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Canada INFO-0597

Canada INFO-0597

INFO 0597 CA9600098

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MOL Atomic Energy Commission de contrôle Control Board de l'énergie atomique INFO-0597

Faulting in Unconsolidated Sediments and Bedrock East of — Phase 1

by

C. Rogojina I A.A. Mohajerand N. Eyles Seismican Geophysical Limited I 1 I Prepared for I the Atomic Energy Control Board under its Regulatory Research and Support Program I , Canada

I AECB Project No. 2.263. I October 1995 I I Atomic Energy Commission de contrôle 1*1 Control Board de l'énergie atomique Canada I | NEXT PAGE(S) left BLANK FAULTING IN UNCONSOLIDATED SEDIMENTS AND BEDROCK EAST OF TORONTO — PHASE 1

A report prepared by C. Rogojina, A.A. Mohajer and N. Eyles, Seismican Geophysical Limited, under contract to the Atomic Energy Control Board.

ABSTRACT

Increasing concern with the potential hazard in southern has focused attention on neotectonic structures affecting bedrock. Within the boundaries of the area (632 km2), about 2500 fracture orientations have been measured in more than 70 bedrock outcrops. An east-northeast systematic fracture set and an abutting orthogonal truncated fracture set constitute the most commonly-oriented fundamental fracture system in the study area. The east-northeast systematic fracture set may be the product of the current compressive stress field combined with regional uplift, but this should be confirmed by further field investigation. Anomalous fracture patterns were identified at the periphery of Metropolitan Toronto, specifically along the West Humber and Rouge rivers. Four post- glacial pop-ups were identified within Metro Toronto. Careful mapping and description of these pop-ups show a possible relationship with the contemporary principal stresses in the area and the local fracture pattern.

RESUME

Les préoccupations croissantes que soulève le danger de tremblements de terre dans le sud de l'Ontario ont attiré l'attention sur les structures néotectoniques qui affectent le substratum rocheux. Dans la région métropolitaine de Toronto (632 km2), on a mesuré environ 2 500 orientations de fractures dans plus de 70 affleurements de substratum rocheux. Un ensemble systématique de fractures est-nord-est et un ensemble adjacent de fractures tronquées orthogonales constituent le réseau fondamental de fractures à l'orientation la plus commune dans la zone d'étude. L'ensemble systématique de fractures est-nord-est peut avoir été produit par le champ actuel de contraintes de compression combiné à un soulèvement régional, mais il faudrait le confirmer par d'autres analyses^sur place. Des configurations anormales de fractures ont été repérées à la périphérie de la région métropolitaine de Toronto, plus précisément le long des rivières West Humber et Rouge. On a repéré quatre structures de soulèvement postglaciales dans la région métropolitaine de Toronto. Une description et une cartographie minutieuses de ces structures de soulèvement indiquent un lien possible avec les principales contraintes contemporaines dans la région et la configuration locale des fractures.

DISCLAIMER

The Atomic Energy Control Board is not responsible for the accuracy of the statements made or opinions expressed in this publication and neither the Board nor the authors assume liability with respect to any damage or loss incurred as a result of the use made of the information contained in this publication.

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"ftFYTPAGE(S)lëftBLANK, TABLE OF CONTENTS

ABSTRACT iii

1.0 INTRODUCTION 1

2.0 GEOLOGICAL AND TECTONIC SETTING OF THE STUDY AREA 2

3.0 BEDROCK FRACTURES ACROSS METROPOLITAN TORONTO 3

3.1 Methods of Investigation and Data Analysis 3

3.2 Terminology 3

3.3 Description of Fractures 4

3.4 Relationship of Fracture System in Metropolitan Toronto to Regional

Fracture Systems in 4

4.0 POP-UPS, MONOCLINES AND FOLDS 5

4.1 Locations 5

4.2 Structural Features and Age of Pop-ups 6

5.0 CONCLUSIONS 7

REFERENCES CITED 8

FIGURE CAPTIONS 12 TERMS OF REFERENCE:

Newly found faults in the unconsolidated sediments and the underlying bedrock were reported along the valley, near Pickering, east of Metro Toronto (Mohajer, et al., 1992). This finding received considerable attention because of concerns that the faults may have potential implications for assessment at the site of the Pickering nuclear power plant, located less than 7 km to the east. The scope of the present work is to continue the search for, and to document the presence of, faults in the young sedimentary deposits in Metro Toronto. This work was supported by the Atomic Energy Control Board under contract No. 2.263.1 dated September 17, 1992.

