unless theresults are published, the results are useless to future scientists whoneed them in order to build on what has been done before. As we understandmore, new questions arise and new techniques to resolve them are developed. Science is never finished. There is a continuing evolution of thought and accomplishment.
The process is long, as shown by the following examples.
When I first visited the Prince Rupert area in 1969, the only priorwork available was that done by the Geological Survey of Canada, under thedirection of the late W W. Hutchison. In the framework of his reconnaissance map, which was based on his work in the 60's, but not published until 1983, I made a collection of samples from the shores of Khtada Lake in 1971. By 1973, examination of these samples was complete and preliminary oral presentations were made at scientific meetings held during the mid 70's. At these presentations, other scientists made creative suggestions for improvements. A manuscript was prepared that was submitted to an international journal for review. After some months, reviews were returned, with helpful comments. The manuscript was revised and resubmitted, and accepted by the journal in early 1977. It finally appeared in print in 1979.
Based on these results, applications were made to funding agenciesand a program of research that involved support of graduate students wasbegun. Other scientists were attracted by the new findings and brought their particular specialties to scientific questions posed by the work onthe Coast Mountains. Eventually, by the early 90's, we were able to organize the seismic ship that came into the local waters late in 1994.Again, to give an idea of the time from experiment to publication, the first manuscript based on the 1994 experiment has just been (early 1997) submitted for publication. The seismic experiment was conceived in 1991,done in 1994, and first publication of results will be in 1997 or 1998.
ACCRETE is an example of basic research, and governments support most basic research. There is no sure way to assure success or relevancy of any program or project, but the way that gives the best results is through support of what individuals find interesting and want to do. No group of government officials or university officials anywhere in the world can know enough to wisely decide what science should be done and by whom. Ultimately, it comes down to the individual to create and argue for his/her project, and the courage of someone in a position of responsibility for distribution of funds to say, yes, we will support this project even thoughwe cannot foresee how this will affect humanity.
A final word.
ACCRETE was an experiment and a miracle. Nobody before had tried to use a seismic ship in inland waterways combined with a dense array of on-shore portable seismometers. Most scientists had well-considered reasons why the seismic component of the study would not work. Well, itdid, and it worked much better than even we, its promoters, thought it would. We have, in fact, some of the best seismic data ever obtained forthe continental crust. This will give seismologists years of data toprocess and interpret. The Prince Rupert area will become known amongstgeologists around the world for providing new scientific insights.
The miracle was that we were able to get the permits to do the experiment. The people in the Prince Rupert region were understandably worried that the airguns we used to make the seismic signals would harm marine life. Fortunately the people of Prince Rupert were able to share the vision of the scientists in what could be done, and had the patience to learn and understand the technique and judge for themselves that our equipment would cause no damage. We caused no damage, and I am grateful to the people of the Prince Rupert region for their support.
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