TORONTO — North American stock markets had their biggest one-day tumble since early February on Thursday but analysts were hard-pressed to identify a single reason for the drop.The S&P/TSX composite index in Toronto fell 194.08 points to 15,330.74, as a number of big Canadian corporations missed earnings forecasts, but the index remained up about 1,736 points since the beginning of the year.New York’s Dow Jones industrials plunged 317.06 points to 16,563.3, leaving the index below where it started the year by about a dozen points. The Nasdaq lost 93.13 points to 4,369.77 and the S&P 500 index declined 39.4 points to 1,930.67.The loonie closed down 0.02 of a cent to 91.71 cents US.The U.S. Federal Reserve indicated Wednesday that it will keep short-term interest rates low “for a considerable time” after it ends its bond purchases, likely in October. The Fed is expected to start hiking rates mid-2015, but stronger than expected economic growth in the second quarter has investors concerned that the Fed could raise rates sooner.Argentina moved into a debt default for the second time in 13 years after a deadline of midnight Wednesday night came and went without a deal with bondholders.And volume is less than usual with many market participants on holidays.But the stock market declines also come at a time when many investors have registered substantial gains. “You get thinner markets and it doesn‘t take much to move things around,” said Wes Mills, chief investment officer Scotia Private Client Group.“Clearly everyone has made good money and there is no evidence that people are taking money off the table yet. It‘s probably just an overdue correction in a thin summer market with a combination of factors.”Valeant Pharmaceuticals International Inc., which is making a hostile takeover bid for Botox maker Allergan, posted a quarterly net profit of $126 million or 37 cents a share. Adjusted income was $651 million, or $1.91 per share, missing estimates of $1.98 a share, and its shares fell $9.59 or 7% to $127.83.Barrick Gold Corp. delivered a US$269-million quarterly net loss and $159-million of adjusted earnings in the second quarter, missing analyst estimates on both counts. The adjusted profit amounted to 14 cents US per share, two cents below estimates. Barrick shares dipped 44 cents to $19.70.Suncor Energy Inc. posted net income of $211 million, or 14 cents per share, compared to $680 million, or 45 cents per share, a year earlier. Operating earnings, which strip out the effects of unusual items, came in at $1.14 billion or 77 cents a share, 20 cents below estimates and its shares were down $1.02 to $44.77.First Quantum Minerals fell 4.33% to $25.86 as copper sales increased but earnings per share came in at 23 cents, three cents short of forecasts.Going against the grain was corporate software company Open Text, up 14.5% to $60.64 as its quarterly profit doubled while revenue improved by more than 40%.And Bombardier Inc. reported its adjusted earnings were US$192 million, or 10 cents — a cent ahead of estimates and its shares climbed six cents to $3.73.The TSX energy sector was down 1.75% as September oil moved $2.10 lower to US$98.17 a barrel.The metals and mining sector fell 2.27% with September copper off a penny at US$3.23 a pound.The gold sector fell about 2.2% while December gold faded $14.10 to US$1,282.80 an ounce.