joint analysis in grenville province
TRANSCRIPT
Joint Analysis of the Grenville Outcrop
Research Project in Environmental Science EESD09
By: Iain Ching 999131390
Supervisor: Heidi Daxberger
October 20, 2016
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Abstract
The Central Metasedimentary Belt joint fractures contain substantial information regarding the
tectonic activity in Southern Ontario. Joint analysis was conducted in northeastern Peterborough to
determine the cause of deformation. Rose diagrams of joint strike direction measurements taken from 4
different sites show NNW, ESE, NE, SE and ENE trending joint sets. The joint sets comparison show
similar joint trajectories to prior studies. The NE and ENE joint sets may have been formed from the
active E-W extension of the Mid-Atlantic ridge, NNW joint set are suggested to have formed during the
Alleghanian deformation. The SE set likely formed from tectonic loading events, and the ESE joint set
has formed from the breakup of Pangea by ESE trending extension rifting. This study has affirmed
aforementioned work done regarding the joint sets and trajectories in Southern Ontario. NE and ENE joint
sets should be studies further since the Atlantic spreading is currently active and still has influence in the
local tectonic setting.
Introduction
Southeastern Ontario is sitting well beyond any major faults and has been considered tectonically
stable in light of its intraplate position; however, recent studies that have examined intraplate tectonism
has eroded this assumption (Sbar & Sykes, 1972). Interest in studying the Grenville Province has been
ignited by potential reactivation of basement faults, creating apprehension of future earthquakes in the
region as a result (Wallach, 1998). Environmental conditions and processes, such as post-glacial isostatic
readjustment, alignment of structures with contemporary tectonic stress field, and post glacial-tectonic
processes, are likely to be the cause of seismic activity in southern Ontario (Dineva, 2004; Ma, et al.,
2008). The purpose of this study is to determine possible the tectonic activity in the general area.
Insight into the region’s activity can be established by examining faults, folds, foliations and joint
fractures in the Paleozoic - Precambrian outcrop. In this study joint fractures were analysed; joint
structure is indicative of their creation and therefore the region’s geologic activity can be determined
(Andjelkovic, et al., 1997). Joints may form in response to the environmental conditions and processes
stated earlier. Joint measurements were taken at 4 sites in the northeastern Peterborough region, recording
the strike direction, dip angle and joint quality. The data was analysed and compared to similar studies
with analogous goals but set in differing locations (Andjelkovic, et al., 1997; Andjelkovic, et al., 1996;
Andjelkovic & Cruden 1998; Cruden, 2011; Mitchell, 2007; Williams, et al., 1985).
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Background
The Grenville province in southern Ontario, Canada has a complex structure and was forged from
million years of accretion tectonism accompanied by metamorphism. The Grenville province formed 1.5
billion to 1.0 billion years ago during an orogeny associated with the formation of the supercontinent
Rodinia, specifically along the eastern margin of the Laurentian continent. Tectonic movement has
resulted in a multitude of collisions and accretionary events creating the Grenville Orogeny (Hoffman,
1988). Over time erosion and weathering diminished the orogen; subsequently Paleozoic-Quaternary
sedimentary rocks, such as sandstones, shales, limestones, dolostones and some evaporates from quiet
marine and fluvial environments were deposited on top of the former orogen (Sloss, 1988). Much of the
sedimentary cover rock was eroded away by the Wisconsin glaciation 10,000 years ago, leaving behind
exposed Precambrian bedrock. After the retreat of the Wisconsin glacier, post-glacial rebound occurred.
Presently, the orogen covers a significant portion of the North American continent, from Mexico
to Newfoundland, and is also found in parts of Scotland (Figure 1). The two major features of the
province are belts and terranes (or domains), which will be described below. The Bancroft and Elzevir
terranes were previously volcanic arcs and subsequently collided into the region to form the terranes
(Easton, 1991); while the Frontenac terrane was previously a Laurentian mountain belt (Easton 1991). It
is subdivided into the three regions, the Central Metasedimentary Belt (CMB), the Grenville front tectonic
zone (GFTZ), and the Central Gneiss Belt (CGB). The three regions are contained within the Precambrian
basement, and as the name suggests, it is comprised of rock of ages pertaining to the Precambrian eon
(1.4 – 1 Ga). Because this study took place on the CMB, the region will be discussed in more detail.
Figure 1. Map showing the extent of the Grenville Orogeny in the present, expanding from Canada down to parts in
Mexico, and also found in Scotland. Retrieved from Tollo et al. 2004.