1.0 INTRODUCTION

In the last two decades, increasing awareness of quasi-continuous crustal response to compressional stresses generated by plate motions has focused attention on brittle and semi-brittle structures at shallow depths. In eastern , Sbar and Sykes (1973) have shown that high regional compressive stress commonly controls the reactivation of unhealed old faults and their associated modern seismicity. In , Sanford et al. (1985) found that plate movements and their epeirogenic responses (arch and basin development) were intermittently active throughout much of time and that some segments of the fault-bounded megablocks might be still technically and seismically active.

Southern Ontario is underlain by complexly-structured mid-Proterozoic crystalline . Several discrete structural zones and associated aeromagnetic and gravimetric anomalies pass under Metropolitan Toronto and the surrounding area (Wallach and Mohajer, 1990). The most prominent structure is the Central Metasedimentary Belt Boundary Zone (CMBBZ) (Ontario Geological Survey, 1991a, b; Milkereit et al.,1992). Geological and geophysical data suggest a history of intermittent Phanerozoic reactivation (e.g. Sanford et al., 1985; Eyles et al., 1993). Unfortunately, relatively thick glacigenic sediments blanket wide areas of the Paleozoic rocks in southern Ontario, so that any neotectonic structures, if present, are either buried or confused with glaciotectonic or sedimentary structures. There is a critical need for studies of near-surface tectonic features that may provide valuable information on the pattern of contemporary stress, megablock boundaries and shallow expressions of deep reactivated faults and fractures (Saull and Williams, 1974; Sanford et al., 1985; Mollard, 1988; Wallach and Mohajer, 1990).

The present study reports on deformation (fractures, pop-ups) in bedrock in metropolitan Toronto. A systematic regional fracture set, oriented 070°, has been identified, and is most likely related to anisotropic horizontal compressive stresses (Gross and Engelder, 1992). Fractures and pop-ups have also been documented in the Paleozoic sequence between and outcropping rocks (White and Russell, 1982; Daniels, 1990; Rutty and Cruden, in press); the latter are indicative of contemporary high crustal stresses and some members of the former may be as .

2.0 GEOLOGICAL AND TECTONIC SETTING OF THE STUDY AREA

The of southern Ontario consists of Lower Paleozoic bedrock and Quaternary strata covering the Precambrian basement. Precambrian metamorphic and igneous rocks, exposed in the northern part of the , comprise part of the Grenville Province of the . Within the Grenville Province, the Central Belt and the Central Metasedimentary Belt are separated by the Central Metasedimentary Belt Boundary Zone (CMBBZ) that extends southward into the study area under a cover of Paleozoic and Quaternary strata (Fig. 1).

The CMBBZ has been interpreted as a complex shear zone, with a dominant history of northwestward thrusting (Hanmer, 1988). A fault, coincident with the southward extension of the CMBBZ (Ontario Geological Survey, 1991a), is superimposed on the Niagara Pickering Linear Zone (NPLZ), which consists of prominant linear aeromagnetic and gravimetric anomalies (Wallach and Mohajer, 1990) (Fig. 1). This structure continues to the south, beneath western Lake Ontario and eastern (Milkereit et al., 1992). A fault and geophysical lineament along the median of the western part of the Lake Ontario (Ontario Geological Survey, 1991a; Mohajer et al., 1992) (Fig. 1) may belong to the southwestward extension of the St. Lawrence rift system (Kumarapeli and Saull, 1966; Adams and Basham, 1989). Other lineaments, such as the Linear Zone (Wallach, 1990; Wallach and Mohajer, 1990) and the Toronto Hamilton Seismic Zone (Mohajer et al., 1992), might also be associated with faults.

The Algonquin Arch and Findlay Arch separate the and Appalachian Basins. Arch rejuvenation took place on, and marginal to, a northeast-southwest axis during Paleozoic time and was expressed as fault-block readjustments in southwestern Ontario (Sanford et al., 1985). Concentrations of recorded along the Georgian Bay Linear Zone and Toronto Hamilton Seismic Zone (Wallach, 1990; Mohajer et al., 1992), and the presence of pop-ups in areas adjacent to the NPLZ (Rutty and Cruden, in press), suggest that tectonic activity is continuing in the region encompassing the study area.

