Peter Bird, Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567 Geochemistry Geophysics Geosystems, 4(3), 1027, doi:10.1029/2001GC000252, 2003
##Introduction Plate boundary curves and the curves which outline orogens are approximated by sequences of discrete points. The intention is that adjacent points should be connected by short arcs of great circles. However, since no arc between adjacent points is longer than 111 km, it will not cause significant additional error if maps derived from this data set show the points as connected by straight lines, regardless of the map projection.
Each point is given as a (longitude, latitude) pair, with coordinates in units of decimal degrees. Latitude is positive in the northern hemisphere and negative in the southern hemisphere. Longitude is generally positive to the east of the Greenwich meridian, and negative to the west, but some points in the western hemisphere are represented with positive longitudes in the range 180 to 360. All coordinates are given with 6 significant digits, so that round-off error in positions does not exceed 60 m; however, accuracy never equals precision.
In most of the digital files presenting model PB2002, sequences of points are grouped into "segments", whose ends are indicated by a marker record, "*** end of line segment ***". Preceding the list of sequential points is a title record for the segment; in these records, the essential information is contained in the first 2-8 bytes, and notations following byte 27 (if any) are to give attribution to the source of the information or opinion.
##Data files
-
PB2002_boundaries.dig.txt contains the plate boundary curves in the most compact form, and should be used to add plate boundaries to maps. It contains 6,048 points grouped into 229 segments. The title record for each segment has 5 bytes, in which the first two bytes give the identifier of the plate on the left (as one travels along the segment, looking down from outside the Earth) and bytes 4-5 give the identifier of the plate on the right. In byte 3, the symbol "/" indicates that the right-hand plate subducts under the left-hand plate, while symbol "" indicates the opposite polarity of subduction. All non-subducting plate boundary segments have a hyphen "-" in byte 3. The number of segments exceeds the number of plate boundaries for several reasons: (i) because a single plate boundary may include both subduction and non-subduction segments, which require different titles; (ii) because different parts of a single plate boundary may be credited to different sources; (iii) for convenience in digitizing long plate boundaries which did not fit onto a single map.
-
PB2002_plates.dig.txt For some applications it is necessary to represent plates by closed outlines. They include computing the areas of plates, determining which plate a given point lies within, and mapping plates as regions of contrasting color. For such applications, file PB2002_plates.dig is provided. It contains 52 segments, each titled with the two-letter identifier of a plate. Each segment is a closed curve outlining that plate in the counterclockwise direction (as seen from outside the Earth). The last point in the segment is identical to the first point. Because each plate boundary necessarily appears twice in this file, it is about twice as large as the first.
-
PB2002_orogens.dig.txt contains 13 segments which give the outlines of the orogens. Each outline is digitized in the counterclockwise direction, as seen from outside the Earth. The format is the same as in the file above, except that orogens are given spelled-out names (e.g., "Alps", "Persia-Tibet-Burma") to help prevent confusion between orogens and plates. See the initial sections of the associated paper for definitions of plate and orogen, and also for clarication of the overlay relationship between the set of orogens and the set of plates.
-
PB2002_steps.dat.txt For studies of seismicity and geochemical cycles it is useful to know how much plate boundary is of spreading, transform, or subduction class, and how fast these boundaries slip. But a single class cannot always be specified for an entire plate boundary curve, or even for an entire plate boundary segment, because the class may change repeatedly with local changes in the azimuth of the boundary. A similar problem arises with computed relative velocities, which change smoothly in both magnitude and azimuth along every boundary. Therefore, file PB2002_steps.dat presents detailed information for each "digitization step". (A digitization step is the short great- circle arc between adjacent digitized plate boundary points.) There are 5,819 steps in model PB2002, with mean length of 44.7 km and length range from 1 km to 109 km. Table 2 of the associated paper details the information which is computed at the midpoint of every step.
The 7 classes of plate boundary step contained in this file are defined as follows. Subduction zones (SUB) are plate boundary steps with a convergent (component of) velocity and a Benioff zone of intermediate to deep earthquakes and/or a parallel volcanic arc with Quaternary activity. (These criteria exclude the Alps, Zagros, Pamirs, Himalaya, Tien Shan, and Taiwan.) The designation of Benioff zones and/or volcanic arcs with the title symbols "/" or "" was manual, with the intention that small gaps in a Benioff zone or volcanic arc should not cause unrealistic high-frequency changes in the interpreted step character. Because so many subduction zones lie along continental margins, or contain thickened arc crust which may be considered to have some "continental" character, subduction zones are not divided into continental and oceanic types.
Other plate boundaries are classified into one of 6 types according to whether they are oceanic or continental, and whether their relative velocity is divergent, strike-slip, or convergent. Oceanic plate boundary steps are those lying entirely: (i) within sea floor whose age is known from linear magnetic anomalies to be less than 180 Ma [Mueller et al., 1997]; and/or (ii) at water depths exceeding 2000 m [ETOPO5, Anonymous, 1988]. Continental plate boundary steps are any that are not oceanic. (Thus, transitional steps are considered continental.) Boundary steps are considered strike-slip if the computed relative velocity (based on the Euler poles of Table 1) has an azimuth within 20 degrees of the azimuth of the plate boundary step. (This tolerance is intended to allow for random local errors in plate boundary azimuth, as well as systematic errors in model velocity azimuth caused by errors in Euler pole positions.) Combining these two tests yields the 6 non-subduction types: oceanic spreading ridge (OSR), oceanic transform fault (OTF), oceanic convergent boundary (OCB), continental rift boundary (CRB), continental transform fault (CTF), and continental convergent boundary (CCB).
-
PB2002_steps_desc.txt Description of the fields/formats for PB2002_steps.dat.txt. This file was added to this collection by Hugo Ahlenius on June 26 2014, and acquired from the supporting information for the published paper.
-
PB2002_steps.csv The file PB2002_steps.dat.txt converted to csv format by Hugo Ahlenius, June 26 2014. This document, as well as PB2002_steps_desc.txt explains the fields (as well as the published paper).
References cited may be found in the bibliography of the associated paper.
This document is slightly modified (mostly formatting) from the original resource, available at http://peterbird.name/oldFTP/PB2002/