Name Lab Section work in groups of two, but each person will turn in their own lab GEOSCIENCE 001 PLATE TECTONICS LAB: REUNITE GONDWANA! modified from Adam Maloof INTRODUCTION A combination of the Cold War, a large defense budget, and the influence of a Navy admiral who was also a geophysicist led to a massive bathymetric and magnetic exploration of the ocean basins. A surprising discovery was a nearly continuous ridge near the centers of many of the ocean basins; the sea floor on either side of these ridges dropped in a symmetrical, exponential fashion. An even more surprising revelation from this exploration was the discovery that magnetic anomalies (deviations in the Earth’s magnetic field) formed linear patterns parallel to and symmetric about those ridges. Armed with the observation from terrestrial rock magnetism that the earth’s magnetic field switches polarity sporadically, researchers proposed that these magnetic anomalies were due to the magnetization of rocks formed at mid-ocean spreading ridges. New crust, cooling through the Curie temperature, would record the Earth’s magnetic field at the time of cooling, and then this ocean crust magnetization would interact with the Earth’s present day magnetic field, leading to the anomalies. This hypothesis was further strengthened by correlation of the reversal pattern as seen on the sea floor with that observed in sequences of terrestrial lava flows that spanned the last 20 Ma or so. Thus, geoscientists were able to develop maps of the age of the sea floor based on magnetic anomalies. In this lab, we will use magnetic anomaly data to reconstruct the paleogeography of the Indian Ocean and its surrounding continents, which collectively comprise Gondwana, the southern portion of the supercontinent Pangea. The theory behind this is very simple: to determine the geography at a given time in the past, remove all the ocean crust which has been formed since that time. All that remains then is to close the gaps. We will do this for three discrete stages in the tectonic evolution of the Indian Ocean: 35Ma, 80Ma, and 120Ma (Ma=mega annum, i.e., a million years into the past). We will also explore in more detail the origins of the Seychelles Islands, where something interesting happened about 64 Ma. First: Study the Maps Examine the set of 4 identical maps provided. You will use one of the copies to color on, and the other three to cut-up. You should be able to recognize the continental margins of Africa, Madagascar, Arabia, India, Indonesia, Australia, Asia, and Antarctica. The heavy black line represents the spreading ridges — the Carlsberg Ridge, the Southeast Indian Ridge, and the Southwest Indian Ridge. The intersection of these ridges is called a Triple Junction. Perpendicular to the ridges are a series of fracture zones (thin black lines), which represent either fossil or active strike-slip boundaries between plates. As such, they record the direction of relative (not, absolute) motion between two adjacent plates. Fracture zones are parallel to plate motion and age changes. Notice that there are some other fracture zones that have no obvious relation to the present-day ridges. Parallel to the ridges (in most spots) and perpendicular to fracture zones you’ll notice another set of lines called isochrons, or lines of constant age. The isochron labeled 35 represents ocean crust that is 35 Ma — this is the same as magnetic anomaly 13 as seen onthe larger map of the world’s magnetic lineations (note that at the bottom of this map, there is a chart that shows the ages of the major magnetic anomalies). There are two separate isochrons drawn, corresponding to the different ages we are going to reconstruct. (The third age — 120 Ma — corresponds to the coastlines.) Notice that the isochrons further away from the modern mid-ocean ridge (e.g. 80 Ma) have no clear geometrical relationship with the ridges active today. Perhaps spreading in the past was oriented differently than spreading is today. Finally, note the dashed lines which criss-cross the ocean. These are ship-tracks along which a magnetometer was towed. Previous workers have completed the difficult task of interpreting the magnetic data from these cruises and identified the magnetic anomalies and thus isochrons on the tracks. EXERCISES 1. Age Map of the Indian Ocean Your first task is to use the isochrons to create an colorized age map of the Indian Ocean sea floor. Remember that the age of the ocean crust increases in a direction perpendicular and way from the ridge, and parallel to the fracture zones. Color in this map so that you at least 5 different color bands representing different age ranges (e.g., 0-20Ma, 20-35Ma, and so on). You may find it helpful to consult the large map of all the magnetic anomalies. It is common to label young oceanic crust with warm colors, and old oceanic crust with cool colors. Be sure to include a color key on this map. 2. 35 Ma Reconstruction a) Leaving the colored map intact, cut out all sea floor younger than 35Ma on one of your other maps. Assume in this and all subsequent stages that Asia and Antarctica do not move (i.e. their absolute motion is zero, which could be tested by looking at hotspot tracks). Now, move the remaining pieces back to their positions before the sea floor you just removed was created. Remember that you must move pieces of crust along fracture zones. Notice that you cannot accomplish this task and keep both Asia and Antarctica stationary without making a couple more cuts — consult a map of present day plate boundaries to decide where to make these cuts. Do this carefully, keeping in mind that you need to separate the Indian/Australian Plate from the Asian Plate, and the African Plate from the Ind/Aus Plate and the Asian Plate. Tape your reconstruction down on a blank sheet of paper — label it 35 Ma Reconstruction and turn it in with the rest of your lab. b) There should be some large areas where there is no material at all. What do these areas represent? c) What has happened to this missing material?d) Using a red pencil, trace out the spreading ridges that were active at 35 Ma. Do these ridges correspond exactly to the ones in the present-day map? Describe the similarities and differences. What does this tell you about the nature of spreading centers? 3. The Seychelles The Seychelles Islands,