The Paleozoic sedimentary cover commences with Lower and Middle Ordovician strata (along the boundary with the Grenville Province), and thickens toward the southwest (Fig. 1; Liberty, 1969). A thick and extensive Quaternary cover is represented by glacial sediments deposited during the last glacial-interglacial cycle spanning approximately the last 100,000 years (Karrow, 1984). In the Metropolitan Toronto area, along the main river valleys and lakeshore, bedrock is composed of Late Ordovician shales (Whitby, Georgian Bay Formations) that dip to the southwest at about 5 m/km (Karrow, 1967; Liberty, 1969). Shales contain thin (<30cm) sandstone beds (Kerr and Eyles, 1991).

3.0 BEDROCK FRACTURES ACROSS METROPOLITAN TORONTO

3.1 Methods of Investigation and Data Analysis

The strikes and dips of more than 2500 bedrock fractures were measured and observations were made of cross-cutting relationships and vein-filling material (if any). At any one site, the minimum number of fracture measurements, deemed to be statistically significant, varies between 50 and 500 (Babcock, 1973; Hoist and Foote, 1981; Stauffer and Gendzwill, 1986; etc.). In this study, the number of measurements at an outcrop varied between 15 and 250; about 70 exposures were investigated.

Since all of the fractures identified in this study are vertical, fracture data have been processed as strike frequency distributions (e.g. Babcock, 1973; Daniels, 1990; Gross and Engelder, 1991). Rose diagrams at 10° intervals were chosen for a better visual impact. Bi-directional rose diagrams of the bedrock fractures have been obtained using an orientation analysis application program (Vector Rose 1.0 developed by P.A. Zippi, ).

3.2 Terminology

The use of the composite term "neotectonic fractures" requires a brief discussion of terminology. Hancock (1988) defined neotectonics as "the study of the movements of late Cenozoic age, especially those in harmony with contemporary horizontal and vertical crustal motions". "Neotectonic fractures" is a term used by Hancock and Engelder (1989) to describe all fractures which evolved in the late Cenozoic stress field.

Groups of parallel and subparallel oriented fractures, locally found as en-echelon arrays, are widely known as fracture sets (e.g. Terzaghi, 1965; Mollard, 1988). Two or more sets displaying consistent angular relationships over large areas are called systems (e.g. Mollard, 1988) and an orthogonal fracture system, characteristic of flat-layered rocks, is composed of three mutually perpendicular fracture sets, two vertical and one horizontal and parallel to the bedding surfaces (Fig. 2a). A regional fracture set is a set characterised by uniform orientations over large areas (Terzaghi, 1965; Nickelsen and Hough, 1967; Lorenz et al., 1991). Where fractures show a crossing relationship, the qualitative descriptive scheme of Hodgson (1961) has been employed (Fig. 2a). This scheme recognises two major groups, systematic and non-systematic. Systematic fractures refer to those fractures grouped in a set that across fractures of other sets. Non-systematic fractures, developed between two systematic fractures, are characterized by irregular fracture traces and a criss-crossing relationship among themselves. Hodgson's (1961) "cross fractures", renamed "truncatedfractures " by Nickelsen and Hough (1967), are a distinct variety of non-systematic fractures. Truncated fractures cross the interval between two systematic fractures, but terminate at the systematic fractures. The normal relationship between a systematic and a non-systematic fracture set defines a particular fracture system that was named "fundamentalfracture system " by Nickelsen and Hough (1967).

3.3 Description of Fractures

Fractures in bedrock (Fig. 3) display a well-developed orthogonal system which occurs throughout the Metropolitan Toronto area. This fracture system is composed of a systematic fracture set and a truncated fracture set (Fig. 3). The systematic fractures are predominantly vertical, show an average orientation of 070° (Fig. 3), and are spaced from a few centimeters to a few meters apart. The members of the truncated fracture set occur normal to the systematic fractures and have irregular trajectories, lengths and strikes. Irregular lengths are a consequence of the inconsistent spacing of systematic fractures. The truncated fracture set strikes about 160° throughout most of the Metropolitan Toronto area, perpendicular to the systematic fracture set. Most of the fracture planes lack any kind of shearing marks and generally do not display remnants of plumose structures. Films of iron oxides and calcite commonly obliterate any mechanical marks on fracture surfaces.