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Throughout the CMB a combination of marble, meta-volcanic rock and clastic metasedimentary
rock can be found. Supracrustal rocks have been penetrated by various, syntectonic, late and post-tectonic
plutonic intrusions, all the while being metamorphosed at varied degrees from greenschist to granulite
facies (Easton, 1991). The CMB is further subdivided from east to west into two parts, terranes;
Frontenac terrane, Elzevir terrane, and Bancroft terrane, and the Central Metasedimentary Belt boundary
zone (CMBBZ) shown in Figure 2 (Pluijim & Carlson, 1989). The terranes are characterised based on
rock assemblages, metamorphic grade, structure and geophysical characteristics (Pluijim & Carlson,
1989); also the terranes are separated by shear zones (Easton & Carr, 2009).
The Bancroft Terrane is composed of deformed carbonate metasedimentary rocks and minimal
volcanic rock along with nepheline syenites and syenites, intruding the suite of rock approximately 1270
to 1220 Ma ago (Easton, 1991). As for its rock composition, it is underlain by dolomite and calcite
marble, with some quartzite, rusty pelitic gneiss (metamorphic rock derived from fine-grained
sedimentary protolith) and quartzofeldspathic gneiss (Davidson, 1995). Within the terrane, the strike
directions of layers tend to strike northeast and have a moderate to steep dip (Easton & Carr, 2009).
The Elzevir Terrane just southeast to Bancroft is characterised by ample amounts of metavolcanic
rocks interlayered with meta-carbonates as well as fine grained quartzofeldpathic metasedimentary rocks
(Davidson, 1995); with the majority of the terrane preserved in greenschist facies (Easton, 1992).
Volcanism and sedimentation occurred between 1300 and 1250 Ma, and was succeeded by plutonism and
metamorphism between 1250 to 1230 Ma and 1130 to 1070 Ma respectively (Easton, 1991). This terrane
is subdivided into several subterranes, the Elzevir, the Maxinaw and Sharbot Lake terranes, which all
have similar rock types, structural and metamorphic history (Easton, 1991).
The Frontenac Terrane comprises no volcanic rock, but rather consists of marbles, quartzites and
quartzofeldspathic gneisses which were intruded by plutonic rocks and undergone metamorphism
approximately from 1170 to 1160 Ma (Easton, 1991). The metamorphic rocks are at granulite facies tier
(Easton, 1991). Throughout the three terranes of the CMB exists an array of 1090 to 1070 MA syenite
intrusions, which is thought to have intruded the rocks during the time in which the three terranes began
to act as a single tectonic entity (Corriveau, et al., 1990).
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Figure 2. Map of the Central Metasedimentary Belt various terranes and belts. Note the major terranes; Bancroft,
Elzevir, Sharbot Lake and Fronteac and the Metasedimentary Belt boundary Zone (MBBZ). Retrieved from
Streepey, M.M. et al. 2000.
Method
Field work was conducted on September 22, 2016. The field work was carried out to obtain the
structural orientation data on joint and fractures within bed- and cover rock; while taking note of physical
characteristics of each site, such as rock description, foliation, fossil presence and other potential causes
of fracturing. Joint measurements (Table 1) include, strike direction, dip angle and joint quality (scale
from 1 to 3, 1 being the highest and 3 being the lowest based on visibility of dip angle and width), and
were collected using a Silva Ranger compass-clinometer and recorded in a notebook, later to be entered
into a Microsoft Excel spreadsheet and Stereonet 9. The sites are, located north-east of Peterborough, and
UTM coordinates were recorded with a Garmin eTrex (Table 1). The sites were at least 2 kilometers
apart, in close proximity to the road and the measured joints were roughly 1-2m apart to minimise
sampling bias.
The software packages used for this study were Microsoft Excel, Stereonet 9, Google Earth Pro,
and Adobe Photoshop. Furthermore, Gmap4 an online tool was used to overlay Google Maps with
topographic maps. Joint measurements were imported to an Excel spreadsheet an inputted into the
Stereonet 9 software, where the data was plotted in the form of stereonets and rose diagrams. UTM
coordinates were used in Google Earth Pro to examine their location relative to one another and geologic
map was used within the program to give a detailed account of outcrop rock types. Adobe Photoshop
along with Gmap4 were used to create aerial photographs of each site along with the rose diagrams of the
joints in the given area.
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Results
A total of 65 joint fractures were measured throughout the northeastern Peterborough region. The
joint measurements can be found on table 1 and 2; also the corresponding rose diagrams for each site on
figure 3, 5, 6, 7 and 8. Most measured joints can be configured into grouped sets, defined by the
frequency of particular orientations. Rose diagrams were used to confirm joint sets within each site.
Figure 3. Map of all the sites with respect to Peterborough. Map taken from Google Earth Pro.