Fractures exposed along the West , in the northwestern part of the study area, deviate from the regional pattern identified above (Fig. 4). The fracture pattern along the Humber would also be classified as a fundamental fracture system in that it comprises a systematic fracture set at 020° and a truncated fracture set at 110°. Fractures having an azimuth of 070° cut across both fracture sets of the fundamental fracture system (Figs. 2, 3). A second area, where the fracture orientation deviates from the regional pattern, was identified in the eastern part of the Metropolitan Toronto area, in bedrock outcrops along the Rouge River and the Little Rouge Creek valleys (Fig. 4). There, the fundamental fracture system is composed of a systematic fracture set striking 135° and a truncated fracture set with an azimuth of 045°. An additional fracture set, composed of fractures with an average orientation of 100°, crosses both sets of the fundamental fracture system.

3.4. Relationship of Fracture System in Metropolitan Toronto to Regional Fracture Systems in Southern Ontario

There are few published data on fractures in southern Ontario, but many from State (Nickelsen and Hough, 1967; Fakundiny et al., 1978; Engelder, 1982; Hancock and Engelder, 1989). Most fracture studies in southern Ontario have been concentrated in the Niagara Peninsula where bedrock is widely exposed (Scheidegger, 1977; Williams, et al., 1985; Daniels, 1991). Scheidegger (1977) identified two fracture sets with average trends of 085° and 180° and Williams et al. (1985) documented four sets with azimuths at 005°, 045°, 085° and 135°. In the same area, Daniels (1991) identified two major fracture sets trending towards 045° and 315°, along with the presence of two additional fracture sets oriented north-south and east-west. Along a segment of the , Gross and Engelder (1991) measured two fracture sets oriented 060° and 100°.

Throughout the region there is a well-defined east-northeast trending regional fracture set across southern Ontario. Scheidegger (1977) recognized the existence of a dominant fracture pattern across south- defined by orthogonal fractures. Daniels (1991) studied outcrops from Hamilton to Pickering and reported a dominant northwest- trending fracture direction, with many northeast-trending fractures. This fracture pattern has been identified over wide areas of the Appalachian Plateau of New York and other areas of the northern United States (Engelder, 1982; Gross and Engelder, 1991). The east- northeast trending fracture set, identified by Scheidegger (1977), Williams et al. (1985) and Daniels (1991), as well as the systematic fracture set characteristic of the Metropolitan Toronto area (Fig. 4), appear to be part of the same fracture system.

4.0. POP-UPS, MONOCLINES AND FOLDS

During the course of this investigation four pop-ups or bedrock warps were identified in the Metropolitan Toronto . There is an increasing interest in bedrock pop-ups, which are considered to be indicative of high horizontal stress and present-day seismicity (e.g. Fakundiny et al., 1978; Wallach, 1990; Wallach et al., 1993). However, besides brief references given by White and Russell (1982) and Rutty and Cruden (in press), no systematic documentation regarding pop-ups in the Metropolitan Toronto area is available.

4.1. Locations

1) The right bank of , 700 m north of Bloor Street, on the property of the Markland Wood Country Club, exposes a pop-up about 1 m high (Fig. 5, location 1; Fig. 6A). The axis of the structure is buried under 0.5 m-thick coarse alluvial deposits and has an azimuth direction of 092°.

2) , 1500 m upstream from the Queen Elizabeth Highway behind the Etobicoke School of the Arts, exposes a monocline in the bedrock (Fig. 5, location 2) with a fold axis trend of 064°. The structure is associated with small reverse faults (oriented from left to right, Fig. 6B, 141766°N, 125750°N, 139737°N, and 118°/10°N) that contain a thick (up to 10 cm), hard dark grey matrix and angular shale fragments. Delicate polished surfaces within the gouge suggest oblique-slip displacements with small apparent offsets.

3) A well-defined bedrock pop-up is located in the bed of the Humber River, 30 m upstream from (Fig 5, location 3). It occurs as a 30-40 cm high ridge with an axial orientation of 126°. The ridge is characterized by a zigzag fracture trace generated by alternations of fracture-bounded slabs dipping 10-20° in opposite directions away from the ridge crest (Fig. 7). Slabs are bounded by fractures belonging to the orthogonal sets oriented 070° and 160° trending sets, respectively (Fig. 5).