Site 1
This site (Figures 5) was located at UTM 18, 264356 easting and 493841 northing; the site was
located a few meters off of the road and road blasting did not seem to influence jointing. The rock found
within site 1 is pink granite rock, lacking foliations, while the general area is composed of various felsic
plutonic rocks types (e.g. granodiorite, tonalite, monzogranite, syenogranite; derived gneisses and
migmatites; Ontario Geological Survey, 1991). 20 joints measured, with the majority (80%) dipping
vertically or subvertically. En echelons offset joints are present on site (Figure 4), which can represent
tensile fractures that form within a broader deformation zone and are parallel to another. Dominate joint
sets in this site trend north-northwest (approximately 334°) and east-southeast (approximately 103°) as
shown in the rose diagram on Figure 5.
Figure 4. Photograph taken of en echelons found at site 1, evidence for tensile fractures. Rock is pink granite
without foliations.
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Site 2
Site 2 (Figure 6) was located at UTM 17, 738057 easting and 493877 northing; the site was in
close proximity to the road so road blasting could have created recent fractures in the outcrop. The rock
found within site 2 are granite gneisses with foliations, while the general area consists commonly of
undeformed felsic plutonic rocks. There were 7 joint measured (Table 1), with all being vertical or
subvertical. The measurement of foliations is 054°, 77° the strike direction and dip angle respectively.
The dominate joint set in this site trends southeast (approximately 135°) according to the rose diagram in
Figure 6.
Site 3
Site 3 (Figure 7) was located at UTM 18, 269995 easting and 493458 northing. The
measurements were taken close to the road, where road blasting may have affected the integrity of the
outcrop. The rocks in the site were biotite, fine grained amphibolite and course grained plagioclase, with
oxidized iron as well as foliations. A total of 16 joints were measured (Table 2), most being vertical or
subvertical (81%). The foliation was measured to strike at 244° with a dip angle of 70°. The joint set
found in this site trends northeast (approximately 38°).
Site 4
Site 4 (Figure 8) was located at UTM 18, 270725 easting and 494298 northing. This site is part of
the Paleozoic cover strata, comprised of limestone deposited horizontally. Endoceras and orthoceras
fossils were found in the limestone, representing a marine setting in its history. A total of 22 joints were
measured (Table 2), with all of them being vertical or subvertical. It also should be noted that there is
evidence of karst erosion of the joints. The dominate joint set at this location trends east-northeast
(approximately 77°).
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Table 1. Joint measurements for Sites 1 & 2. Joint quality describes the physical appearance of the joints, 1 being
very defined, and 3 being poor quality. Site 1 had NNW and WNW trending joint sets, and site 2 had a NNE trending
joint set, all were mostly vertical and subvertical fractures.
Site 2: UTM 17 – 738057 East - 493877 North
Joint Strike
Direction Dip Angle
Dip
Quadrant
Joint
Quality
(1-3)
1 143 84 W 1
2 166 90 W 1
3 312 90 N 1
4 30 82 E 1
5 350 90 E 1
6 312 76 N 1
7 54 77 S 1
Average 195 84
Site 1: UTM 18 – 264356 East - 493841 North
Joint Strike
Direction Dip Angle
Dip
Quadrant
Joint
Quality
(1-3)
1 332 90 E 1
2 105 82 S 1
3 337 90 E 1
4 277 90 N 1
5 280 90 N 1
6 334 90 E 1
7 318 90 E 1
8 282 90 N 3
9 283 90 N 2
10 266 90 N 1
11 266 90 N 2
12 162 70 W 1
13 154 48 W 1
14 114 30 S 1
15 158 90 W 2-3
16 104 66 S 2
17 356 90 E 3
18 10 90 E 2-3
19 76 68 S 1
20 50 90 S 3
Average 213 81
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Table 2. Joint measurements for Sites 3 & 4. Joint quality describes the physical appearance of the joints, 1 being
very defined, and 3 being poor quality. Site 3 did not have a clear joint set; site 4 had a well-defined ENE joint set.
Most joints within the joint sets were vertical or subvertical.
Site 3: UTM 18 – 269995 East - 493458 North
Joint Strike
Direction Dip Angle
Dip
Quadrant
Joint
Quality
(1-3)
1 312 90 N 1
2 250 80 N 1
3 32 74 E 1
4 24 57 E 1
5 18 90 E 1
6 46 82 S 1
7 309 36 N 1
8 49 84 S 1
9 68 72 S 1
10 130 78 S 1
11 30 80 E 1
12 52 90 S 1
13 294 90 N 1
14 160 36 W 1
15 150 74 W 1
16 110 90 S 1
Average 127 75
Site 4: UTM 18 – 270725 East - 494298 North
Joint Strike
Direction Dip Angle
Dip
Quadrant
Joint
Quality
(1-3)
1 22 82 E 1
2 308 90 N 1
3 324 90 E 1
4 292 90 N 2
5 252 90 N 2
6 140 90 W 2
7 248 90 N 1
8 200 90 W 1
9 70 90 S 1
10 64 90 S 1
11 78 90 S 1
12 150 90 W 2
13 188 90 W 2
14 74 90 S 2
15 162 90 W 2
16 258 90 N 1
17 152 90 W 1
18 72 90 S 2
19 181 90 W 3
20 136 90 W 3
21 64 90 S 2
22 111 90 S 3
Average 161 89
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Figure 5. Aerial photograph of site 1 along with the rose diagram of the joints measured, number of joints
measured was 20. Site contains pink granite lacking foliations and presence of a few en echelon.