4) The Rouge River, about 1000 m upstream from its confluence with the Little Rouge Creek, shows an overturned fold and bedrock warp in the southern bank (Fig. 5, location 4; Fig. 6C). This structure deforms overlying Pleistocene sediments of the last interglacial age and is characterized by an axial orientation of 125°. The structure is associated with small shear fractures and faults (Fig. 6c), which converge about 80 cm below the bedrock surface. A vertical, 3-cm-thick dyke fills a bedrock fissure having an orientation of 110°. In the surrounding undeformed shales, the fracture system is represented by systematic fractures trending 125° and a truncated fracture set trending 010°).

Orientations of the principal structural elements of all pop-ups and monocline folds located during this study are shown in Figs. 5a and b.

4.2 Structural Features and Age of Pop-ups

Data from Ontario suggest that at least some pop-ups formed after the last glaciation, which ended c. 13,000 years ago (e. g. White and Russell, 1982; Wallach, 1990). In the study area, pop-ups either buckle up and offset overlying Pleistocene sediments (location 4, Fig. 5 and 6C), are covered with recent alluvial material (locations 1 and 2, Figs. 5 and 6A and 6B) or are freshly exposed (location 3, Figs. 5 and 7).

It is generally accepted that bedrock buckles (pop-ups) are triggered by near-surface contemporary horizontal compressive stresses (White et al., 1973; Saull and Williams, 1974; Fakundiny et al., 1978; Lo, 1978; Asmis and Lee, 1980; Adams, 1982; Wallach, 1990). Another important pre-condition for pop-up formation is the existence of horizontal planes of weakness such as bedding planes (Lo, 1978; Wallach, 1990, Wallach, etal., 1993).

According to Fakundiny et al. (1978), pop-ups can be classified as simple or complex types. A simple pop-up is defined as a 'tent-like' anticlinal structure in surface strata. In contrast, complex bedrock pop-ups are associated with faults. Both types of pop-ups are most commonly controlled by pre-existing fractures (White et al., 1973; Fakundiny et al., 1978; White and Russell, 1982). This observation is supported by the present study from Metropolitan Toronto.

The axial orientation of the pop-up at location 3 (092°) is sub-parallel to the trend of surrounding systematic fractures (070°). The ridge shows the distinct zigzag trace of a stepped fracture developed between tilted, fracture bounded slabs which dip away from the crest of the ridge (Fig. 7). At location 1 (Fig. 5 and Fig. 6A) the axial fracture is oriented at 067° and belongs to the systematic fracture set observed in surrounding shales, clearly indicating that pop-up development was controlled by pre-existing systematic fractures.

Other reported examples of a similar relationship between pop-ups and associated bedrock fracture patterns can be cited from the literature. At Milton, Ontario, a clear relationship between the strike of the pop-up crests and the strike of fractures is reported by White et al., (1973). Descriptions and photographs of bedrock pop-ups from the Thousands Islands area, New York State (Cushing et al., 1910 in Sbar and Sykes, 1973), indicate a similar fracture-controlled origin.

The complex pop-ups in metropolitan Toronto (e.g. Mimico Creek, Rouge River) show no simple relationship between axial trend and the systematic fracture set oriented at (070°). In the Rouge Valley the presence of the anomalous fracture domain (see above), the pop-up and surface faulting (Mohajer et al., 1992) require further detailed field investigations in order to determine whether or not there is a cause-effect relationship. It should be noted, in addition, that Wallach et al. (1993) suggested that both pop-ups and the fault movements responsible for earthquakes are kinematically congruent, therefore, evidence of stress application on the surface implies stress application at depth. They further concluded that the presence of pop-ups in an area should be considered as indicating the potential for moderate to large earthquakes to occur in that area.

5.0 CONCLUSIONS

In Late Ordovician shale bedrock of the Metropolitan Toronto area, a fundamental fracture system can be identified. This is composed of a systematic vertical fracture set oriented 070° and an abutting perpendicular fracture set. The systematic fracture set belongs to a regional fracture set that is present across a large area of eastern North America (e.g. Engelder, 1982; Hancock and Engelder, 1989; Gross and Engelder, 1991).

Along the eastern and western boundaries of Metropolitan Toronto, in the valleys of the Humber and Rouge rivers, the bedrock fracture pattern is characterized by a marked shift in the trend of the fundamental fracture system. The reason for this change is not clear and is currently being investigated. The change in the fracture orientation may be related to basement structures below the Paleozoic cover.

Bedrock buckles (pop-ups) in Metropolitan Toronto, can be classified into simple and complex types and appear to be largely controlled by the preexisting orthogonal fracture pattern. Since, some authors regard pop-ups as indicating the epicentral area of major earthquakes (e.g. Wallach et al., 1993) further detailed investigations are warranted.

REFERENCES CITED

Adams, J., 1982. Stress-relief buckles in the McFarland quarry, Ottawa. Canadian Journal of Earth Sciences, 19, p. 1883-1887.

Adams, J., and Basham, P., 1989. The seismicity and seismotectonics of Canada East of the Cordillera. Geoscience Canada,16,l, p. 3-16.

Asmis, H.W., and Lee, CF., 1980. Mechanistic modes of stress accumulation and relief in Ontario rocks. In Underground engineering, C. M. Special Volume, 22, p. 51- 58.

Babcock, E. A., 1973. Regional jointing in southern Alberta. Canadian Journal of Earth Sciences, 10, 1769-1781.

Daniels, S. D.,1990. Regional jointing patterns within the surficial glacial sediments and bedrock of South-central Ontario. M. Se. Thesis, Mc.Master University, Hamilton.

Engelder, T., 1982. Is there a genetic relationship between selected regional joints and contemporary stress within the lithosphère of North America ? Tectonics, 1, 2, p. 161-177.

Eyles, N., Boyce, J., and Mohajer, A.A., 1993. The bedrock surface of the western Lake Ontario region: evidence of the reactivated basement structures? Géographie physique et Quaternaire, in press.

Fakundiny, R. H., Myers, J. T., Pomeroy, P. W., Pferd, J. W., and Nowak, T.A., 1978. Structural instability features in the vicinity of the Clarendon-Linden fault system, western New York and Lake Ontario. In Advances in analysis of geotechnical instabilities, University of Waterloo Press, SM Study No.13, Paper 4, p.121-178.

Gross M. R., and Engelder, T., 1991. A case for neotectonic joints along the Niagara Escarpment. Tectonics, 10, 3, p. 631-641.

Hancock, P. L., 1988. Neotectonics. Geology Today, 2, p. 57-61.

Hancock, P. L., and Engelder, T., 1989. Neotectonic joints. Geological Society of America Bulletin, 101, p. 1197-1208. Hanmer, S., 1988. Ductile thrusting at mid-crustal level, southwestern Grenville Province. Canadian Journal of Earth Sciences, 25, p. 1049-1059.

Hodgson, R. A., 1961. Regional study of jointing in Comb Ridge-Navajo area, Arizona and Utah. American Association of Petroleum Geologists Bulletin, 45, 1, p. 1-38.

Hoist, T. B., and Foote, H.R., 1981. Joint orientation in rocks in the northern portion of the lower peninsula of Michigan. Geological Society of America Bulletin, 1, 92, p. 85-93.

Karrow, P. F.,1967. Pleistocene geology of the Scarborough area. Ontario Department of Mines, Geological Report, 46,108 p.

Kerr, M. and Eyles, N., 1991. Storm-deposited (tempestites) and related ichnofossils of the Late Ordovician Georgian Bay Formation, southern Ontario, Canada. Canadian Journal of Earth Sciences, 28, p. 266-282.

Kumarapeli, P. S. and Saull, V. A., 1966. The St. Lawrence valley system: a North American equivalent of the East African rift valley system. Canadian Journal of Earth Sciences, 3, p. 639-657.

LaPointe, P. R., and Hudson, J. A., 1985. Characterization and interpretation of rock mass joint patterns. Geological Society of America Special Paper, 199, 37p.

Liberty, B. A., 1969. Palaeozoic geology of the area. Geological Survey of Canada, Memoir 355,173p.

Lo, K.Y., 1978. Regional distribution of/// situ horizontal stresses in rocks of Southern Ontario. Canadian Geotechnical Journal, 15, p. 371- 381.