Figure 6. Aerial photograph of site 2with rose diagram of joint measurements, 7 joints were measured in total. Site
contains granite, gneiss with foliations derived from igneous rock. Joints measured in close proximity to the road,
meaning road blasting may have been the cause of some joints. Foliation was also measured to be 054/ 77°.
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Figure 7. Aerial photograph of site 3with rose diagram of 16 measured joints. Site contains many varying rock
types; biotite, amphibolite and plagioclaise (both fine and coarse grained). Foliation was present along with some
oxidized, rusty magnesium iron. Foliation was measured to be 244 / 70°.
Figure 8. Aerial photograph of site 4 with rose diagram of 22 measured joints. Site contained grey/beige Paleozoic
limestone with horizontal bedding. Limestone joints showed signs of dissolving, possible large scale karst
topography. Contains plethora of fossils, endoceras and orthoceras.
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Discussion
As presented earlier, the trends of each joint set are the following; NNW (approximately 334°),
ESE (approximately 103°), SE (approximately 135°), NE (approximately 38°) and ENE (approximately
77°). The joint sets seem to be locally developed since they are present only within their corresponding
sites. However, these findings are consistent with similar large-scale joint orientation trajectory studies in
the region (Figure 9) (Andjelkovic, et al., 1997; Andejelkovic & Cruden, 2005) and support the notion
that these joint sets follow a regional pattern (Figure 10). The jointing is found throughout the
Precambrian basement and Paleozoic cover rock and it is suggested that these are related (Andjelkovic, et
al., 1997). Furthermore, because of the complex history of uplift and tectonic activity, it can be suggested
that some joint sets occurring in the Precambrian formed before the Ordovician sedimentation and some
Paleozoic joint sets were inherited from pre-existing ones in the Precambrian (Andjelkovic, et al., 1997).
The vertical structure of the joints support joint formation are suggested to originating from tectonic
loading events and vertical compaction and burial diagenesis under conditions of high pore fluid pressure
(Frizzell, et al., 2011), rather than solely post-glacial rebound.
Figure 9. Rose diagram (left) of all 4 sites of this study shows similar trends to the regional (from Madoc township
to Westport village) rose diagram (right) done by Andejelkovic (1997).
Figure 10. Diagram showing joint trajectories in the Precambrian-Paleozoic Contact vicinity. Note the NNW, ESE,
SE, ENE, and NE trending sets. Taken from Andjelkovic, et al., 1997.
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The NNW, ESE, NE, SE and ENE joint sets can be categorized by time of creation (Mitchell,
2007). NNW directed joints formed during the Alleghanian deformation occurring around 300 to 250
million years ago (Figure 11). ESE-directed extension caused rifting of Pangea (around 200 million years
ago) which led to normal fault formation by the Central Metasedimentary Belt Boundary thrust zone and
finally resulting in the ESE joint set (Mitchell, 2007). NE and ENE joint sets are neotectonic in origin,
formed during the current tectonic stress regime attributed to the mid-Atlantic ridge extension (from the
World Stress Map (Heidbach, et al., 2016) interpreted by Andjelkovic (2005) and Mitchell (2007) (Figure
12)). The SE set likely formed from tectonic loading events, such as extension stress, in the foreland of
the Appalachian orogeny (480 million years ago) (Andjelkovic & Cruden, 2005). With all that
considered, mid-Atlantic ridge extension is the mechanism that is currently active and may influence the
investigated area’s tectonics; more research should be done regarding the impact this has in the area.
Figure 11. Series of diagrams showing various time periods and their associated major tectonic stresses. Note t2-3
and t6; t2-3 shows the major jointing caused by the Alleghanian deformation, and t6 shows the current stresses and
the joints sets associated with them. Taken from Mitchel, 2007.
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Figure 12. The tectonic activity in the area of study, thought to be caused by the extension of the Atlantic Ocean.
Taken from The World Stress Map (Heidibach, O., et al., 2016).
Conclusion
Within the 4 sites 5 joint sets were found; NNW, ESE, NE, SE and ENE. The NE and ESE joint
sets carry implications of active tectonics because of the active extending Atlantic ridge, and should be
studied to further understand its influence on the region’s seismicity. The NNW, ENE and SE joint sets
were formed prior to the Wisconsin Glaciation and their causes are not explicitly presently active;
although reactivation of basement faults that caused these joints are also important to consider when
examining seismicity in the area.
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