Lorenz, J. C, Teufel, L. W., and Warpinski, N. R., 1991. Regional fractures I: A mechanism for the formation of regional fractures at depth in flat-lying reservoirs. American Association of Petroleum Geologists Bulletin, 75, 11, p. 1714-1734.

Milkereit, B., Forsyth, D. A., Green, A. G., Davidson, A., Hanmer, S., Hutchinson, D. R., Hinze, W. J., and Mereu, R. F., 1992. Seismic images of a Grenvillan terrane boundary. Geology, 20, p. 1027-1030.

Mohajer, A., Eyles, N., and Rogojina, C.,1992. Neotectonic faulting in metropolitan Toronto; Implications for earthquake hazard assessment in the Lake Ontario region. Geology, 20, p. 1003-1006. I

Mollard, J. D., 1988. First R. M. Hardy Memorial Lecture: Fracture lineament research I and applications on the western Canadian plains. Canadian Geotechnical Journal, 25, • p. 749-767. f

Nickelsen, R. P., and Hough, V. N. H., 1967. Jointing in the Appalachian Plateau of g Pennsylvania. Geological Society of America Bulletin, 78, p. 609-630. |

Ontario Geological Survey, 1991. Bedrock , southern sheet. Ontario I Geological Survey, Map 2544, scale 1:1 000 000. I

Ontario Geological Survey, 1991. Bedrock geology of Ontario, explanatory notes and I legend. Ontario Geological Survey, Map 2545. •

Rutty, A. L., and Cruden, A. R. (in press). Neotectonic stress release features and the I basement fracture pattern in the area. Géographie physique et • Quaternaire.

Saull, V. A., and Williams, D. A., 1974. Evidence for recent deformation in the Montreal Area. Canadian Journal of Earth Sciences, 11, p. 1625-1624. .

Sanford, B.V., Thompson, F. J., and McFall, G. H., 1985. Plate tectonics- a possible controlling mechanism in the development of hydrocarbon traps in southwestern i Ontario. Bulletin of Canadian Petroleum Geology, 33, 1, p. 52-71. |

Sbar, M. L., and Sykes, L. R., 1973. Contemporary compressive stress and seismicity in eastern North America: An example of intra- plate tectonics. Geological Society of America Bulletin, 84, p. 1861-1882.

Scheidegger, A. E., 1977. Joints in Ontario, Canada. Revista Italiana de Geofïsica, 4, 1/2, p. 1-10.

Siauffer, M. R., and Gendzwill, D. J., 1987. Fractures in the northern plains, stream patterns, and the midcontinent stress field. Canadian Journal of Earth Sciences, 24, p. 1086-1097.

Terzaghi, R. D., 1965. Sources of error in joint surveys. Geotechnique, 15, p.287-304.

Wallach, J. L.,1990. Newly discovered geological structures and their potential impact on Darlington and Pickering. Report, AECB INFO-0342., Ottawa. 20 p.

Wallach, J. L., McFall, G. M., Bowlby, J. R., and Mohajer, A. A., 1992. Pop-ups and seismic hazard in intraplate eastern North America (abs). In Morner,N-A., Owen, L. A., Stewart, I. and Vita-Finzi, C. (eds), Neotectonics-Recent Advances: Abstract Volume. Quaternary Research Association, Cambridge, p.74.

10 Wallach, J. L., and Mohajer, A. A., 1990. Integrated geoscientific data relevant to assessing seismic hazard in the vicinity of the Darlington and Pickering nuclear power plants. Proceedings, Canadian Geotechnical Conference, oct, 10-12 1990, City. p. 679-686.

Wallach, J.L., A.A. Mohajer, G.H. McFall, J.R. Bowlby, M. Pearce, and D.A. McKay, 1993. Pop-ups as geological indicators of earthquake-prone areas in intraplate eastern North America, in Quaternary Proceedings.

White, 0. L., Karrow, P. F., and Mcdonald, J. R., 1973. Residual stress relief phenomena in southern Ontario. //; Proceedings, 9tn Canadian Rock Mechanics Symposium, Montreal, p. 323-348.

White, O.L., and Russell, D. J., 1982. High horizontal stresses in southern Ontario-their orientation and their origin, hi Proceedings, 4tn Congress, International Association of , New Delhi, 5, p. 39-54.

Williams, H. R., Corkery, D., and Lorek, E. G., 1985. A study of joints and stress-release buckles in Paleozoic rocks of the Niagara Peninsula, southern Ontario. Canadian Journal of Earth Sciences, 22, p. 296-300.

11 Figure 1. Simplified bedrock geology, major structures and geophysical lineaments of southern Ontario (after compilations by Wallach and Mohajer, 1990; Wallach, 1990; Ontario Geological Survey, 1991 a, b; Mohajer et al., 1992). Abbreviations: GBLZ = Georgian Bay Linear Zone; NPLZ = Niagara Pickering Linear Zone; THSZ = Toronto Hamilton Structural Zone; OML = Oshawa Magnetic Lineament. Inset: location of study area in the region.

Figure 2. Definitions and terminology, a) Orthogonal fracture system constituted of two vertical and a bedding-parallel horizontal fracture sets. The systematic fractures and abutting perpendicular truncated fractures are grouped in two fracture sets that belong to a fundamental fracture system (terminology and definitions after Hodgson, 1961; Nickelsen and Hough, 1967). Lithology and fracture relationships are characteristic for the bedrock in the Metropolitan Toronto area. Observe coexistence of two systematic fracture sets confined within the thicker /siltstone bed. b) Fracture mechanics terminology (modified from Pollard and Aydin, 1988). Arrows show directions of displacement. Modes of displacement (I, II or III) are defined relative to the fracture surface and fracture front orientation.

Figure 3. Mean strike of bedrock fracture sets in the study area. Bedrock blocks exemplify the types of fracture trace relationships in the Metropolitan Toronto area (MT) and anomalous domains West Humber (WH) and Rouge (R) (for simplicity, horizontal fractures parallel with bedding are incompletely represented).

12 Figure 4. Bedrock fracture patterns in the Metropolitan Toronto area (MT) and anomalous domains West Humber (WH) and Rouge (R) (interpretation of the fracture set orientations from Fig. 3 and crisscrossing relationships). Sets a (systematic) and b (truncated) constitute a fundamental fracture system.

Figure 5. Pop-up locations in the study area and relationships between main tectonic features. Insets: a) all ridge (dashed line) and buckle axis (continuous line) orientations; b) stereographic projection of all fault plane orientations (lower hemisphere equal area projections).

Figure 6. Simple (A) and complex (B and C) pop-up structures in vertical sections. A = location 1, Etobicoke Cr.; B = location 2, Mimico Cr.; C = location 4, Rouge River ( location map at Fig. 7).

Figure 7. Schematic block-diagram of a simple pop-up on the Humber River (location 3, fig. 5).

Figure 8. Modern drainage pattern and location of pop-ups.

13 /////////////

LAKE LAKE ONTARIO HURON Niagara Falls

STRATIGRAPHY Upper Devonian Middle Devonian Upper Middle and Lower Silurian Structural Geophysical lineaments Upper Ordovician arches a.Blue Mountain Form. b.Georgian Bay and Findley GBLZ Queenston Form. NPLZ Igonquin Middle Ordovician THSZ OML Precambrian

Figure 1 fundamental joint system

joint front orientation

Mode 11 (shearing)

Mode I (opening)

Mode 111 (shearing)

sandstones/sil tstones'

shales. b

Figure 2 79°40' 79°20'

43°50

km

43°40' LEGEND

Systematic fracture set Truncated fracture set

Metropolitan Toronto

Figure 3 79°40' 79° 20'

43°50'

km

LEGEND 43°40' Systematic fracture set Truncated fracture set Mixed fracture and vein set outline of the Metropolitan Toronto study area

Figure 4 79°40' 79°20"

43°S01 r- -^=^^— —

LEGEND 4340' outline of the Metropolitan Toronto study area Orientations : • systematic fracture set 4 * truncated fracture set -• buckle axis •*• fault strike « pop-up ridge

Figure 5 I

rO

ç c 3 à •a

3 O\

CO 1 11 1 '«'I.!* ! ?,!i''l Ordov cian Pleistocene ridge axis zigzaged trace of stepped fracture mixed mode II + III shearing

mixed mode I +111 opening

potential shear and gouge zone ridge-parallel fractures i

systematic fractures

truncated fractures

cm 50-i

25- SCALE

0- 0 0.5 1.0 2.0 m

Figure 7 0 10 km N Pop-ups t Anomalous Fracture Domains

Figure 8

Top View